The aim of this study was to analyze the factors affecting the occurrence of floods and to prepare a zoning map of flood potential in two sub-watersheds of Arghavan and Kalanar, sub-watersheds of Ilam watershed, using AHP method. For this purpose, slope, vegetation, geology, soil permeability, drainage density and watershed shape as effective parameters in flooding in the studied sub-watersheds were selected. After the experts familiar with the watershed completed the questionnaire, these parameters were weighted. Using the results of weighting, the flooding potential zoning map was then calculated. Based on the results, the slope and shape indices of the watersheds had the highest (0.293) and lowest weight (0.072) and effects on flooding, respectively. According to the flood zoning plan, zones with very high flooding class are 1.45% of the area in Arghavan watershed and 14.77% in the sub-watershed of Kalanar, and zones with very low flooding class are 2.86% in Arghavan watershed and 1.25% of the watershed area in Kolanar. According to the final flood potential zoning map, Kolanar sub-watershed has a higher flood potential due to its high slope, impermeable formations, low vegetation and relatively round shape, as well as high specific discharge (0.52 m³/s.km²) compared to Arghavan sub-watershed (0.29 m³/s.km²) and the Arghavan sub-watershed contains a larger volume of floods due to its larger surface area than the Kalanar sub-watershed. As a result, experts should pay more attention to the Kalanar sub-watershed, which has a higher flood potential.
Extended Abstract
1. Introduction
There are several parameters in the catchments that cause high peak flows, as a consequence of which nature will be degraded. Among these factors, land-use change, vegetation degradation, soil infiltration, rainfall intensity, slope, and infiltration rate of the catchment are the most important.
Watershed management activity is one of the effective methods for increasing the time concentration of the catchments. Flood hazard zoning of the catchment can also provide suitable information about floods and their effects on floodplains, which can be a strong tool for flood warning. Flood potential zoning determines the area with higher potential runoff. This calculation can be done by comparing hydrologic and hydrogeologic factors of the watersheds and then flood potential of all areas can be evaluated. Using this method, the areas with high potential flood risk could be identified. These results enable us to predict probable flood events. Nowadays, using new technologies like GIS and RS can help decision-makers to have a quick evaluation of natural flood hazards.
2. Materials and Methods
For the aims of this study, first, the main effective factors for zoning flood potential have been collected using documents, expert ideas, and other common techniques. Using GIS all required maps and factors have been prepared including slope, geology, pedology, vegetation index, drainage density and form factor. Then, for weighting parameters, AHP method is used. Combining this method with GIS can provide a strong tool for calculating flood hazard zones. AHP is a suitable tool for final weighting parameters which use comparing matrix and has been done in Expert choice software. At the end, the final weights of parameters were overlaid in ArcGIS and a potential flood hazard zone map was prepared for both catchments. These maps include the areas with very low, low, moderate, high and very high classes. For comparing flood hazard zonation of the, specific high flows were calculated using Fuller's method. Specific high flow is the number of peak flows divided by the area of the watershed that is a suitable factor for comparing two areas related to discharge. Hence, by calculating specific high flows of both watersheds, it is possible to specify the watershed with higher flood potential between Arghavan and Kal-Anar subwatersheds.
3. Results
The final flood potential zonation of both subwatersheds shows that there are five classes of flood potential in both subwatersheds including very high, high, moderate, low and very low. Investigations show that 1.45% of Arghavan and 14.77% of Kal-Anar subwatersheds are in the very high flood potential class, 18.11% of Arghavan and 35.00% of Kal-Anar subwatersheds are in the high flood potential class, 58.23% of Arghavan and 40.08% of Kal-Anar subwatersheds are in moderate flood potential class, 19.35% of Arghavan and 8.90% of Kal-Anar subwatersheds are in low flood potential class and 2.86% of Arghavan and 1.25% of Kal-Anar subwatersheds are in very low flood potential class.
4. Discussion and conclusion
Results indicate that based on the final flood potential zonation of the subwatersheds, because of high slope, impermeable geology, low-density vegetation and watershed shape and higher specific peak flows, Kal-Anar subwatershed has higher flood potential related to Arghavan watershed. On the other hand, Arghavan watershed has more volume of flood in the same storm. Furthermore, based on AHP method, slope with 29.3% and watershed shape with 7.2% have the most and the least effects on flood potential, respectively.
Finally, it was concluded that experts and government decision-makers should focus more on Kal-Anar which is in a higher risk of flood and prepare a suitable plan for combating flood in this subwatershed. Paying more attention to mentioned subwatershed minimizes different kinds of damage.