2016-04-25-minutes-public-works-study-session

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4. Hydrologic Analysis and Channel Regime Width Estimates WSE will estimate the magnitude of the 1.5- to 2 -year annual instantaneous peak discharge using regional regression equations. This event is typically considered a °bankfull" flow. Knowing it along the slope of the longitudinal profile will allow WSE to use published channel regime width formulas to estimate the natural equilibrium channel width. This will serve as a check to validate the widths measured during the field survey. In estimating the 1.5- to 2 -year event, it will take no additional effort to also estimate the 25- and 100 -year discharges, both of which are needed to complete the hydraulic design (Part 2). Assuming WSDOT hydraulic culvert design criteria will be used, the proposed culvert will need to pass the 25 -year flow with approximately one foot of freeboard and also pass the 100 -year event without scouring the stream bed within the culvert or overtopping the road. S. Memorandum —Culvert Width Recommendation WSE will prepare a technical memorandum which will recommend a culvert width based upon the measured top width of the natural stream channel, a WDFW requirement. The memorandum will contain sufficient technical information to justify the recommended width, knowing that WDFW personnel will review the document. 6. Site Meeting with WDFW and County Personnel If desired by WDFW and/or County, WSE staff will participate in a site visit to discuss the recommended culvert width. 7. Administration -- Part 1 WSE will maintain frequent communication with the county and submit monthly invoices which will include brief progress reports. Pam 2 — Completion of Hydraulic Design 8. Hydraulic Analysis WSE will use survey data collected in Task 2, to create a simple HEC -RAS hydraulic model of the stream to evaluate the hydraulic capacity of proposed crossing altematives (i.e. to refine the design of the culvert to ensure that it can safely pass the 25 -year and 100 -year floods in accordance with WSDOT hydraulic design requirements). High water mark and flow data suitable to calibrate the model are not available; therefore, model parameters will be set based upon engineering judgement and experience. Model results will also aid in design of the streambed material gradations required to backfill the culvert floor, to assess scour and erosion potential, and to design scour and erosion countermeasures should they be needed.