Stream Rehabilitation Equivalency for Alternative Compliance
Long-term reversal of hydromodification effects requires movement away from reliance on site-based approaches to more integrated watershed-based strategies. Through a holistic approach to mitigate watershed hydromodification impacts at a regional scale, stakeholders can make a difference in improving watershed health. As part of developing watershed-based strategies to improve watershed health, San Diego County copermittees are currently developing approaches to use stream rehabilitation for alternative compliance. Stream rehabilitation projects provide multiple benefits by improving water quality, controlling hydromodification, augmenting water supply via groundwater recharge, and/or restoring habitat within watersheds. In addition to addressing future impacts caused by development projects in the watershed, stream rehabilitation projects also address legacy impacts. By reducing sediment and nutrient loading that occurs with unstable streams, stream rehabilitation projects can also help meet water quality goals such as total maximum daily loads and achieve other local water quality benefits that result in reduced costs for governments.
Stream rehabilitation projects are indicated in the San Diego MS4 permit as one of options that may be eligible to qualify as a “candidate” project in an alternative compliance program. Candidate projects must establish “equivalency” between onsite and offsite impacts. San Diego County copermittees developed a two part methodology for allowing stream rehabilitation projects to be used as alternative compliance projects: 1) to identify sensitive stream segments that require rehabilitation; and 2) to determine equivalency credits generated from the stream rehabilitation activity that can be used by development projects in a watershed. This presentation will detail the methodology developed to perform equivalency calculations for hydromodification management. The methodology generally follows the following steps:
• Step 1: Identify the scope of mitigation required through stream rehabilitation.
• Step 2: Perform channel screening to estimate the geomorphic stability of the receiving waters in a watershed.
• Step 3: Estimate the geomorphic impact on the receiving waters in the watershed for full built out condition.
• Step 4: Compare the geomorphic impact and geomorphic stability to identify the sensitive stream segments in the watershed.
• Step 5: Develop stream rehabilitation plans for the sensitive stream segments in the watershed.
• Step 6: Estimate credits generated by implementing the designed stream rehabilitation projects.
The above methodology was presented to and received concurrence from the Technical Advisory Committee (TAC) and Stakeholder Advisory Group (SAG) formed to support the development of an alternative compliance program for the San Diego Region. The TAC was comprised of local experts in the fields of engineering, planning, biology, chemistry, law, and academia; as well as staff from the Regional Water Quality Control Board. The SAG was comprised of a boarder audience of local interested professionals. The proposed methodology can be adapted for other regions to allow and use stream rehabilitation projects as regional solutions for hydromodification management to mitigate for both future and legacy impacts.
The authors will present case studies with site photos depicting application of the methodology to better convey the steps involved. Presentation of case studies is intended to engage the audience through recognition of similar stream systems or to inquire about how to implement for different types of systems.
The theme of this year’s conference “Stormwater – Are We Making a Difference?” is especially relevant to this topic. Application and implementation of stream rehabilitation projects identified by the methodology will result in immediate and measurable improvement in receiving water health.