Low-head dams can be built in ephemeral streambeds to trap sediments which can store water or serve as sand reserves for other uses. For sand dams to provide sustainable and dependable water supplies, or to provide valuable sand for other purposes, these reservoirs should primarily fill with coarse sand rather than fine sediments. The problem of sand dams being negatively impacted by an excess of fine sediments is a widespread issue. In Kenya, 40-60 percent of sand dams are reported to be affected by this problem, which can limit their ability to recharge and provide recoverable water. We describe a novel approach to preventing collection of fine sediments by geomorphic management of reservoir sedimentation. Specifically, we suggest building dams with “Eiffel Tower” shaped outlets (broad at the base and narrowing with height) to remain open until the reservoir is sediment filled. The opening is designed to provide constant Rouse number of 2.5 for 0.125 mm grains so that regardless of flow, only sand of size greater than 0.125 mm will accumulate. Considering the limitations of 1-dimensional simulations in capturing edge effects, a stage discharge relationship acquired through HEC-RAS simulation is utilized to correct the opening. Numerical modeling confirmed that these outlets maintain constant bed shear stress, and thus promote the deposition of uniform coarse sediments within the reservoir regardless of riverine flow rate. The findings of the HEC-RAS simulation demonstrate that bottom-notch openings, especially those of the “Eiffel Tower” shape, exhibit superior performance with an MSE value of less than 1% when determining the deviation between the desired Rouse number (2.5) and the calculated Rouse number.

Sand dams, a water harvesting system built in arid or semi-arid regions, collect and store water in saturated sands to increase water availability in dry seasons, while avoiding evaporation and reducing water-borne disease vectors. The capacity of the dam to store water depends on the texture of the sediment accumulated in the reservoir. An ideal sand dam is expected to wash fine-grained particles, especially silt and clay out of the reservoir, collecting only coarse particles to provide for maximum open pore space and minimum capillary retention (the water is typically extracted via an open well). Although conceptually simple, sand dam commonly failed due to the retention of fine particles. It has been recommended to build sand dams in stages to overcome this problem, with each stage low enough so that the shear forces of flow will keep silt and clay mobile, and pass them out of the reservoir. Although it is effective, this method is not preferred in terms of cost and time spent (repeatedly re-mobilizing a team to add to the dam). We present a new approach to the siltation problem by seeking an ideal shape (weir) cut in the face of a sand dam designed to provide shear forces such that coarse particles accumulation while washing the finer materials such as silt and clay. This will be done by finding a relation between the cut-outs of in the face of the dam and the sediment transport rates. Challenges in predicting sediment transport rate are investigated using both numerical and experimental modeling. We seek to reduce the failure rate of sand dams, and also provide for a method to re-establish sediment fills behind existing dams.