Hydrologic Analysis¶
Hydrologic analysis translates site data into the quantitative inputs that drive BMP selection and sizing. This stage determines how much runoff the site generates, how it compares to pre-development conditions, and what design volumes and flow rates the stormwater management system must address.
Stage Purpose¶
New Jersey's stormwater regulations (N.J.A.C. 7:8) require compliance with three independent standards — water quality, groundwater recharge, and flood control — each requiring its own hydrologic computation. This stage produces the numerical targets (volumes and peak flow rates) that define the engineering problem. Without a defensible hydrologic analysis, BMP sizing has no basis and permit applications will be rejected.
Engineering Tasks¶
Curve Number (CN) Determination¶
The NRCS Curve Number method is the standard approach for computing runoff volumes in New Jersey stormwater design.
- Assign CN values by Hydrologic Soil Group (HSG) and land use category from the NJ BMP Manual tables. The 2026 edition updates CN values for several categories:
| Land Use | HSG B (2023) | HSG B (2026) | Notes |
|---|---|---|---|
| Conventional lawn | 69 | 66 | Reduced based on NJ monitoring data |
| Single-family residential (variable lot sizes) | Various | Adjusted | Check updated tables for specific lot size |
| Paved / impervious surfaces | 98 | 98 | Unchanged across all HSGs |
- Calculate the area-weighted composite CN for the entire contributing drainage area in both pre-development and post-development conditions.
- Projects designed under 2023 standards being revised must verify whether updated 2026 CN values apply, as changes affect WQV and recharge calculations.
Water Quality Volume (WQV) Calculation¶
The WQV is the controlling design volume for the water quality standard:
WQV = P x Rv x A
Where:
- P = 1.25 inches (NJ Water Quality Design Storm, 2-hour duration)
- Rv = volumetric runoff coefficient derived from the post-development composite CN
- A = contributing drainage area
The WQV defines the minimum storage or infiltration volume that each BMP must provide for water quality compliance. Under the 2026 framework, if a GI BMP achieves full volumetric reduction (VRv >= WQV per Chapter 14), no separate TSS removal calculation is required.
Groundwater Recharge Volume (Rev) Calculation¶
Rev = SUM[ (Rrate_pre x A_pre) - (Rrate_post x A_post) ]
- Annual recharge rates are from NJ BMP Manual Table 6-1 by HSG and land use.
- The 2026 edition updates recharge rates for turf/lawn on HSG A soils (revised upward).
- Impervious surfaces receive zero recharge credit regardless of slope or adjacency to pervious areas.
- The resulting deficit volume must be provided by infiltration BMPs on site.
Peak Flow Analysis (Flood Control)¶
Flood control analysis uses the NRCS TR-55 methodology (or equivalent accepted hydrologic model) with NJ-specific rainfall data:
| Design Storm | Approximate Depth (24-hr) | Purpose |
|---|---|---|
| 2-year | 3.2 - 3.6 in (varies by location) | Peak rate comparison, channel protection |
| 10-year | 5.0 - 6.5 in (varies by location) | Conveyance system design |
| 100-year | 9 - 13 in (varies by location) | Flood control compliance |
- Rainfall depths must be sourced from NOAA Atlas 14 IDF data for the specific project location. The representative values above are reference ranges only.
- Storm distribution: SCS Type III for most of New Jersey.
- Calculate peak discharge rates for both pre-development and post-development conditions at each point of analysis.
- Post-development peak rates must not exceed pre-development rates for the 2-year and 100-year storms (N.J.A.C. 7:8-5.6).
Pre- vs. Post-Development Comparison¶
Prepare a summary comparison table documenting the hydrologic impact of development:
| Parameter | Pre-Development | Post-Development | Delta |
|---|---|---|---|
| Composite CN | — | — | — |
| Runoff volume (WQ storm) | — | — | = WQV |
| Annual recharge volume | — | — | = Rev deficit |
| 2-yr peak discharge (cfs) | — | — | Must be <= 0 |
| 100-yr peak discharge (cfs) | — | — | Must be <= 0 |
This comparison table is a required element of the Stormwater Management Report (SWMR) submitted with permit applications.
Relevant BMPs¶
Hydrologic analysis results determine the type and scale of BMP required:
- WQV-driven designs: Bioretention, pervious pavement, rain gardens — sized to capture and treat the 1.25-inch storm.
- Recharge-driven designs: Infiltration basins, dry wells, pervious pavement — sized to the annual recharge deficit.
- Flood-control-driven designs: Extended detention, wet ponds, underground detention — sized to attenuate peak flows for the 2-yr through 100-yr storms.
Supporting Regulations¶
- N.J.A.C. 7:8-5.4 — Groundwater recharge standard; defines the recharge volume requirement. See Regulatory Explorer.
- N.J.A.C. 7:8-5.5 — Stormwater quality standard; establishes the 1.25-inch WQV design storm and 80% TSS removal requirement.
- N.J.A.C. 7:8-5.6 — Flood control standard; requires post-development peak rates not to exceed pre-development for the 2-yr and 100-yr storms.
- NJ BMP Manual Chapter 5 — Computational methodology for WQV, CN tables, and flood control analysis.
- NJ BMP Manual Chapter 6 — Recharge rate tables (Table 6-1) and recharge volume calculation methodology.
- NJ BMP Manual Chapter 14 (2026) — Volumetric reduction accounting for GI BMPs.
Key Deliverables from This Stage¶
- CN Worksheet — Tabulation of land use areas, HSG classifications, and area-weighted composite CN for pre- and post-development.
- WQV Calculation — For each drainage sub-area contributing to a BMP.
- Recharge Volume Calculation — Annual recharge deficit using Table 6-1 rates.
- Peak Flow Summary — Pre/post comparison at each point of analysis for the 2-yr, 10-yr, and 100-yr storms.
- Hydrologic Comparison Table — Consolidated pre/post summary for the SWMR.
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