2018-12-10-minutes-public-works-study-session

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DES To �r U KITT rAS COi7N'fY 3kKIT'rITgF.3 Co DEPARTMENT OF PUBLIC WORKS historical aerial photos, flood data, bridge maintenance records, and anecdotal information on past flood events from County maintenance staff. Assumptions: • County will supply relevant data they possess TASK 4.20 — Site Inspection WSE will conduct a one -day field inspection to examine the crossing and Big Boulder Creek. WSE will examine hydraulic and geomorphic characteristics, sediment transport, debris flow potential, lateral erosion, and scour. Assumptions: • NTP is received in time to allow WSE to visit the site before snow fall. TASK 4.30 — Channel Survey WSE will survey up to eight cross sections and a longitudinal profile of the stream channel as part of the site inspection task above. WSE will merge the stream channel data with the County's survey to develop a single topographic base map required for the hydraulic investigation. Assum tip ons: 0 County will or has collected detailed topographic data within the road right-of-way. TASK 4.40 — HydroIogy WSE will use USGS regional regression equations to estimate annual instantaneous peak design flow rates. The design event for flow and scour will be the 100 -year flood and the check flood for potential damage will be the 500 -year event. The 500 -year event is not the design event, it is solely used to assess risk in -order to identify any remaining significant erosion or scour issues that might need to be addressed in the design. Assum tions: • USGS regression equations will provide reasonable estimates for the 100 -year and 500 - year annual instantaneous peak flows. If they do not, WSE will use alternative methods to estimate the flows. TASK 4.50 — Hydraulic Analysis WSE will develop a simple HEC -RAS model of Big Boulder Creek in the vicinity of the bridge crossing using topographic base map. The model will not be calibrated because observed