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BMP Selection

BMP selection is the process of matching site constraints and hydrologic requirements to specific stormwater management practices. This stage applies the constraint-based filtering logic from the Engineering Constraints module and the design volumes from the Hydrologic Analysis stage to identify a feasible set of BMP candidates for the project.

Stage Purpose

Under the 2026 NJ BMP Manual framework, BMP selection follows a defined hierarchy: Green Infrastructure (GI) BMPs must be evaluated first, and Non-GI BMPs may only be proposed when GI is documented as infeasible. This stage ensures that the designer considers the full range of available practices, applies site constraints systematically, and develops defensible alternatives before advancing to detailed sizing. The 2026 edition requires a formal GI feasibility analysis as a pre-condition to proposing any Non-GI alternative.

Engineering Tasks

GI vs. Non-GI Hierarchy

The NJ regulatory framework establishes a clear preference hierarchy:

  1. GI BMPs providing full volumetric reduction (VRv >= WQV) — preferred compliance pathway; exempt from separate TSS removal calculation.
  2. GI BMPs providing partial volumetric reduction (VRv < WQV) — acceptable; residual volume (WQV - VRv) must be managed by a compliant BMP with documented 80% TSS removal.
  3. Non-GI BMPs — permitted only when GI is demonstrated infeasible due to site constraints (HSG D soils, SHWT < 2 ft, contamination, hotspot classification, or insufficient footprint).

The designer must document which tier applies and why, as part of the Stormwater Management Report (SWMR).

Constraint-Based Filtering

Apply the site data gathered during Site Constraints Analysis to eliminate infeasible BMP types:

Constraint BMPs Eliminated BMPs Remaining
HSG D soils Infiltration basin, dry well, unlined bioretention Lined bioretention w/ underdrain, detention, manufactured devices
SHWT < 2 ft from BMP bottom All unlined infiltrating systems Raised systems, lined bioretention, surface filtration
Hotspot classification All infiltrating BMPs Surface treatment, manufactured treatment devices, lined systems
Available area < 500 sq ft Basins, wetlands, extended detention Tree trenches, dry wells, manufactured devices
Contributing area > 5 acres Small-scale GI practices Wet ponds, constructed wetlands, extended detention
Karst geology Concentrated infiltration practices Distributed practices, lined systems, surface treatment
Steep slopes (> 15%) Ponding BMPs on slopes Terraced systems, vegetated filter strips, grass swales

See Engineering Constraints for the full constraint classification framework.

Decision Tree Approach

The NJ BMP Manual (Agents/14) provides structured decision trees for BMP selection. The general decision sequence is:

  1. Is GI feasible? Evaluate soils (HSG A/B/C), SHWT separation (>= 2 ft), absence of hotspot classification, and available permeable area.
  2. Can full volumetric reduction be achieved? Can the proposed GI BMP capture and retain the entire WQV with no surface discharge?
  3. If partial GI only: What residual volume requires treatment? What secondary BMP addresses the residual?
  4. If GI is infeasible: Document the technical infeasibility finding with supporting soil investigation data, and select the most effective Non-GI alternative.

Developing Alternatives

Both editions of the manual recommend developing at least two design alternatives for comparison. Each alternative should document:

  • BMP type(s) proposed and their location on the site plan
  • Drainage area served by each BMP
  • Estimated storage or treatment volume provided
  • Estimated compliance status for each standard (WQ, recharge, flood control)
  • Relative construction cost and long-term maintenance burden
  • Consistency with LID site planning principles

Alternatives analysis strengthens the permit application and provides the design team with fallback options if the preferred alternative encounters unforeseen constraints during detailed design.

Treatment Train Considerations

Where a single BMP cannot meet all three standards, a treatment train — multiple BMPs in series — may be required. Common treatment train configurations:

Configuration Application
Grass swale + bioretention Pretreatment + volumetric reduction
Bioretention + extended detention WQ compliance + flood control
Pervious pavement + underground detention Source reduction + peak flow attenuation
Green roof + cistern + bioretention ET reduction + reuse + infiltration

When calculating combined TSS removal for a treatment train, use the joint probability method specified in Chapter 4 — not simple additive removal percentages.

Relevant BMPs

The full BMP Library is the reference for this stage:

Supporting Regulations

  • N.J.A.C. 7:8-5.3 — GI BMP preference hierarchy; requires documented GI feasibility analysis before Non-GI alternatives. See Regulatory Explorer.
  • N.J.A.C. 7:8-5.5 — Water quality standard; 80% TSS removal or full volumetric reduction.
  • NJ BMP Manual Chapter 4 — TSS removal performance tables and treatment train calculation methodology (joint probability method).
  • NJ BMP Manual Chapter 14 (2026) — Volumetric reduction standard and GI BMP compliance pathway definitions.

Key Deliverables from This Stage

  1. GI Feasibility Analysis — Documented evaluation of GI applicability with supporting soil investigation data; required before proposing Non-GI alternatives (2026).
  2. BMP Alternatives Comparison — Minimum two alternatives with estimated compliance status, cost, and maintenance implications.
  3. Selected BMP Configuration — Preferred alternative with rationale for advancement to detailed sizing.

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