WSP12053 - Laserfiche WebLink

Home Browse Search

002622 Mean-Annual Total-Sediment Loads The mean-annual total-sediment load for each of the selected gaging stations in the Yampa River basin was computed by combining the dally total- sediment load versus daily mean-discharge relations with the respective flow- duration curves to give sediment-load-duration curves. The latter curves were integrated and the sums were multiplied by 365 days per year. The average-annual total-sediment loads and water discharges for the 17 gaging stations are summarized In table 2. Sediment loads and water discharges for an additional station, the Yampa River at Deerlodge Park (fig. 1), were computed by summing the sediment and water discharges for the Little Snake River near Lily and the Yampa River near Maybell, and adjusting for the intervening ungaged drainage area. The estimated mean-annuai total-sediment load and water discharge of the Yampa River at Deerlodge Park are 2.0 million tons (1.8 million t) and 2,125 ft3/s (60.2 m3/s), respectively. Accuracy of Computational Method Annual sediment loads computed by the flow-duration, sediment-transport- curve method are less accurate than the values which would be obtained from continuous or daily measurements. Commonly, there is a considerable scatter of observations about the mean relation between sediment load and water discharge. Furthermore, there are occasionally seasonal shifts In the sediment-load, water-discharge relation, Consequently, It is unlikely that a .single sediment-transport curve will accurately represent the actual sediment-load curves during an extended period of time. A comparison by Miller (1951) indicated an error of only 4 percent during a 19-year period between the measured average annual sediment loads and the sediment loads computated by the flow-duration, sediment-transport- curve method. Larger errors of 10 to 20 percent were computed by Colby (1956) and Bennett and Sabol (1973). Measurements and computations by Wal- ling (1977) Indicate that the flow-duration, sediment-transport-curve method may underestimate the annual sediment load by as much as 30 percent in small drainage basins where sediment loads consist predominantly of silt and clay. Mean-annua I tota l-sed i ment loads computed by the flow-durati on, sediment-transport-curve method compare very well with those computed from measured daily sediment loads for the Yampa River near Maybell and the Little Snake River near Lily. For the Yampa River near Maybell, the flow-duration, sediment-transport curve estimate was 11 percent greater than the 7-year average load based on measured daily sediment loads. The flow-duration, sediment-transport-curve estimate of the average annual sediment load of the Little Snake River near Lily was only 4 percent greater than the average annual loads based on nearly 6 years of measured daily sediment loads. These comparisons indicate a somewhat greater accuracy for the flow-duration, sediment-transport-curve method than previous studies. In general, however, the uncertainties In the computed annual sediment loads for streams in the Yampa River basin are probably larger; perhaps 10 to 20 percent, as found by Colby (1956) and Bennett and Sabol (1973). 11 . ,,~