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HomeMy WebLinkAboutRS_BankUsePlan_Memo_DRAFT_20171102_v1.pdf1 T ECHNICAL M EMORANDUM Date: November 2, 2017 To: Rebecca Saur, Stantec From: Hugh Mortensen, Tess Brandon, and Sarah Sandstrom Project Number: 131113 Project Name: Thunder Hills Creek Sanitary Sewer Interceptor Rehabilitation Subject: Springbrook Creek Mitigation Bank Use Plan The proposed Thunder Hills Creek Sanitary Sewer Interceptor Project will have unavoidable direct wetland impacts. The applicant, the City of Renton Public Works, proposes to mitigate for these direct wetland impacts at the Springbrook Creek Wetland and Habitat Mitigation Bank (Springbrook Bank). This memorandum describes how the Thunder Hills Creek sanitary sewer interceptor rehabilitation project complies with the appropriate process for use of the Springbrook Creek mitigation bank, and is intended to serve as the bank use plan for that transaction. The contents of this plan are based on Using Credits from Wetland Mitigation Banks: Guidance to Applicants on Submittal Contents for Bank Use Plans (Interagency Review Team for Washington State 2009) and Springbrook Creek Wetland and Habitat Mitigation Bank: Mitigation Banking Instrument (WSDOT and City of Renton 2006). 1. Project Description Thunder Hills Sanitary Sewer Interceptor was installed in 1965 and, like many sewer projects of that time, was installed in a stream valley to take advantage of favorable gravity-flow conditions. It consists of approximately 3,600 lineal feet of 10-inch to 18- inch concrete pipe that runs beneath or adjacent to Thunder Hills Creek. The depth of the pipe varies, and may be greater than 30 feet in some areas. Currently, access to the existing sewer is poor. Erosion along the creek has compromised the sewer interceptor in the past; portions of the stream bank were stabilized with gabion baskets in the early 1980s, and more repairs that used large quantities of rip-rap armoring were done in 2008 and 2009. The City proposes to improve system components and access for the sanitary sewer interceptor through a combination of Cured-In-Place-Pipe (CIPP) technologies and construction of replacement sewer line segments. From the northern end of the project area south, proposed improvements include adding an equipment turnaround at Station 2 6+50,1 building a new access road across PSE property at Station 15+00, and rebuilding the entrance road at Station 30+00. A new parallel sewer will be installed from approximately Station 3+28 to 11+20 and 13+08 to 15+44 underneath the new/improved access roads. The remaining portion of the existing sewer will be rehabilitated in place using CIPP technology. Existing rock buttresses and rockeries south of Station 6+00 will be rebuilt to protect the existing/new sewer and access road. Construction associated with proposed sewer interceptor rehabilitation will result in some disturbance to on-site wetlands and their associated buffers. Impacts from proposed project activities include temporary impacts (vegetation clearing) from construction, direct wetland impacts (vegetation clearing and wetland fill) from new access roads, and impacts to wetland buffers (vegetation clearing) from new access roads. The sewer interceptor rehabilitation work itself will not result in permanent impacts to wetlands or wetland buffers. Impact areas are shown on project site plan drawings submitted with this memorandum. 2. Existing Conditions of Wetlands and Buffers Six small wetlands were delineated in the project area. The wetlands were named alphabetically based on location along the creek, from upstream to downstream. Wetland A was categorized as a Category III slope/depressional wetland with a habitat score of four and a total score of 17. Wetland B was categorized as a Category III riverine wetland with a habitat score of six and a total score of 19. Wetland C was categorized as a Category IV slope wetland with a habitat score of five and a total score of 14. Wetland D was categorized as a Category III riverine wetland with a habitat score of five and a total score of 18. Wetland E was categorized as a Category III depressional wetland with a habitat score of five and a total score of 17. Wetland F was categorized as a Category III slope wetland with a habitat score of five and a total score of 16. All wetlands are located adjacent to Thunder Hills Creek, with the exception of Wetland A, which is collocated with the tributary stream (Stream E) at the upstream end of the project area. A summary of existing wetlands is provided in Table 1. Detailed information on wetlands delineated in the project area, and a map of all delineated features, are provided in the Stream & Wetland Delineation Report (The Watershed Company 2015). 1 Station numbers refer to location markers found on project site plan drawings. 3 Table 1: Existing Conditions of Wetlands Feature Ecology Rating Habitat Score HGM classification Standard Buffer Width (ft) Wetland A III 4 Slope/Depressional 75 Wetland B III 6 Riverine 100 Wetland C IV 5 Slope/Depressional 50 Wetland D III 5 Riverine 100 Wetland E III 5 Depressional 100 Wetland F III 5 Slope 100 Non-wetland areas are dominated mainly by a native deciduous forest with a few coniferous trees. The forest has an understory that includes typical shade-tolerant native shrubs and groundcovers. Patches of locally-dominant invasive weeds such as Himalayan blackberry and English ivy are also present throughout the forest (Figure 5). Dense monoculture of Himalayan blackberry and Japanese knotweed predominate where powerlines cross the stream and along forested edges. Thunder Hills Creek is a perennial stream that flows north-northwest through the entire project area. Bed and bank armoring have been placed along significant portions of the stream channel in an effort to prevent erosion. Large, loose riprap occurs throughout the channel, and gabion baskets armor sections in the northern (downstream) portion of the project area (Figure 3). As described in the Critical Areas Report (The Watershed Company 2017), multiple barriers downstream prevent salmonid fish use on-site in Thunder Hills Creek. Seven presumed seasonal tributary streams join Thunder Hills Creek along its course through the project area. 3. Wetland Impacts 3.1 Avoidance and Minimization of Wetland Impacts The entire project would be located within stream and wetland buffers designated by the City of Renton; therefore, strict avoidance of buffer impacts is not feasible. Nevertheless, several project elements minimize the area of wetland and wetland buffer impacted. These minimization measures are described below: 4 • CIPP technologies are used to repair the existing sewer interceptor where it crosses Thunder Hills Creek; • Where replacement pipe segments are proposed, construction those segments under the access road to minimize the footprint of construction and maintenance impacts; • Locating the improved/new access roads over the existing unmaintained trail/roads to minimize impacts to vegetation in the combined wetland and stream buffer; • Using a narrower width path (seven feet compared to 12-foot road) for specialized equipment on the northern portion of the project area; • Implementing a Temporary Erosion and Sediment Control plan, provided with the project site plan drawings; and • Timing construction to avoid the wet season when sedimentation and erosion would be anticipated. 3.2 Unavoidable Wetland Impacts The project will result in unavoidable wetland impacts. Temporary impacts to wetlands and wetland buffers (20,703 square feet) will be restored to their pre-existing conditions following construction. Permanent wetland buffer and stream buffer impacts (41,000 square feet) will be mitigated on-site. Direct wetland impacts include placement of a new gravel access road within Wetland F, and construction of a new gravel turnaround that extends partially into Wetland E at the southernmost extent of the project area. These direct wetland impacts total 1,332 square feet (0.03 acres) (Table 2). Wetland Category Area of Impact Credits Required F III Slope 1,315 square feet (0.029 acres) 0.025 E III Depressional 17 square feet (0.001 acres) 0.001 Total III 1,332 square feet (0.03 acres) 0.026 3.3. Impacted Wetland Functions Wetland E Wetland E was classified as a Category III depressional wetland with a habitat score of five and a total score of 17. Only a small area of Wetland E will be directly affected by the project. Wetland E is a disturbed wetland that has formed on top of fill material and is located on what once was likely the stream channel of Thunder Hills Creek. The 5 culvert that carries Thunder Hills Creek under I-405 is under Wetland E; the culvert inlet is located just south of the wetland. Scrub-shrub and emergent vegetation in the wetland may filter runoff from adjacent development before it reaches Thunder Hills Creek and downstream impaired waters of the Black River. Due to the limited area of impact, this project is not expected to affect water storage potential of the wetland significantly. Similarly, the small area of impact will not significantly affect diversity of habitat structures within the wetland. Wetland F Wetland F was categorized as a Category III slope wetland with a habitat score of five and a total score of 16. The project will fill the majority of Wetland F, effectively eliminating its current functions, described below. Because the average slope of the wetland is greater than five percent and the vegetation has a limited potential for trapping sediments and pollutants from adjacent development before it reaches Thunder Hills Creek and downstream impaired waters of the Black River. The nearby residential development may generate flashy runoff associated with impervious surface coverage. The wetland position and vegetative composition would have limited potential to slow or store floodwaters. However, and flood reduction potential would be beneficial to downstream development, which encompasses the urban core of the City of Renton. The wetland itself has limited site complexity in terms of vegetation, hydroperiods, and special habitat features. Similarly, given the proximity of adjacent high-intensity development, there is limited potential to support habitat in the surrounding landscape. However, the adjacent Thunder Hills Creek provides instream, riparian, and downed wood habitats, which increase the habitat value of the wetland. 4. Springbrook Creek Mitigation Bank Use 4.1 Wetland Mitigation Site Selection Rationale The project applicant considered several alternatives for mitigating for permanent wetland impacts. Given the steep nature of the project site and the lack of public ownership, on-site wetland mitigation outside of existing wetland areas was not a feasible option. Additionally, The Environmental Protection Agency (EPA) and the U.S. 6 Army Corps of Engineers (Corps) have established a preference for use of mitigation banks or in-lieu-fee programs over on-site, permittee-responsible mitigation (33 CFR §325 and 332, 40 CFR §230). The City of Renton has an approved wetland mitigation bank on Springbrook Creek and King County operates an in lieu fee program through its Mitigation Reserves Program. The applicant proposes to purchase wetland credits at the Springbrook Mitigation Bank (Springbrook Bank) to offset permanent wetland impacts. The Springbrook Bank is owned by the City of Renton, and its Service Area includes portions of the Green- Duwamish Watershed (WRIA 9) including the project site. The Springbrook Bank was selected over the King County Mitigation Reserves Program because of its proximity to the impact site and location within the City of Renton and WRIA 9. 4.2 Wetland Functions Provided at Wetland Mitigation Bank The Springbrook Bank created approximately 18 acres of new wetland area, and rehabilitated or enhanced approximately 85 acres of wetlands. The proximity of the Springbrook Bank to the lower reaches of Springbrook Creek and the Lower Green River provides an opportunity for the wetland mitigation bank to improve water quality, hydrologic, and riparian functions. Dense native vegetation will help filter water quality. In addition to water quality, riparian vegetation supports detritus and shading, which supports instream habitat for salmonids. Springbrook Creek floodwater is captured by the bank wetlands during high water events because parts of the manmade levees were removed. This additional flood storage capacity reduces downstream flood elevations. Additionally, because the Springbrook Bank is located within an otherwise densely developed area, similar to the impact site, the mitigation bank protects and enhances the remaining area available for wildlife populations. 4.3 Wetland Functions Not Mitigated at Wetland Mitigation Bank Temporary impacts to wetlands and wetland buffers will be mitigated on-site rather than at the mitigation bank. This on-site restoration is warranted, as it will help to maintain existing functions present at the site to the maximum extent feasible. 7 Additionally, permanent impacts to wetland buffers will be mitigated on-site. The on- site wetland mitigation is warranted to maintain buffering functions for the stream and existing wetland areas that will remain. 4.4 Proposed Mitigation Credits The project proposes to mitigate for unavoidable permanent impacts to Wetland E and Wetland F using the Springbrook Creek Mitigation Bank. Because Wetland and Wetland F are Category III wetlands, 0.85 credits at the Springbrook Creek Mitigation Bank are required per impact acre (WSDOT and City of Renton 2006). Therefore, permanent impact to 0.03 acres to Wetland E and Wetland F requires 0.026 mitigation credits. 4.5 Credit Purchase or Transfer Timing Credits will be purchased immediately upon receiving permit approvals from regulatory agencies. References Interagency Review Team for Washington State. 2009. Using Credits from Wetland Mitigation Banks: Guidance to Applicants on Submittal Contents for Bank Use Plans. The Watershed Company. 2015. Stream & Wetland Delineation Report Thunder Hills Sanitary Sewer Interceptor Alternatives Analysis, Renton, WA. Prepared for City of Renton. The Watershed Company. 2017. Critical Areas Report Thunder Hills Sanitary Sewer Line- Renton, WA. Prepared for Stantec Consulting Services, Inc. Washington State Department of Transportation (WSDOT). 2006. Springbrook Creek Wetland and Habitat Mitigation Bank Prospectus. Washington State Department of Transportation (WSDOT) and City of Renton. 2006. Springbrook Creek Wetland and Habitat Mitigation Bank: Mitigation Banking Instrument.