Site Constraints Analysis¶
The Site Constraints Analysis is the foundational stage of stormwater design. Before any BMP can be selected or sized, the engineer must characterize the physical, environmental, and regulatory conditions of the project site. Incomplete or inaccurate site data is the most common root cause of design revisions, permit delays, and post-construction performance failures.
Stage Purpose¶
This stage exists to establish the factual basis for every downstream design decision. The 2026 NJ BMP Manual elevates site investigation from an informal pre-design task to a formal, documented deliverable — particularly the Soil Investigation Protocol (Chapter 6, 2026), which sets procedural minimums for borings, SHWT determination, and hydraulic conductivity testing. Without verified site data, BMP feasibility screening and sizing calculations have no defensible foundation.
Engineering Tasks¶
Soils Investigation¶
- Classify soils by Hydrologic Soil Group (HSG) using NRCS SSURGO data for screening, confirmed by on-site borings.
- Conduct in-situ permeability testing (falling-head or constant-head) at each proposed BMP location. Design saturated hydraulic conductivity (K_sat) is the geometric mean of tested values with a 50% safety factor applied (2026 requirement).
- Minimum boring density: one per BMP footprint up to 5,000 sq ft; one additional per each additional 5,000 sq ft.
- Boring depth: minimum 72 inches below proposed BMP bottom.
Seasonal High Water Table (SHWT) Determination¶
- Document SHWT depth through field observation of redoximorphic features in soil borings or test pits, conducted by a licensed soil scientist.
- SSURGO database estimates are permitted for first screening only; borings are required for design confirmation (2026 edition explicitly).
- Verify minimum 2-foot vertical separation between BMP bottom and SHWT for all infiltrating practices.
Drainage Area Delineation and Topography¶
- Delineate contributing drainage areas to each proposed BMP location using topographic survey and grading plans.
- Calculate existing and proposed impervious cover for each sub-area.
- Identify natural drainage features (swales, depressions, stream corridors) to be preserved or integrated into the stormwater design.
Utility and Infrastructure Mapping¶
- Map all underground utilities (water, sewer, gas, electric, telecom) within and adjacent to proposed BMP locations.
- Confirm setback distances from structures and infrastructure:
| Feature | Minimum Setback |
|---|---|
| Building foundations / footings | 10 ft |
| Private wells | 25 ft (verify with local authority) |
| Septic system leach fields | 50 ft |
| Property lines | 10 ft |
| Slopes > 20% | Not directly upslope |
Environmental Screening¶
- Stormwater hotspot assessment: Determine if the site or contributing drainage area includes hotspot land uses (fueling stations, vehicle maintenance, hazardous material storage). Infiltration is prohibited in classified hotspot areas.
- Karst geology screening: Evaluate whether the site is in a known karst region; infiltration BMPs in karst areas require geotechnical evaluation to prevent sinkhole formation.
- Contaminated soils: Review environmental site assessment records. Infiltration is not permitted through contaminated soils.
Relevant BMPs¶
Site constraint findings directly control which BMPs advance to the selection stage. Key relationships:
| Site Condition | BMP Implications |
|---|---|
| HSG A/B soils, SHWT > 4 ft | Full suite of infiltrating GI BMPs feasible |
| HSG C soils, SHWT 2-4 ft | Bioretention with underdrain; extended drawdown periods expected |
| HSG D soils or SHWT < 2 ft | Infiltration infeasible; Non-GI BMPs or lined systems required |
| Hotspot classification | No infiltration; surface treatment BMPs only |
| Limited footprint | Small-scale GI: bioretention, tree trenches, dry wells |
| Large available area | Infiltration basins, constructed wetlands, extended detention |
See the full Engineering Constraints module for the constraint-to-BMP filtering framework.
Supporting Regulations¶
- N.J.A.C. 7:8-5.4 — Groundwater recharge standard; requires soil investigation to confirm infiltration feasibility. See Regulatory Explorer.
- N.J.A.C. 7:8-5.5 — Stormwater quality standard; site characterization determines whether GI or Non-GI compliance pathway applies.
- NJ BMP Manual Chapter 6 (2026) — Soil Investigation Protocol establishing minimum boring density, depth, K_sat testing, and SHWT documentation requirements.
- NJ BMP Manual Chapter 6, Table 6-2 (2026) — Consolidated SHWT separation requirements for all infiltrating BMP types.
Key Deliverables from This Stage¶
- Soil Investigation Report — Boring logs, redoximorphic feature documentation, K_sat test results, design K_sat with safety factor, SHWT depth determination.
- Drainage Area Map — Pre- and post-development sub-area delineation with impervious cover tabulation.
- Site Constraints Summary — Tabulated findings for soils, SHWT, setbacks, hotspot status, and utility conflicts, formatted for use in BMP feasibility screening.
Next stage: Hydrologic Analysis