Presenter/s: Yong Lai
Symposium Session: 2024 - 09 Different Dam Problems with Different Dam Solutions
Topics covered: adaptive management and monitoring, instream structure (culvert/bridge/dam), and water quality
ABSTRACT
Flow releases from dams are often used to mitigate downstream effects on a stream’s biology. The releases in summer are normally higher than pre-dam flows to provide colder water temperatures. For example, the flow release in July-September from Lewiston Dam on the Trinity River, California is maintained at or above 450 ft3/s (12.7 m3/s) with temperatures in the range of 9 to 11 oC to provide cold water for spring Chinook holding. In contrast, the unregulated mean flow before Lewiston Dam was about 191 ft3/s (5.4 m3/s) with the mean temperature of 20 oC. The higher flow and colder water provide holding habitat for spring Chinook, but might be detrimental to the river biome by causing spatial uniformity in cold temperatures. This can render formerly suitable habitats unusable or sub-optimal for organisms that compose the stream food base, which can hinder juvenile salmonid growth and survival to adult.
This research explores the hypothesis that water releases from dams may be adaptively lowered, even during Summer, and used as a river restoration tool to provide greater environmental benefits than the existing practice. In particular, we propose that flow releases from Lewiston Dam may be lowered in Summer so that river pools may maintain temperature stratification to provide habitat for spring Chinook holding while also achieving other biotic and water resource benefits.
We developed a three-dimensional (3D), computational fluid dynamics (CFD) model and conducted a modeling study on two pools on the Trinity River to prove the feasibility of the hypothesis. In specific, the release rate that is required to maintain thermal stratification in the pools is quantified along with important thermal processes identified. The study is further strengthened by field measurements. We show that the CFD model results closely agree with the measured data, providing validation that the model may be used in other pools on the Trinity River. In such applications, the model can be used to determine the critical flow under which pool stratification is formed and maintained, evaluate the potential for temperature stratification, and develop flow schedules on regulated streams that maintain cold water in pools and temperature diversity in other areas without depleting reservoir storage.
With the hypothesis confirmed, we are currently applying the CFD model to a dozen pools of the Trinity River to understand how pool bathymetry influences the formation and persistence of thermal stratification under variable flows and water temperatures. This knowledge will be used to expand the results to all 36 pools in a 64 km reach below Lewiston Dam. From this, rating curves will be developed to relate discharge to the volume of pool habitat that provides springers the temperature, velocity and depths. Flow releases that meet these goals will then be evaluated for their influence on indicator species.
Take home message: pool temperature stratification may be explored to increase reservoir summer water releases for the benefits of both adult salmon and other species.