First Advisor

Scott Wells

Term of Graduation

Spring 2020

Date of Publication

7-1-2020

Document Type

Thesis

Degree Name

Master of Science (M.S.) in Civil & Environmental Engineering

Department

Civil and Environmental Engineering

Language

English

Subjects

Water quality -- Washington (State) -- Cedar River Watershed (King County) -- Mathematical models, Water quality -- Washington (State) -- Chester Morse Lake -- Mathematical models

DOI

10.15760/etd.7397

Physical Description

1 online resource (xvii, 220 pages)

Abstract

The laterally averaged, two-dimensional model CE-QUAL-W2 was used to develop a water quality model of the Cedar River Municipal Watershed as a reservoir management and climate change scenario tool. The 90,638-acre watershed, located 56 kilometers southeast of Seattle, WA, provides drinking water to over 1.4 million people. The watershed relies on two waterbodies for storage, Chester Morse Lake and the Masonry Pool. The Masonry Dam is the main storage structure in the watershed. The Cedar River flows downstream from the Masonry Dam for 57 kilometers to Lake Washington. The reservoir model simulated Chester Morse Lake and the Masonry Pool. The river model simulated the Cedar River from the Masonry Dam for 21 kilometers to the Landsburg Diversion Dam. Model inputs included bathymetric data, stream inflows and temperatures, outflows from the Masonry Dam, water quality constituent concentrations, and meteorological data. The system was modeled over two separate time periods: January 1, 2005 to December 31, 2008 and January 1 to December 31, 2015. Water level calibration was completed by comparing observed water surface elevations in Chester Morse Lake and the Masonry Pool. Flow calibration was completed by comparing streamflow gages in the Cedar River. Water temperature calibration used temperature data from twelve locations for the 2005-2008 model and six locations for the 2015 model. Water quality calibration used data from five locations for the 2005-2008 model and ten locations for the 2015 model. The model simulated water temperature on the hourly timescale with an RMSE of 0.60-0.65°C in the reservoir models and an RMSE of 0.48-0.71°C in the river models. The model simulated dissolved oxygen profile concentrations in Chester Morse Lake with an RMSE of 0.51-0.66 mg/L in the reservoir models and dissolved oxygen discrete sample concentrations in the Cedar River with an RMSE of 0.32-0.36 mg/L in the river models. Other water quality parameters were simulated within observed ranges for all parameters.

Three climate change scenarios considered changes in meteorological data and inflow data. Two reservoir management scenarios considered changes in reservoir storage and spring refill level. The scenario with the greatest predicted impact was Scenario 1, in which air temperature and water temperature were increased by a uniform 3°C. Temperature change of the average monthly temperature at noon increased by 1.0-2.6°C across the watershed. Dissolved oxygen concentrations less than 6 mg/L were predicted 9 percent of the time for the reservoir from May to September compared to 0 percent in the base model and all other scenarios. Other water quality parameters did not experience significant change under any of the modeled scenarios.

The impact on fish habitat under each scenario was determined for the reservoir model and the river model. Non-lethal growth conditions for bull trout in Scenario 1 decreased by 2 to 20 percent of reservoir volume from June through October. Core summer salmonid habitat decreased by 1 to 13 percent of river volume from mid June through mid September under Scenario 1.

The calibrated Cedar River Municipal Watershed model provides a watershed management tool to help implement new management scenarios and prepare for the impacts of climate change. Current model limitations include a reliance on the historically observed Masonry Pool water levels to control outflows from the reservoir in management scenarios. Operation logic that de couples the model from the historically observed water surface elevations should be developed. This would allow the model to be a more useful management tool based on operation logic rather than observed operation strategy.

Rights

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Persistent Identifier

https://archives.pdx.edu/ds/psu/33631

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