Graduate Thesis Or Dissertation


Preventing the Accumulation of Fine Sediments in Sand Dam Reservoirs: An Analytical Framework and Flume Experiments Public Deposited

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  • Sand dams – low-head dams built in ephemeral streambeds that store water within saturated sediments – are a proven means to increase water supplies for rural communities in arid and semiarid regions. Water percolates into the reservoir-impounded sediments during wet season rains and can subsequently be extracted during the dry season through shallow wells or hand-dug holes. For sand dams to provide optimal, sustainable, and dependable water supplies, reservoirs should primarily fill with uniform, coarse sand, and with minimal fine sediments such as silt, which can reduce the porosity and hydraulic conductivity and increase capillary retention of the deposit. The accumulation of fine sediments in sand dam reservoirs (siltation) is a widespread problem impacting more than half of all sand dams by some estimates. I applied sediment transport analysis based on particle Rouse number to develop a novel approach to preventing siltation: building sand dams with strategically shaped apertures. I designed these apertures to modify the reservoir’s stage-discharge relationship such that, regardless of riverine flowrate, bed shear stress in the reservoir is sufficient to keep fine sediments in suspension while allowing primarily coarse sediments to accumulate. Scaled flume experiments were conducted to assess this approach in a laboratory setting. Results from flume experiments largely support analytical predictions. The median particle size (D50) of particles accumulated during flume experiments with strategically shaped apertures were 4 to 26 percent coarser than those from experiments with typical broad-crested weir dams. Flume experiments also revealed that deposit grain size characteristics were influenced by behaviors not predicted through Rouse number-based analysis, including particle-particle interaction and the segregation of particles by size into distinct stratigraphic layers. Based on these results, I recommend that the strategically shaped aperture approach be tested at larger scales and under a wider range of experimental conditions and see promise that it could one day play a role in reducing the prevalence of sand dam siltation.
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