Sedimentation and Drains' Clogging in Recharge-Flood Protection Dams in Arid Areas: Lessons Learned from Oman

Multipurpose reservoirs for flood control and recharging groundwaterare essential for secured development by replenishing water resources. The harvested floodwater continuously recharges the aquifers beforedispersing in the sea or the desert plateaus.As of 2022 statistics, 174 dams with a total storage capacity of 432 Mm³ were constructed in Oman for various purposes;three flood protection dams, 56 groundwater recharge dams, and 115 surface storage. Unlike dams in humid climates, these structures in Oman, UAE, Saudi Arabia, and other arid regions are usually "dry dams," i.e., their reservoirs are only occasionally filled by flash-flood water pulses. Although the dams in Oman proved their efficiency, like elsewhere globally, they face challenges that threaten their functions and safety. One of the most common difficulties confronted is the sedimentation of aging reservoirs. As a result, the infiltration rate of the dam bed decreases, evaporation losses increase, and the original reservoir storage capacity is reduced. Such reduction causes overflowing the dam crest and transporting Suspended Fine Sediment (SFS) particles to the downstream zone. This paper presents the research outputs of several studies conducted for dams in Oman. Various approaches used include experimental (sandbox and column), analytical and numerical models, and field experiments. Our laboratory and field experiments proved that the SFS particles, entrained by pulses of infiltrating water, are vertically translocated into the vadose zone. Over time, it causes a concealed but pernicious physical clogging (lessivage and colmation) of the recharge basin, reducing the infiltration rate by up to 10 times compared with "pristine," pre-overflowing conditions. For example, about 3.4 million cubic meters (Mm³) of sediments have been deposited in the Al-Khoud reservoir (Oman) since 1985. There is no regular detailed monitoring of Omani reservoirs' siltation rates and reservoir bathymetries. Still, recently few dams were equipped with scale bars to assess the sedimentation volume after each flash flood. Low-cost multispectral satellite data for mapping siltation has also been utilized. Also, the deposited sediments may gradually clog the toe (blanket) drains in the embankments of earth-filled dams. Seepage in the embankments is essentially 2D and induces a spatially non-uniform and transient suffusion. The "pristine" embankment dam (including its clay core) is washed away downstream and deposited as a cake on the top of the drain. That defeats the drain filter's standard geotechnical design and action (the Terzaghi-type gradation). Under high hydraulic gradients in the drain's vicinity, the fine particulates are entrained into the body of the initially coarse material such that the "internal colmation" occurs. Our sandbox experiments studied the impact of drain clogging regimesfor different reservoir water levels. A dramatic rise of the phreatic surface in the dam was detected, albeit accompanied by a drop in the seepage flow rate. The former is insidious for the dam's stability because the phreatic surface can cause uplifting force on the tail-water slope, and the structure may collapse. We discuss the engineering and dam-management methods of alleviating dynamic clogging-caking and recuperating already mechanically clogged reservoir beds and drains.