Tables: Groundwater Table Hydraulic Impact Assessments for Infiltration BMPs
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Table fragments retain their source context. Refer to the original page for layout, figures, merged headings and notes.
1. Table 1
BMP 2026, Ch 13, p. 8
| Input Values |
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| 1.000 |
R |
Recharge rate (permeability rate) (in/hr) |
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| 0.150 |
Sy |
Specific yield, Sy (dimensionless)
default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
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| 1.00 |
Kh |
Horizontal hydraulic conductivity (in/hr)
Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
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| 50.000 |
x |
1/2 length of basin (x direction, in feet) |
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| 50.000 |
y |
1/2 width of basin (y direction, in feet) |
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| 24.00 |
t |
Duration of infiltration period (hours) |
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| 10.000 |
hi(0) |
Initial thickness of saturated zone (feet) |
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Source context and notes
$$
\text{Duration of infiltration period, } t = \frac{\text{Volume of runoff to be infiltrated (cf)} \times 12 \text{ in/ft}}{\text{Infiltration area (sf)} \times \text{Recharge rate (in/hr)}} = \frac{5,000 \text{ (cf)} \times 12 \text{ in/ft}}{2,500 \text{ (sf)} \times 1 \text{ (in/hr)}} = 24 \text{ hours}
$$
Since 1 in/hr is used as the infiltration rate in the design of the BMP, the 1 in/hr is also used as the recharge rate in the Hantush Spreadsheet. Furthermore, assuming the basin is not in the Coastal Plain, the horizontal hydraulic conductivity rate also uses the value of recharge rate as 1 in/hr by default. The inputs for the Hantush Spreadsheet are thus based on the above information and are shown below:
### Input Section
| Input Values | | | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 1.000 | R | Recharge rate (permeability rate) (in/hr) | | | | | | |
| 0.150 | Sy | Specific yield, Sy (dimensionless)
default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | | | | |
| 1.00 | Kh | Horizontal hydraulic conductivity (in/hr)
Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | | | | | |
| 50.000 | x | 1/2 length of basin (x direction, in feet) | | | | | | |
| 50.000 | y | 1/2 width of basin (y direction, in feet) | | | | | | |
| 24.00 | t | Duration of infiltration period (hours) | | | | | | |
| 10.000 | hi(0) | Initial thickness of saturated zone (feet) | | | | | | |
2. Table 2
BMP 2026, Ch 13, p. 9
| 23.24 |
h(max) |
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
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| 13.24 |
Δh(max) |
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
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| Ground-water Mounding, in feet |
Distance from center of basin in x direction, in feet |
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| 13.237 |
0 |
Re-Calculate Now |
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| 13.179 |
10 |
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| 12.927 |
20 |
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| 12.212 |
30 |
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| 10.495 |
40 |
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| 6.659 |
50 |
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| 2.408 |
60 |
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| 0.652 |
70 |
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| 0.129 |
80 |
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| 0.023 |
90 |
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Source context and notes
# Groundwater Table Hydraulic Impact Assessments for Infiltration BMPs
Source revision: January 2026.
The results section is depicted below.
## Results Section
| 23.24 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 13.24 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | | | | |
| Ground-water Mounding, in feet | Distance from center of basin in x direction, in feet | | | | | | | | | | |
| 13.237 | 0 | Re-Calculate Now | | | | | | | | | |
| 13.179 | 10 | | | | | | | | | | |
| 12.927 | 20 | | | | | | | | | | |
| 12.212 | 30 | | | | | | | | | | |
| 10.495 | 40 | | | | | | | | | | |
| 6.659 | 50 | | | | | | | | | | |
| 2.408 | 60 | | | | | | | | | | |
| 0.652 | 70 | | | | | | | | | | |
| 0.129 | 80 | | | | | | | | | | |
| 0.023 | 90 | | | | | | | | | | |
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The mounding height is 13.24 feet based on a recharge rate, i.e. the design permeability rate, of 1 in/hr.
If, instead of the design permeability rate, the tested permeability rate of 2 in/hr is used in the Hantush Spreadsheet, what mounding height will result? If the tested permeability rate, 2 in/hr, is used, the duration of infiltration will be recalculated as:
$$
\text{Duration of infiltration period, } t \left( hr \right) = \frac{5,000 \left( cf \right) \times 12 \text{ in}/ft}{2,500 \left( sf \right) \times 2 \left( in/hr \right)} = 12 \text{ hours}
$$
3. Table 3
BMP 2026, Ch 13, p. 9
| Input Values |
|
|
|
|
|
|
|
|
|
|
|
| 2.000 |
R |
Recharge rate (permeability rate) (in/hr) |
|
|
|
|
|
|
|
|
|
| 0.150 |
Sy |
Specific yield, Sy (dimensionless) default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
|
|
|
|
|
|
| 2.00 |
Kh |
Horizontal hydraulic conductivity (in/hr) Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
|
|
|
|
|
|
|
|
|
| 50.000 |
x |
1/2 length of basin (x direction, in feet) |
|
|
|
|
|
|
|
|
|
| 50.000 |
y |
1/2 width of basin (y direction, in feet) |
|
|
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|
|
| 12.00 |
t |
Duration of infiltration period (hours) |
|
|
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|
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| 10.000 |
hi(0) |
Initial thickness of saturated zone (feet) |
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Source context and notes
$$
\text{Duration of infiltration period, } t \left( hr \right) = \frac{5,000 \left( cf \right) \times 12 \text{ in}/ft}{2,500 \left( sf \right) \times 2 \left( in/hr \right)} = 12 \text{ hours}
$$
The recharge rate now uses the tested permeability rate, 2 in/hr. The horizontal hydraulic conductivity in the Hantush Spreadsheet uses the recharge rate as 2 in/hr. The inputs, shown below, and the results, shown on the following page, based on the tested permeability rate are now recalculated as:
## Input Section
| Input Values | | | | | | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 2.000 | R | Recharge rate (permeability rate) (in/hr) | | | | | | | | | |
| 0.150 | Sy | Specific yield, Sy (dimensionless) default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | | | | | | | |
| 2.00 | Kh | Horizontal hydraulic conductivity (in/hr) Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | | | | | | | | |
| 50.000 | x | 1/2 length of basin (x direction, in feet) | | | | | | | | | |
| 50.000 | y | 1/2 width of basin (y direction, in feet) | | | | | | | | | |
| 12.00 | t | Duration of infiltration period (hours) | | | | | | | | | |
| 10.000 | hi(0) | Initial thickness of saturated zone (feet) | | | | | | | | | |
4. Table 4
BMP 2026, Ch 13, p. 10
| 23.24 |
h(max) |
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
|
|
|
| 13.24 |
Δh(max) |
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
|
|
|
| Ground-water Mounding, in feet |
Distance from center of basin in x direction, in feet |
|
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|
|
| 13.237 |
0 |
Re-Calculate Now |
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| 13.179 |
10 |
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| 12.927 |
20 |
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| 12.212 |
30 |
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| 10.493 |
40 |
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| 6.659 |
50 |
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| 2.408 |
60 |
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| 0.632 |
70 |
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| 0.129 |
80 |
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| 0.023 |
90 |
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Source context and notes
# Groundwater Table Hydraulic Impact Assessments for Infiltration BMPs
Source revision: January 2026.
# Results Section
| 23.24 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 13.24 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | | | | |
| Ground-water Mounding, in feet | Distance from center of basin in x direction, in feet | | | | | | | | | | |
| 13.237 | 0 | Re-Calculate Now | | | | | | | | | |
| 13.179 | 10 | | | | | | | | | | |
| 12.927 | 20 | | | | | | | | | | |
| 12.212 | 30 | | | | | | | | | | |
| 10.493 | 40 | | | | | | | | | | |
| 6.659 | 50 | | | | | | | | | | |
| 2.408 | 60 | | | | | | | | | | |
| 0.632 | 70 | | | | | | | | | | |
| 0.129 | 80 | | | | | | | | | | |
| 0.023 | 90 | | | | | | | | | | |
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| | | | | | | | | | | | |
The mounding height is still 13.24 ft and the mounding curve is exactly the same as the curve based on the design permeability rate.
If the bioretention basin is now located in the Coastal Plain, the horizontal hydraulic conductivity rate will be 5 times that of the recharge rate used in the Hantush Spreadsheet. Screenshots of the Hantush Spreadsheet based on design permeability rate of 1 in/hr (recharge rate 1 in/hr and horizontal hydraulic conductivity rate 5 in/hr) and the tested permeability rate of 2 in/hr (recharge rate 2 in/hr and horizontal hydraulic conductivity rate 10 in/hr) are shown below or on the next page:
## Calculation Based on the Design Permeability Rate
5. Table 5
BMP 2026, Ch 13, p. 10
| Input Values |
|
|
|
|
|
|
|
|
|
| 1.000 |
R |
Recharge rate (permeability rate) (in/hr) |
|
|
|
|
|
|
|
| 0.150 |
Sy |
Specific yield, Sy (dimensionless) default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
|
|
|
|
| 5.00 |
Kh |
Horizontal hydraulic conductivity (in/hr) Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
|
|
|
|
|
|
|
| 50.000 |
x |
1/2 length of basin (x direction, in feet) |
|
|
|
|
|
|
|
| 50.000 |
y |
1/2 width of basin (y direction, in feet) |
|
|
|
|
|
|
|
| 24.00 |
t |
Duration of infiltration period (hours) |
|
|
|
|
|
|
|
| 10.000 |
hi(0) |
Initial thickness of saturated zone (feet) |
|
|
|
|
|
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| 20.48 |
h(max) |
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
|
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|
|
| 10.48 |
Δh(max) |
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
|
Source context and notes
| | | | | | | | | | | | |
The mounding height is still 13.24 ft and the mounding curve is exactly the same as the curve based on the design permeability rate.
If the bioretention basin is now located in the Coastal Plain, the horizontal hydraulic conductivity rate will be 5 times that of the recharge rate used in the Hantush Spreadsheet. Screenshots of the Hantush Spreadsheet based on design permeability rate of 1 in/hr (recharge rate 1 in/hr and horizontal hydraulic conductivity rate 5 in/hr) and the tested permeability rate of 2 in/hr (recharge rate 2 in/hr and horizontal hydraulic conductivity rate 10 in/hr) are shown below or on the next page:
## Calculation Based on the Design Permeability Rate
| Input Values | | | | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 1.000 | R | Recharge rate (permeability rate) (in/hr) | | | | | | | |
| 0.150 | Sy | Specific yield, Sy (dimensionless) default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | | | | | |
| 5.00 | Kh | Horizontal hydraulic conductivity (in/hr) Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | | | | | | |
| 50.000 | x | 1/2 length of basin (x direction, in feet) | | | | | | | |
| 50.000 | y | 1/2 width of basin (y direction, in feet) | | | | | | | |
| 24.00 | t | Duration of infiltration period (hours) | | | | | | | |
| 10.000 | hi(0) | Initial thickness of saturated zone (feet) | | | | | | | |
| | | | | | | | | | |
| 20.48 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | | | | |
| 10.48 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | | |
6. Table 6
BMP 2026, Ch 13, p. 11
| Input Values |
|
|
|
|
|
|
|
|
|
| 2.000 |
R |
|
Recharge rate (permeability rate) (in/hr) |
|
|
|
|
|
|
| 0.150 |
Sy |
|
Specific yield, Sy (dimensionless)default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
|
|
|
| 10.00 |
Kh |
|
Horizontal hydraulic conductivity (in/hr)Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
|
|
|
|
|
|
| 50.000 |
x |
|
1/2 length of basin (x direction, in feet) |
|
|
|
|
|
|
| 50.000 |
y |
|
1/2 width of basin (y direction, in feet) |
|
|
|
|
|
|
| 12.00 |
t |
|
Duration of infiltration period (hours) |
|
|
|
|
|
|
| 10.000 |
hi(0) |
|
Initial thickness of saturated zone (feet) |
|
|
|
|
|
|
| 20.48 |
h(max) |
|
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
| 10.48 |
Δh(max) |
|
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
Source context and notes
# Groundwater Table Hydraulic Impact Assessments for Infiltration BMPs
Source revision: January 2026.
Calculation Based on the Tested Permeability Rate
| Input Values | | | | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 2.000 | R | | Recharge rate (permeability rate) (in/hr) | | | | | | |
| 0.150 | Sy | | Specific yield, Sy (dimensionless)default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | | | | |
| 10.00 | Kh | | Horizontal hydraulic conductivity (in/hr)Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | | | | | |
| 50.000 | x | | 1/2 length of basin (x direction, in feet) | | | | | | |
| 50.000 | y | | 1/2 width of basin (y direction, in feet) | | | | | | |
| 12.00 | t | | Duration of infiltration period (hours) | | | | | | |
| 10.000 | hi(0) | | Initial thickness of saturated zone (feet) | | | | | | |
| 20.48 | h(max) | | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | | | |
| 10.48 | Δh(max) | | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | |
Once more, the mounding analysis using the Hantush Spreadsheet for a specific basin will be the same whether based on the design permeability rate or the tested permeability rate.
This same relationship will be maintained even if the horizontal hydraulic conductivity entered does not equal the default value. For example, if the tested permeability rate for the vertical direction is 2 in/hr and the tested horizontal hydraulic conductivity rate is 8 in/hr, the design rates computed, after the application of the factor of safety = 2, will yield 1 in/hr for the vertical direction design permeability and 4 in/hr for the design horizontal hydraulic conductivity. The corresponding screenshots of the Hantush Spreadsheet for each of the above rates are shown below and discussed on the next page:
Calculation for $R = 2$ in/hr Tested Vertical Permeability Rate and Kh = 8 in/hr Tested Horizontal Hydraulic Conductivity Rate
7. Table 7
BMP 2026, Ch 13, p. 11
| Input Values |
|
|
|
|
|
|
|
| 2.000 |
R |
|
Recharge rate (permeability rate) (in/hr) |
|
|
|
|
| 0.150 |
Sy |
|
Specific yield, Sy (dimensionless)default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
|
| 8.00 |
Kh |
|
Horizontal hydraulic conductivity (in/hr)Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
|
|
|
|
| 50.000 |
x |
|
1/2 length of basin (x direction, in feet) |
|
|
|
|
| 50.000 |
y |
|
1/2 width of basin (y direction, in feet) |
|
|
|
|
| 12.00 |
t |
|
Duration of infiltration period (hours) |
|
|
|
|
| 10.000 |
hi(0) |
|
Initial thickness of saturated zone (feet) |
|
|
|
|
| 21.12 |
h(max) |
|
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
|
|
|
|
| 11.12 |
Δh(max) |
|
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
|
|
|
|
Source context and notes
| 10.48 | Δh(max) | | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | |
Once more, the mounding analysis using the Hantush Spreadsheet for a specific basin will be the same whether based on the design permeability rate or the tested permeability rate.
This same relationship will be maintained even if the horizontal hydraulic conductivity entered does not equal the default value. For example, if the tested permeability rate for the vertical direction is 2 in/hr and the tested horizontal hydraulic conductivity rate is 8 in/hr, the design rates computed, after the application of the factor of safety = 2, will yield 1 in/hr for the vertical direction design permeability and 4 in/hr for the design horizontal hydraulic conductivity. The corresponding screenshots of the Hantush Spreadsheet for each of the above rates are shown below and discussed on the next page:
Calculation for $R = 2$ in/hr Tested Vertical Permeability Rate and Kh = 8 in/hr Tested Horizontal Hydraulic Conductivity Rate
| Input Values | | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- |
| 2.000 | R | | Recharge rate (permeability rate) (in/hr) | | | | |
| 0.150 | Sy | | Specific yield, Sy (dimensionless)default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | | |
| 8.00 | Kh | | Horizontal hydraulic conductivity (in/hr)Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | | | |
| 50.000 | x | | 1/2 length of basin (x direction, in feet) | | | | |
| 50.000 | y | | 1/2 width of basin (y direction, in feet) | | | | |
| 12.00 | t | | Duration of infiltration period (hours) | | | | |
| 10.000 | hi(0) | | Initial thickness of saturated zone (feet) | | | | |
| 21.12 | h(max) | | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | |
| 11.12 | Δh(max) | | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | |
Calculation for $R = 1$ in/hr Design Vertical Permeability Rate and Kh = 4 in/hr Design Horizontal Hydraulic Conductivity Rate
| Input Values | | | | | | |
| --- | --- | --- | --- | --- | --- | --- |
| 1.000 | R | | Recharge rate (permeability rate) (in/hr) | | | |
| 0.150 | Sy | | Specific yield, Sy (dimensionless)default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | |
| 4.00 | Kh | | Horizontal hydraulic conductivity (in/hr)Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | | |
| 50.000 | x | | 1/2 length of basin (x direction, in feet) | | | |
| 50.000 | y | | 1/2 width of basin (y direction, in feet) | | | |
| 24.00 | t | | Duration of infiltration period (hours) | | | |
| 10.000 | hi(0) | | Initial thickness of saturated zone (feet) | | | |
| 21.12 | h(max) | | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | |
| 11.12 | Δh(max) | | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | |
8. Table 8
BMP 2026, Ch 13, p. 11
| Input Values |
|
|
|
|
|
|
| 1.000 |
R |
|
Recharge rate (permeability rate) (in/hr) |
|
|
|
| 0.150 |
Sy |
|
Specific yield, Sy (dimensionless)default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
| 4.00 |
Kh |
|
Horizontal hydraulic conductivity (in/hr)Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
|
|
|
| 50.000 |
x |
|
1/2 length of basin (x direction, in feet) |
|
|
|
| 50.000 |
y |
|
1/2 width of basin (y direction, in feet) |
|
|
|
| 24.00 |
t |
|
Duration of infiltration period (hours) |
|
|
|
| 10.000 |
hi(0) |
|
Initial thickness of saturated zone (feet) |
|
|
|
| 21.12 |
h(max) |
|
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
|
|
|
| 11.12 |
Δh(max) |
|
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
|
|
|
Source context and notes
| 50.000 | y | | 1/2 width of basin (y direction, in feet) | | | | |
| 12.00 | t | | Duration of infiltration period (hours) | | | | |
| 10.000 | hi(0) | | Initial thickness of saturated zone (feet) | | | | |
| 21.12 | h(max) | | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | |
| 11.12 | Δh(max) | | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | |
Calculation for $R = 1$ in/hr Design Vertical Permeability Rate and Kh = 4 in/hr Design Horizontal Hydraulic Conductivity Rate
| Input Values | | | | | | |
| --- | --- | --- | --- | --- | --- | --- |
| 1.000 | R | | Recharge rate (permeability rate) (in/hr) | | | |
| 0.150 | Sy | | Specific yield, Sy (dimensionless)default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | |
| 4.00 | Kh | | Horizontal hydraulic conductivity (in/hr)Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | | |
| 50.000 | x | | 1/2 length of basin (x direction, in feet) | | | |
| 50.000 | y | | 1/2 width of basin (y direction, in feet) | | | |
| 24.00 | t | | Duration of infiltration period (hours) | | | |
| 10.000 | hi(0) | | Initial thickness of saturated zone (feet) | | | |
| 21.12 | h(max) | | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | |
| 11.12 | Δh(max) | | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | |
9. Table 9
BMP 2026, Ch 13, p. 12
| Summary of Model Input Parameters |
|
| Input Parameter |
Value for Initial Model Run |
| Recharge Rate, R |
½ the lowest tested permeability rate, determined in accordance with Chapter 12: Soil Testing Criteria, of the most hydraulically restrictive layer located between the basin bottom and the SHWT. Values are between 0.5 in/hr to 10 in/hr. |
| Specific Yield, Sy |
Default value 0.15 and up to a maximum value 0.2, with supporting soil test results |
| Horizontal Hydraulic Conductivity, Kh |
In the Coastal Plain: 5 x recharge rate Outside the Coastal Plain: 1 x recharge rate |
| Basin length, x |
½ the proposed basin length |
| Basin width, y |
½ the proposed basin width |
| Duration of Infiltration Period, t |
Time for infiltrating the runoff volume through the BMP using the recharge rate above. Maximum value is 72 hr. |
| Initial Thickness of Saturated Zone, hi(0) |
Default value 10 ft and up to a maximum value of 75 ft* with supporting continuous boring test results. |
Source context and notes
# Groundwater Table Hydraulic Impact Assessments for Infiltration BMPs
Source revision: January 2026.
The results are the same again. The above demonstration dispels the misconception that using the design permeability rate in the Hantush Spreadsheet will produce a different outcome from the tested permeability rate. To maintain consistency, always use the design permeability rate in both sizing the BMP and in performing a Groundwater Mounding Analysis.
| Summary of Model Input Parameters | |
| --- | --- |
| Input Parameter | Value for Initial Model Run |
| Recharge Rate, R | ½ the lowest tested permeability rate, determined in accordance with Chapter 12: Soil Testing Criteria, of the most hydraulically restrictive layer located between the basin bottom and the SHWT. Values are between 0.5 in/hr to 10 in/hr. |
| Specific Yield, Sy | Default value 0.15 and up to a maximum value 0.2, with supporting soil test results |
| Horizontal Hydraulic Conductivity, Kh | In the Coastal Plain: 5 x recharge rate Outside the Coastal Plain: 1 x recharge rate |
| Basin length, x | ½ the proposed basin length |
| Basin width, y | ½ the proposed basin width |
| Duration of Infiltration Period, t | Time for infiltrating the runoff volume through the BMP using the recharge rate above. Maximum value is 72 hr. |
| Initial Thickness of Saturated Zone, hi(0) | Default value 10 ft and up to a maximum value of 75 ft* with supporting continuous boring test results. |
*The maximum is based on the preliminary results of research conducted by the USGS. The exact value may be subject to revision based on the final results of the research.
The table below is an index of important steps to be addressed in a groundwater mounding analysis
| Groundwater Mounding Analysis Steps | Page No. |
| --- | --- |
| Creation of the Initial Model | 13 |
| Adjustment of the Recharge Rate and the Duration of Infiltration Period | 14 |
| Adjustment of the Basin Size and Depth of Stormwater Runoff to be Infiltrated | 17 |
| Evaluate Impacts to Nearby Underground Structures | 17 |
| Calculate the Height of Groundwater Mounding for Two or More Nearby Basins | 17 |
10. Table 10
BMP 2026, Ch 13, p. 12
| Groundwater Mounding Analysis Steps |
Page No. |
| Creation of the Initial Model |
13 |
| Adjustment of the Recharge Rate and the Duration of Infiltration Period |
14 |
| Adjustment of the Basin Size and Depth of Stormwater Runoff to be Infiltrated |
17 |
| Evaluate Impacts to Nearby Underground Structures |
17 |
| Calculate the Height of Groundwater Mounding for Two or More Nearby Basins |
17 |
Source context and notes
| Basin width, y | ½ the proposed basin width |
| Duration of Infiltration Period, t | Time for infiltrating the runoff volume through the BMP using the recharge rate above. Maximum value is 72 hr. |
| Initial Thickness of Saturated Zone, hi(0) | Default value 10 ft and up to a maximum value of 75 ft* with supporting continuous boring test results. |
*The maximum is based on the preliminary results of research conducted by the USGS. The exact value may be subject to revision based on the final results of the research.
The table below is an index of important steps to be addressed in a groundwater mounding analysis
| Groundwater Mounding Analysis Steps | Page No. |
| --- | --- |
| Creation of the Initial Model | 13 |
| Adjustment of the Recharge Rate and the Duration of Infiltration Period | 14 |
| Adjustment of the Basin Size and Depth of Stormwater Runoff to be Infiltrated | 17 |
| Evaluate Impacts to Nearby Underground Structures | 17 |
| Calculate the Height of Groundwater Mounding for Two or More Nearby Basins | 17 |
11. Table 11
BMP 2026, Ch 13, p. 13
| Initial Model Input Parameters |
|
| Input Parameter |
Value for Initial Model Run |
| Recharge Rate, R |
½ the lowest tested permeability rate, determined in accordance with Chapter 12, of the most hydraulically restrictive layer located between the basin bottom and the SHWT. |
| Specific Yield, Sy |
0.15 |
| Horizontal Hydraulic Conductivity, Kh |
In the Coastal Plain: 5 x recharge rate Outside the Coastal Plain: 1 x recharge rate |
| Basin length, x |
½ the proposed basin length |
| Basin width, y |
½ the proposed basin width |
| Duration of Infiltration Period, t |
Time for infiltrating the runoff volume through the BMP using the recharge rate above |
| Initial Thickness of Saturated Zone, hi(0) |
10 ft |
Source context and notes
# Groundwater Table Hydraulic Impact Assessments for Infiltration BMPs
Source revision: January 2026.
# Initial Model
The first part of this step is calculating the duration of the infiltration period. Next, use the values shown in the table below as inputs into the Hantush Spreadsheet in the gold shaded cells in rows 7 through 13 of column A of the Hantush Spreadsheet:
| Initial Model Input Parameters | |
| --- | --- |
| Input Parameter | Value for Initial Model Run |
| Recharge Rate, R | ½ the lowest tested permeability rate, determined in accordance with Chapter 12, of the most hydraulically restrictive layer located between the basin bottom and the SHWT. |
| Specific Yield, Sy | 0.15 |
| Horizontal Hydraulic Conductivity, Kh | In the Coastal Plain: 5 x recharge rate Outside the Coastal Plain: 1 x recharge rate |
| Basin length, x | ½ the proposed basin length |
| Basin width, y | ½ the proposed basin width |
| Duration of Infiltration Period, t | Time for infiltrating the runoff volume through the BMP using the recharge rate above |
| Initial Thickness of Saturated Zone, hi(0) | 10 ft |
The Input Section of the Hantush Spreadsheet is shown below for a different BMP.
Input Section of the Hantush Spreadsheet
| | A | B | C | D | E | F | G | H | I | J | K |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 6 | Input Values | | | | | | | | | | |
| 7 | 2.00 | R | | Recharge rate (permeability rate) (in/hr) | | | | | | | |
| 8 | 0.150 | Sy | | Specific yield, Sy (dimensionless) default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | | | | | |
| 9 | 10.00 | Kh | | Horizontal hydraulic conductivity (in/hr) Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | | | | | | |
| 10 | 25.000 | x | | 1/2 length of basin (x direction, in feet) | | | | | | | |
| 11 | 25.000 | y | | 1/2 width of basin (y direction, in feet) | | | | | | | |
| 12 | 12.00 | t | | Duration of infiltration period (hours) | | | | | | | |
| 13 | 10.000 | hi(0) | | Initial thickness of saturated zone (feet) | | | | | | | |
12. Table 12
BMP 2026, Ch 13, p. 13
|
A |
B |
C |
D |
E |
F |
G |
H |
I |
J |
K |
| 6 |
Input Values |
|
|
|
|
|
|
|
|
|
|
| 7 |
2.00 |
R |
|
Recharge rate (permeability rate) (in/hr) |
|
|
|
|
|
|
|
| 8 |
0.150 |
Sy |
|
Specific yield, Sy (dimensionless) default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
|
|
|
|
| 9 |
10.00 |
Kh |
|
Horizontal hydraulic conductivity (in/hr) Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
|
|
|
|
|
|
|
| 10 |
25.000 |
x |
|
1/2 length of basin (x direction, in feet) |
|
|
|
|
|
|
|
| 11 |
25.000 |
y |
|
1/2 width of basin (y direction, in feet) |
|
|
|
|
|
|
|
| 12 |
12.00 |
t |
|
Duration of infiltration period (hours) |
|
|
|
|
|
|
|
| 13 |
10.000 |
hi(0) |
|
Initial thickness of saturated zone (feet) |
|
|
|
|
|
|
|
Source context and notes
| Basin width, y | ½ the proposed basin width |
| Duration of Infiltration Period, t | Time for infiltrating the runoff volume through the BMP using the recharge rate above |
| Initial Thickness of Saturated Zone, hi(0) | 10 ft |
The Input Section of the Hantush Spreadsheet is shown below for a different BMP.
Input Section of the Hantush Spreadsheet
| | A | B | C | D | E | F | G | H | I | J | K |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 6 | Input Values | | | | | | | | | | |
| 7 | 2.00 | R | | Recharge rate (permeability rate) (in/hr) | | | | | | | |
| 8 | 0.150 | Sy | | Specific yield, Sy (dimensionless) default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | | | | | |
| 9 | 10.00 | Kh | | Horizontal hydraulic conductivity (in/hr) Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | | | | | | |
| 10 | 25.000 | x | | 1/2 length of basin (x direction, in feet) | | | | | | | |
| 11 | 25.000 | y | | 1/2 width of basin (y direction, in feet) | | | | | | | |
| 12 | 12.00 | t | | Duration of infiltration period (hours) | | | | | | | |
| 13 | 10.000 | hi(0) | | Initial thickness of saturated zone (feet) | | | | | | | |
On the following page, the corresponding Results Section of the Hantush Spreadsheet is depicted. The values in column B, from row 18 to row 27, are the distances from the center of the infiltration BMP to depict the mounding height. The values can be adjusted to show a larger or smaller distance between stations shown in the result, but it is recommended that the stations all be the same distance apart when modifying the default station locations. However, unless the basin is so large that the distances shown in rows 18 through 27, in column B do not extend beyond the basin limits, the default values do not require any changes. Be aware that a change of the values in the gold-shaded cells in rows 18 through 27 may result in a distorted or cut-off display of the groundwater mounding curve.
13. Table 13
BMP 2026, Ch 13, p. 14
|
A |
B |
C |
D |
E |
F |
G |
H |
I |
J |
K |
L |
| 15 |
16.07 |
h(max) |
|
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
|
|
| 16 |
6.07 |
Δh(max) |
|
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
|
|
| 17 |
Ground-water Mounding, in feet |
Distance from center of basin in x direction, in feet |
|
|
|
|
|
|
|
|
|
|
| 18 |
6.073 |
0 |
|
Re-Calculate Now |
|
|
|
|
|
|
|
|
| 19 |
4.900 |
20 |
|
|
|
|
|
|
|
|
|
|
| 20 |
2.066 |
40 |
|
|
|
|
|
|
|
|
|
|
| 21 |
1.205 |
50 |
|
|
|
|
|
|
|
|
|
|
| 22 |
0.666 |
60 |
|
|
|
|
|
|
|
|
|
|
| 23 |
0.350 |
70 |
|
|
|
|
|
|
|
|
|
|
| 24 |
0.176 |
80 |
|
|
|
|
|
|
|
|
|
|
| 25 |
0.086 |
90 |
|
|
|
|
|
|
|
|
|
|
| 26 |
0.041 |
100 |
|
|
|
|
|
|
|
|
|
|
| 27 |
0.011 |
120 |
|
|
|
|
|
|
|
|
|
|
| 28 |
|
|
|
|
|
|
|
|
|
|
|
|
| 29 |
|
|
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|
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|
|
|
| 30 |
|
|
|
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|
|
|
|
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|
|
| 31 |
|
|
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|
|
|
|
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|
| 32 |
|
|
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|
|
|
|
|
|
|
|
| 33 |
|
|
|
|
|
|
|
|
|
|
|
|
| 34 |
|
|
SHWT → |
|
|
|
|
|
|
|
|
|
| 35 |
|
|
|
|
|
|
|
|
|
|
|
|
Source context and notes
# Groundwater Table Hydraulic Impact Assessments for Infiltration BMPs
Source revision: January 2026.
# Results Section of the Hantush Spreadsheet
| | A | B | C | D | E | F | G | H | I | J | K | L |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 15 | 16.07 | h(max) | | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | | | | | |
| 16 | 6.07 | Δh(max) | | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | | | |
| 17 | Ground-water Mounding, in feet | Distance from center of basin in x direction, in feet | | | | | | | | | | |
| 18 | 6.073 | 0 | | Re-Calculate Now | | | | | | | | |
| 19 | 4.900 | 20 | | | | | | | | | | |
| 20 | 2.066 | 40 | | | | | | | | | | |
| 21 | 1.205 | 50 | | | | | | | | | | |
| 22 | 0.666 | 60 | | | | | | | | | | |
| 23 | 0.350 | 70 | | | | | | | | | | |
| 24 | 0.176 | 80 | | | | | | | | | | |
| 25 | 0.086 | 90 | | | | | | | | | | |
| 26 | 0.041 | 100 | | | | | | | | | | |
| 27 | 0.011 | 120 | | | | | | | | | | |
| 28 | | | | | | | | | | | | |
| 29 | | | | | | | | | | | | |
| 30 | | | | | | | | | | | | |
| 31 | | | | | | | | | | | | |
| 32 | | | | | | | | | | | | |
| 33 | | | | | | | | | | | | |
| 34 | | | SHWT → | | | | | | | | | |
| 35 | | | | | | | | | | | | |
The user then clicks on the blue button adjacent to the words "Recalculate Now." The spreadsheet will recalculate the height of the mound, measured upward from the SHWT, which is at elevation 0.00. The calculated mounding height corresponding to a certain distance from the center of the basin in the x direction will appear in the red cells under the label "Groundwater Mounding, in feet," i.e., the cells within the range A18:A27. If the model shows that the groundwater mounding height will not reach the bottom surface of the proposed BMP, then the infiltration in the proposed BMP will have no adverse impacts on the performance of the BMP. However, the evaluation of the hydraulic impact to the water table may be required if the nearby underground structures are fully or partially within the extent of the mounding curve. The discussion of the hydraulic impact to nearby underground structures is addressed on Page 17 and Examples 5 through 8 of this chapter.
# Adjustment of the Recharge Rate and the Duration of Infiltration Period
In the event the height of the groundwater mounding reaches or exceeds the elevation of the bottom surface of the BMP, the design must be adjusted and a new model created. This is because the elevated groundwater table below the BMP will reduce the vertical hydraulic gradient, meaning the recharge rate will be less than the design value. Adjustment of the recharge rate to a lower value allows slower infiltration of runoff and more time for the infiltrated runoff to dissipate into the saturated zone. A reduced recharge rate will have a smaller mounding height but result in a longer duration of infiltration period. Several trials of different reduced recharge rates may be needed to find which recharge rate works. The duration of infiltration period based on the reduced recharge rate may not exceed 72 hours. If the adjustment of the recharge rate still does not allow the proposed BMP to be drained within 72 hours using the designed dimensions of the BMP with the intended runoff infiltration volume, adjustment of the BMP size or an alternative design using another type of BMP may be required to avoid groundwater mounding issues.
14. Table 14
BMP 2026, Ch 13, p. 15
| 2.00 |
R |
Recharge rate (permeability rate) (in/hr) |
|
|
|
| 0.150 |
Sy |
Specific yield, Sy (dimensionless, default value is 0.15; max value is 0.2 only if proved with lab test data of 0.2 or higher) |
|
|
|
| 2.00 |
Kh |
Horizontal hydraulic conductivity (in/hr) |
|
|
|
| 75.000 |
x |
Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
|
|
|
| 50.000 |
y |
1/2 length of basin (x direction, in feet) |
|
|
|
| 12.00 |
t |
Duration of infiltration period (hours) |
|
|
|
| 10.000 |
hi(0) |
Initial thickness of saturated zone (feet) |
|
|
|
| 29.294 |
h(max) |
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
|
|
|
| 13.286 |
Δh(max) |
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
|
|
|
Source context and notes
Source revision: January 2026.
In the event that the recharge rate needs to be reduced, the duration of infiltration period must be increased proportionally, without exceeding the 72 hours maximum allowable timeframe.
Consider a groundwater mounding analysis with the initial model shown as below.
## Initial Model
| 2.00 | R | Recharge rate (permeability rate) (in/hr) | | | |
| --- | --- | --- | --- | --- | --- |
| 0.150 | Sy | Specific yield, Sy (dimensionless, default value is 0.15; max value is 0.2 only if proved with lab test data of 0.2 or higher) | | | |
| 2.00 | Kh | Horizontal hydraulic conductivity (in/hr) | | | |
| 75.000 | x | Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | | |
| 50.000 | y | 1/2 length of basin (x direction, in feet) | | | |
| 12.00 | t | Duration of infiltration period (hours) | | | |
| 10.000 | hi(0) | Initial thickness of saturated zone (feet) | | | |
| 29.294 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | |
| 13.286 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | |
In the initial model, if the stormwater runoff volume calculated to take 12 hours to infiltrate for a recharge rate of 2 in/hr results in a groundwater mounding height that would project above the bottom surface of the BMP, the model could be adjusted with a recharge rate reduced by 50% to 1 in/hr and a duration of infiltration period increased by 200% from 12 hours to 24 hours to maintain the proportionality. No changes would be made to any other inputs at this time.
## Adjusted Model
| Input Values | | | | | | |
| --- | --- | --- | --- | --- | --- | --- |
| 1.00 | R | Recharge rate (permeability rate) (in/hr) | | | | |
| 0.150 | Sy | Specific yield, Sy (dimensionless)
default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | | |
| 2.00 | Kh | Horizontal hydraulic conductivity (in/hr)
Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | | | |
| 75.000 | x | 1/2 length of basin (x direction, in feet) | | | | |
| 50.000 | y | 1/2 width of basin (y direction, in feet) | | | | |
| 24.00 | t | Duration of infiltration period (hours) | | | | |
| 10.000 | hi(0) | Initial thickness of saturated zone (feet) | | | | |
| 22.934 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | |
| 12.936 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | |
15. Table 15
BMP 2026, Ch 13, p. 15
| Input Values |
|
|
|
|
|
|
| 1.00 |
R |
Recharge rate (permeability rate) (in/hr) |
|
|
|
|
| 0.150 |
Sy |
Specific yield, Sy (dimensionless)
default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
|
| 2.00 |
Kh |
Horizontal hydraulic conductivity (in/hr)
Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
|
|
|
|
| 75.000 |
x |
1/2 length of basin (x direction, in feet) |
|
|
|
|
| 50.000 |
y |
1/2 width of basin (y direction, in feet) |
|
|
|
|
| 24.00 |
t |
Duration of infiltration period (hours) |
|
|
|
|
| 10.000 |
hi(0) |
Initial thickness of saturated zone (feet) |
|
|
|
|
| 22.934 |
h(max) |
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
|
|
|
|
| 12.936 |
Δh(max) |
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
|
|
|
|
Source context and notes
| 10.000 | hi(0) | Initial thickness of saturated zone (feet) | | | |
| 29.294 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | |
| 13.286 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | |
In the initial model, if the stormwater runoff volume calculated to take 12 hours to infiltrate for a recharge rate of 2 in/hr results in a groundwater mounding height that would project above the bottom surface of the BMP, the model could be adjusted with a recharge rate reduced by 50% to 1 in/hr and a duration of infiltration period increased by 200% from 12 hours to 24 hours to maintain the proportionality. No changes would be made to any other inputs at this time.
## Adjusted Model
| Input Values | | | | | | |
| --- | --- | --- | --- | --- | --- | --- |
| 1.00 | R | Recharge rate (permeability rate) (in/hr) | | | | |
| 0.150 | Sy | Specific yield, Sy (dimensionless)
default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | | |
| 2.00 | Kh | Horizontal hydraulic conductivity (in/hr)
Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | | | |
| 75.000 | x | 1/2 length of basin (x direction, in feet) | | | | |
| 50.000 | y | 1/2 width of basin (y direction, in feet) | | | | |
| 24.00 | t | Duration of infiltration period (hours) | | | | |
| 10.000 | hi(0) | Initial thickness of saturated zone (feet) | | | | |
| 22.934 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | |
| 12.936 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | |
Note that the horizontal hydraulic conductivity remains constant and is not affected by an elevated groundwater table because it is less affected by changes in the vertical hydraulic gradient. Therefore, input values for the horizontal hydraulic conductivity must not be altered, even in instances in which the input value for the recharge rate is adjusted to evaluate the possibility of reducing the height of groundwater mounding. At this point in the analysis, the user may wish to create a table to record the input values of the required parameters along with the results and any notes. Example 2, which begins on Page 20, includes a sample recordkeeping table or spreadsheet, depicted on Page 23. Screen captures may prove useful for inclusion in the groundwater mounding analysis report.
- If the adjusted model shows that the groundwater mounding height will not reach the bottom surface of the proposed BMP, then groundwater mounding will have no adverse impacts on the BMP.
- Alternatively, the duration of infiltration can be further extended, or stretched, up to the 72 hr maximum, measured from the end of the precipitation event, provided the recharge rate is
16. Table 16
BMP 2026, Ch 13, p. 16
| 0.75 |
R |
Recharge rate (permeability rate) (in/hr) |
| 0.150 |
Sy |
Specific yield, Sy (dimensionless, default value is 0.15; max value is 0.2 only if proved with lab test data of 0.2 or higher) |
| 2.00 |
Kh |
Horizontal hydraulic conductivity (in/hr) Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
| 75.000 |
x |
1/2 length of basin (x direction, in feet) |
| 50.000 |
y |
1/2 width of basin (y direction, in feet) |
| 32.00 |
t |
Duration of infiltration period (hours) |
| 10.000 |
hi(0) |
Initial thickness of saturated zone (feet) |
| 22.599 |
h(max) |
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
| 12.599 |
Δh(max) |
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
Source context and notes
For the example shown on the previous page, the initial model begins with a recharge rate of 2 in/hr by using the design permeability rate obtained from soil testing and a duration of infiltration period of 12 hr. The calculated mounding height is 13.284 ft. The time frame may be extended, with the corresponding recharge rate pro-rated, as follows:
For a desired t value, $\mathrm{R}_{\text {t\_new }} = \mathrm{R}_{\text {design }} x \left[ \frac{\mathrm{t}_{\text {initial }}(\mathrm{hr})}{\mathrm{t}_{\text {desired }}(\mathrm{hr})} \right]$ , such that for
- $t = {32}$ hours, ${\mathrm{R}}_{32} = 2 \ln /{hr}x\left( {{12hr}/{32hr}}\right) = {0.75}\ln /{hr}$
Initial Model with Adjusted $t = 32$ hr
| 0.75 | R | Recharge rate (permeability rate) (in/hr) |
| --- | --- | --- |
| 0.150 | Sy | Specific yield, Sy (dimensionless, default value is 0.15; max value is 0.2 only if proved with lab test data of 0.2 or higher) |
| 2.00 | Kh | Horizontal hydraulic conductivity (in/hr) Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
| 75.000 | x | 1/2 length of basin (x direction, in feet) |
| 50.000 | y | 1/2 width of basin (y direction, in feet) |
| 32.00 | t | Duration of infiltration period (hours) |
| 10.000 | hi(0) | Initial thickness of saturated zone (feet) |
| 22.599 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
| 12.599 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
- $t = {48}$ hours, ${\mathrm{R}}_{48} = 2 \ln /{hr}x\left( {{12hr}/{48hr}}\right) = {0.5}\ln /{hr}$ .
Initial Model with Adjusted $t = 48$ hr
| 0.50 | R | Recharge rate (permeability rate) (in/hr) |
| --- | --- | --- |
| 0.150 | Sy | Specific yield, Sy (dimensionless, default value is 0.15; max value is 0.2 only if proved with lab test data of 0.2 or higher) |
| 2.00 | Kh | Horizontal hydraulic conductivity (in/hr) Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
| 75.000 | x | 1/2 length of basin (x direction, in feet) |
| 50.000 | y | 1/2 width of basin (y direction, in feet) |
| 48.00 | t | Duration of infiltration period (hours) |
| 10.000 | hi(0) | Initial thickness of saturated zone (feet) |
| 21.883 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
| 11.883 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
17. Table 17
BMP 2026, Ch 13, p. 16
| 0.50 |
R |
Recharge rate (permeability rate) (in/hr) |
| 0.150 |
Sy |
Specific yield, Sy (dimensionless, default value is 0.15; max value is 0.2 only if proved with lab test data of 0.2 or higher) |
| 2.00 |
Kh |
Horizontal hydraulic conductivity (in/hr) Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
| 75.000 |
x |
1/2 length of basin (x direction, in feet) |
| 50.000 |
y |
1/2 width of basin (y direction, in feet) |
| 48.00 |
t |
Duration of infiltration period (hours) |
| 10.000 |
hi(0) |
Initial thickness of saturated zone (feet) |
| 21.883 |
h(max) |
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
| 11.883 |
Δh(max) |
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
Source context and notes
| 10.000 | hi(0) | Initial thickness of saturated zone (feet) |
| 22.599 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
| 12.599 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
- $t = {48}$ hours, ${\mathrm{R}}_{48} = 2 \ln /{hr}x\left( {{12hr}/{48hr}}\right) = {0.5}\ln /{hr}$ .
Initial Model with Adjusted $t = 48$ hr
| 0.50 | R | Recharge rate (permeability rate) (in/hr) |
| --- | --- | --- |
| 0.150 | Sy | Specific yield, Sy (dimensionless, default value is 0.15; max value is 0.2 only if proved with lab test data of 0.2 or higher) |
| 2.00 | Kh | Horizontal hydraulic conductivity (in/hr) Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
| 75.000 | x | 1/2 length of basin (x direction, in feet) |
| 50.000 | y | 1/2 width of basin (y direction, in feet) |
| 48.00 | t | Duration of infiltration period (hours) |
| 10.000 | hi(0) | Initial thickness of saturated zone (feet) |
| 21.883 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
| 11.883 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
As shown in the lower red shaded cells of the above two images, the calculated mounding heights are lower than that of the initial model; e.g., for a reduced recharge rate of 0.75 in/hr with a proportionally increased duration of infiltration period of 32 hr, the calculated height of groundwater mounding is 12.599 ft. Likewise, for a reduced recharge rate of 0.5 in/hr and a duration of infiltration period of 48 hr, the calculated height of groundwater mounding is 11.883 ft. As previously stated, the duration of infiltration period may be stretched only if the recharge rate is proportionally reduced and other inputs are not changed.
18. Table 18
BMP 2026, Ch 13, p. 18
| Example No. |
Scenario Description |
Page No. |
| 1 |
No adverse hydraulic impact to groundwater |
18 |
| 2 |
Adverse hydraulic impact to groundwater and adjustment of infiltration period |
20 |
| 3 |
Adverse hydraulic impact to groundwater and adjustment of basin size |
24 |
| 4 |
Rectangular or round infiltration BMP |
29 |
| 5 |
Underground perforated pipe |
31 |
| 6 |
Drywell |
34 |
| 7 |
Replacement of soil below an infiltration BMP |
35 |
| 8 |
Small-scale Infiltration Basin (Green Infrastructure BMP) in ‘B’ Soils |
39 |
Source context and notes
# Groundwater Table Hydraulic Impact Assessments for Infiltration BMPs
Source revision: January 2026.
# Examples
The following examples illustrate how to use the groundwater mounding analysis spreadsheet (USGS's Hantush Spreadsheet) to evaluate the hydraulic impact to groundwater from infiltration practices.
| Example No. | Scenario Description | Page No. |
| --- | --- | --- |
| 1 | No adverse hydraulic impact to groundwater | 18 |
| 2 | Adverse hydraulic impact to groundwater and adjustment of infiltration period | 20 |
| 3 | Adverse hydraulic impact to groundwater and adjustment of basin size | 24 |
| 4 | Rectangular or round infiltration BMP | 29 |
| 5 | Underground perforated pipe | 31 |
| 6 | Drywell | 34 |
| 7 | Replacement of soil below an infiltration BMP | 35 |
| 8 | Small-scale Infiltration Basin (Green Infrastructure BMP) in ‘B’ Soils | 39 |
# Example 1: No adverse hydraulic impact to groundwater scenario
A development located in the Coastal Plain proposes an infiltration basin, whose bottom footprint measures 50 ft by 50 ft, to infiltrate 5,000 cf of runoff generated by the Water Quality Design Storm (WQDS). The maximum depth of stormwater runoff to be infiltrated is designed to be 2 feet. Soil testing was performed in accordance with Chapter 12. The soil testing report shows that the Seasonal High Water Table (SHWT) is 7.5 ft below the 6 in thick bottom sand layer of the basin (8 feet below the bottom surface of the basin). The tested soil permeability rate of the most restrictive soil horizon below the basin is 4 in/hr. No nearby underground structure is present in this scenario.
## Step 1: Calculate the duration of infiltration period
19. Table 19
BMP 2026, Ch 13, p. 19
|
A |
B |
C |
D |
E |
F |
G |
H |
I |
J |
K |
| 6 |
Input Values |
|
|
|
|
|
|
|
|
|
|
| 7 |
2.00 |
R |
|
Recharge rate (permeability rate) (in/hr) |
|
|
|
|
|
|
|
| 8 |
0.150 |
Sy |
|
Specific yield, Sy (dimensionless)default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
|
|
|
|
| 9 |
10.00 |
Kh |
|
Horizontal hydraulic conductivity (in/hr)Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
|
|
|
|
|
|
|
| 10 |
25.000 |
x |
|
1/2 length of basin (x direction, in feet) |
|
|
|
|
|
|
|
| 11 |
25.000 |
y |
|
1/2 width of basin (y direction, in feet) |
|
|
|
|
|
|
|
| 12 |
12.00 |
t |
|
Duration of infiltration period (hours) |
|
|
|
|
|
|
|
| 13 |
10.000 |
hi(0) |
|
Initial thickness of saturated zone (feet) |
|
|
|
|
|
|
|
Source context and notes
Duration of infiltration period (t) $= 12$ hr
Initial thickness of saturated zone $(\mathrm{hi}(0)) = 10$ ft
The graphic below depicts the input section of the Hantush Spreadsheet with the parameter values shown above entered.
Example 1: Input Section of the Hantush Spreadsheet
| | A | B | C | D | E | F | G | H | I | J | K |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 6 | Input Values | | | | | | | | | | |
| 7 | 2.00 | R | | Recharge rate (permeability rate) (in/hr) | | | | | | | |
| 8 | 0.150 | Sy | | Specific yield, Sy (dimensionless)default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | | | | | |
| 9 | 10.00 | Kh | | Horizontal hydraulic conductivity (in/hr)Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | | | | | | |
| 10 | 25.000 | x | | 1/2 length of basin (x direction, in feet) | | | | | | | |
| 11 | 25.000 | y | | 1/2 width of basin (y direction, in feet) | | | | | | | |
| 12 | 12.00 | t | | Duration of infiltration period (hours) | | | | | | | |
| 13 | 10.000 | hi(0) | | Initial thickness of saturated zone (feet) | | | | | | | |
The results, depicted below, show the maximum height of groundwater mounding, 6.07 ft, that will occur at the end of the duration of infiltration period. Since the SHWT is 8 ft below the surface of the sand layer, the temporarily elevated groundwater table, located at 6.07 ft above the SHWT elevation, will not reach the bottom surface of the BMP. Therefore, the assessment shows no adverse hydraulic impact to the groundwater table from the proposed infiltration BMP.
Example 1: Results Section of the Hantush Spreadsheet
| 15 | 16.07 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 16 | 4.07 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | | | | |
| 17 | Ground-water Mounding, in feet | Distance from center of basin in x direction, in feet | | | | | | | | | | |
| 18 | 6.073 | 0 | Re-Calculate Now | | | | | | | | | |
| 19 | 4.900 | 20 | | | | | | | | | | |
| 20 | 2.066 | 40 | | | | | | | | | | |
| 21 | 1.205 | 50 | | | | | | | | | | |
| 22 | 0.666 | 60 | | | | | | | | | | |
| 23 | 0.350 | 70 | | | | | | | | | | |
| 24 | 0.176 | 80 | | | | | | | | | | |
| 25 | 0.086 | 90 | | | | | | | | | | |
| 26 | 0.041 | 100 | | | | | | | | | | |
| 27 | 0.011 | 120 | | | | | | | | | | |
| 28 | | | | | | | | | | | | |
| 29 | | | | | | | | | | | | |
| 30 | | | | | | | | | | | | |
| 31 | | | | | | | | | | | | |
| 32 | | | | | | | | | | | | |
| 33 | | | SHWT→0.000 | | | | | | | | | |
| 34 | | | | | | | | | | | | |
| 35 | | | | | | | | | | | | |
20. Table 20
BMP 2026, Ch 13, p. 19
| 15 |
16.07 |
h(max) |
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
|
|
|
| 16 |
4.07 |
Δh(max) |
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
|
|
|
| 17 |
Ground-water Mounding, in feet |
Distance from center of basin in x direction, in feet |
|
|
|
|
|
|
|
|
|
|
| 18 |
6.073 |
0 |
Re-Calculate Now |
|
|
|
|
|
|
|
|
|
| 19 |
4.900 |
20 |
|
|
|
|
|
|
|
|
|
|
| 20 |
2.066 |
40 |
|
|
|
|
|
|
|
|
|
|
| 21 |
1.205 |
50 |
|
|
|
|
|
|
|
|
|
|
| 22 |
0.666 |
60 |
|
|
|
|
|
|
|
|
|
|
| 23 |
0.350 |
70 |
|
|
|
|
|
|
|
|
|
|
| 24 |
0.176 |
80 |
|
|
|
|
|
|
|
|
|
|
| 25 |
0.086 |
90 |
|
|
|
|
|
|
|
|
|
|
| 26 |
0.041 |
100 |
|
|
|
|
|
|
|
|
|
|
| 27 |
0.011 |
120 |
|
|
|
|
|
|
|
|
|
|
| 28 |
|
|
|
|
|
|
|
|
|
|
|
|
| 29 |
|
|
|
|
|
|
|
|
|
|
|
|
| 30 |
|
|
|
|
|
|
|
|
|
|
|
|
| 31 |
|
|
|
|
|
|
|
|
|
|
|
|
| 32 |
|
|
|
|
|
|
|
|
|
|
|
|
| 33 |
|
|
SHWT→0.000 |
|
|
|
|
|
|
|
|
|
| 34 |
|
|
|
|
|
|
|
|
|
|
|
|
| 35 |
|
|
|
|
|
|
|
|
|
|
|
|
Source context and notes
| 11 | 25.000 | y | | 1/2 width of basin (y direction, in feet) | | | | | | | |
| 12 | 12.00 | t | | Duration of infiltration period (hours) | | | | | | | |
| 13 | 10.000 | hi(0) | | Initial thickness of saturated zone (feet) | | | | | | | |
The results, depicted below, show the maximum height of groundwater mounding, 6.07 ft, that will occur at the end of the duration of infiltration period. Since the SHWT is 8 ft below the surface of the sand layer, the temporarily elevated groundwater table, located at 6.07 ft above the SHWT elevation, will not reach the bottom surface of the BMP. Therefore, the assessment shows no adverse hydraulic impact to the groundwater table from the proposed infiltration BMP.
Example 1: Results Section of the Hantush Spreadsheet
| 15 | 16.07 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 16 | 4.07 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | | | | |
| 17 | Ground-water Mounding, in feet | Distance from center of basin in x direction, in feet | | | | | | | | | | |
| 18 | 6.073 | 0 | Re-Calculate Now | | | | | | | | | |
| 19 | 4.900 | 20 | | | | | | | | | | |
| 20 | 2.066 | 40 | | | | | | | | | | |
| 21 | 1.205 | 50 | | | | | | | | | | |
| 22 | 0.666 | 60 | | | | | | | | | | |
| 23 | 0.350 | 70 | | | | | | | | | | |
| 24 | 0.176 | 80 | | | | | | | | | | |
| 25 | 0.086 | 90 | | | | | | | | | | |
| 26 | 0.041 | 100 | | | | | | | | | | |
| 27 | 0.011 | 120 | | | | | | | | | | |
| 28 | | | | | | | | | | | | |
| 29 | | | | | | | | | | | | |
| 30 | | | | | | | | | | | | |
| 31 | | | | | | | | | | | | |
| 32 | | | | | | | | | | | | |
| 33 | | | SHWT→0.000 | | | | | | | | | |
| 34 | | | | | | | | | | | | |
| 35 | | | | | | | | | | | | |
21. Table 21
BMP 2026, Ch 13, p. 20
|
A |
B |
C |
D |
E |
F |
G |
H |
I |
J |
K |
| 6 |
Input Values |
|
|
|
|
|
|
|
|
|
|
| 7 |
2.00 |
R |
|
Recharge rate (permeability rate) (in/hr) |
|
|
|
|
|
|
|
| 8 |
0.150 |
Sy |
|
Specific yield, Sy (dimensionless) default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
|
|
|
|
| 9 |
10.00 |
Kh |
|
Horizontal hydraulic conductivity (in/hr) Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
|
|
|
|
|
|
|
| 10 |
44.000 |
x |
|
1/2 length of basin (x direction, in feet) |
|
|
|
|
|
|
|
| 11 |
44.000 |
y |
|
1/2 width of basin (y direction, in feet) |
|
|
|
|
|
|
|
| 12 |
11.62 |
t |
|
Duration of infiltration period (hours) |
|
|
|
|
|
|
|
| 13 |
10.000 |
hi(0) |
|
Initial thickness of saturated zone (feet) |
|
|
|
|
|
|
|
Source context and notes
Duration of infiltration period (t) = 11.62 hr
Initial thickness of saturated zone (hi(0)) = 10 ft
The graphic shown below depicts the input section of the Hantush Spreadsheet with the parameter values shown above entered.
Example 2: Input Section of the Hantush Spreadsheet
| | A | B | C | D | E | F | G | H | I | J | K |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 6 | Input Values | | | | | | | | | | |
| 7 | 2.00 | R | | Recharge rate (permeability rate) (in/hr) | | | | | | | |
| 8 | 0.150 | Sy | | Specific yield, Sy (dimensionless) default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | | | | | |
| 9 | 10.00 | Kh | | Horizontal hydraulic conductivity (in/hr) Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | | | | | | |
| 10 | 44.000 | x | | 1/2 length of basin (x direction, in feet) | | | | | | | |
| 11 | 44.000 | y | | 1/2 width of basin (y direction, in feet) | | | | | | | |
| 12 | 11.62 | t | | Duration of infiltration period (hours) | | | | | | | |
| 13 | 10.000 | hi(0) | | Initial thickness of saturated zone (feet) | | | | | | | |
22. Table 22
BMP 2026, Ch 13, p. 22
|
A |
B |
C |
D |
E |
F |
G |
H |
I |
J |
K |
| 6 |
Input Values |
|
|
|
|
|
|
|
|
|
|
| 7 |
1.00 |
R |
|
Recharge rate (permeability rate) (in/hr) |
|
|
|
|
|
|
|
| 8 |
0.150 |
Sy |
|
Specific yield, Sy (dimensionless)default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
|
|
|
|
| 9 |
10.00 |
Kh |
|
Horizontal hydraulic conductivity (in/hr)Kh = 5xRecharge Rate (R) in the coastal plain: Kh=R outside the coastal plain |
|
|
|
|
|
|
|
| 10 |
44.000 |
x |
|
1/2 length of basin (x direction, in feet) |
|
|
|
|
|
|
|
| 11 |
44.000 |
y |
|
1/2 width of basin (y direction, in feet) |
|
|
|
|
|
|
|
| 12 |
23.24 |
t |
|
Duration of infiltration period (hours) |
|
|
|
|
|
|
|
| 13 |
10.000 |
hi(0) |
|
Initial thickness of saturated zone (feet) |
|
|
|
|
|
|
|
Source context and notes
# Groundwater Table Hydraulic Impact Assessments for Infiltration BMPs
Source revision: January 2026.
The graphic below depicts the input section of the Hantush Spreadsheet with the parameter values, adjusted as shown on the previous page, entered.
Example 2: Revised Input Section of the Hantush Spreadsheet
| | A | B | C | D | E | F | G | H | I | J | K |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 6 | Input Values | | | | | | | | | | |
| 7 | 1.00 | R | | Recharge rate (permeability rate) (in/hr) | | | | | | | |
| 8 | 0.150 | Sy | | Specific yield, Sy (dimensionless)default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | | | | | |
| 9 | 10.00 | Kh | | Horizontal hydraulic conductivity (in/hr)Kh = 5xRecharge Rate (R) in the coastal plain: Kh=R outside the coastal plain | | | | | | | |
| 10 | 44.000 | x | | 1/2 length of basin (x direction, in feet) | | | | | | | |
| 11 | 44.000 | y | | 1/2 width of basin (y direction, in feet) | | | | | | | |
| 12 | 23.24 | t | | Duration of infiltration period (hours) | | | | | | | |
| 13 | 10.000 | hi(0) | | Initial thickness of saturated zone (feet) | | | | | | | |
The results, depicted below, show the maximum height of groundwater mounding, 7.33 ft, that will occur at the end of the duration of infiltration period. Since the SHWT is 7.5 ft below the bottom of the sand layer or 8 feet below the basin bottom, the temporarily elevated groundwater table, located at 7.33 ft above the SHWT elevation, will not reach the bottom surface of the basin. Therefore, the assessment shows no adverse hydraulic impact to the groundwater table from the proposed infiltration BMP. As calculated above, the infiltration basin now needs 23.24 hours to drain. Since the drain time is still within 72 hours, the design of the proposed infiltration basin still meets the design criteria set forth in the BMP manual. Note that if there are underground structures near the BMP, an assessment of the hydraulic impact will need to be performed. See examples 5 and 6 for the assessment.
Example 2: New Results Section of the Hantush Spreadsheet
| 15 | 11.00 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 16 | 7.33 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | | | | |
| 17 | Ground-water Mounding, in feet | Distance from center of basin in x direction, in feet | | | | | | | | | | |
| 18 | 2.325 | 0 | Re-Calculate Now | | | | | | | | | |
| 19 | 7.210 | 10 | | | | | | | | | | |
| 20 | 8.853 | 20 | | | | | | | | | | |
| 21 | 8.220 | 30 | | | | | | | | | | |
| 22 | 9.264 | 40 | | | | | | | | | | |
| 23 | 3.906 | 50 | | | | | | | | | | |
| 24 | 2.850 | 60 | | | | | | | | | | |
| 25 | 1.985 | 70 | | | | | | | | | | |
| 26 | 1.339 | 80 | | | | | | | | | | |
| 27 | 0.877 | 90 | | | | | | | | | | |
| 28 | | | | | | | | | | | | |
| 29 | | | | | | | | | | | | |
| 30 | | | | | | | | | | | | |
| 31 | | | | | | | | | | | | |
| 32 | | | | | | | | | | | | |
| 33 | | | SHWT → | | | | | | | | | |
| 34 | | | | | | | | | | | | |
| 35 | | | | | | | | | | | | |
23. Table 23
BMP 2026, Ch 13, p. 22
| 15 |
11.00 |
h(max) |
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
|
|
|
| 16 |
7.33 |
Δh(max) |
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
|
|
|
| 17 |
Ground-water Mounding, in feet |
Distance from center of basin in x direction, in feet |
|
|
|
|
|
|
|
|
|
|
| 18 |
2.325 |
0 |
Re-Calculate Now |
|
|
|
|
|
|
|
|
|
| 19 |
7.210 |
10 |
|
|
|
|
|
|
|
|
|
|
| 20 |
8.853 |
20 |
|
|
|
|
|
|
|
|
|
|
| 21 |
8.220 |
30 |
|
|
|
|
|
|
|
|
|
|
| 22 |
9.264 |
40 |
|
|
|
|
|
|
|
|
|
|
| 23 |
3.906 |
50 |
|
|
|
|
|
|
|
|
|
|
| 24 |
2.850 |
60 |
|
|
|
|
|
|
|
|
|
|
| 25 |
1.985 |
70 |
|
|
|
|
|
|
|
|
|
|
| 26 |
1.339 |
80 |
|
|
|
|
|
|
|
|
|
|
| 27 |
0.877 |
90 |
|
|
|
|
|
|
|
|
|
|
| 28 |
|
|
|
|
|
|
|
|
|
|
|
|
| 29 |
|
|
|
|
|
|
|
|
|
|
|
|
| 30 |
|
|
|
|
|
|
|
|
|
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|
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| 31 |
|
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|
|
| 32 |
|
|
|
|
|
|
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|
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|
|
| 33 |
|
|
SHWT → |
|
|
|
|
|
|
|
|
|
| 34 |
|
|
|
|
|
|
|
|
|
|
|
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| 35 |
|
|
|
|
|
|
|
|
|
|
|
|
Source context and notes
| 11 | 44.000 | y | | 1/2 width of basin (y direction, in feet) | | | | | | | |
| 12 | 23.24 | t | | Duration of infiltration period (hours) | | | | | | | |
| 13 | 10.000 | hi(0) | | Initial thickness of saturated zone (feet) | | | | | | | |
The results, depicted below, show the maximum height of groundwater mounding, 7.33 ft, that will occur at the end of the duration of infiltration period. Since the SHWT is 7.5 ft below the bottom of the sand layer or 8 feet below the basin bottom, the temporarily elevated groundwater table, located at 7.33 ft above the SHWT elevation, will not reach the bottom surface of the basin. Therefore, the assessment shows no adverse hydraulic impact to the groundwater table from the proposed infiltration BMP. As calculated above, the infiltration basin now needs 23.24 hours to drain. Since the drain time is still within 72 hours, the design of the proposed infiltration basin still meets the design criteria set forth in the BMP manual. Note that if there are underground structures near the BMP, an assessment of the hydraulic impact will need to be performed. See examples 5 and 6 for the assessment.
Example 2: New Results Section of the Hantush Spreadsheet
| 15 | 11.00 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 16 | 7.33 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | | | | |
| 17 | Ground-water Mounding, in feet | Distance from center of basin in x direction, in feet | | | | | | | | | | |
| 18 | 2.325 | 0 | Re-Calculate Now | | | | | | | | | |
| 19 | 7.210 | 10 | | | | | | | | | | |
| 20 | 8.853 | 20 | | | | | | | | | | |
| 21 | 8.220 | 30 | | | | | | | | | | |
| 22 | 9.264 | 40 | | | | | | | | | | |
| 23 | 3.906 | 50 | | | | | | | | | | |
| 24 | 2.850 | 60 | | | | | | | | | | |
| 25 | 1.985 | 70 | | | | | | | | | | |
| 26 | 1.339 | 80 | | | | | | | | | | |
| 27 | 0.877 | 90 | | | | | | | | | | |
| 28 | | | | | | | | | | | | |
| 29 | | | | | | | | | | | | |
| 30 | | | | | | | | | | | | |
| 31 | | | | | | | | | | | | |
| 32 | | | | | | | | | | | | |
| 33 | | | SHWT → | | | | | | | | | |
| 34 | | | | | | | | | | | | |
| 35 | | | | | | | | | | | | |
Creating a table, as mentioned in the discussion on Page 15, to record the inputs and results, may be helpful. On the following page is an example of such a table.
24. Table 24
BMP 2026, Ch 13, p. 23
| Model No. |
Depth (ft) |
Depth (in) |
R (in/hr) |
( S_y ) |
( K_h ) (in/hr) |
x (ft) |
y (ft) |
t (hr) |
hi(0) (ft) |
( \Delta h(max) ) (ft) |
| Initial |
2 |
24 |
2 |
0.15 |
10 |
44 |
44 |
11.62 |
10 |
9.51 |
| Adjusted |
2 |
24 |
1 |
0.15 |
10 |
44 |
44 |
23.24 |
10 |
7.33 |
Source context and notes
# Groundwater Table Hydraulic Impact Assessments for Infiltration BMPs
Source revision: January 2026.
| Model No. | Depth (ft) | Depth (in) | R (in/hr) | \( S_y \) | \( K_h \) (in/hr) | x (ft) | y (ft) | t (hr) | hi(0) (ft) | \( \Delta h(max) \) (ft) |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| Initial | 2 | 24 | 2 | 0.15 | 10 | 44 | 44 | 11.62 | 10 | 9.51 |
| Adjusted | 2 | 24 | 1 | 0.15 | 10 | 44 | 44 | 23.24 | 10 | 7.33 |
A comparison of the mounding effects is shown below. The red dotted line represents the mounding effect from the infiltration basin assuming a recharge rate of 2 in/hr and infiltrating the runoff in 11.62 hr. The blue solid line represents the infiltration basin assuming a recharge rate of 1 in/hr and infiltrating the runoff in 23.24 hr.
[Figure: img-4.jpeg (original PDF, p. 23)](SOURCE.pdf#page=23)
Example 2: A Comparison of Mounding Effects Caused by Different Recharge Rates
25. Table 25
BMP 2026, Ch 13, p. 25
|
A |
B |
C |
D |
E |
F |
G |
H |
I |
J |
K |
| 6 |
Input Values |
|
|
|
|
|
|
|
|
|
|
| 7 |
0.50 |
R |
Recharge rate (permeability rate) (in/hr) |
|
|
|
|
|
|
|
|
| 8 |
0.150 |
Sy |
Specific yield, Sy (dimensionless) default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
|
|
|
|
|
| 9 |
0.50 |
Kh |
Horizontal hydraulic conductivity (in/hr) Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
|
|
|
|
|
|
|
|
| 10 |
40.000 |
x |
1/2 length of basin (x direction, in feet) |
|
|
|
|
|
|
|
|
| 11 |
40.000 |
y |
1/2 width of basin (y direction, in feet) |
|
|
|
|
|
|
|
|
| 12 |
48.00 |
t |
Duration of infiltration period (hours) |
|
|
|
|
|
|
|
|
| 13 |
10.000 |
hi(0) |
Initial thickness of saturated zone (feet) |
|
|
|
|
|
|
|
|
Source context and notes
# Groundwater Table Hydraulic Impact Assessments for Infiltration BMPs
Source revision: January 2026.
The graphic below depicts the input section of the Hantush Spreadsheet with the parameter values shown above entered.
Example 3: Input Section of the Hantush Spreadsheet
| | A | B | C | D | E | F | G | H | I | J | K |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 6 | Input Values | | | | | | | | | | |
| 7 | 0.50 | R | Recharge rate (permeability rate) (in/hr) | | | | | | | | |
| 8 | 0.150 | Sy | Specific yield, Sy (dimensionless) default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | | | | | | |
| 9 | 0.50 | Kh | Horizontal hydraulic conductivity (in/hr) Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | | | | | | | |
| 10 | 40.000 | x | 1/2 length of basin (x direction, in feet) | | | | | | | | |
| 11 | 40.000 | y | 1/2 width of basin (y direction, in feet) | | | | | | | | |
| 12 | 48.00 | t | Duration of infiltration period (hours) | | | | | | | | |
| 13 | 10.000 | hi(0) | Initial thickness of saturated zone (feet) | | | | | | | | |
The results depicted below show the maximum height of groundwater mounding, 12.94 ft, that will occur at the end of the duration of infiltration period. Since the SHWT is 4 ft below the bottom of the sand layer, the temporarily elevated groundwater table, located at 12.94 ft above the SHWT elevation, intersects and exceeds the elevation of the bottom surface of the basin, interfering with the infiltration capability. Adjustment of the basin design is needed to reduce the impact of groundwater mounding.
Example 3: Results Section of the Hantush Spreadsheet
| 15 | 22.94 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 16 | 12.94 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | | | |
| 17 | Ground-water Mounding, in feet | Distance from center of basin in x direction, in feet | | | | | | | | | |
| 18 | 12.939 | 0 | Re-Calculate Now | | | | | | | | |
| 19 | 12.089 | 20 | | | | | | | | | |
| 20 | 6.656 | 40 | | | | | | | | | |
| 21 | 2.394 | 50 | | | | | | | | | |
| 22 | 0.628 | 60 | | | | | | | | | |
| 23 | 0.129 | 70 | | | | | | | | | |
| 24 | 0.023 | 80 | | | | | | | | | |
| 25 | 0.007 | 90 | | | | | | | | | |
| 26 | 0.005 | 100 | | | | | | | | | |
| 27 | 0.004 | 120 | | | | | | | | | |
| 28 | | | | | | | | | | | |
| 29 | | | | | | | | | | | |
| 30 | | | | | | | | | | | |
| 31 | | | | | | | | | | | |
| 32 | | | | | | | | | | | |
| 33 | | | | | | | | | | | |
| 34 | | | SHWT | | | | 0 | 20 | 40 | 60 | 80 |
| 35 | | | | | | | | | | | 140 |
26. Table 26
BMP 2026, Ch 13, p. 25
| 15 |
22.94 |
h(max) |
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
|
|
| 16 |
12.94 |
Δh(max) |
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
|
|
| 17 |
Ground-water Mounding, in feet |
Distance from center of basin in x direction, in feet |
|
|
|
|
|
|
|
|
|
| 18 |
12.939 |
0 |
Re-Calculate Now |
|
|
|
|
|
|
|
|
| 19 |
12.089 |
20 |
|
|
|
|
|
|
|
|
|
| 20 |
6.656 |
40 |
|
|
|
|
|
|
|
|
|
| 21 |
2.394 |
50 |
|
|
|
|
|
|
|
|
|
| 22 |
0.628 |
60 |
|
|
|
|
|
|
|
|
|
| 23 |
0.129 |
70 |
|
|
|
|
|
|
|
|
|
| 24 |
0.023 |
80 |
|
|
|
|
|
|
|
|
|
| 25 |
0.007 |
90 |
|
|
|
|
|
|
|
|
|
| 26 |
0.005 |
100 |
|
|
|
|
|
|
|
|
|
| 27 |
0.004 |
120 |
|
|
|
|
|
|
|
|
|
| 28 |
|
|
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|
|
|
|
|
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| 29 |
|
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| 30 |
|
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| 31 |
|
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|
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|
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| 32 |
|
|
|
|
|
|
|
|
|
|
|
| 33 |
|
|
|
|
|
|
|
|
|
|
|
| 34 |
|
|
SHWT |
|
|
|
0 |
20 |
40 |
60 |
80 |
| 35 |
|
|
|
|
|
|
|
|
|
|
140 |
Source context and notes
| 11 | 40.000 | y | 1/2 width of basin (y direction, in feet) | | | | | | | | |
| 12 | 48.00 | t | Duration of infiltration period (hours) | | | | | | | | |
| 13 | 10.000 | hi(0) | Initial thickness of saturated zone (feet) | | | | | | | | |
The results depicted below show the maximum height of groundwater mounding, 12.94 ft, that will occur at the end of the duration of infiltration period. Since the SHWT is 4 ft below the bottom of the sand layer, the temporarily elevated groundwater table, located at 12.94 ft above the SHWT elevation, intersects and exceeds the elevation of the bottom surface of the basin, interfering with the infiltration capability. Adjustment of the basin design is needed to reduce the impact of groundwater mounding.
Example 3: Results Section of the Hantush Spreadsheet
| 15 | 22.94 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 16 | 12.94 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | | | |
| 17 | Ground-water Mounding, in feet | Distance from center of basin in x direction, in feet | | | | | | | | | |
| 18 | 12.939 | 0 | Re-Calculate Now | | | | | | | | |
| 19 | 12.089 | 20 | | | | | | | | | |
| 20 | 6.656 | 40 | | | | | | | | | |
| 21 | 2.394 | 50 | | | | | | | | | |
| 22 | 0.628 | 60 | | | | | | | | | |
| 23 | 0.129 | 70 | | | | | | | | | |
| 24 | 0.023 | 80 | | | | | | | | | |
| 25 | 0.007 | 90 | | | | | | | | | |
| 26 | 0.005 | 100 | | | | | | | | | |
| 27 | 0.004 | 120 | | | | | | | | | |
| 28 | | | | | | | | | | | |
| 29 | | | | | | | | | | | |
| 30 | | | | | | | | | | | |
| 31 | | | | | | | | | | | |
| 32 | | | | | | | | | | | |
| 33 | | | | | | | | | | | |
| 34 | | | SHWT | | | | 0 | 20 | 40 | 60 | 80 |
| 35 | | | | | | | | | | | 140 |
# Step 3: Adjust basin size
The excessive height of groundwater mounding seen in the results from Step 2 indicates the design is concentrating the infiltration of runoff into a footprint that is too small. The adjustment will have to evaluate a larger basin area, which infers a shallower maximum depth of stormwater runoff to be infiltrated, in order to reduce the height of groundwater mounding under the center of the BMP. For this example, the area of the basin bottom will be increased from 80 ft by 80 ft to 160 ft by 160 ft; i.e., the area increases from 6,400 sf to 25,600 sf. Accordingly, the maximum depth of stormwater runoff to be infiltrated is proportionally decreased:
27. Table 27
BMP 2026, Ch 13, p. 26
| 6 |
Input Values |
|
|
|
|
|
|
| 7 |
0.50 |
R |
|
Recharge rate (permeability rate) (in/hr) |
|
|
|
| 8 |
0.150 |
Sy |
|
Specific yield, Sy (dimensionless)default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
| 9 |
0.50 |
Kh |
|
Horizontal hydraulic conductivity (in/hr)Kh = SxRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
|
|
|
| 10 |
80.000 |
x |
|
1/2 length of basin (x direction, in feet) |
|
|
|
| 11 |
80.000 |
y |
|
1/2 width of basin (y direction, in feet) |
|
|
|
| 12 |
12.00 |
t |
|
Duration of infiltration period (hours) |
|
|
|
| 13 |
10.000 |
hi(0) |
|
Initial thickness of saturated zone (feet) |
|
|
|
Source context and notes
Duration of infiltration period (t) = 12 hr
Initial thickness of saturated zone (hi(0)) = 10 ft
The graphic below depicts the input section of the Hantush Spreadsheet with the parameter values shown above entered.
Example 3: Revised Input Section of the Hantush Spreadsheet
| 6 | Input Values | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- |
| 7 | 0.50 | R | | Recharge rate (permeability rate) (in/hr) | | | |
| 8 | 0.150 | Sy | | Specific yield, Sy (dimensionless)default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | |
| 9 | 0.50 | Kh | | Horizontal hydraulic conductivity (in/hr)Kh = SxRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | | |
| 10 | 80.000 | x | | 1/2 length of basin (x direction, in feet) | | | |
| 11 | 80.000 | y | | 1/2 width of basin (y direction, in feet) | | | |
| 12 | 12.00 | t | | Duration of infiltration period (hours) | | | |
| 13 | 10.000 | hi(0) | | Initial thickness of saturated zone (feet) | | | |
The results, depicted on the following page, show the maximum height of groundwater mounding, 3.33 ft, that will occur at the end of the duration of infiltration period. Since the SHWT is 4 ft below the bottom of the sand layer, the temporarily elevated groundwater table, located at 3.33 ft above the SHWT elevation, will not reach the bottom surface of the basin. Therefore, the assessment shows no adverse hydraulic impact to the groundwater table from the redesigned infiltration BMP. As calculated above, the infiltration basin now needs 12 hours to drain. Since the drain time is still within 72 hours, the design of the proposed infiltration basin still meets the design criteria set forth in the BMP manual.
28. Table 28
BMP 2026, Ch 13, p. 28
| 6 |
Input Values |
|
|
|
|
|
| 7 |
0.50 |
R |
|
Recharge rate (permeability rate) (in/hr) |
|
|
| 8 |
0.150 |
Sy |
|
Specific yield, Sy (dimensionless) |
|
|
|
|
|
|
default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
| 9 |
0.50 |
Kh |
|
Horizontal hydraulic conductivity (in/hr) |
|
|
|
|
|
|
Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
|
|
| 10 |
73.000 |
x |
|
1/2 length of basin (x direction, in feet) |
|
|
| 11 |
73.000 |
y |
|
1/2 width of basin (y direction, in feet) |
|
|
| 12 |
14.40 |
t |
|
Duration of infiltration period (hours) |
|
|
| 13 |
10.000 |
hi(0) |
|
Initial thickness of saturated zone (feet) |
|
|
Source context and notes
$\frac{1}{2}$ length of the basin $= 146ft / 2$ $= 73ft$
t $= \frac{12,800cf\times 12in / ft}{21,333sf\times 0.5in / hr} =$ 14.4 hrs
The new adjusted parameter values are entered into the Hantush Spreadsheet.
Example 3: Revised Input Section of the Hantush Spreadsheet
| 6 | Input Values | | | | | |
| --- | --- | --- | --- | --- | --- | --- |
| 7 | 0.50 | R | | Recharge rate (permeability rate) (in/hr) | | |
| 8 | 0.150 | Sy | | Specific yield, Sy (dimensionless) | | |
| | | | | default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | |
| 9 | 0.50 | Kh | | Horizontal hydraulic conductivity (in/hr) | | |
| | | | | Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | |
| 10 | 73.000 | x | | 1/2 length of basin (x direction, in feet) | | |
| 11 | 73.000 | y | | 1/2 width of basin (y direction, in feet) | | |
| 12 | 14.40 | t | | Duration of infiltration period (hours) | | |
| 13 | 10.000 | hi(0) | | Initial thickness of saturated zone (feet) | | |
29. Table 29
BMP 2026, Ch 13, p. 30
| 6 |
Input Values |
|
|
|
|
|
|
| 7 |
0.50 |
R |
|
Recharge rate (permeability rate) (in/hr) |
|
|
|
| 8 |
0.150 |
Sy |
|
Specific yield, Sy (dimensionless) default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
| 9 |
0.50 |
Kh |
|
Horizontal hydraulic conductivity (in/hr) Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
|
|
|
| 10 |
160.000 |
x |
|
1/2 length of basin (x direction, in feet) |
|
|
|
| 11 |
40.000 |
y |
|
1/2 width of basin (y direction, in feet) |
|
|
|
| 12 |
12.00 |
t |
|
Duration of infiltration period (hours) |
|
|
|
| 13 |
10.000 |
hi(0) |
|
Initial thickness of saturated zone (feet) |
|
|
|
Source context and notes
# Groundwater Table Hydraulic Impact Assessments for Infiltration BMPs
Source revision: January 2026.
Example 4: Input Section of the Hantush Spreadsheet – Long Side Evaluation
| 6 | Input Values | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- |
| 7 | 0.50 | R | | Recharge rate (permeability rate) (in/hr) | | | |
| 8 | 0.150 | Sy | | Specific yield, Sy (dimensionless) default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | |
| 9 | 0.50 | Kh | | Horizontal hydraulic conductivity (in/hr) Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | | |
| 10 | 160.000 | x | | 1/2 length of basin (x direction, in feet) | | | |
| 11 | 40.000 | y | | 1/2 width of basin (y direction, in feet) | | | |
| 12 | 12.00 | t | | Duration of infiltration period (hours) | | | |
| 13 | 10.000 | hi(0) | | Initial thickness of saturated zone (feet) | | | |
The results show the extent of the mounding along the direction of the long side:
Example 4: Results Section of the Hantush Spreadsheet – Long Side Evaluation
| 15 | 13.33 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 16 | 3.33 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | | | | |
| 17 | Ground-water Mounding, in feet | Distance from center of basin in x direction, in feet | | | | | | | | | | |
| 18 | 3.333 | 0 | Re-Calculate Now | | | | | | | | | |
| 19 | 3.333 | 20 | | | | | | | | | | |
| 20 | 3.333 | 40 | | | | | | | | | | |
| 21 | 3.333 | 60 | | | | | | | | | | |
| 22 | 3.333 | 80 | | | | | | | | | | |
| 23 | 3.333 | 100 | | | | | | | | | | |
| 24 | 3.333 | 120 | | | | | | | | | | |
| 25 | 3.333 | 140 | | | | | | | | | | |
| 26 | 1.667 | 160 | | | | | | | | | | |
| 27 | 0.000 | 180 | | | | | | | | | | |
| 28 | | | | | | | | | | | | |
| 29 | | | | | | | | | | | | |
| 30 | | | | | | | | | | | | |
| 31 | | | | | | | | | | | | |
| 32 | | | | | | | | | | | | |
| 33 | | | | | | | | | | | | |
| 34 | | | | | | | | | | | | |
| 35 | | | | | | | | | | | | |
30. Table 30
BMP 2026, Ch 13, p. 30
| 15 |
13.33 |
h(max) |
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
|
|
|
| 16 |
3.33 |
Δh(max) |
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
|
|
|
| 17 |
Ground-water Mounding, in feet |
Distance from center of basin in x direction, in feet |
|
|
|
|
|
|
|
|
|
|
| 18 |
3.333 |
0 |
Re-Calculate Now |
|
|
|
|
|
|
|
|
|
| 19 |
3.333 |
20 |
|
|
|
|
|
|
|
|
|
|
| 20 |
3.333 |
40 |
|
|
|
|
|
|
|
|
|
|
| 21 |
3.333 |
60 |
|
|
|
|
|
|
|
|
|
|
| 22 |
3.333 |
80 |
|
|
|
|
|
|
|
|
|
|
| 23 |
3.333 |
100 |
|
|
|
|
|
|
|
|
|
|
| 24 |
3.333 |
120 |
|
|
|
|
|
|
|
|
|
|
| 25 |
3.333 |
140 |
|
|
|
|
|
|
|
|
|
|
| 26 |
1.667 |
160 |
|
|
|
|
|
|
|
|
|
|
| 27 |
0.000 |
180 |
|
|
|
|
|
|
|
|
|
|
| 28 |
|
|
|
|
|
|
|
|
|
|
|
|
| 29 |
|
|
|
|
|
|
|
|
|
|
|
|
| 30 |
|
|
|
|
|
|
|
|
|
|
|
|
| 31 |
|
|
|
|
|
|
|
|
|
|
|
|
| 32 |
|
|
|
|
|
|
|
|
|
|
|
|
| 33 |
|
|
|
|
|
|
|
|
|
|
|
|
| 34 |
|
|
|
|
|
|
|
|
|
|
|
|
| 35 |
|
|
|
|
|
|
|
|
|
|
|
|
Source context and notes
| 11 | 40.000 | y | | 1/2 width of basin (y direction, in feet) | | | |
| 12 | 12.00 | t | | Duration of infiltration period (hours) | | | |
| 13 | 10.000 | hi(0) | | Initial thickness of saturated zone (feet) | | | |
The results show the extent of the mounding along the direction of the long side:
Example 4: Results Section of the Hantush Spreadsheet – Long Side Evaluation
| 15 | 13.33 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 16 | 3.33 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | | | | |
| 17 | Ground-water Mounding, in feet | Distance from center of basin in x direction, in feet | | | | | | | | | | |
| 18 | 3.333 | 0 | Re-Calculate Now | | | | | | | | | |
| 19 | 3.333 | 20 | | | | | | | | | | |
| 20 | 3.333 | 40 | | | | | | | | | | |
| 21 | 3.333 | 60 | | | | | | | | | | |
| 22 | 3.333 | 80 | | | | | | | | | | |
| 23 | 3.333 | 100 | | | | | | | | | | |
| 24 | 3.333 | 120 | | | | | | | | | | |
| 25 | 3.333 | 140 | | | | | | | | | | |
| 26 | 1.667 | 160 | | | | | | | | | | |
| 27 | 0.000 | 180 | | | | | | | | | | |
| 28 | | | | | | | | | | | | |
| 29 | | | | | | | | | | | | |
| 30 | | | | | | | | | | | | |
| 31 | | | | | | | | | | | | |
| 32 | | | | | | | | | | | | |
| 33 | | | | | | | | | | | | |
| 34 | | | | | | | | | | | | |
| 35 | | | | | | | | | | | | |
## Step 2: Switch the x and y inputs and re-analyze
Next, switch the x and y inputs, and evaluate the results, meaning the short side will be the x direction and the long side will be the y direction.
$$
\begin{array}{l}
1/2 \text{ length of basin (x direction)} = 40 \text{ ft} \\
1/2 \text{ length of basin (y direction)} = 160 \text{ ft} \\
\end{array}
$$
31. Table 31
BMP 2026, Ch 13, p. 30
| 6 |
Input Values |
|
|
|
|
|
|
| 7 |
0.50 |
R |
|
Recharge rate (permeability rate) (in/hr) |
|
|
|
| 8 |
0.150 |
Sy |
|
Specific yield, Sy (dimensionless) default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
| 9 |
0.50 |
Kh |
|
Horizontal hydraulic conductivity (in/hr) Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
|
|
|
| 10 |
40.000 |
x |
|
1/2 length of basin (x direction, in feet) |
|
|
|
| 11 |
160.000 |
y |
|
1/2 width of basin (y direction, in feet) |
|
|
|
| 12 |
12.00 |
t |
|
Duration of infiltration period (hours) |
|
|
|
| 13 |
10.000 |
hi(0) |
|
Initial thickness of saturated zone (feet) |
|
|
|
Source context and notes
\begin{array}{l}
1/2 \text{ length of basin (x direction)} = 40 \text{ ft} \\
1/2 \text{ length of basin (y direction)} = 160 \text{ ft} \\
\end{array}
$$
Example 4: Input Section of the Hantush Spreadsheet – Short Side Evaluation
| 6 | Input Values | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- |
| 7 | 0.50 | R | | Recharge rate (permeability rate) (in/hr) | | | |
| 8 | 0.150 | Sy | | Specific yield, Sy (dimensionless) default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | |
| 9 | 0.50 | Kh | | Horizontal hydraulic conductivity (in/hr) Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | | |
| 10 | 40.000 | x | | 1/2 length of basin (x direction, in feet) | | | |
| 11 | 160.000 | y | | 1/2 width of basin (y direction, in feet) | | | |
| 12 | 12.00 | t | | Duration of infiltration period (hours) | | | |
| 13 | 10.000 | hi(0) | | Initial thickness of saturated zone (feet) | | | |
32. Table 32
BMP 2026, Ch 13, p. 32
| 6 |
Input Values |
|
|
|
|
|
|
| 7 |
4.00 |
R |
|
Recharge rate (permeability rate) (in/hr) |
|
|
|
| 8 |
0.150 |
Sy |
|
Specific yield, Sy (dimensionless)
default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
| 9 |
20.00 |
Kh |
|
Horizontal hydraulic conductivity (in/hr)
Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
|
|
|
| 10 |
1.500 |
x |
|
1/2 length of basin (x direction, in feet) |
|
|
|
| 11 |
250.000 |
y |
|
1/2 width of basin (y direction, in feet) |
|
|
|
| 12 |
24.00 |
t |
|
Duration of infiltration period (hours) |
|
|
|
| 13 |
10.000 |
hi(0) |
|
Initial thickness of saturated zone (feet) |
|
|
|
Source context and notes
Horizontal hydraulic conductivity (Kh) = 20 in/hr
½ length of basin (x direction) = 1.5 ft
½ length of basin (y direction) = 250 ft
Duration of infiltration period (t) = 24 hr
Initial thickness of saturated zone (hi(0)) = 10 ft
Example 5: Input Section of the Hantush Spreadsheet
| 6 | Input Values | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- |
| 7 | 4.00 | R | | Recharge rate (permeability rate) (in/hr) | | | |
| 8 | 0.150 | Sy | | Specific yield, Sy (dimensionless)
default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | |
| 9 | 20.00 | Kh | | Horizontal hydraulic conductivity (in/hr)
Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | | |
| 10 | 1.500 | x | | 1/2 length of basin (x direction, in feet) | | | |
| 11 | 250.000 | y | | 1/2 width of basin (y direction, in feet) | | | |
| 12 | 24.00 | t | | Duration of infiltration period (hours) | | | |
| 13 | 10.000 | hi(0) | | Initial thickness of saturated zone (feet) | | | |
Step 3: Analyze the results
The results, depicted on the following page, show the maximum height of groundwater mounding, 1.66 ft, that will occur at the end of the duration of infiltration period. The discussion continues after the graphic.
33. Table 33
BMP 2026, Ch 13, p. 34
| Input Values |
|
|
|
|
|
|
|
|
|
|
| 2.00 |
R |
Recharge rate (permeability rate) (in/hr) |
|
|
|
|
|
|
|
|
| 0.150 |
Sy |
Specific yield, Sy (dimensionless) |
|
|
|
|
|
|
|
|
|
|
default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
|
|
|
|
|
| 2.00 |
Kh |
Horizontal hydraulic conductivity (in/hr) |
|
|
|
|
|
|
|
|
|
|
Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
|
|
|
|
|
|
|
|
| 2.500 |
x |
1/2 length of basin (x direction, in feet) |
|
|
|
|
|
|
|
|
| 2.500 |
y |
1/2 width of basin (y direction, in feet) |
|
|
|
|
|
|
|
|
| 42.00 |
t |
Duration of infiltration period (hours) |
|
|
|
|
|
|
|
|
| 10.000 |
hi(0) |
Initial thickness of saturated zone (feet) |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| 11.11 |
h(max) |
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
|
|
| 1.11 |
Δh(max) |
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
|
|
Source context and notes
½ length of basin (y direction) = 2.5 ft
Duration of infiltration period (t) = 42 hr
Initial thickness of saturated zone (hi(0)) = 10 ft
## Example 6: Input Section of the Hantush Spreadsheet
| Input Values | | | | | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 2.00 | R | Recharge rate (permeability rate) (in/hr) | | | | | | | | |
| 0.150 | Sy | Specific yield, Sy (dimensionless) | | | | | | | | |
| | | default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | | | | | | |
| 2.00 | Kh | Horizontal hydraulic conductivity (in/hr) | | | | | | | | |
| | | Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | | | | | | | |
| 2.500 | x | 1/2 length of basin (x direction, in feet) | | | | | | | | |
| 2.500 | y | 1/2 width of basin (y direction, in feet) | | | | | | | | |
| 42.00 | t | Duration of infiltration period (hours) | | | | | | | | |
| 10.000 | hi(0) | Initial thickness of saturated zone (feet) | | | | | | | | |
| | | | | | | | | | | |
| 11.11 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | | | | | |
| 1.11 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | | | |
## Step 2: Analyze the results
The results, depicted on the following page, show the maximum height of groundwater mounding, 1.11 ft, that will occur at the end of the duration of infiltration period at the center of the drywell. The temporary groundwater mounding does not encroach upon the bottom of the drywell, where it would have impaired infiltration. The discussion continues below the figure.
34. Table 34
BMP 2026, Ch 13, p. 36
| 6 |
Input Values |
|
|
|
|
|
|
|
|
|
| 7 |
10.00 |
R |
|
Recharge rate (permeability rate) (in/hr) |
|
|
|
|
|
|
| 8 |
0.150 |
Sy |
|
Specific yield, Sy (dimensionless) |
|
|
|
|
|
|
| 9 |
0.25 |
Kh |
|
default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
|
|
|
| 10 |
17.500 |
x |
|
Horizontal hydraulic conductivity (in/hr) |
|
|
|
|
|
|
| 11 |
17.500 |
y |
|
Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain |
|
|
|
|
|
|
| 12 |
2.40 |
t |
|
1/2 width of basin (y direction, in feet) |
|
|
|
|
|
|
| 13 |
10.00 |
hi(0) |
|
Duration of infiltration period (hours) |
|
|
|
|
|
|
| 14 |
|
|
|
Initial thickness of saturated zone (feet) |
|
|
|
|
|
|
| 15 |
20.000 |
h(max) |
|
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
| 16 |
10.000 |
Δh(max) |
|
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
Source context and notes
$$
\text{Initial thickness of saturated zone (hi(0))} = 10 \text{ ft}
$$
The graphic below depicts the input section of the Hantush Spreadsheet with the parameter values shown above entered.
Example 7: Input Section of the Hantush Spreadsheet
| 6 | Input Values | | | | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 7 | 10.00 | R | | Recharge rate (permeability rate) (in/hr) | | | | | | |
| 8 | 0.150 | Sy | | Specific yield, Sy (dimensionless) | | | | | | |
| 9 | 0.25 | Kh | | default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | | | | |
| 10 | 17.500 | x | | Horizontal hydraulic conductivity (in/hr) | | | | | | |
| 11 | 17.500 | y | | Kh = 5xRecharge Rate (R) in the coastal plain; Kh=R outside the coastal plain | | | | | | |
| 12 | 2.40 | t | | 1/2 width of basin (y direction, in feet) | | | | | | |
| 13 | 10.00 | hi(0) | | Duration of infiltration period (hours) | | | | | | |
| 14 | | | | Initial thickness of saturated zone (feet) | | | | | | |
| 15 | 20.000 | h(max) | | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | | | |
| 16 | 10.000 | Δh(max) | | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | |
The results, shown above in the red cell in Row 16, indicate that the basin will not drain using a recharge rate of 10 in/hr. The height of groundwater mounding determined above will reduce the vertical hydraulic gradient, which then prevents the infiltration of runoff. Therefore, the recharge
35. Table 35
BMP 2026, Ch 13, p. 37
| 6 |
Input Values |
|
|
|
|
|
|
|
|
|
| 7 |
5.00 |
R |
Recharge rate (permeability rate) (in/hr) |
|
|
|
|
|
|
|
|
|
|
Specific yield, 5y (dimensionless) |
|
|
|
|
|
|
|
| 8 |
0.150 |
5y |
default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
|
|
|
|
|
|
|
Horizontal hydraulic conductivity (in/hr) |
|
|
|
|
|
|
|
| 9 |
0.25 |
Kh |
Kh = 5xRecharge Rate (R) in the coastal plain: Kh=R outside the coastal plain |
|
|
|
|
|
|
|
| 10 |
17.500 |
x |
1/2 length of basin (x direction, in feet) |
|
|
|
|
|
|
|
| 11 |
17.500 |
y |
1/2 width of basin (y direction, in feet) |
|
|
|
|
|
|
|
| 12 |
4.80 |
t |
Duration of infiltration period (hours) |
|
|
|
|
|
|
|
| 13 |
10.00 |
hi(0) |
Initial thickness of saturated zone (feet) |
|
|
|
|
|
|
|
| 14 |
|
|
|
|
|
|
|
|
|
|
| 15 |
23.333 |
h(max) |
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
|
| 16 |
13.333 |
Δh(max) |
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
|
Source context and notes
rate will be greatly reduced, meaning it must be adjusted and rechecked to determine if the basin can still drain within 72 hours.
# Step 3: Adjust the recharge rate and the duration of infiltration period
Adjust the recharge rate to 5 in/hr (half of the initial rate). The duration of the infiltration is also proportionally adjusted to 4.8 hours since the recharge rate is reduced in half. The results show that groundwater mounding is lowered but the basin is still not working properly.
Example 7: Revised Input Section of the Hantush Spreadsheet
| 6 | Input Values | | | | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 7 | 5.00 | R | Recharge rate (permeability rate) (in/hr) | | | | | | | |
| | | | Specific yield, 5y (dimensionless) | | | | | | | |
| 8 | 0.150 | 5y | default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | | | | | |
| | | | Horizontal hydraulic conductivity (in/hr) | | | | | | | |
| 9 | 0.25 | Kh | Kh = 5xRecharge Rate (R) in the coastal plain: Kh=R outside the coastal plain | | | | | | | |
| 10 | 17.500 | x | 1/2 length of basin (x direction, in feet) | | | | | | | |
| 11 | 17.500 | y | 1/2 width of basin (y direction, in feet) | | | | | | | |
| 12 | 4.80 | t | Duration of infiltration period (hours) | | | | | | | |
| 13 | 10.00 | hi(0) | Initial thickness of saturated zone (feet) | | | | | | | |
| 14 | | | | | | | | | | |
| 15 | 23.333 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | | | | |
| 16 | 13.333 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | | |
# Step 4: Extend the duration of infiltration period to 72 hours and proportionally adjust the recharge rate
Adjust the duration of infiltration period to 72 hours, and pro-rate the recharge rate to 0.33 in/hr (= 5 in/hr x 72 / 4.8). The results show that the groundwater mounding is reduced but this adjustment is still not sufficient enough to make the basin work, i.e., the mounding height of 9.77 ft is still greater than the 7 ft depth of the SHWT, as shown in the red cell in Row 16 of the image shown below:
Example 7: Re-adjusted Input Section of the Hantush Spreadsheet
36. Table 36
BMP 2026, Ch 13, p. 37
| 6 |
Input Values |
|
|
|
|
|
|
|
|
| 7 |
0.33 |
R |
Recharge rate (permeability rate) (in/hr) |
|
|
|
|
|
|
|
|
|
Specific yield, 5y (dimensionless) |
|
|
|
|
|
|
| 8 |
0.150 |
5y |
default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
|
|
|
|
|
|
Horizontal hydraulic conductivity (in/hr) |
|
|
|
|
|
|
| 9 |
0.25 |
Kh |
Kh = 5xRecharge Rate (R) in the coastal plain: Kh=R outside the coastal plain |
|
|
|
|
|
|
| 10 |
17.500 |
x |
1/2 length of basin (x direction, in feet) |
|
|
|
|
|
|
| 11 |
17.500 |
y |
1/2 width of basin (y direction, in feet) |
|
|
|
|
|
|
| 12 |
72.00 |
t |
Duration of infiltration period (hours) |
|
|
|
|
|
|
| 13 |
10.00 |
hi(0) |
Initial thickness of saturated zone (feet) |
|
|
|
|
|
|
| 14 |
|
|
|
|
|
|
|
|
|
| 15 |
19.769 |
h(max) |
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
| 16 |
9.769 |
Δh(max) |
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
Source context and notes
| 16 | 13.333 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | | |
# Step 4: Extend the duration of infiltration period to 72 hours and proportionally adjust the recharge rate
Adjust the duration of infiltration period to 72 hours, and pro-rate the recharge rate to 0.33 in/hr (= 5 in/hr x 72 / 4.8). The results show that the groundwater mounding is reduced but this adjustment is still not sufficient enough to make the basin work, i.e., the mounding height of 9.77 ft is still greater than the 7 ft depth of the SHWT, as shown in the red cell in Row 16 of the image shown below:
Example 7: Re-adjusted Input Section of the Hantush Spreadsheet
| 6 | Input Values | | | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 7 | 0.33 | R | Recharge rate (permeability rate) (in/hr) | | | | | | |
| | | | Specific yield, 5y (dimensionless) | | | | | | |
| 8 | 0.150 | 5y | default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | | | | |
| | | | Horizontal hydraulic conductivity (in/hr) | | | | | | |
| 9 | 0.25 | Kh | Kh = 5xRecharge Rate (R) in the coastal plain: Kh=R outside the coastal plain | | | | | | |
| 10 | 17.500 | x | 1/2 length of basin (x direction, in feet) | | | | | | |
| 11 | 17.500 | y | 1/2 width of basin (y direction, in feet) | | | | | | |
| 12 | 72.00 | t | Duration of infiltration period (hours) | | | | | | |
| 13 | 10.00 | hi(0) | Initial thickness of saturated zone (feet) | | | | | | |
| 14 | | | | | | | | | |
| 15 | 19.769 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | | | |
| 16 | 9.769 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | |
# Step 5: Reconfigure the basin geometry, adjust input parameters and analyze results
Since the basin will not drain at the maximum allowable drain time of 72 hours, the basin must be reconfigured to provide a shallower depth of stormwater runoff to be infiltrated, i.e., reduce the current maximum depth from 2 ft to 1 ft. To lower the mounding height, the specified volume of runoff must be spread out over a larger area. If the square basin shape is to be maintained, adjusting the length and width of the basin to 50 ft each creates 2,500 sf of area for infiltration. Assuming the
37. Table 37
BMP 2026, Ch 13, p. 38
| 6 |
Input Values |
0 |
10 |
15 |
20 |
25 |
30 |
35 |
40 |
45 |
50 |
| 7 |
0.50 |
R |
Recharge rate (permeability rate) (in/hr) |
|
|
|
|
|
|
|
|
| 8 |
0.150 |
Sy |
Specific yield, Sy (dimensionless) |
|
|
|
|
|
|
|
|
| 9 |
0.25 |
Kh |
default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
|
|
|
|
|
| 10 |
25.000 |
x |
Horizontal hydraulic conductivity (in/hr) |
|
|
|
|
|
|
|
|
| 11 |
25.000 |
y |
Kh = 5xRecharge Rate (R) in the coastal plain: Kh=R outside the coastal plain |
|
|
|
|
|
|
|
|
| 12 |
23.52 |
t |
Duration of infiltration period (hours) |
|
|
|
|
|
|
|
|
| 13 |
10.00 |
hi(0) |
Initial thickness of saturated zone (feet) |
|
|
|
|
|
|
|
|
| 14 |
|
|
|
|
|
|
|
|
|
|
|
| 15 |
16.517 |
h(max) |
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
|
|
| 16 |
6.517 |
Δh(max) |
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
|
|
Source context and notes
Duration of infiltration period, t (hours) $= \frac{\text{Volume of runoff to be infiltrated (cf)} \times 12 \, \text{ft/in}}{\text{Infiltration area (sf)} \times \text{Recharge rate (in/hr)}}$
t $= \frac{2,450\,cf \times 12\,ft/in}{2,500\,sf \times 0.5\,in/hr} = 23.52\,hr$
The inputs and resulting height of groundwater mounding are depicted below.
Example 7: Further Revised Input Section of the Hantush Spreadsheet
| 6 | Input Values | 0 | 10 | 15 | 20 | 25 | 30 | 35 | 40 | 45 | 50 |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 7 | 0.50 | R | Recharge rate (permeability rate) (in/hr) | | | | | | | | |
| 8 | 0.150 | Sy | Specific yield, Sy (dimensionless) | | | | | | | | |
| 9 | 0.25 | Kh | default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | | | | | | |
| 10 | 25.000 | x | Horizontal hydraulic conductivity (in/hr) | | | | | | | | |
| 11 | 25.000 | y | Kh = 5xRecharge Rate (R) in the coastal plain: Kh=R outside the coastal plain | | | | | | | | |
| 12 | 23.52 | t | Duration of infiltration period (hours) | | | | | | | | |
| 13 | 10.00 | hi(0) | Initial thickness of saturated zone (feet) | | | | | | | | |
| 14 | | | | | | | | | | | |
| 15 | 16.517 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | | | | | |
| 16 | 6.517 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | | | |
The redesigned basin with a bottom area measuring 50 ft x 50 ft, a maximum depth of stormwater runoff to be infiltrated of 0.98 ft and a drain time of 23.52 hr may work since the SHWT is 7 ft below the proposed bottom of the basin and the height of the groundwater mounding is 6.52 ft. This exercise demonstrates that replacement of the underlying soil below an infiltration basin may work for a site having low permeability soils. If a shallower depth of native soils were replaced, the lowest tested field permeability rate of the remaining native soil would be used in calculating the recharge rate as it would be the limiting factor.
Regardless, one must always remember that soil replacement does not change the horizontal hydraulic conductivity of the soils outside the basin. The low horizontal hydraulic conductivity will still limit the ability of the runoff to be dissipated into the saturated zone and may result in high groundwater mounding in the unsaturated zone. Therefore, the use of sand replacement in basin designs must still be carefully evaluated with a groundwater mounding analysis.
38. Table 38
BMP 2026, Ch 13, p. 39
| Design Storm |
Pre-Construction Flow Rates |
Post-Construction Allowable Discharge |
|
Post-Construction (without Quantity Control) |
|
|
Peak flow (cfs) |
Allowable % of Existing |
Peak flow (cfs) |
Peak flow (cfs) |
Volume (cf) |
| 2-year |
0.30 |
50% |
0.15 |
9.51 |
28,920 |
| 10-year |
1.76 |
75% |
1.32 |
14.93 |
46,216 |
| 100-year |
6.49 |
80% |
5.19 |
25.85 |
81,308 |
Source context and notes
A major development located in Ocean County proposes a small-scale infiltration basin to meet the water quality, quantity and groundwater recharge standards for the runoff from a 2.5 ac impervious parking lot. Small-scale infiltration basins are designed to infiltrate runoff generated by the Water Quality Design Storm (WQDS) and to attenuate the runoff from the 2-, 10- and 100-year Design Storms through one or more discharge outlets. Additionally, in a small-scale infiltration basin, exfiltration is allowed for routing all storm events. The proposed 2.5 ac parking lot measures 330 ft by 330 ft and will be located on an existing wooded area where hydrologic soil group (HSG) ‘B’ soils are present. Soil testing was performed in accordance with Chapter 12, and the report shows that the SHWT is 20 ft below the proposed bottom sand layer of the basin. The tested soil permeability rate of the most restrictive soil horizon below the basin is 10 in/hr. Testing confirmed the initial thickness of the saturated zone is 30 ft. Assume the sides of the basin are vertical in all calculations.
Step 1: Calculate the volume and basin size for the WQDS
The WQDS consists of 1.25 inches of rain falling over 2 hours. The rainfall depths for the 2-, 10- and 100-year design storms are 3.42, 5.33 and 9.2 in, respectively. The Curve Number for woods in 'good' condition, as required in N.J.A.C. 7:8-5.7(a)2, on 'B' Soils is 55. The hydraulically most distant point in the existing woods is determined to be 405 ft from the point of analysis. The existing land has an average slope of 5%. The Manning's roughness coefficient for woods is 0.4. The sheet flow length is assumed to be 100 feet for the pre-construction condition.
The time of concentration (Tc) for the existing condition is calculated to be 23.7 minutes, of which 14.6 minutes is the sheet flow segment using a Manning's roughness coefficient of 0.4 and 14.5 minutes is the remaining segment, i.e., 305 ft, of shallow concentrated flow. For the post-construction condition, a Tc of 6 minutes is assumed for the impervious parking lot. In accordance with the information provided above, runoff volumes and rates for the pre- and post-construction conditions are calculated using hydrologic modeling software and shown in the table found below:
| Design Storm | Pre-Construction Flow Rates | Post-Construction Allowable Discharge | | Post-Construction (without Quantity Control) | |
| --- | --- | --- | --- | --- | --- |
| | Peak flow (cfs) | Allowable % of Existing | Peak flow (cfs) | Peak flow (cfs) | Volume (cf) |
| 2-year | 0.30 | 50% | 0.15 | 9.51 | 28,920 |
| 10-year | 1.76 | 75% | 1.32 | 14.93 | 46,216 |
| 100-year | 6.49 | 80% | 5.19 | 25.85 | 81,308 |
The proposed 2.5 acres of regulated motor vehicle impervious surface produces 9,388.74 cf of stormwater runoff generated by the WQDS and this specific volume is therefore used to initially size the small-scale infiltration basin. The tested permeability rate of the soil is 10 in/hr, which is converted to the design infiltration rate, as well as the exfiltration rate, of 5 in/hr.
For the WQDS, a small-scale surface infiltration basin has a maximum allowable depth of stormwater runoff to be infiltrated of 2 ft. With exfiltration incorporated into the routing calculation, the basin bottom area, or footprint, required is 5,000 sf. When modeling the BMP, the invert elevation of the
39. Table 39
BMP 2026, Ch 13, p. 40
| Inflow Area = 108,900 sf,100.00% Impervious, Inflow Depth = 1.03" for Custom event |
|
|
|
|
| Inflow = 7.240 cfs @ 1.09 hrs, Volume= 9,389 cf |
|
|
|
|
| Outflow = 0.428 cfs @ 0.85 hrs, Volume= 9,389 cf Atten= 94%, Lag= 0.0 min |
|
|
|
|
| Discarded = 0.428 cfs @ 0.85 hrs, Volume= 9,389 cf |
|
|
|
|
| Primary = 0.000 cfs @ 0.00 hrs, Volume= 0 cf |
|
|
|
|
| Routing by Stor-Ind method, Time Span= 0.00-72.00 hrs, dt= 0.05 hrs |
|
|
|
|
| Peak Elev= 1.46' @ 1.86 hrs Surf.Area= 5,000 sf Storage= 7,282 cf |
|
|
|
|
| Plug-Flow detention time= 151.0 min calculated for 9,382 cf (100% of inflow) |
|
|
|
|
| Center-of-Mass det. time= 151.1 min (221.4 - 70.3) |
|
|
|
|
| Volume Invert Avail.Storage Storage Description |
|
|
|
|
| #1 0.00' 38,796 cf Custom Stage Data (Prismatic) Listed below (Recalc) |
|
|
|
|
| Elevation Surf.Area Inc.Store Cumb.Size (cubic-feet) |
|
|
|
|
| (feet) (sq-ft) |
|
|
|
|
| 0.00 5,000 0 0 |
|
|
|
|
| 1.00 5,000 5,000 5,000 |
|
|
|
|
| 2.00 5,000 5,000 10,000 |
|
|
|
|
| 3.00 5,776 5,388 15,388 |
|
|
|
|
| 4.00 7,056 6,416 21,804 |
|
|
|
|
| 5.00 8,464 7,760 29,564 |
|
|
|
|
| 6.00 10,000 9,232 38,796 |
|
|
|
|
| Device Routing Invert Outlet Devices |
|
|
|
|
| #1 Primary 2.00' 2.5" Vert. Orifice/Grate C= 0.600 |
|
|
|
|
| #2 Discarded 0.00' 0.428 cfs Exfiltration at all elevations |
|
|
|
|
| #3 Primary 5.71' 20.0' long x 0.5' breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 Coef. (English) 2.80 2.92 3.08 3.30 3.32 |
|
|
|
|
| #4 Primary 3.00' 6.0" Vert. Orifice/Grate C= 0.600 |
|
|
|
|
| Discarded OutFlow Max=0.428 cfs @ 0.85 hrs HW=0.08' (Free Discharge) -2=Exfiltration (Exfiltration Controls 0.428 cfs) |
|
|
|
|
| Primary OutFlow Max=0.000 cfs @ 0.00 hrs HW=0.00' (Free Discharge) -1=Orifice/Grate (Controls 0.000 cfs) -3=Broad-Crested Rectangular Weir (Controls 0.000 cfs) -4=Orifice/Grate (Controls 0.000 cfs) |
|
|
|
|
Source context and notes
# Groundwater Table Hydraulic Impact Assessments for Infiltration BMPs
Source revision: January 2026.
first orifice is therefore set at a distance of 2 ft above the bottom surface of the basin. Note that flow through the outlet control structure is labeled as "Primary" in the graphic below and "Discarded" is used to denote runoff that is exfiltrated. Results for the WQDS are shown below as reprints from a hydrologic modelling software package using the NRCS methodology:
Initial WQDS Summary Report
| Inflow Area = 108,900 sf,100.00% Impervious, Inflow Depth = 1.03" for Custom event | | | | |
| --- | --- | --- | --- | --- |
| Inflow = 7.240 cfs @ 1.09 hrs, Volume= 9,389 cf | | | | |
| Outflow = 0.428 cfs @ 0.85 hrs, Volume= 9,389 cf Atten= 94%, Lag= 0.0 min | | | | |
| Discarded = 0.428 cfs @ 0.85 hrs, Volume= 9,389 cf | | | | |
| Primary = 0.000 cfs @ 0.00 hrs, Volume= 0 cf | | | | |
| Routing by Stor-Ind method, Time Span= 0.00-72.00 hrs, dt= 0.05 hrs | | | | |
| Peak Elev= 1.46' @ 1.86 hrs Surf.Area= 5,000 sf Storage= 7,282 cf | | | | |
| Plug-Flow detention time= 151.0 min calculated for 9,382 cf (100% of inflow) | | | | |
| Center-of-Mass det. time= 151.1 min (221.4 - 70.3) | | | | |
| Volume Invert Avail.Storage Storage Description | | | | |
| #1 0.00' 38,796 cf Custom Stage Data (Prismatic) Listed below (Recalc) | | | | |
| Elevation Surf.Area Inc.Store Cumb.Size (cubic-feet) | | | | |
| (feet) (sq-ft) | | | | |
| 0.00 5,000 0 0 | | | | |
| 1.00 5,000 5,000 5,000 | | | | |
| 2.00 5,000 5,000 10,000 | | | | |
| 3.00 5,776 5,388 15,388 | | | | |
| 4.00 7,056 6,416 21,804 | | | | |
| 5.00 8,464 7,760 29,564 | | | | |
| 6.00 10,000 9,232 38,796 | | | | |
| Device Routing Invert Outlet Devices | | | | |
| #1 Primary 2.00' 2.5" Vert. Orifice/Grate C= 0.600 | | | | |
| #2 Discarded 0.00' 0.428 cfs Exfiltration at all elevations | | | | |
| #3 Primary 5.71' 20.0' long x 0.5' breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 Coef. (English) 2.80 2.92 3.08 3.30 3.32 | | | | |
| #4 Primary 3.00' 6.0" Vert. Orifice/Grate C= 0.600 | | | | |
| Discarded OutFlow Max=0.428 cfs @ 0.85 hrs HW=0.08' (Free Discharge) -2=Exfiltration (Exfiltration Controls 0.428 cfs) | | | | |
| Primary OutFlow Max=0.000 cfs @ 0.00 hrs HW=0.00' (Free Discharge) -1=Orifice/Grate (Controls 0.000 cfs) -3=Broad-Crested Rectangular Weir (Controls 0.000 cfs) -4=Orifice/Grate (Controls 0.000 cfs) | | | | |
Source: HydroCAD® Summary Report; HydroCAD is a register trademark of HydroCAD Software Solutions LLC. Used with permission.
40. Table 40
BMP 2026, Ch 13, p. 41
| 6 |
Input Values |
|
|
|
|
|
|
|
|
|
|
| 7 |
5.00 |
R |
|
Recharge rate (permeability rate) (in/hr) |
|
|
|
|
|
|
|
| 8 |
0.150 |
Sy |
|
Specific yield, Sy (dimensionless) |
|
|
|
|
|
|
|
| 9 |
25.00 |
Kh |
|
default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
|
|
|
|
| 10 |
35.360 |
x |
|
Horizontal hydraulic conductivity (in/hr) |
|
|
|
|
|
|
|
| 11 |
35.360 |
y |
|
Kh = 5xRecharge Rate (R) in the coastal plain: Kh=R outside the coastal plain |
|
|
|
|
|
|
|
| 12 |
4.50 |
t |
|
1/2 length of basin (x direction, in feet) |
|
|
|
|
|
|
|
| 13 |
10.000 |
hi(0) |
|
1/2 width of basin (y direction, in feet) |
|
|
|
|
|
|
|
| 14 |
|
|
|
Duration of infiltration period (hours) |
|
|
|
|
|
|
|
| 15 |
18.01 |
h(max) |
|
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
|
| 16 |
4.01 |
Δh(max) |
|
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
|
|
|
|
|
|
|
Source context and notes
$\frac{1}{2}$ length of basin (y direction) = 35.36 ft
Duration of infiltration period (t) = 6 hr
Initial thickness of saturated zone (hi(0)) = 10 ft
## Example 8: Input Section of the Hantush Spreadsheet and Calculated $\Delta h(\max)$
| 6 | Input Values | | | | | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 7 | 5.00 | R | | Recharge rate (permeability rate) (in/hr) | | | | | | | |
| 8 | 0.150 | Sy | | Specific yield, Sy (dimensionless) | | | | | | | |
| 9 | 25.00 | Kh | | default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | | | | | |
| 10 | 35.360 | x | | Horizontal hydraulic conductivity (in/hr) | | | | | | | |
| 11 | 35.360 | y | | Kh = 5xRecharge Rate (R) in the coastal plain: Kh=R outside the coastal plain | | | | | | | |
| 12 | 4.50 | t | | 1/2 length of basin (x direction, in feet) | | | | | | | |
| 13 | 10.000 | hi(0) | | 1/2 width of basin (y direction, in feet) | | | | | | | |
| 14 | | | | Duration of infiltration period (hours) | | | | | | | |
| 15 | 18.01 | h(max) | | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | | | | |
| 16 | 4.01 | Δh(max) | | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | | |
With these input values, the Hantush Spreadsheet yields a maximum height of groundwater mounding equal to 8.01 ft. Since the SHWT is 20 ft below the bottom of the sand layer, the elevated groundwater table will not reach the bottom surface of the basin. Therefore, this basin configuration can infiltrate the post-construction stormwater runoff generated by the WQDS.
41. Table 41
BMP 2026, Ch 13, p. 43
| Inflow Area = 108,900 sf,100.00% Impervious, Inflow Depth = 8.96" for 100 year event |
|
|
|
|
| Inflow = 22.062 cfs @ 12.09 hrs, Volume= 81,308 cf |
|
|
|
|
| Outflow = 4.917 cfs @ 12.49 hrs, Volume= 81,308 cf. Atten= 78%, Lag= 24.4 min |
|
|
|
|
| Discarded = 0.428 cfs @ 7.00 hrs, Volume= 42,739 cf |
|
|
|
|
| Primary = 4.489 cfs @ 12.49 hrs, Volume= 38,569 cf |
|
|
|
|
| Routing by Stor-Ind method, Time Span= 0.00-72.00 hrs, dt= 0.05 hrs |
|
|
|
|
| Peak Elev= 5.84' @ 12.49 hrs Surf.Area= 9,753 sf Storage= 37,209 cf |
|
|
|
|
| Plug-Flow detention time= 237.8 min calculated for 81,308 cf (100% of inflow) |
|
|
|
|
| Center-of-Mass det. time= 237.7 min (977.3 - 739.6) |
|
|
|
|
| Volume |
Invert |
Avail.Storage |
Storage Description |
|
| #1 |
0.00' |
38,796 cf |
Custom Stage Data (Prismatic) Listed below (Recalc) |
|
| Elevation (feet) |
Surf.Area (sq-ft) |
Inc.Store (cubic-feet) |
Cum.Store (cubic-feet) |
|
| 0.00 |
5,000 |
0 |
0 |
|
| 1.00 |
5,000 |
5,000 |
5,000 |
|
| 2.00 |
5,000 |
5,000 |
10,000 |
|
| 3.00 |
5,776 |
5,388 |
15,388 |
|
| 4.00 |
7,056 |
6,416 |
21,804 |
|
| 5.00 |
8,464 |
7,760 |
29,564 |
|
| 6.00 |
10,000 |
9,232 |
38,796 |
|
| Device |
Routing |
Invert |
Outlet Devices |
|
| #1 |
Primary |
2.00' |
2.5" Vert. Orifice/Grate C= 0.600 |
|
| #2 |
Discarded |
0.00' |
0.428 cfs Exfiltration at all elevations |
|
| #3 |
Primary |
5.71' |
20.0' long x 0.5' breadth Broad-Crested Rectangular Weir
Head (feet) 0.20 0.40 0.60 0.80 1.00
Coef. (English) 2.80 2.92 3.08 3.30 3.32 |
|
| #4 |
Primary |
3.00' |
6.0" Vert. Orifice/Grate C= 0.600 |
|
Discarded OutFlow Max=0.428 cfs @ 7.00 hrs HW=0.06' (Free Discharge)
+2=Exfiltration (Exfiltration Controls 0.428 cfs) |
|
|
|
|
Primary OutFlow Max=4.415 cfs @ 12.49 hrs HW=5.84' (Free Discharge)
+1=Orifice/Grate (Orifice Controls 0.317 cfs @ 9.30 fps)
-3=Broad-Crested Rectangular Weir (Weir Controls 2.577 cfs @ 1.00 fps)
-4=Orifice/Grate (Orifice Controls 1.521 cfs @ 7.75 fps) |
|
|
|
|
Source context and notes
# Groundwater Table Hydraulic Impact Assessments for Infiltration BMPs
Source revision: January 2026.
# 100-year Design Storm Summary Report
| Inflow Area = 108,900 sf,100.00% Impervious, Inflow Depth = 8.96" for 100 year event | | | | |
| --- | --- | --- | --- | --- |
| Inflow = 22.062 cfs @ 12.09 hrs, Volume= 81,308 cf | | | | |
| Outflow = 4.917 cfs @ 12.49 hrs, Volume= 81,308 cf. Atten= 78%, Lag= 24.4 min | | | | |
| Discarded = 0.428 cfs @ 7.00 hrs, Volume= 42,739 cf | | | | |
| Primary = 4.489 cfs @ 12.49 hrs, Volume= 38,569 cf | | | | |
| Routing by Stor-Ind method, Time Span= 0.00-72.00 hrs, dt= 0.05 hrs | | | | |
| Peak Elev= 5.84' @ 12.49 hrs Surf.Area= 9,753 sf Storage= 37,209 cf | | | | |
| Plug-Flow detention time= 237.8 min calculated for 81,308 cf (100% of inflow) | | | | |
| Center-of-Mass det. time= 237.7 min (977.3 - 739.6) | | | | |
| Volume | Invert | Avail.Storage | Storage Description | |
| #1 | 0.00' | 38,796 cf | Custom Stage Data (Prismatic) Listed below (Recalc) | |
| Elevation (feet) | Surf.Area (sq-ft) | Inc.Store (cubic-feet) | Cum.Store (cubic-feet) | |
| 0.00 | 5,000 | 0 | 0 | |
| 1.00 | 5,000 | 5,000 | 5,000 | |
| 2.00 | 5,000 | 5,000 | 10,000 | |
| 3.00 | 5,776 | 5,388 | 15,388 | |
| 4.00 | 7,056 | 6,416 | 21,804 | |
| 5.00 | 8,464 | 7,760 | 29,564 | |
| 6.00 | 10,000 | 9,232 | 38,796 | |
| Device | Routing | Invert | Outlet Devices | |
| #1 | Primary | 2.00' | 2.5" Vert. Orifice/Grate C= 0.600 | |
| #2 | Discarded | 0.00' | 0.428 cfs Exfiltration at all elevations | |
| #3 | Primary | 5.71' | 20.0' long x 0.5' breadth Broad-Crested Rectangular Weir
Head (feet) 0.20 0.40 0.60 0.80 1.00
Coef. (English) 2.80 2.92 3.08 3.30 3.32 | |
| #4 | Primary | 3.00' | 6.0" Vert. Orifice/Grate C= 0.600 | |
| Discarded OutFlow Max=0.428 cfs @ 7.00 hrs HW=0.06' (Free Discharge)
+2=Exfiltration (Exfiltration Controls 0.428 cfs) | | | | |
| Primary OutFlow Max=4.415 cfs @ 12.49 hrs HW=5.84' (Free Discharge)
+1=Orifice/Grate (Orifice Controls 0.317 cfs @ 9.30 fps)
-3=Broad-Crested Rectangular Weir (Weir Controls 2.577 cfs @ 1.00 fps)
-4=Orifice/Grate (Orifice Controls 1.521 cfs @ 7.75 fps) | | | | |
Source: HydroCAD® Summary Report; HydroCAD is a register trademark of HydroCAD Software Solutions LLC. Used with permission.
42. Table 42
BMP 2026, Ch 13, p. 44
| 6 |
Input Values |
|
|
|
|
|
|
| 7 |
5.00 |
R |
Recharge rate (permeability rate) (in/hr) |
|
|
|
|
|
|
|
Specific yield, Sy (dimensionless) |
|
|
|
|
| 8 |
0.150 |
Sy |
default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
|
|
|
|
|
|
|
Horizontal hydraulic conductivity (in/hr) |
|
|
|
|
| 9 |
25.00 |
Kh |
Kh = 5xRecharge Rate (R) in the coastal plain: Kh=R outside the coastal plain |
|
|
|
|
| 10 |
35.360 |
x |
1/2 length of basin (x direction, in feet) |
|
|
|
|
| 11 |
35.360 |
y |
1/2 width of basin (y direction, in feet) |
|
|
|
|
| 12 |
20.51 |
t |
Duration of infiltration period (hours) |
|
|
|
|
| 13 |
10.000 |
hi(0) |
Initial thickness of saturated zone (feet) |
|
|
|
|
| 14 |
|
|
|
|
|
|
|
| 15 |
23.82 |
h(max) |
Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
|
|
|
|
| 16 |
13.82 |
Δh(max) |
Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
|
|
|
|
Source context and notes
$$
t = \frac {42739cf \times 12 \ in/ft}{5,000 \ sf \times 5 \ in/hr} = 20.51 \ hr
$$
The resulting value for $t$ is 20.51 hr. The length and width of the basin for the purpose of inputs to Hantush Spreadsheet are assumed to be 70.72 ft and 70.72 ft for the footprint 5,000 sf. As done previously, the values for $x$ and $y$ are one-half the basin length and width and the horizontal conductivity remains at 25 in/hr, with no change to the values for the specific yield and the initial thickness of saturated zone. In accordance with the results from the Hantush Spreadsheet, the maximum height of groundwater mounding produced by the 100-year design storm is 13.82 ft, as shown below.
Example 8: Input Section of the Hantush Spreadsheet and Calculated $\Delta h(\max)$
| 6 | Input Values | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- |
| 7 | 5.00 | R | Recharge rate (permeability rate) (in/hr) | | | | |
| | | | Specific yield, Sy (dimensionless) | | | | |
| 8 | 0.150 | Sy | default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | | |
| | | | Horizontal hydraulic conductivity (in/hr) | | | | |
| 9 | 25.00 | Kh | Kh = 5xRecharge Rate (R) in the coastal plain: Kh=R outside the coastal plain | | | | |
| 10 | 35.360 | x | 1/2 length of basin (x direction, in feet) | | | | |
| 11 | 35.360 | y | 1/2 width of basin (y direction, in feet) | | | | |
| 12 | 20.51 | t | Duration of infiltration period (hours) | | | | |
| 13 | 10.000 | hi(0) | Initial thickness of saturated zone (feet) | | | | |
| 14 | | | | | | | |
| 15 | 23.82 | h(max) | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | |
| 16 | 13.82 | Δh(max) | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | |
Recall that the SHWT is 20 ft below the proposed basin bottom. The results indicate the temporary elevation of the groundwater table will be 6.18 ft below the proposed basin bottom. Therefore, the exfiltration of runoff during the 100-year storm will not have an adverse hydraulic impact on the proposed small-scale infiltration basin. If the calculated height of the groundwater mounding had been higher than 20 ft, the basin would be hydraulically impacted by the elevated groundwater table, and a trial and error approach using reduced infiltration rates, as illustrated in Example 5-8 of Chapter 5: Stormwater Management Quantity and Quality Standards and Computations, would be needed to redesign the basin size and outlets in order to avoid the adverse hydraulic impact.
43. Table 43
BMP 2026, Ch 13, p. 45
| Inflow Area = |
108,900 sf,100.00% Impervious, Inflow Depth = 3.19" for 2 year event |
|
|
| Inflow = |
8.118 cfs @ 12.09 hrs, Volume= |
28,920 cf |
|
| Outflow = |
0.531 cfs @ 13.55 hrs, Volume= |
28,920 cf, Atten= 93%, Lag= 88.0 min |
|
| Discarded = |
0.428 cfs @ 10.45 hrs, Volume= |
27,610 cf |
|
| Primary = |
0.103 cfs @ 13.55 hrs, Volume= |
1,309 cf |
|
| Routing by Stor-Ind method, Time Span= 0.00-72.00 hrs, dt= 0.05 hrs |
|
|
|
| Peak Elev= 2.50' @ 13.55 hrs Surf.Area= 5,388 sf Storage= 12,598 cf |
|
|
|
| Plug-Flow detention time= 223.5 min calculated for 28,899 cf (100% of inflow) |
|
|
|
| Center-of-Mass det. time= 223.5 min (978.5 - 755.0) |
|
|
|
| Volume |
Invert |
Avail.Storage |
Storage Description |
| #1 |
0.00' |
38,796 cf |
Custom Stage Data (Prismatic) Listed below (Recalc) |
| Elevation (feet) |
Surf.Area (sq-ft) |
Inc.Store (cubic-feet) |
Cum.Store (cubic-feet) |
| 0.00 |
5,000 |
0 |
0 |
| 1.00 |
5,000 |
5,000 |
5,000 |
| 2.00 |
5,000 |
5,000 |
10,000 |
| 3.00 |
5,776 |
5,388 |
15,388 |
| 4.00 |
7,056 |
6,416 |
21,804 |
| 5.00 |
8,464 |
7,760 |
29,564 |
| 6.00 |
10,000 |
9,232 |
38,796 |
| Device |
Routing |
Invert |
Outlet Devices |
| #1 |
Primary |
2.00' |
2.5" Vert. Orifice/Grate C= 0.600 |
| #2 |
Discarded |
0.00' |
0.428 cfs Exfiltration at all elevations |
| #3 |
Primary |
5.71' |
20.0' long x 0.5' breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 Coef. (English) 2.80 2.92 3.08 3.30 3.32 |
| #4 |
Primary |
3.00' |
6.0" Vert. Orifice/Grate C= 0.600 |
| Discarded OutFlow Max=0.428 cfs @ 10.45 hrs HW=0.06' (Free Discharge) |
|
|
|
| +2=Exfiltration (Exfiltration Controls 0.428 cfs) |
|
|
|
| Primary OutFlow Max=0.103 cfs @ 13.55 hrs HW=2.50' (Free Discharge) |
|
|
|
| +1=Orifice/Grate (Orifice Controls 0.103 cfs @ 3.03 fps) |
|
|
|
| -3=Broad-Crested Rectangular Weir (Controls 0.000 cfs) |
|
|
|
| -4=Orifice/Grate (Controls 0.000 cfs) |
|
|
|
Source context and notes
Source revision: January 2026.
# Step 5: BMP Stormwater Routings for the 2- and 10-year design storms and Groundwater Mounding Analysis
We must perform the routing analyses for the 2- and, 10-year design storms to check if the primary discharge flows exceed the respective allowable design storm peak flow rates:
## 2-year Design Storm Summary Report
| Inflow Area = | 108,900 sf,100.00% Impervious, Inflow Depth = 3.19" for 2 year event | | |
| --- | --- | --- | --- |
| Inflow = | 8.118 cfs @ 12.09 hrs, Volume= | 28,920 cf | |
| Outflow = | 0.531 cfs @ 13.55 hrs, Volume= | 28,920 cf, Atten= 93%, Lag= 88.0 min | |
| Discarded = | 0.428 cfs @ 10.45 hrs, Volume= | 27,610 cf | |
| Primary = | 0.103 cfs @ 13.55 hrs, Volume= | 1,309 cf | |
| Routing by Stor-Ind method, Time Span= 0.00-72.00 hrs, dt= 0.05 hrs | | | |
| Peak Elev= 2.50' @ 13.55 hrs Surf.Area= 5,388 sf Storage= 12,598 cf | | | |
| Plug-Flow detention time= 223.5 min calculated for 28,899 cf (100% of inflow) | | | |
| Center-of-Mass det. time= 223.5 min (978.5 - 755.0) | | | |
| Volume | Invert | Avail.Storage | Storage Description |
| #1 | 0.00' | 38,796 cf | Custom Stage Data (Prismatic) Listed below (Recalc) |
| Elevation (feet) | Surf.Area (sq-ft) | Inc.Store (cubic-feet) | Cum.Store (cubic-feet) |
| 0.00 | 5,000 | 0 | 0 |
| 1.00 | 5,000 | 5,000 | 5,000 |
| 2.00 | 5,000 | 5,000 | 10,000 |
| 3.00 | 5,776 | 5,388 | 15,388 |
| 4.00 | 7,056 | 6,416 | 21,804 |
| 5.00 | 8,464 | 7,760 | 29,564 |
| 6.00 | 10,000 | 9,232 | 38,796 |
| Device | Routing | Invert | Outlet Devices |
| #1 | Primary | 2.00' | 2.5" Vert. Orifice/Grate C= 0.600 |
| #2 | Discarded | 0.00' | 0.428 cfs Exfiltration at all elevations |
| #3 | Primary | 5.71' | 20.0' long x 0.5' breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 Coef. (English) 2.80 2.92 3.08 3.30 3.32 |
| #4 | Primary | 3.00' | 6.0" Vert. Orifice/Grate C= 0.600 |
| Discarded OutFlow Max=0.428 cfs @ 10.45 hrs HW=0.06' (Free Discharge) | | | |
| +2=Exfiltration (Exfiltration Controls 0.428 cfs) | | | |
| Primary OutFlow Max=0.103 cfs @ 13.55 hrs HW=2.50' (Free Discharge) | | | |
| +1=Orifice/Grate (Orifice Controls 0.103 cfs @ 3.03 fps) | | | |
| -3=Broad-Crested Rectangular Weir (Controls 0.000 cfs) | | | |
| -4=Orifice/Grate (Controls 0.000 cfs) | | | |
Source: HydroCAD® Summary Report; HydroCAD is a register trademark of HydroCAD Software Solutions LLC. Used with permission.
44. Table 44
BMP 2026, Ch 13, p. 46
| Inflow Area = 108,900 sf,100.00% Impervious, Inflow Depth = 5.09" for 10 year event |
|
|
|
|
| Inflow = 12.738 cfs @ 12.09 hrs, Volume= 46,216 cf |
|
|
|
|
| Outflow = 1.352 cfs @ 12.79 hrs, Volume= 46,216 cf. Atten= 89%, Lag= 42.2 min |
|
|
|
|
| Discarded = 0.428 cfs @ 9.00 hrs, Volume= 35,289 cf |
|
|
|
|
| Primary = 0.924 cfs @ 12.79 hrs, Volume= 10,927 cf |
|
|
|
|
| Routing by Stor-Ind method, Time Span= 0.00-72.00 hrs, dt= 0.05 hrs |
|
|
|
|
| Peak Elev= 3.81' @ 12.79 hrs Surf.Area= 6,817 sf Storage= 20,510 cf |
|
|
|
|
| Plug-Flow detention time= 248.3 min calculated for 46,184 cf (100% of inflow) |
|
|
|
|
| Center-of-Mass det. time= 248.3 min ( 995.3 - 747.0 ) |
|
|
|
|
| Volume |
Invert |
Avail.Storage |
Storage Description |
|
| #1 |
0.00' |
38,796 cf |
Custom Stage Data (Prismatic) Listed below (Recalc) |
|
| Elevation (feet) |
Surf.Area (sq-ft) |
Inc.Store (cubic-feet) |
Cum.Store (cubic-feet) |
|
| 0.00 |
5,000 |
0 |
0 |
|
| 1.00 |
5,000 |
5,000 |
5,000 |
|
| 2.00 |
5,000 |
5,000 |
10,000 |
|
| 3.00 |
5,776 |
5,388 |
15,388 |
|
| 4.00 |
7,056 |
6,416 |
21,804 |
|
| 5.00 |
8,464 |
7,760 |
29,564 |
|
| 6.00 |
10,000 |
9,232 |
38,796 |
|
| Device |
Routing |
Invert |
Outlet Devices |
|
| #1 |
Primary |
2.00' |
2.5" Vert. Orifice/Grate C= 0.600 |
|
| #2 |
Discarded |
0.00' |
0.428 cfs Exfiltration at all elevations |
|
| #3 |
Primary |
5.71' |
20.0' long x 0.5' breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 Coef. (English) 2.80 2.92 3.08 3.30 3.32 |
|
| #4 |
Primary |
3.00' |
6.0" Vert. Orifice/Grate C= 0.600 |
|
| Discarded OutFlow Max=0.428 cfs @ 9.00 hrs HW=0.06' (Free Discharge) |
|
|
|
|
| 1-2=Exfiltration (Exfiltration Controls 0.428 cfs) |
|
|
|
|
| Primary OutFlow Max=0.924 cfs @ 12.79 hrs HW=3.81' (Free Discharge) |
|
|
|
|
| 1=Orifice/Grate (Orifice Controls 0.215 cfs @ 6.29 fps) |
|
|
|
|
| 3=Broad-Crested Rectangular Weir ( Controls 0.000 cfs) |
|
|
|
|
| 4=Orifice/Grate (Orifice Controls 0.710 cfs @ 3.61 fps) |
|
|
|
|
Source context and notes
# Groundwater Table Hydraulic Impact Assessments for Infiltration BMPs
Source revision: January 2026.
# 10-year Design Storm Summary Report
| Inflow Area = 108,900 sf,100.00% Impervious, Inflow Depth = 5.09" for 10 year event | | | | |
| --- | --- | --- | --- | --- |
| Inflow = 12.738 cfs @ 12.09 hrs, Volume= 46,216 cf | | | | |
| Outflow = 1.352 cfs @ 12.79 hrs, Volume= 46,216 cf. Atten= 89%, Lag= 42.2 min | | | | |
| Discarded = 0.428 cfs @ 9.00 hrs, Volume= 35,289 cf | | | | |
| Primary = 0.924 cfs @ 12.79 hrs, Volume= 10,927 cf | | | | |
| Routing by Stor-Ind method, Time Span= 0.00-72.00 hrs, dt= 0.05 hrs | | | | |
| Peak Elev= 3.81' @ 12.79 hrs Surf.Area= 6,817 sf Storage= 20,510 cf | | | | |
| Plug-Flow detention time= 248.3 min calculated for 46,184 cf (100% of inflow) | | | | |
| Center-of-Mass det. time= 248.3 min ( 995.3 - 747.0 ) | | | | |
| Volume | Invert | Avail.Storage | Storage Description | |
| #1 | 0.00' | 38,796 cf | Custom Stage Data (Prismatic) Listed below (Recalc) | |
| Elevation (feet) | Surf.Area (sq-ft) | Inc.Store (cubic-feet) | Cum.Store (cubic-feet) | |
| 0.00 | 5,000 | 0 | 0 | |
| 1.00 | 5,000 | 5,000 | 5,000 | |
| 2.00 | 5,000 | 5,000 | 10,000 | |
| 3.00 | 5,776 | 5,388 | 15,388 | |
| 4.00 | 7,056 | 6,416 | 21,804 | |
| 5.00 | 8,464 | 7,760 | 29,564 | |
| 6.00 | 10,000 | 9,232 | 38,796 | |
| Device | Routing | Invert | Outlet Devices | |
| #1 | Primary | 2.00' | 2.5" Vert. Orifice/Grate C= 0.600 | |
| #2 | Discarded | 0.00' | 0.428 cfs Exfiltration at all elevations | |
| #3 | Primary | 5.71' | 20.0' long x 0.5' breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 Coef. (English) 2.80 2.92 3.08 3.30 3.32 | |
| #4 | Primary | 3.00' | 6.0" Vert. Orifice/Grate C= 0.600 | |
| Discarded OutFlow Max=0.428 cfs @ 9.00 hrs HW=0.06' (Free Discharge) | | | | |
| 1-2=Exfiltration (Exfiltration Controls 0.428 cfs) | | | | |
| Primary OutFlow Max=0.924 cfs @ 12.79 hrs HW=3.81' (Free Discharge) | | | | |
| 1=Orifice/Grate (Orifice Controls 0.215 cfs @ 6.29 fps) | | | | |
| 3=Broad-Crested Rectangular Weir ( Controls 0.000 cfs) | | | | |
| 4=Orifice/Grate (Orifice Controls 0.710 cfs @ 3.61 fps) | | | | |
Source: HydroCAD® Summary Report; HydroCAD is a register trademark of HydroCAD Software Solutions LLC. Used with permission.
The peak, or primary, flow rates for the 2- and 10-year design storms are 0.103 cfs and 0.924 cfs, respectively, which are less than the allowable peak flow rates, i.e., 0.15 cfs and 1.32 cfs, respectively. Therefore, the stormwater runoff quantity control requirements are met.
In Step 4, the groundwater mounding analysis for the 100-year design storm shows that the elevated groundwater level resulting from the 100-year design storm runoff does not have an adverse impact to the proposed BMP. Since the 2- and 10-year design storms produce lower runoff volumes to be infiltrated by the BMP, the mounding heights will each be less than the mounding height produced by infiltration of stormwater runoff generated by the 100-year design storm. Therefore, the calculations using the Hantush Spreadsheet are not necessary.
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