2012/03/21 Council Agenda Packet

City of Everett ! March 12,2012 Page 4 J is Develop critical cross sections and slope stabilization schematics designed to support the existing banks and work with the design team to integrate the geotechnical measures with the geomorphic and hydraulic solutions for up to three different design alternatives; and ■ Provide an estimate of the long-term stability and angle of repose of the slope based on the stream stabilization design for up to three different design alternatives. Assumptions s Slope stabilization schematics will include multiple variations on the elevation of the final stream channel and resulting factors of safety and estimated long-term angle of repose of the adjacent banks. ■ Design work does not include design of structural walls or unusual slope protection that might be required to avoid impacts to private property. Deliverables • Text and schematics to be included in the Draft/Final Basis of Design report outlining key findings of bank stability analysis for up to three different project alternatives, and recommendations regarding a preferred alternative and stable bank height and slope. TASK 4- HYDROLOGY AND HYDRAULIC ANALYSIS GeoEngineers will estimate a range of peak flood recurrence intervals for current conditions within the watershed from bankfull to the 100-year event(2-, 10-, 25-, and 100-year) employing the US Army Corps of Engineers Hydrologic Modeling System (HEC-HMS). The estimated flows will account for the storm water ponds that discharge to the stream channel within 1,000 feet upstream of the project reach. The purpose of the hydrologic analysis is to develop estimate of peak streamfiows within the tributary channel of the project reach to facilitate the design of stream stabilization measures. HEC-HMS utilizes a single event rainfall runoff hydrologic simulation designed to simulate runoff for dendritic watersheds. We will develop runoff estimates at the upstream and downstream extent of the project area for each for the following rainfall eventsbased on the 2012 City of Everett rainfall time series developed by HDR Inc.: ■ 2 year, 24 hour rainfall ■ 10 year, 24 hour rainfall. • 25 year, 24 hour rainfall ■ 100 year,24 hour rainfall Following the hydrologic analysis,GeoEngineers will develop a hydraulic model for the project reach using a one-dimensional (1-D) HEC-RAS computer model. Information from recent topographic survey, LiDAR data, and available analyses performed by others will be incorporated into the model. The purpose of the hydraulic analysis will be to provide all the appropriate hydraulic parameters needed to assess scour and design stream stabilization measures to withstand the expected hydraulic forces within the project reach during the design flow conditions. These parameters include flow velocities, shear stresses, water surface elevations(depths),and flow widths through the project reach location. The HEC-RAS hydraulic model will be first used to evaluate existing hydraulic conditions within the tributary channel and provide hydraulic data to support the geomorphic assessment. Following the GEOENGINEERSI17 File No.0661-086-02 35 5