Advantages of an Erosion Potential Hydromodification Management Standard
The current state of the practice for hydromodification management (HM) in California for new and redevelopment is to mimic pre-development hydrology on the project site. The theory is that if the pre-development distribution of in-stream flows is maintained, then the baseline capacity to transport sediment, a proxy for the geomorphic condition, will be maintained as well. A popular method of mimicking the pre-development flow regime is by maintaining the pre-development distribution of runoff, known as flow duration control (FDC). This can be done onsite by routing post-development runoff through structural stormwater BMPs such that runoff is stored and slowly released to match pre-development flow duration characteristics.
While, theoretically FDC maintains the pre-project sediment transport capacity for the full distribution of erosive flows, in practice it is difficult to achieve a good match for the entire range of flows evaluated if typical onsite LID BMPs are used. This is because the outlet structure configuration of a typical LID BMP consists of a simple overflow weir and a low flow orifice (if needed). However, in order to get a good match of the flow duration curve with passive controls such as LID BMPs, a more complicated system of intermediate weirs and/or orifices is often required. As a result, LID BMPs sized for FDC can over-mitigate site runoff and the consequence can be larger BMPs than necessary. Other limitations of FDC include that it: can only be used for out-of-stream (i.e., Onsite and Regional) HM Controls, and not in-stream measures; is a hydrologic metric that does not account for alterations to channel form (i.e., geometry and bed/bank material) of a receiving stream nor can it account for losses of bed sediment supply; and does not directly quantify hydromodification benefit of green infrastructure retrofits, for which there may not be sufficient room to achieve a given HM performance standard.
To avoid potential over-mitigation and over-sizing of LID-type BMPs, an Erosion Potential (Ep) numeric control standard is being implemented by the Contra Costa Clean Water Program (CCCWP) to update their LID Design Guide in compliance with MS4 permit requirements. Additionally, an Ep-based HM standard has been included in a growing number of California MS4 permits (i.e., Bay Area, Central Valley, Ventura County, and Los Angeles County) to facilitate the sizing of HM controls. This presentation:
• Summarizes the regulatory, technical, and design practicability advantages of an Ep HM Standard;
• Describes the application of the Ep methodology for on-site LID BMPs, regional detention/retention basins, in-stream rehabilitation, or a combination thereof;
• Provides results of a robust BMP sizing sensitivity analysis performed by CCCWP; and
• Presents valuable lessons learned for other municipalities and clean water programs in California who are in the process of protecting their creeks from the effects of hydromodification while complying with MS4 requirements in a cost-effective manner.