Sediment Supply NPDES Permit Requirement for Hydromodification Management
Watershed health and function are strongly influenced by hydrological and geomorphological processes occurring in the watershed. Land development modifies the hydrologic and geomorphic processes by introducing impervious surfaces and drainage infrastructure that increases runoff discharge magnitude and duration. These changes caused by land use modifications are referred to as hydromodification. The impact of hydromodification can manifest itself through adjustment of stream morphology via channel incision, widening, planform alteration, or coarsening of the bed material. The state of the practice for hydromodification management in California this past decade has been to mimic pre-development site hydrology. A standard method of mimicking the pre-development site hydrology is by maintaining the pre-development frequency distribution of runoff, known as flow duration control. 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. One issue with focusing only on hydrology, however, is that this approach assumes there are no changes in sediment supply, bed/bank material composition, and channel geometry due to land development. Changes to these additional factors may also impact the geomorphic stability of streams.
Recognizing the important role that sediment supply plays in geomorphic stability, the Regional Municipal Separate Storm Sewer System (MS4) Permit, adopted by the San Diego Regional Water Quality Control Board (RWQCB), requires land development applicants to avoid known critical sediment yield areas or implement measures that allow critical coarse sediment to be discharged to receiving waters such that there is no net impact. The MS4 Permit factsheet states this requirement is necessary because the availability of coarse sediment supply is as much an issue for causing erosive conditions to receiving streams as are accelerated flows.
The County of San Diego and the City of San Diego, through a Technical Advisory Committee process, developed a four-step process for land development applicants to meet the sediment supply MS4 Permit requirement. The steps include:
Step 1. Identify: Land development applicants must identify critical coarse sediment yield areas (CCSYAs) located onsite and/or upstream of the project site. If no CCSYAs are identified in this step, no further consideration of critical coarse sediment supply is necessary.
Step 2. Avoid: Applicants are encouraged to avoid impacts to onsite CCSYAs through effective site design techniques as feasible.
Step 3. Bypass: Applicants should bypass bed sediment from onsite and/or upstream CCSYAs to downstream receiving waters.
Step 4. Demonstrate No Net Impact: When impacts to CCSYAs are not avoided or passed through, the applicant must demonstrate that the project generates no net impact to the receiving water.
The presenters will provide an overview of this process designed to meet the permit requirements and answer questions pertaining to its development, both from the technical aspect and the stakeholder process aspect. Consistent with this year’s conference theme, this project highlights the theme that sediment, should be managed, as should stormwater, as a resource to support stream stability and overall watershed health.