Joint Federal Interagency Sedimentation Conferenet and Federal Interagency Hydrologic Modeling Conference 2015

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Reconciliation of Flux-based and Morphologic-based Sediment Budgets

Technical Paper
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Prediction of river channel morphologic change in response to changes in streamflow and/or sediment regime requires understanding the relationship between the reach-scale sediment budget and local morphologic responses. Sediment budgets are typically computed from measurements of sediment flux that are made at a different scale than measurements of local changes in channel morphology. Flux measurements are typically made at widely spaced gaging stations whereas measurements of mor-phologic change are made for specific study reaches. Although conservation of mass requires that the measurements made by different methods agree, uncertainty often makes reconciliation of the results difficult. Uncertainty derives from many sources including measurement error, sampling design, and interpolation error. Some uncertainties, such as unmeasured sediment inputs, may be completely unquantified. These uncertainties may result in large differences between the segment-scale (~50 km) sediment budget based on measurements of sediment flux and reach-scale sediment budget (~5 km) based on measurements of morphologic change. In some cases, uncertainty may be so large that the budget is indeterminate and it is impossible to predict morphologic change with any degree of confidence. At what scale must one measure morphologic change in order to track trends in sediment storage that are consistent with a segment-scale sediment budget computed from measurements of sediment influx and efflux? Alternatively, to what extent can morphologic change be inferred from a flux-based sediment budget? These questions are important because scientists are often asked to make predictions of channel morphologic change (or stability) based on a computed (or estimated) sediment budget. We draw from a rare dataset that consists of companion measurements of sediment flux and morphologic change for a 50-km segment of the Colorado River in Marble Canyon. The flux measurements were made at 15-minute intervals at the upstream and downstream ends of the study reach. Morphologic measurements were made at the beginning and end of the study period over more than 80% of the study reach. We use these results to explore the range of spatial and temporal scales over which flux-based and morphologic-based sediment budgets are consistent.

Author(s):

Paul Grams    
Grand Canyon Monitoring and Research Center
USGS

Daniel Buscombe    
Grand Canyon Monitoring and Research Center
USGS

David Topping    
Grand Canyon Monitoring and Research Center
USGS

Joseph Hazel    
Northern Arizona University

Matt Kaplinski    
Northern Arizona University

 

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