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Exhibit A-1 Scope of Work <br />Continued <br />interaction of Riverton Creek and the Duwamish River wherein the hydraulic conditions at the <br />confluence are governed by the timing of flows in the creek and stages on the river. However, due <br />to the configuration of the model that includes a storage area at the upstream side of the existing <br />outfall, detailed hydraulic information (velocities, etc.) in the design reach is not available. The <br />hydraulic analysis presented here builds on the NHC hydraulic analysis to obtain the necessary <br />hydraulic design information as described in this task. <br />6. I Data Review <br />Review the previous hydraulic model and report (NHC, 2010). <br />6.2 Statistical Analysis <br />Perform a statistical analysis of the time series data from the NHC hydraulic analysis to develop the <br />joint probability of flows in Riverton Creek and stages on the Duwamish River. This will provide a <br />range of plausible flow scenarios for the hydraulic analysis of Riverton Creek at the confluence. <br />Deliverables: <br />• Results of the statistical analysis and recommended flows and stages for use in the hydraulic <br />analysis of Riverton Creek. <br />6.3 Hydraulic Analysis <br />Construct a detailed one-dimensional (1-D) steady flow HEC -RAS model of Riverton Creek for the <br />study reach that extends from the confluence with the Duwamish River to the northbound SR -599 <br />on ramp. Cross sections along Riverton Creek will be based on available topographic survey data of <br />the study reach. The model will be configured for existing and proposed conditions and run for a <br />range of design flows and Duwamish River stages based on the joint probability analysis performed <br />for Task 6.2. <br />Assumptions: <br />• Available survey data are sufficient for the development of accurate cross sections for use in <br />the hydraulic analysis <br />Deliverables: <br />• Electronic files of the hydraulic model <br />6.4 Channel Stability Analysis and Hydraulic Design of Scour Countermeasures <br />Utilize results of the hydraulic analysis to assess the impacts of the flap gate removal on hydraulic <br />conditions and the stability of Riverton Creek, and to develop the hydraulic design of any necessary <br />scour countermeasures at the site of the flap gate removal and replacement pedestrian bridge. Any <br />W:APW Eng\PROJECTS\A- DR Projects\Riverton Flap Gate Removal (99830103 98-DR03)\Consultant Selection\Phase <br />2\contract\Amendment #1\Riverton SOW_Amend 1_17_1027.docx 3 93 <br />