Closure of sediment budgets: tractable task or elusive goal?

Closure of a sediment mass balance - successfully resolving the difference between inputs and outputs with measured changes in storage - is a frequently touted goal of fluvial sediment budgeting, but is rarely achieved. In the rare case when all components of the mass balance are measured, inherent imprecision and uncertainties in measurement techniques often result in budgets that are indeterminate. Here, we present the results of a sediment mass balance that was developed for a discrete flood event on an aggrading gravel bed river. Gravel flux rates were precisely measured at the upstream and downstream ends of the 4-km study reach on the Provo River, Utah during a dam-controlled flood. Thus, we measured the change in storage from the difference of the inputs and outputs of gravel. We also collected high-resolution measurements of channel topography before and after the controlled flood for the entire reach using a suite of tools. Prior to the flood event, bathymetry and floodplain topography were documented using aerial LiDAR and multispectral imagery. Relations between multispectral imagery and flow depth were calibrated using ground-based surveys. Post-flood, the study reach was completely resurveyed in high detail using total station and RTK-GPS. Uncertainties in the surface representation of this topography with a digital elevation model were robustly characterized using a spatially-variable approach. Utilizing topographic data from these disparate sources provided the rare opportunity to precisely quantify all components of the sediment mass balance for a single bed-mobilizing flood event for the entire study area. Despite this significant effort, matching measured changes in storage with calculated differences between inputs and outputs remains a difficult and elusive goal for several reasons. First, in settings where morphologic adjustment is diffuse, changes in storage are masked by measurement uncertainties and the uncertainty that comes from interpolation of topographic data. Second, quantification of inputs and outputs is inevitably limited by the imprecision of the transport relations. Third, even when sampling is ‘event-based’, there are inherent inconsistencies between the change in time terms for flux measurements versus change in storage measurements.