Author(s): Ali Al-Maktoumi; Anvar Kacimov; Shahad Al-Yaqoubi; Said Al-Ismaily; Sameh A. Kantoush; Mohamed Saber; Tetsuya Sumi
Linked Author(s): Ali Al-Maktoumi, Tetsuya Sumi
Keywords: Recharged dams, sedimentations, arid zone, clogging, Oman
Abstract: Multipurpose reservoirs for groundwater recharge and flood protection are effectively managed and replenished water resources, especially aquifers, by using floodwater, otherwise lost to the sea and desert. As of 2018 statistics, there are 155 dams in Oman: flood protection (3 dams), recharge (46 having a storage capacity of 101.1 Mm3), and surface storage (106 – 100.52 Mm3). Unlike dams in humid climates, these structures in Oman and other arid countries are usually "dry," i.e., 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 of suspended fine sediment particles to the downstream zone. The results based on our field experiments proved that these 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 Mm3 of sediments have been deposited in the Al-Khoud reservoir (Oman) since 1985. Unfortunately, detailed measurements of the siltation rates of Omani reservoirs are missing. Still, recently few dams were equipped with gauging sticks to assess sedimentation volume after each flash flood. The use of low-cost multispectral satellite data for mapping siltation has also been started. Also, the deposited sediments may gradually clog the toe (blanket) drains in the embankments of earth-filled dams. Seepage is essentially 2D and induces a spatially non-uniform and transient suffusion. The fine soil fractions of the "pristine" embankment dam (including its clay core) are washed away downstream and deposited as a cake on the top of the drain. That defeats a standard geotechnical design and action (the Terzaghi-type gradation) of the drain filter. 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. In our sandbox experiments, the impact of drain clogging regimes was studied for different reservoir water levels. A dramatic rise of the phreatic surface in the dam was detected, albeit accompanied by a drop of the seepage flow rate. The former is insidious for the dam's stability because the phreatic surface can outcrop on the tail-water slope, and the dam may collapse. We discuss the engineering and dam-management methods of alleviating dynamic clogging-caking and recuperating already mechanically fouled reservoir beds and drains.