Ephemeral Tributary Sediment Loads in the Arid West
How can sediment loads from ephemeral non-armored tributaries be estimated accurately and reliably, and what field data is necessary to adequately estimate these sediment loads.
Need and Benefit
One common source of uncertainty in mobile-bed sediment transport modeling of a river is lateral sediment volumes delivered by tributaries. The uncertainty and error associated with tributary sediment loads is much larger for ephemeral non-armored tributaries as compared to perennial armor-bed systems. Many of the rivers in the western United States, especially in the southwest and central plains, have ephemeral tributaries which can be significant sources of sediment for the mainstem river. This is especially true in situations where an upstream dam cuts off mainstem sediment loads. These tributary sediment loads are very important for sediment management and river restoration purposes, and are often poorly estimated and can involve order of magnitude errors in their quantification.
Current methods to estimate sediment transport involve estimating either: (1) the sediment supply or (2) the sediment capacity. Sediment supply estimates are usually of the Universal Soil Loss Equation types (USLE, MUSLE, RUSLE) and are understood to include significant uncertainties. Sediment transport capacity is typically estimated using a transport equation, which can be effective when the appropriate equation is selected to match the specific conditions. However, nearly all transport equations have been developed based on the energy in perennial systems, and bed load flux in an ephemeral channel has been shown to be "several orders of magnitude higher than maxima measured at similar levels of stream power in perennial counterparts" (Reid, Laronne, 1995). It is imperative that stream restoration studies, sediment management actions, and reservoir sedimentation studies conducted by or for Reclamation include tributary sediment estimates that are more accurate than what is currently being used.
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Ephemeral Tributary Sediment Loads (final, PDF,
By David Varyu
Report completed on September 30, 2013