BASMAA Guidance for Sizing Green Infrastructure Facilities in Street Projects (June 2019)

BASMAA Guidance for Sizing Green Infrastructure Facilities in Street Projects (June 2019)

with companion analysis: Green Infrastructure Facility Sizing for Non-Regulated Street Projects

NOTE: This guidance is to be used together with its companion analysis: Green Infrastructure Facility Sizing for Non-Regulated Street Projects, which is attached to the end of the guidance. 

Provision C.3.j. in the reissued Municipal Regional Stormwater Permit[1](MRP) requires each Permittee to “complete and implement a Green Infrastructure (GI) Plan for the inclusion of low impact development drainage design into storm drain infrastructure on public and private lands, including streets, roads, storm drains, parking lots, building roofs, and other storm drain infrastructure elements.”

Provision C.3.j.i.(g) further mandates that these plans include:

Requirements that projects be designed to meet the treatment and hydromodification sizing requirements in Provisions C.3.c. and C.3.d. For street projects not subject to Provision C.3.b.ii. (i.e., non-Regulated Projects) Permittees may collectively propose a single approachwith their Green Infrastructure Plans for how to proceed should project constraints preclude fully meeting the C.3.d. sizing requirements. The single approach can include different options to address specific issues or scenarios. That is, the approach shall identify the specific constraints that would preclude meeting the sizing requirements and the design approach(es) to take in that situation. The approach should also consider whether a broad effort to incorporate hydromodification controls into green infrastructure, even where not otherwise required, could significantly improve creek health and whether such implementation may be appropriate, plus all other information as appropriate (e.g., how to account for load reduction for the PCBs or mercury TMDLs).

This document represents the “single approach” collectively proposed by the Permittees for how to proceed when constraints on GI projects affect facility sizing in street projects. For other types of projects, information on hydraulic sizing is provided in the technical guidance manuals for Provision C.3 developed by each countywide stormwater program.


[1]Order R2-2015-0049

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