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COMPARISON OF BAHM AND CONTRA COSTA APPROACHES TO HYDROMODIFICATION MANAGEMENT PLAN REQUIREMENTS

The San Francisco Bay Regional Water Quality Control Board (Water Board) has issued municipal stormwater permit amendments that contain requirements for stormwater programs to implement Hydromodification Management (HM) requirements. The control standards established in the municipal permits generally require that post-project runoff shall not exceed pre-project rates or durations over a defined range of storm event sizes from one-tenth of the 2-year recurrence flow to the 10-year flow. The change in hydrology associated with development must be evaluated over a long timeframe using a continuous simulation hydrologic model. The results of the modeling are used to size control measures to match the pre-project flow duration patterns.