پژوهشنامه مدیریت حوزه آبخیز (Jul 2024)
Prioritization of factors affecting the expansion of debris and preparing its susceptibility map using density area and DSI Models (Case study: Haraz Valley, Plour-Baijan)
Abstract
Extended Abstract Background: Debris is one of the important phenomena of slope movements in mountainous areas. Harsh climatic conditions and the availability of tectonic and geological structures in the central Alborz Mountain heights have caused a significant amount of debris to form on the slopes. Studying this process by relying on two opportunity and threat patterns is particularly important. Therefore, the current research tries to analyze the susceptibility of the occurrence of the Haraz Valley debris flow in the range of Plour to Baijan by providing a suitable index to prioritize the factors affecting the formation and expansion of debris to achieve its more accurate zoning. Methods: The effects of six layers of information, including lithology, height, slope, aspect, distance from the fault, vegetation, and land use, were investigated on the occurrence and spread of debris flow. First, the debris location in the area was identified using the geological map of Damavand, with a scale of 1:100,000 and Google Earth images. Then, the distribution map of the debris was prepared after field survey and recording points by a GPS device and transferred to the GIS environment. Maps of independent and influential variables on the spread of debris, including lithology and distance from the fault, were prepared from the Damavand geological map, and the height, slope, and direction of the range were prepared using the ASTER digital elevation model. The vegetation and land use maps were prepared from the national coverage map of Iran by processing Sentinel images on the Google Earth Engine Cloud platform. In the next step, the distribution map of the debris was combined with each map of the affecting factors, and the weight of each class of independent variables was obtained based on the density area model. Then, a zoning map of the susceptibility of the spread of debris was prepared in five classes of very high, high, moderate, low, and very low susceptibility. The average effective weight of the deposit susceptibility index was also calculated to determine the prioritization of affecting factors and prepare a more accurate map for the zoning of the expansion of debris. Results: The analysis of factors affecting the spread of debris in the studied area using the density area model shows that Melafir stone units (weathered basalts) have the most weight among all the stone units and all factors affecting the spread of debris in the region. After that, biogenic limestone, alluvial defenses, and Lar Formation are more important factors. The investigation of the elevation factor shows that the frequency of debris increases from 2100 meters up, and the elevation class of 2400-2700 meters is more sensitive. In terms of the slope factor, most of the debris in the region was observed in the slope between 10 and 40 degrees, and the largest deposit cover is related to the slope layer of 20-30 degrees. The calculation of the area density of the debris in different aspects shows that the northwest and north slopes have the most weight and susceptibility. The process of freezing and melting snow is evident in the destruction of rocks and the creation of most of the debris on these slopes. Examining the distance from the fault also showed that the frequency of debris is significant up to a distance of 1000 meters, and the highest weight of this layer, based on the density area, belongs to the zero-500-m layer of the faults. In addition to transmitting seismic stresses, faults cause the penetration of water caused by melting snow into rocks and are influential in destroying rocks and causing debris. In terms of vegetation and land use, pasture lands are the most susceptible to debris. The results of the statistical analysis of the density area and preparation of the regional zoning map showed that 5, 21.15, 29.78, 29.53, and 14.54% of the area of the region have very high, high, medium, and low susceptibility, respectively, to expand debris. According to the Debris Susceptibility Index (DSI) model, land use, lithology, and slope have the highest priorities with average effective weights of 21.04, 20.12, and 18.72, respectively, and are the main factors controlling the spread of debris in the area. The factors of slope, elevation, aspect, and distance from the fault were the next priorities. Conclusion: In the current research, the distribution of debris in different classes of independent variables was analyzed using the Density Area Model, and the importance of each class of parameters affecting the spread of debris was determined based on the weighting of the layers relative to each other. To evaluate the effic iency of the results of the density area model and verify its performance, a zoning map of the expansion of debris was prepared using the DSI model. The evaluation of the accuracy of the models using the empirical probability (P) shows that the density area and DSI models are suitable for zoning the susceptibility of debris in the area with probabilities of 0.79 and 0.80, respectively. The layers of high susceptibility to very high expansion of the deposit cover about 26% of the area, which is considered an environmental resource from one point of view. Hence, it can be exploited as a sedimentary deposit for use in engineering structures by forming a volume reserve, and from another point of view, it is a potential risk in agricultural fields, residential areas, and hydraulic structures.
