Evaluation of Water Year 2011 Glen Canyon Dam FLow Release Scenarios on Downstream Sand Storage along the Colorado River in Arizona

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Evaluation of Water Year 2011 Glen Canyon Dam Flow Release Scenarios on Downstream Sand Storage along the Colorado River in Arizona By Scott A. Wright and Paul E. Grams Abstract This report describes numerical modeling simulations of sand transport and sand budgets for reaches of the Colorado River below Glen Canyon Dam. Two hypothetical Water Year 2011 annual release volumes were each evaluated with six hypothetical operational scenarios. The six operational scenarios include the current operation, scenarios with modifications to the monthly distribution of releases, and scenarios with modifications to daily flow fluctuations. Uncertainties in model predictions were evaluated by conducting simulations with error estimates for tributary inputs and mainstem transport rates. The modeling results illustrate the dependence of sand transport rates and sand budgets on the annual release volumes as well as the within year operating rules. The six operational scenarios were ranked with respect to the predicted annual sand budgets for Marble Canyon and eastern Grand Canyon reaches. While the actual WY 2011 annual release volume and levels of tributary inputs are unknown, the hypothetical conditions simulated and reported herein provide reasonable comparisons between the operational scenarios, in a relative sense, that may be used by decision makers within the Glen Canyon Dam Adaptive Management Program. Introduction Physical characteristics of the riverine ecosystem of the Colorado River in Glen Canyon National Recreation Area and Grand Canyon National Park are affected by the existence and operations of Glen Canyon Dam (GCD) upstream (Schmidt and Graf, 1990; Wright and others, 2005; Hazel and others, 2006; Grams and others, 2007). The dam has blocked the upstream supply of sand and finer sediment since completion in 1963, and dam operations determine the transport capacity of the Colorado River, which affects the magnitude of sediment retention along the bed and banks versus the magnitude of sediment export downstream to Lake Mead. Sediment that is retained may be stored on the channel bed, along the channel margins, or in zones of lateral recirculating flow or eddies (Schmidt, 1990). Sediment within eddies, if deposited by high flows that are sufficiently greater than base flow, creates sandbars that are valued as recreational campsites (Kearsley and others, 1994), backwater aquatic habitat that may be used by native fish (Valdez and others, 2001), and substrate for riparian vegetation (Ralston, 2005). One of the goals of the Glen Canyon Dam Adaptive Management Program (GCDAMP) is to manage the dam to promote sand retention and sandbar deposition (Bureau of Reclamation, 2001). Monitoring sediment flux and sandbar size provides information on how dam operations have affected sand retention and storage. Numerical modeling tools developed and tested with the monitoring data are now available to provide managers with predictions on how future dam operations are likely to affect sediment retention and, thereby, sandbar characteristics. Results from previous modeling and analyses have varied from predictions of persistent sand erosion (Laursen and others, 1976) to likely sand retention (Howard and Dolan, 1981; U.S. Department of the Interior, 1995). Most recently, a simplified modeling approach based on assumptions of steady dam releases and a stable suspended sand rating curve