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Exhibit A-1 Scope of Work <br />Continued <br />New Scope Items <br />The following task list is new tasks that are to be added to the Contract. Following these tasks is a <br />section pertaining to Contract items that are to be modified because of changes in scope. <br />Task 2 — Intake Review <br />2.5 Alternatives Analysis and Meetings (New) <br />Otak will complete additional work related to development of conceptual plans and cost estimates <br />for two alternatives for the proposed open confluence of Riverton Creek and the Duwamish River. <br />Alternative 1 will include a pedestrian bridge with a revised channel section to allow for channel side <br />slopes less steep than the 1.25'H : 1'V slopes proposed in the 75% design by others. Alternative 2 <br />will include a box culvert and wingwalls. Otak's subconsultant, GeoEngineers, will review the <br />concept designs and provide comments on stability and recommendations for geotechnical <br />evaluation. <br />Task 4 — Permitting, Environmental Review and Coordination <br />4.8 Water Quality Monitoring (New) <br />In order to provide an initial assessment of potential stormwater impacts to salmonids, Otak will <br />evaluate water quality conditions in the system using the WSDOT Hi -Run model and field <br />verification of existing conditions. The model and field verification will extend from the confluence <br />of Riverton Creek and the Duwamish River upstream to the culverts conveying Riverton Creek <br />under SR 599. Work for the Hi -Run modeling effort will involve collecting information on the <br />discharge area for contributing stormwater, precipitation data, existing baseline conditions, end -of - <br />pipe -loading subroutine, and receiving water dilution subroutine. Otak will prepare a technical <br />memorandum describing and interpreting the modelling and field verification results. <br />Task 6 — Hydraulic Analysis (New) <br />This task includes the hydraulic analysis necessary to support the design of the removal of two flap <br />gates and creek channel restoration for Riverton Creek at the confluence with the Duwamish River. <br />A previous hydraulic analysis was conducted by Northwest Hydraulic Consultants (NHC) to <br />evaluate any potential increased flood risk from the flap gate removal. The analysis was based on a <br />long-term (52 -year) HEC -RAS unsteady flow simulation to evaluate potential changes in the <br />frequency of water levels along the creek. The long-term simulation accounts for the complex <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 2 92 <br />