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~TTITA..S OOUN"T'Y <br />DEPARTMENT OF PUBLIC WORKS <br />historical aerial photos, flood data, bridge maintenance records, and anecdotal information on <br />past flood events from County maintenance staff. <br />Assumptions: <br />• County will supply relevant data they possess <br />TASK 4.20 -Site Inspection <br />WSE will conduct a one-day field inspection to examine the crossing and Big Boulder Creek. <br />WSE will examine hydraulic and geomorphic characteristics, sediment transport, debris flow <br />potential, lateral erosion, and scour. <br />Assumptions: <br />• NTP is received in time to allow WSE to visit the site before snow fall. <br />TASK 4.30 -Channel Survey <br />WSE will survey up to eight cross sections and a longitudinal profile of the stream channel as <br />part of the site inspection task above. WSE will merge the stream channel data with the <br />County's survey to develop a single topographic base map required for the hydraulic <br />investigation. <br />Assumptions: <br />• County will or has collected detailed topographic data within the road right-of-way. <br />TASK 4.40 -Hydrology <br />WSE will use USGS regional regression equations to estimate annual instantaneous peak design <br />flow rates. The design event for flow and scour will be the 100-year flood and the check flood <br />for potential damage will be the 500-year event. The 500-year event is not the design event, it is <br />solely used to assess risk in-order to identify any remaining significant erosion or scour issues <br />that might need to be addressed in the design. <br />Assumptions: <br />• USGS regression equations will provide reasonable estimates for the 100-year and 500- <br />year annual instantaneous peak flows. If they do not, WSE will use alternative methods <br />to estimate the flows. <br />TASK 4.50 -Hydraulic Analysis