Graduate Thesis Or Dissertation

Spillway Staging and Selective Sediment Deposition in Sand Storage Dams

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  • Rainfall scarcity and variability present serious challenges to water security for many rural communities throughout the world's drylands. Sand dams--weirs built across ephemeral or seasonal rivers--provide an appropriate water harvesting and storage option for many regions. The structures quickly fill with sediment during rainy season flow events and store water underground in interstitial pores, thereby limiting evaporation, contamination and the prevalence of disease-carrying vectors. The size of deposited sediment particles largely determines sand dam effectiveness; fine materials do not transmit or yield usable quantities of water under a dam's modest head conditions. Many researchers and practitioners propose building sand dams in stages to limit capture of fine particles. I apply field and remotely-sensed data and statistical analysis to evaluate hypotheses about the catchment and reach-scale conditions required to optimize sediment deposition. I also use the results of unsteady HEC-RAS flow models to quantify the sensitivity of sedimentation processes to spillway height. The results of the statistical analyses show a negative correlation between mean catchment slope and median particle size collected by sand dams. Modeled results indicate that sedimentation is relatively more sensitive to variations in spillway height than to changes in the hydrograph, especially when a dam is short. However, sensitivities to a given modeled parameter vary by site. Based on the results, I recommend that the need for spillway staging be evaluated on a site-by-site basis, accounting for costs and expected benefits, and that designs incorporate progressively taller stages.
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