QUANTITATIVE ASSESSMENT OF SEDIMENT TRANSPORT AND CHANNEL MAINTENANCE CAPACITY OF RIVERS

Restoration of rivers may require the restoration of streamflows. A quantitative approach to the assessment of the capacity of a river to transport sediment uses a power equation for sediment load in rivers of the form Sl = a [Q-Qcrt)**b] Q as the basis of the assessment process. In the equation Sl is the sediment load, Q is the discharge, Qcrt is a critical discharge below which the load is zero or the channel is not maintained, and a and b are parameters. There are two uses of the technique: 1) look at the variation of sediment transport capacity of a river, and 2) the variation in the ability of the river to maintain its channel. The meaning of the Qcrt and a and b are different depending on the specific issue being investigated. One application of the technique is to investigate the capacity of a river to transport sediment and the variation in the capacity. A simple application of the approach is made using data for the Rio Mameyes in Puerto Rico Eighty percent of the variation in annual sediment loads is explained but there is a significant bias. A more complicated application is made using data for the Chehalis River in Washington State. Ninety four percent of the variation in annual sediment loads is explained with much less bias than the application to the Rio Mameyes. The second application of the technique is to investigate the ability of a river to maintain its channel. One approach to evaluating channel maintenance is done using only streamflow information. In this case the channel maintenance capacity is calculated using discharges over a critical discharge determined from an analysis of maximum streamflows. The second used both hydraulic and streamflow information together. The channel maintenance capacity is the shear stress over a critical shear stress. A major use of both the sediment transport index and the channel maintenance index is to compare alternative water management projects on the potential of the river to maintain its channel and transport sediment. Both the sediment transport and channel maintenance techniques were used to investigate the impact of a proposed project on the Cache la Poudre River in Colorado. The project will have significant impacts on the capacity of the river to move sediment and maintain its channel.