Geometric Characteristics of watersheds are very important in study and assessment of floods. They play the most significant role in hydrologic assessment of the missing data on watersheds. Watershed diversity in our country is a major obstacle to the correct assessment of flood forecasting in terms of climatic conditions, vegetation, soil, geology and lack of hydrometric stations. Flood is a natural phenomenon which nations adopted as an inevitable event. However, flood event, size and frequency result from several changing factors in terms of climatic, natural and geographical conditions in areas. This is why the relation between rainfall and runoff is noticeably different in every basin. Not only every basin but also every sub-basin has its unique conditions which must be independently discussed. Since at present we can’t change atmospheric elements in order to prevent these harmful phenomena, we should seek any alternative and solution on the earth and especially in watershed basins. In this regard, areas that have high potentials in creating floods should be identified. Therefore, the first step to reduce flood danger is flood managing in its origin, that is, watersheds. Certainly, we need to identify flood-producing areas within the basin because it isn’t possible to perform executive and reparatory operations across the basin due to the large extent and spread of the watersheds, and even if it isn’t studied precisely, peak debit can be intensified by changing the synchronization of peak debit in sub-basins. At present, the flood phenomenon in our country is the result of disturbing natural balance and geographical conditions (physical and morphological) in flood-producing areas rather than the incidence of heavy precipitation, which is why common rainfalls cause flowing floods. Factors like vegetation wastage due to improper knowledge of present resources management, lack of places for it in great policy makings and economical conditions dominant on this scope and overusing natural capacities of existing resources provided conditions under which occasionally we see flood-producing and its destructive effects and losing millions of tons of rich soil. Therefore, one of the main challenges in managing our country and Ilam province is to relatively reduce and control flood dangers, and one of the approaches in reducing flood damage is zoning the flood-producing potentials of watersheds.
Materials and Methods
In this study, we used topography maps 1:50000 of Armed Forces Geographical Organization, geological map 1:250000, vegetation Atlas, land use, area soil and the studies done in the basin level and multi-variable statistical methods of factor analysis and cluster analysis by SPSS software and Arc GIS software package. We obtained rainfall statistics from Ilam province Weather Bureau and Iran Meteorology Organization and obtained debit statistics from the Ministry of Power (Tamab). In this research, Konjancham watershed was classified into 29 sub-basins. In view of the objectives of this study, the maximum instant debit, daily rainfall, date of basin floods during the statistical period of 1995-2007 were selected. And for other indexes used in this study, at first we calculated the effective indexes in producing flood like geometric variables, physiographic, permeability and climatic variables for Konjancham watershed sub-basins by Arc GIS software. Then they were analyzed using statistical factors and cluster analysis methods by SPSS statistical software.
Results and Discussion
In this study, Konjancham watershed in Ilam province is divided into 29 sub-basins and the intensity of flood producing of sub-basins is classified into five classes. Considering the purposes of this study, instant peak flow rate, daily rainfall and the date of basin floods during statistical period of 1995-2007 are selected. Using Arc GIS software, we calculated 28 geometric, physiographic, permeability and climatic parameters such as area, perimeter, length and slope of the main stream, length and slope of the watershed, concentration time, form of coefficient and precipitation variables, vegetation canopy, CN, flow rate and etc.. Statistical methods of cluster and factor analysis were used to determine the flood potential in the sub-basins of Konjancham watershed and the data for 28 sub-basins variables were processed by SPSS statistical software and summarized in the form of 5 major factors (form, stream, slope, drainage and runoff) and finally, the flood producing potentials zoning map was drawn in five levels: low, moderate, relatively low, relatively high and high. In addition, qualitative variables effective in occurrence of flood have been descriptively analyzed within Konjancham river sub-basins.
The results indicate that the form factor with specific value of 9.75 is the most important factor in the studied basin flood production. Factors like stream, slope, drainage and runoff with specific values of 6.55, 3.45, 2.51 and 2.26 respectively are grouped into the next ranks in terms of priority. Then according to the factor score, the region was divided into 5 parts including high, relatively high, moderate, relatively low and low flood producing, and as a result, the zoning map of flood producing potentials was drawn in GIS for sub-basins and a descriptive analysis of qualitative variables effective on flood occurrence was done in Konjancham river basin.