BMP Sizing¶
BMP sizing converts the governing source standards into actual storage, flow-control, infiltration, and maintenance-ready dimensions. In the live source set, the sizing step has to keep water-quality, recharge, runoff-quantity, and 2026 volumetric-reduction checks separate instead of collapsing them into one shortcut formula.
Structured citation evidence for this page lives in the sibling claim manifest.
Core Sizing Inputs¶
The current source-backed workflow uses four distinct inputs:
- the Water Quality Design Storm for runoff-quality sizing
- the recharge standard where infiltration into subsoil is part of the design
- the runoff-quantity standard for peak-rate control
- the volumetric-reduction standard in the 2026 rule set where GI retention is being used
That separation matters. A BMP can satisfy one standard without automatically satisfying the others.
Source-Backed Sizing Checks¶
At the page level, the stable checks are:
- Confirm the runoff-quality target using the Water Quality Design Storm.
- Check recharge and infiltration assumptions against Chapter 12 design permeability and the Chapter 13 hydraulic-impact framework.
- Evaluate runoff-quantity control against the governing design storms and peak-rate benchmarks.
- For 2026 projects using the retention pathway, calculate and document volumetric reduction as its own demonstration.
Sizing Output Matrix¶
| Sizing lane | Typical output |
|---|---|
| Runoff quality | Storage, flow path, and treatment sizing tied to the WQDS |
| Recharge / infiltration | Design permeability, separation, and hydraulic-impact-supported infiltration dimensions |
| Runoff quantity | Storage and outlet-control dimensions for the selected quantity path |
| Volumetric reduction | Retained WQDS volume and any residual that must be addressed elsewhere |
Why This Step Cannot Be Shortcutted¶
The older narrative pages tended to flatten sizing into a single worksheet idea. The live source set does not support that shortcut. The governing rule and manual chapters keep the standards distinct, which means:
- infiltration credit depends on the site investigation record, not just the preferred BMP type
- runoff-quality sizing does not by itself resolve runoff-quantity control
- runoff-quantity control does not replace the separate 2026 volumetric-reduction showing
- treatment-train decisions often remain necessary when one BMP does not satisfy every standard
That is the practical point of this stage: it converts the standards into dimensions and controls that can actually be reviewed, built, and maintained.