Resolution_TO1_WatershedScience_WInterchange

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� i WATERSHED '�` SCIENCE x ENGINEERING February 14, 2023 Task 2. Hydraulic Modeling and Analysis Task 1.1. Update Effective Model to HEC -RAS 6.3.1 The current FEMA effective model of Reecer and Currier Creek was completed in 2016 using HEC -RAS version 5.0.7. WSE will update the FEMA HEC -RAS model to the latest version of HEC -RAS (v 6.3.1), and will run the model to simulate the 1 -percent annual exceedance probability (AEP) flood and floodway. Results will be compared to effective model results to identify changes to BFEs or floodway elevations. Results of the updated model will provide the baseline for comparison of project alternative impacts (Task 2.3). Assumptions — Updating the model from 5.0.7 to 6.3.1 will result in only minor changes to BFEs or floodway elevations. Significant updates to the model geometry or floodway will not be required. Task 2.1. Initial Alternatives Development WSE will develop preliminary concept sketches for two (2) channel and floodplain modification alternatives which will provide an inset floodplain corridor along Currier and Reecer Creeks to reduce 1% AEP flood inundation extents by increasing near channel flood conveyance and storage. A goal is to do so in a manner that will provide multiple benefits in the form of reduced flood risk and habitat enhancement. The two alternatives are: • Alternative 1— Maintain the existing Reecer and Currier Creek channel alignments and excavate floodplain benches to create an inset floodplain that will contain the majority of the 1% AEP flood. • Alternative 2 — Realign and/or combine portions of the Reecer and Currier channels and excavate floodplain benches to create an inset floodplain that will contain the majority of the 1% AEP flood. The layout and extent of the combined and realigned channel would be developed based on direction from resource agency and tribal stakeholders. To help conceptualize the alternatives, WSE will participate in up to two, 1.5 -hour virtual meetings with MFA, stakeholders and County staff. Assumptions: The primary goal at this stage in the project is develop floodplain modifications that will reduce flood extents and BFEs so that significant and permittable floodplain development can occur; however, WSE will not attempt to develop alternatives that accommodate a specific development plan. This will be addressed in Task 3 below. Deliverables: Initial concept sketches showing alternative 1 and alternative 2 Final alternative layout sketches to support hydraulic modeling Task 1.3. Hydraulic Modeling WSE will update the hydraulic model to represent the two alternatives developed in Task 2.2. WSE will run the models to simulate the 1% AEP flood and compare results to baseline conditions to identify changes in inundation extents and BFEs. WSE will refine the alternative project layouts to attempt to maximize floodplain benefits and minimize impacts. Following these refinements, WSE will generate figures comparing baseline and proposed condition inundation extents and BFEs. Results will be presented to the