Climate Based Forecasts for Rule Curve Analysis
* Can climate based and newer forecasting technology allow for adjustment of rule curves on the Columbia River Basin system that meet the requirements for flood control and allow for better power production, biological function, and water storage?
Need and Benefit
Delaying the date of maximum flood control drawdown at large storage reservoirs could provide benefits to power generation, ecosystem health, and water supply reliability. This research, if successful, will improve the ability of water managers to predict the onset of spring runoff more than 10 days in advance. Knowledge of the likelihood of occurrence of the spring freshet based in hydrologic circumstances several weeks in advance could be very useful to water managers and power planners.
At Grand Coulee Dam, the Corps of Engineers requires maximum flood control drawdown by April 30 regardless of runoff volume. A similar situation occurs at other Section 7 flood control reservoirs. At Grand Coulee, the fixed drawdown end date (April 30) in combination with an Endangered Species Act (ESA) requirement
that the reservoir be at its flood control elevation on April 10, leads to an excessive drawdown rate that can exceed powerhouse capacity. Spilling water leads to lost power revenues and exceedence of water quality standards for total dissolved gas below the dam. Excessive drawdown rates sometimes exceed bank stability criteria and increase the risk of landslides along the reservoir shoreline. An additional impact is that the big
flood control drawdown ends on April 30, before the peak out-migration period for ESA listed salmon smolts (the out migration peaks in mid May). Having the ability to attenuate the drawdown period beyond April 30 could lessen the risk of landslides, lessen the amount of spill (thus allowing more power generation, and benefit smolt emigration by extending the drawdown release conditions into the period of maximum out migration.
At present, the U.S. Army Corps of Engineers (Corps) dictates flood control draft based on the predicted runoff volume and the amount of flood storage space upstream of Grand Coulee. Because Grand Coulee can not completely protect the lower Columbia River Basin from flooding due to the huge amount of seasonal runoff relative to flood control
space, the flood control space is held vacant until absolutely necessary to minimize flooding downstream. A maximum flood flow objective is called the Initial Control Flow (ICF). The ICF is computed annually and defines the flow at The Dalles above which flood control actions are taken. In theory, the maximum drawdown of Lake Roosevelt could be delayed beyond April 30 until a few days before the ICF is exceeded--signaling the beginning of flood storage and refill.
Current flood control curves for prescribing Columbia River system drawdown were developed using a data set that had limited information relative to what is available today and using forecast technology inferior to modern methods. Given updated information and improved forecasting, we believe it is possible to better define the start of spring runoff and enable water managers to attenuate the period of drawdown. The result would be a lessening of the drawdown rate, and opportunities to increase power generation revenues, improve ecosystem service via avoided fish gassing and extended emigration support, and improve water supply reliability via more efficient storage management.
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