Locating the site of underground dams with decision support system (DSS) in North-West of Kermanshah province.

Introduction One of the proper methods for utilizing water resources is the use of subsurface water in coarse gravel sediments by the construction of underground dams. Underground dam is a structure that blocks flow of subsurface water in alluvial deposits, which makes it easy to store water in porous layers and make the use of this water easier. Locating the site of underground dams requires extensive studies. The importance of locating due to impact of many criteria and factors such as hydrological and meteorological criteria, general geology, stratigraphic layering and permeability, topography, groundwater status and socio-economic factors. It takes a lot of time and money to study these factors using traditional methods. River grade, shape of the valley, volume of river runoff received, subsurface water quantity, depth of the bedrock to the surface of the earth, the high volume of alluvium in determining the location of the underground base are very important Locations with faults, spring and wells are inappropriate areas. The purpose of this research is identify suitable sites for construction of underground dams in the North-west of Kermanshah province as a region with appropriate rainfall in the wet season and abundant water requirement in dry season. In this research, with using decision support system (DSS) method, according to the factors and conditions in the study area, removal of step by step inappropriate places has been done. At the end, among appropriate sites, the suitable sites was determined. In this study, the thickness of sedimentary layers has been calculated using satellite images and trigonometric relationships with the least cost and credible accuracy. Using the drawing of altitudes and surface elevations buffer for streams, valleys with a standard width were identified. Methodology In the first stage of  this study, the digital elevation model (DEM) layer with 12M pixel size for study area was provide. Using it, areas with 0 to 5% slope classes were extracted and other areas were deleted. Using DEM and ARC INFO software, a layer of streams was created and overlaid with the slope layer. Streams with 0 to 5% bed slope were extracted. This new layer overlaid with the fault layer and the streams with fault in their beds were removed. It is economical to construct an underground dam in the local area of ​​the streams, with a maximum width and depth of the valley not more than 15 meters. For greater accuracy, a 60-meters surface buffer (30meters from the center of the stream to each side) was determined in the ARC GIS environment for the streams. Using ARC INFO software, a 15-meters altitude buffer was also drawn up for the streams. These two buffer were overlaid and the interleaved sites were extracted as appropriate valleys. For match the extraction points with the actual situation in the area, using the information layer of the villages, topographic maps and Google Earth software extraction points from the overlying surface of the elevation and surface were examined. In order to identify the sediment surface of the streams and their bedrock, extraction point sites overlaid with a 1: 1000000 geological layer of area and locations in inappropriate geological formations were removed for non-permeability. At the last stage, thickness of the sediments that were considered almost suitable at the field visit stage was calculated using the trigonometric relationships. Results and discussion Based on the initial result of overlaying of altitude and surface privacy (as an innovative operation), 257 locations were identified for the construction underground dams. Since this point of the sites is initial, based on local conditions, were examined. Using the topographic map of 1: 50000 area and villages of the region, points were evaluated. The point of a location located more than 500 meters from residential areas and agricultural lands was removed from the final map. Determination of the distance between the underground dams and residential areas depends on the topographic conditions. Forzieri et al. (2008) considered up to 15 km distance in Mali and Haji Azizi et al. (1390) 5 km distance from residential areas. At the end of this stage, number of suitable locations dropped from 257 to 33. With overlaying of the layers of the appropriate points with geological map, from a total of 33 points, 5 points were eliminated due to presence of dissolved formations (limestone) and the lack of coarse grained sediment for water storage. At the field visit, total of 33 locations were evaluated and verified. In terms of proper valley width to construct underground dams, the accuracy of the whole point of the area was confirmed. Regarding the suitability of seasonal flows for the construction underground dams, in the field survey and survey of hydrometric station data in the region, 10 other points due to being in permanent rivers were eliminated from the total points. Alluvium thickness has a direct and large impact on the volume of water stored in the reservoir (Mohebi e Tafreshi et al., 2014). According to studies, the thickness of 5 to 20 meters of sediment is suitable for construction underground in the study area. So, calculating the thickness of the sediment, 6 other site points were removed due to the unsuitable thickness of the sediments. Conclusion Based on the results of this study, 11 suitable spot locations were determined. By constructing underground dams in these areas, it is possible to store sub-surface water from the rain that goes unused, and in dehydrated seasons for drinking, agricultural irrigation, and the creation of new agricultural lands Used. In field studies, at the foot some of slopes, there were a lot of gravel. If, in one place, all conditions for the construction underground dam without adequate thicknesses are provided, it is possible to carry these deposits to the desired location to provide a suitable reservoir for storing water in that location.