BMP Sizing¶

BMP sizing translates the hydrologic targets established during analysis into specific physical dimensions, storage volumes, and hydraulic configurations for each selected practice. This is the quantitative core of stormwater design — where regulatory requirements become construction-ready specifications.

Stage Purpose¶

Each of the three N.J.A.C. 7:8 standards (water quality, groundwater recharge, flood control) generates a distinct sizing requirement. A BMP must be sized to satisfy all applicable standards simultaneously, or the design must employ a treatment train where each practice addresses a specific standard. The 2026 NJ BMP Manual introduces volumetric reduction accounting (Chapter 14) as a new sizing step that affects the water quality compliance pathway for GI BMPs.

Engineering Tasks¶

Water Quality Volume (WQV) Sizing¶

The WQV — calculated during Hydrologic Analysis — is the primary sizing input for water quality BMPs:

WQV = P x Rv x A

P = 1.25 inches (NJ WQ Design Storm)
Rv = volumetric runoff coefficient from composite CN
A = contributing drainage area

The BMP must provide storage, infiltration, or treatment capacity equal to or exceeding the WQV. For infiltrating GI BMPs, the available storage volume includes:

Storage Component	Typical Range
Surface ponding depth	6 - 12 inches
Filter media porosity	0.20 - 0.40 (depends on media spec)
Aggregate reservoir porosity	0.30 - 0.40
Internal Water Storage Zone (IWZ)	Per Ch. 14 (2026); enhances denitrification

Volumetric Reduction Volume (VRv) — 2026 Framework¶

The 2026 edition introduces a formal volumetric reduction calculation for GI BMPs using Chapter 14 methodology:

Calculate the volume of runoff that the GI BMP captures and retains on-site through infiltration and evapotranspiration (no surface discharge from the WQ storm).
This retained volume is the VRv.
Compare VRv to WQV:
VRv >= WQV: Full compliance with water quality standard. No separate TSS removal calculation required.
VRv < WQV: Residual volume (WQV - VRv) must be treated by a BMP meeting 80% TSS removal per Chapter 4 performance tables.

This two-step evaluation replaces the single-step WQV sizing approach of the 2023 edition and typically favors infiltration-based GI BMPs that can demonstrate full volumetric capture.

Groundwater Recharge Volume Sizing¶

The recharge deficit calculated during hydrologic analysis defines the required infiltration volume:

Rev = SUM[ (Rrate_pre x A_pre) - (Rrate_post x A_post) ]

The infiltrating BMP must provide annual infiltration capacity equal to the Rev deficit.
BMPs receiving recharge credit must not use impermeable liners (absolute prohibition per Chapters 6 and 9-10).
Verify that the BMP's design infiltration rate (using the safety-factored K_sat from the Site Constraints Analysis) can drain the recharge volume within the allowable drawdown period (typically 72 hours).

Flood Control Volume Sizing¶

Flood control sizing addresses peak flow attenuation for the design storms:

Design Storm	Duration	Purpose
2-year, 24-hour	~3.2 - 3.6 in	Peak rate control; channel protection
10-year, 24-hour	~5.0 - 6.5 in	Conveyance system design
100-year, 24-hour	~9 - 13 in	Flood control compliance

Rainfall depths from NOAA Atlas 14 for the project location (SCS Type III distribution for most of NJ).
Route the design storm hydrographs through the proposed BMP using standard stage-storage-discharge relationships.
Post-development peak discharge must not exceed pre-development peak for the 2-year and 100-year storms (N.J.A.C. 7:8-5.6).
Outlet control structures (orifices, weirs, risers) are sized to throttle outflow to the pre-development peak rate at each design storm level.

Integrated Sizing — Meeting All Three Standards¶

In many designs, a single BMP must satisfy multiple standards simultaneously. The sizing process follows this sequence:

Size for WQV — Establish minimum storage/infiltration volume for water quality.
Check recharge — Confirm that the infiltration capacity meets the Rev deficit. If not, add supplemental infiltration (e.g., additional dry wells or pervious pavement).
Size for flood control — Add detention storage above the WQV pool to attenuate peak flows. Design outlet controls for multi-stage release.
Verify all standards met — Confirm compliance with each standard independently in the final configuration.

Drawdown Time Verification¶

All infiltrating BMPs must drain the design volume within specified timeframes:

BMP Type	Maximum Drawdown Time
Bioretention surface ponding	72 hours
Infiltration basin	72 hours
Pervious pavement reservoir	72 hours
Dry well	72 hours
Extended detention (flood control)	24 - 72 hours (per design)

Drawdown is calculated using the design K_sat (geometric mean with 50% safety factor) applied to the BMP bottom area. Failure to drain within the specified period indicates undersizing or inadequate soil infiltration capacity.

Relevant BMPs¶

Sizing methodologies vary by BMP type. Refer to individual BMP pages for practice-specific sizing guidance:

Bioretention — Filter media depth, ponding depth, aggregate reservoir, underdrain sizing
Pervious pavement — Reservoir depth, subgrade infiltration rate, pavement void ratio
Infiltration basin — Basin geometry, forebay volume, outlet configuration
Extended detention — Stage-storage-discharge, multi-stage outlet design
Wet pond — Permanent pool volume, extended detention above pool, outlet works
Underground detention — Chamber volume, orifice sizing, overflow routing

Supporting Regulations¶

N.J.A.C. 7:8-5.4 — Groundwater recharge volume standard. See Regulatory Explorer.
N.J.A.C. 7:8-5.5 — Water quality volume standard (1.25-in WQ storm; 80% TSS or volumetric reduction).
N.J.A.C. 7:8-5.6 — Flood control standard (2-yr and 100-yr peak rate match).
NJ BMP Manual Chapter 5 — SWM standards and computational methodology for WQV, recharge, and flood control.
NJ BMP Manual Chapter 6 — Recharge rate tables, soil investigation requirements, SHWT separation.
NJ BMP Manual Chapter 14 (2026) — Volumetric reduction calculation methodology for GI BMP compliance.

Key Deliverables from This Stage¶

BMP Sizing Calculations — WQV, VRv (2026), Rev, and flood control volumes for each proposed BMP, with supporting worksheets.
Stage-Storage-Discharge Analysis — For detention-based BMPs; routing calculations demonstrating peak flow compliance.
Drawdown Verification — Confirmation that all infiltrating BMPs drain within allowable timeframes using design K_sat values.
Preliminary Site Plan — BMP locations, grading, drainage area boundaries, and setback compliance shown on plan.

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