Check dam suitable locations for erosion control using hierarchical analysis process

1- Introduction Watershed describes an area of land that contains a common set of streams and rivers, which all drain into a single large body of water. For sustainable management of watersheds, soil erosion is a major factor, which accelerates the rate of land degradation and hence influences agricultural productivity, run off movement and sometimes leads flood in the lower basin. Soil is one of the most important elements of natural resources in each country. In areas where erosion is not controlled, soil gradually erodes and loses fertility. There are two methods to maintain soil and prevent erosion: direct method and indirect method. One of the direct methods to prevent erosion is the construction of checking dam. These structures are constructed in floodplains and perpendicular to the flow of reducing water velocity and run off erosion by reducing the bed slope. Check dams are constructed using materials such as wood, stone, rock, and mortar and gravel (gabion). The aim of the present research is to determine the suitable location of mortar and gabion check dam in Ghorchay Ramiyan watershed based on multi-criteria decision-making methods. 2- Methodology Gorkhay basin (one of the sub basins of Gorgan Rood basin) is located in the southern Gorgan Rood basin and in south of the city of Ramiyan, Golestan province. The hydrometric stations of Ramiyan and Nodeh were used for data analysis that located inside and  in the neighbor of study area respectively. Criteria and indicators are chosen for evaluation of topic. The research criteria were also considered as factors influencing the location of check dam construction site in separate clusters for the second layer. The classes or options are classified in the third layer based on the risk classes in a separate cluster. In a hierarchy analysis, the normalization is performed using Saati method on the comparison matrix. In this research, the vector of the weight of the options is calculated using the geometric mean method. Prioritization was done using the AHP model and GIS based on effective criteria such as physiography, hydrology, geology , soil science and socioeconomic factors. 3- Results The weight of socioeconomic, hydrological, physiographic, soil and geological criteria is equal to 0.409, 0.124, 0.0558 and 0.409 respectively, which indicates the impact of criteria on site selecting of gabion and stone-mortar check dam. Based on weights prioritization, priorities 1 to 5 is selected for construction stone-mortar check dam belongs to 14, 10, 13, 6 and 9 sub-basin. Sub-basin No. 14 with a weight of 0.304 is  the first priority and sub-basin No. 1 with weight of 0.129 is the last priority. Among the sub-criteria, the sub- criteria of sensitivity to erosion of geology formation with 0.36 weight has the greatest effect and the sub- criteria of flooding coefficient and slop percent with weight 0.05 have the least effect on the selection of the first priority, that is, sub-basin No. 14. On the basis of prioritization of 62 sub-basins final weights with 2nd grade channel for construction of gobbins, the priorities 1 to 5 belong to the sub-basins 35-35-34-32-57, respectively. Sub-basin No. 36 with a weight of 0.453 as the first priority and sub-basin No. 1 with a weight of 0.113 was proposed as the last priority for construction of gabion. 4- Discussion & Conclusions The rapid growth of technology and economic has transformed human’s life in recent decades and has challenged modern societies with sophisticated decision making. The basic characteristic of such issues is the existence of non-homogeneous and inappropriate criteria or objectives, such as cost, reliability, performance, safety and productivity. Sub-basin No. 14 with weight 0.304 as the first priority (at the end of the basin, which has a slope and high erosion due to formations such as limestone and marn) and sub basin No. 1 is the last priority with weight 0.129 (Shale Formation, Sandstone and coal with high sensitivity to erosion and distance from residential areas and away from resources). Among the sub-criteria, formation erosion sensitivity sub-criteria with weight 0.36 has the greatest effect and the sub-criterion of distance from the road with a weight of 0.01 has the least effect on the selection of the first priority, the sub-basin No. 14. The purpose of this research is to assist managers and decision makers in the Watershed Management Department to select the most appropriate options and to rank the location of structures based on choosing the best decision options and emphasizing on effective decision making by introducing and applying scientific decision-making methods.