Influence of reservoir seepage prevention measures and drainage ditch behind dam on groundwater depth of surrounding farmland

Abstract: Reservoir leakage has a great influence on water-salt dynamic, which is easy to cause soil secondary salinization and swamping in arid area. Groundwater depth of farmland behind dam is reduced by anti-seepage body and drain measure usually. Drainage ditches are set up behind dam, which can cut off seepage water and discharge water, therefore reducing the leakage loss. But how to choose parameters of drain, how to be combined with impervious body, and so on, these questions need to be studied further. Based on unsaturated soil seepage theory, aimed to the farmland around Qiala Reservoir, this paper calculated the combination measures by numerical simulation, and analyzed the relationship between groundwater depth and depth and position of drainage ditch by vertical cutoff wall, horizontal impervious blanket or non anti-seepage measures. Besides, we analyzed the downstream slope stability and adopted the field experiment to validate it. This research showed that the buried depth of farmland groundwater is characterized by a “funnel-shaped” downward trend taking the central axis of drainage ditch as a symmetrical axis in different seepage control schemes. The groundwater depth increased gradually before the drainage ditch, and decreased a little after the drainage ditch. Therefore, the “anti-seepage body and drain measure” reduce seepage into the farmland from the source, which can make farmland maintain a low groundwater depth. Different seepage control schemes of “anti-seepage body and drain measure” were different on the curbing and the effect of reducing the leakage of reservoir to farmland. The hanging cutoff wall and the non-impervious body cannot reduce the leakage of the reservoir effectively, and we found the combination of the former 2 ways has a weaker effect on anti-seepage. By comparing the 3 schemes, it was considered that the horizontal impervious blanket with a length of 22 times water head was better than the suspended cutoff wall and non-impervious body in seepage control. After combining it with the drainage ditch and the drainage in time, we controlled the groundwater depth at about 2.72 m effectively, which was greater than the local critical groundwater depth of 2.45 m, and curbed the trend of soil salinization after dam effectively. Drainage ditch position and depth had some effect on dam; we calculated the actual construction (length of 22 times water head) and found the downstream dam slope anti slide safety coefficient was 1.358, which was larger than the minimum slope anti slide safety coefficient of 1.242, so the downstream slope of the dam was in a safe state. After the drainage ditches were set up, the length of hydrops from the dam toe to drainage ditch was the main reason to the soil swamping. By calculating and measurement in the actual condition, we found the length of hydrops was 0.27 m, and the swamping area was smaller. In addition, calculations also revealed that the minimum depth of drainage ditches corresponding to avoiding field swamping was 5.18 m. Drain depth was 6 m in practical project, and thus the present cutoff wall, the distance between drainage ditch and dam toe, and the drain depth were reasonable. The study also found that water level changing and making sure draining timely have great effects on groundwater depth; drainage ditch was an auxiliary measure, and it can be combined with drainage and impervious body and farmland irrigation system, which can have a more effective influence.