2019-11-12-minutes-public-works-study-session

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3H1Z11VxWAL13 Co DEPARTMENT OF PUBLIC WORKS IUMTAs COUNTY TASK 4.40 — Hydrology 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. Assumptions: • 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 discharge/highwater marks are not available. Instead, WSE will use engineering judgment to select and adjust model parameters. The model will be used to help size the bridge waterway, to provide data required to evaluate lateral erosion and scour potential, and to develop countermeasures. TASK 4.50 — Channel Stability and Scour Assessment WSE will determine if lateral erosion, local scour, ice, or debris flows pose a threat to the proposed crossing. If they do, WSE will work with KPFF to address these hazards in the design. Assumptions: These hazards will be addressed through modifications the structure itself (e.g. deeper foundations, longer bridge, etc.) and not through design and installation of external countermeasures (e.g. riprap). If countermeasures are required, the optional task at the end of this scope will be completed. TASK 4.70 — WDFW and Permit Coordination It is anticipated that WDFW personnel have an idea of the length of crossing they would like to see at the site. WSE will work with WDFW personnel, the County, and Jacobs (the company that will acquire the project permits) to collectively arrive at a bridge length that is acceptable to all parties. An allowance is provided for WSE's lead engineer to participate in one meeting with WDFW personnel in Ellensburg. Assumutions: