Tables: Sand Filters (Large-Scale GI)¶
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1. Table 1¶
| N.J.A.C. 7:8 Stormwater Management Rules – Applicable Design and Performance Standards | ||
|---|---|---|
| Green Infrastructure | Yes | |
| Stormwater Runoff Quantity | Yes, when designed as an on-line system | |
| GR | Groundwater Recharge | Only with a waiver or variance from N.J.A.C. 7:8-5.3 |
| % | Stormwater Runoff Quality | Only with a waiver or variance from N.J.A.C. 7:8-5.3, 80% TSS |
| Stormwater Runoff Quality Mechanisms and Corresponding Criteria | ||
| --- | --- | |
| Settling | ||
| Storage Volume | Entire Water Quality Design Storm Volume | |
| Filtration | ||
| Sand bed minimum thickness | 18 inches | |
| Maximum storage above sand bed | 24 inches | |
| Maximum design permeability rate of sand bed | 2 inches/hour | |
| Minimum topsoil permeability rate, if using optional vegetative cover | 2x the permeability of the subsoil |
Source context and notes
Source revision: March 2021. # 10.3 SAND FILTERS (LARGE-SCALE GI) [Figure: img-0.jpeg (original PDF, p. 1)](SOURCE.pdf#page=1) A sand filter designed to infiltrate into the subsoil is a stormwater management system designed to both maximize the removal of pollutants from stormwater and to promote groundwater recharge as well as address the quantity impacts of land development. It consists of a pre-treatment zone and a treatment zone, which includes the sand bed and its underlying materials. Pollutants are treated through settling, filtration and adsorption by the sand bed prior to discharging stormwater runoff by infiltration into the subsoil. The total suspended solids (TSS) removal rate is 80%. | N.J.A.C. 7:8 Stormwater Management Rules – Applicable Design and Performance Standards | | | | --- | --- | --- | | | Green Infrastructure | Yes | | | Stormwater Runoff Quantity | Yes, when designed as an on-line system | | GR | Groundwater Recharge | Only with a waiver or variance from N.J.A.C. 7:8-5.3 | | % | Stormwater Runoff Quality | Only with a waiver or variance from N.J.A.C. 7:8-5.3, 80% TSS | | Stormwater Runoff Quality Mechanisms and Corresponding Criteria | | | --- | --- | | Settling | | | Storage Volume | Entire Water Quality Design Storm Volume | | Filtration | | | Sand bed minimum thickness | 18 inches | | Maximum storage above sand bed | 24 inches | | Maximum design permeability rate of sand bed | 2 inches/hour | | Minimum topsoil permeability rate, if using optional vegetative cover | 2x the permeability of the subsoil |2. Table 2¶
| Area = | 5 acre |
|---|---|
| CN Value = | 98 (100% Impervious) |
| Calculated Time to Peak = | 1.54 hours |
| Calculated WQDS Runoff Volume = | 18,777 cf |
| Forebay Primary Outflow: | Perforated Riser Pipe |
| Forebay Secondary Outflow: | Broad-crested Weir |
| Tested Subsoil Permeability Rate = | 8 inches/hour |
| Forebay Surface Elevation = | 1.10 ft |
| Sand Bed Surface Elevation = | 0.00 ft |
| Assumed Depth of WQDS Runoff to be infiltrated = | 2 ft above the sand filter surface |
Source context and notes
# Sand Filters (Large-Scale GI) Source revision: March 2021. # Designing A Sand Filter The example below illustrates how to design a sand filter designed to infiltrate to treat the stormwater runoff generated by the WQDS. The following parameters apply: | Area = | 5 acre | | --- | --- | | CN Value = | 98 (100% Impervious) | | Calculated Time to Peak = | 1.54 hours | | Calculated WQDS Runoff Volume = | 18,777 cf | | Forebay Primary Outflow: | Perforated Riser Pipe | | Forebay Secondary Outflow: | Broad-crested Weir | | Tested Subsoil Permeability Rate = | 8 inches/hour | | Forebay Surface Elevation = | 1.10 ft | | Sand Bed Surface Elevation = | 0.00 ft | | Assumed Depth of WQDS Runoff to be infiltrated = | 2 ft above the sand filter surface | ## Step 1: Forebay Sizing The forebay must be sized to hold 10% of the WQDS volume. Assuming the depth of stormwater runoff in the forebay is equal to the depth of the WQDS in the surface of the sand filter system, which is assumed to be 1 ft, a rectangular forebay with a width of 42.4 ft and a length of 42.4 ft will provide adequate storage volume. However, in order to facilitate drainage, the water surface elevation in the forebay must be greater than the water surface elevation in the sand filter; in addition, the perforations in the riser pipe must be designed to ensure that the forebay will drain within 9 hours. In this example, the WQDS water surface elevation (W.S.E.) in the forebay is 2.15 ft; this translates to a depth of 1.05 ft, with the additional 0.05 ft of depth resulting from head loss in the riser pipe. Based on the above elevations, the top of the riser pipe is set at an elevation of 2.20 ft. A grate or trash rack must be installed on the top of the riser to keep floatables from entering the riser pipe during large storm events.Images on table pages¶
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