Res-2022-075 - Laserfiche WebLink

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TASK 2 - HYDROLOGY, HYDRAULICS, AND GEOMORPHOLOGY 2.1 Hydrology The consultant shall develop discharge hydrographs for five flows including four that will be required to update FEMA maps. These are the 10-, 2-, 1- and O.2-percent annual-chance events (commonly referred to as the 10-, 50- and 100- and 500-year floods). The peak discharge for each hydrograph will match the peak value from the effective FEMA study unless the consultant determines that the peaks should be revised. The hydrographs will be applied to run the hydraulic model in unsteady mode which will improve the accuracy of results. The consultant shall also prepare a hydrograph and peak discharge for a 5O-percent annual chance event (2-year flood) which has a significant influence on channel shape and size. The 2-year flood will be run in the hydraulic model to help inform the Geomorphic (channel stability) Assessment. Deliverables for this task will be included in the project report: o Updated annual instantaneous peak flood frequency discharges if they ditfer from those used in the recent FEMA study. o Flood hydrographs for use as input to the hydraulic modeling & analysis. o ldentification and description in the project report of flood events that have caused significant geomorphic change. Assumptions: o The consultant will be able to obtain flow records from the Bureau of Reclamation. 2.2 Hydraulic Modeling The consultant shall use the topographic/bathymetric surface to develop an unsteady HEC-RAS 2D model covering the project reach. Model development will be completed in accordance with FEMA Guidance for Flood Risk Analysis and Mapping to support reach hazard assessment and FEMA map updates. This reach is extremely complex from a hydraulic perspective, and a2D hydraulic model will reflect that complexity to support identification and evaluation of hydraulic impacts, risks, and to prepare new FEMA maps. Key model inputs will include channel and floodplain topography (developed under Task 1 .2 and Task 1 .5) and existing land cover data from the USGS National Land Cover Dataset, The modelwill be calibrated to available high- water mark data (if it exists), then run to simulate the five flood hydrographs developed in Task 2.1. Results will be used to provide data required to evaluate hydraulic patterns and forces in order to predict future geomorphic change including channel avulsion and migration potential. Deliverables: o Model results in the form of GIS figures and tables will be included in the project report. Assumptions: . none 4 WATERSHED€+SCIfI{CE 8 ENGINEEPINC