Comparison of Long-Term Observed Sediment Trap Efficiency with Empirical Equations for Coralville Reservoir, Iowa

Trapping efficiency (TE) is defined as the percent of particles that are retained within an impoundment. TE can vary from ⩽0% indicating no sediment is retained or is lost, to 100% where all the sediment is retained. In this paper a high quality, long-term data set (32years) was used to demonstrate that annual sediment trap efficiencies for a run of the river impoundment are variable. For water years 1973–2005, annual trap efficiencies for Coralville Reservoir ranged from 5.6 to 95.8%, with a mean of 74.7% and a median of 79.9%. Overall trap efficiency for the entire period was determined to be 80.3% and a detention time of 12.4days. A comparison of the observed long-term TE was done against different empirical TE equations that are commonly used (Brown, Brune, Churchill, Dendy, Heinemann, & USDA/Summit Co., Water District). The Churchill equation provided the best fit of the empirical trap efficiency equations (TEC=79.1%); all other empirical TE relationships underestimated the observed trap efficiency with errors ranging from 13.7 to 33.1%. Bathymetric data show that since 1958 the reservoir has lost 11% of its flood storage capacity, and over 62% of its normal pool capacity. For the time studied (1973–2005) the loss of storage is estimated to correspond to 16.9×109kg of sediment deposited in the reservoir, resulting in an annual sedimentation rate of 5.3×108kgyear−1.