Evaluating the Effects of Constriction by Levees on a Dynamic Gravel-Bed River through Morphological Sediment Budgeting and Bed Mobility Studies, Snake River, WY

Abstract:

High-energy gravel-bed rivers are subject to a range of management practices used to control the system’s dynamic behavior. The Snake River, near Jackson, WY, offers an opportunity to study the morphological effects of management practices through a comparison of a reach confined by levees to an unmanaged reach just upstream within Grand Teton National Park (GTNP). I hypothesize that levees have reduced sediment supply by disconnecting the river from its banks and increased transport capacity by increasing flow velocity. Together, these effects accentuate the sediment deficit in the leveed reach. To test this I am developing a morphological sediment budget from GTNP to Wilson, WY, using LiDAR data from 2007 and 2012. This analysis will yield insight as to how sediment transport varies between the relatively natural reach in GTNP and the leveed reach downstream. A problem inherent to morphological budgets is the inability to decipher when change occurs within the budget timeframe. To combat this, a partial mobility study was executed using 300 PIT tagged gravels within the leveed reach. Gravels were relocated to decipher how bed mobility and sediment transport varied with grain size under a range of hydraulic conditions. These results are then used to estimate a critical discharge representing the inception of bed motion and geomorphic change. The critical discharge will be used to reconstruct the timing of bed mobility based on streamflow records and thus deconvolve when morphological change occurred during the sediment budget period. I further hypothesize that a greater imbalance between transport capacity and sediment supply in the leveed reach causes the bed to armor, resulting in larger critical shear stresses and implying that the bed will be mobilized only during greater discharge events. To test this hypothesis I will measure armor ratios within the leveed reach and examine how bed mobility differs between the two reaches by comparing the results of our partial mobility study to a previous tracer study within GTNP.