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Feasibility Study of Re-regulation and/or Modification of Upper and Lower Baker Dams for Skagit River Flood Damage Reduction and PMF Analysis - Concrete, WA

Tetra Tech conducted an incremental evaluation of flood storage volume optimization at the Baker River Hydroelectric Project for Puget Sound Energy (PSE). In addition, Tetra Tech is currently conducting a Probable Maximum Flood (PMF) analysis of the Baker Project as a supporting analysis of the Part 12 inspection. 

The flood control storage is dedicated to and operated by the Corps of Engineers’ Seattle District. The primary objective of the flood storage optimization investigation was to determine downstream flood damage reduction benefits in the Skagit River floodplain. The Baker River Hydroelectric Project consists of the Upper and Lower Baker Dams, which are a tandem power generating reservoir system located on a 300 square mile tributary basin of the main stem Skagit River. Upper Baker is 310 feet high and stores 280,000 acre feet at full pool. Lower Baker dam is 280 feet high and impounds 150,000 acre feet at full pool. The evaluation consists of hydrologic, hydraulic, flood damage, and economic analyses, all of which follow USACE guidelines and methodologies in fulfillment of the requirements for a feasibility level study. The study is a parallel effort in cooperation with a related study being conducted by the Seattle District Corps of Engineers.

As part of this contract with PSE, Tetra Tech staff also contributed to an independent technical review of the Seattle District USACE’s hydrologic and hydraulic analysis of the Skagit River basin. One of the primary focal points of the review was the hydrologic analysis and methodology that was used to develop the balanced inflow hydrographs into the two reservoirs.

The PMF analysis for the Baker River hydroelectric projects is currently being conducted using FERC Engineering Guidelines and the hydrometeorological guidelines contained in HMR-57. The PMF analysis is being conducted under FERC review and as well under the review of a three member Board of Consultant (BOC) team. Detailed hydrologic analysis of the watershed is being conducted using the Stochastic Event Flood Model (SEFM) in conjunction with HEC-1. Snowmelt calculations are conducted offline of HEC-1 using USACE snowmelt equations and empirical formulas developed by the USBR to account for the effect of snow compaction on runoff during rain-on-snow events. Historic rainfall, stream gage and snowpack data are being used to calibrate and verify the watershed model.  Spatial and temporal characteristics of the historical storms were determined using a precipitation analysis software package (SPAS). The HEC-5 model is being used to route the PMF hydrographs through the controlled spillways at both dams.  As part of the PMF study, a global sensitivity analysis, using standard Monte Carlo sampling procedures, is being conducted to provide a thorough understanding of the model(s) response to variation in the magnitude of the input parameters.