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Data Collection and Review <br />WSE will collect existing data and information needed to complete hydraulic design. Items may include but <br />are not limited to, existing watershed and stream topographic data, LIDAR, current and historical aerial <br />photographs, existing crossing plans, maintenance reports, and discharge gage records for neighboring <br />streams. <br />Site Reconnaissance <br />WSE staff will examine each crossing and stream to determine whether there are unique geomorphic or <br />hydraulic conditions that need to be addressed during design. WSE staff will inspect each site for the <br />presence of scour, erosion, sediment deposition, grade control features, etc. WSE staff will attempt speak <br />with adjacent landowners to seek anecdotal information on the performance of each crossings. Key field <br />observations will be documented in the project hydraulic design report. <br />Hydrologic analysis <br />WSE will estimate the magnitude of the 2-,100-, and 200 -year annual instantaneous peak flows for each <br />crossing. The design discharge for each crossing will be the 100 -year event with the 200 -year flood used <br />to assess scour potential. The 2 -year flood will be used to aid in the assessment of future channel stabiiity. <br />Stream discharge gaging records are not available for the four streams, therefore, annual instantaneous <br />peak flood frequency discharges will first be estimated using published regional regression equations <br />(USGS (1998) "Magnitude and Frequency of Floods in Washington" Water -Resources Investigations <br />Report 97-4277, S.S. Sumioka, D.L. Kresch, and K.D. Kasnick, United States Geological Survey). If <br />discharge gaging records are available for a neighboring stream that has similar basin characteristics, WSE <br />will create a flood frequency curve from the data. This curve will be adjusted to each crossing site and will <br />be used to estimate discharges. The two data sets will be compared and if similar the gage generated <br />values will be used. If they are not, WSE will use experience and professional judgment to determine <br />which set of estimates to use. <br />WSE will estimate discharges likely to be present within each stream during the typical construction season <br />— July through October. <br />Hydraulic Analysis <br />WSE will use topographic survey data to create a HEC -RAS hydraulic model of each stream and the <br />axlsting crossing. High water mark and flow data suitable to calibrate each model are not available; <br />therefore, model parameters will be set based upon engineering judgment and experience. Each model <br />will be run to evaluate the performance of the existing crossing as well as crossing replacement concepts. <br />Model results will aid in design refinements for the preferred concept, assessment of scour and erosion <br />potential, and the design of scour and erosion countermeasures. <br />Page 3 <br />