پهنه‌بندی خطر وقوع سیلاب با استفاده از مدل هیدرودینامیکی Hec – Ras، (مطالعه موردی: حوضه آبخیز قره‌سو _ استان کرمانشاه)

Abstract
Introduction
Rivers have always played a significant role in the emergence and development of human society and the creation of various civilizations. The history of using the river as a source of supply for part of human needs goes back to the beginning of human civilization. According to history, the oldest civilizations have been formed and developed along the rivers; The first steps in the field of river exploitation, which is typically in the field of river engineering activities, coincided with the beginning of the formation of civilized societies along the rivers (Derakhshan, 1398). In recent decades, thousands of lives have been lost directly or indirectly to floods. In fact, of all the natural hazards, floods pose the greatest natural threat to life today (Alaghmand and others, 2010). Flood is a hydrological phenomenon whose occurrence depends mainly on climatological and geological factors (Jabbari and Khazaei, 2011). Floods are one of the natural hazards that cause great damage to the inhabitants of floodplains every year. Preparation of flood zoning maps for different return periods is one of the common methods used to show the potential of flood hazards (Rezaei Moghadam et al., 1397).
Methodology
In order to perform zoning, cross sections and river banks were identified in ArcGIS software using the HecGeoRAS toolbar. Return periods in CUMFREQ software were calculated based on the amount of rainfall in Dubai in the last 30 years. Then, according to the discharge values and return periods (Table1), the information is transferred to HEC-RAS software; Zoning was prepared for 15, 25, 50 and 100 years.
Analysis of LANDSAT 8 satellite images shows that the vegetation in this area is very poor. A large part of the river passes through the city and the river area was not observed during construction. The study of soil in the area also indicates a weak or nonexisstaent potential to maintain a good amount of runoff. The longitudinal profile of the zones, Figures (3 to 7) show the expansion of the flood width in the return periods. According to Figure (8), the area of flood spread in the 15-year return period is 1.11 km. Due to the location of the study area in the residential sector, the risk; The financial and human damage to human societies is significantly low. The floodplain area of the Qarasu River is 1.20 km according to Figure (9). This return period, like the 15-year return period, is not dangerous for human societies and is more effective in terms of geomorphological changes in the riverbed than human changes and damage. The area of flood zone for QarahSu river with a return period of 50 years according to Figure (10), was calculated to be 1.40 km. Bcause of the location of residential areas around the river and the amount of flood zone calculated, it is dangerous for human settlements and will lead to changes around the river and the destruction of urban settlements. The results obtained from the flood zone with a return period of 100 years according to Figure (11), show an increase in the area of the flood zone compared to the return periods of 15, 25 and 50 years with a value of 1.60 km and in terms of risk for human settlements. It is more favorable and will cause a lot of casualties. Flood zoning of the region in the return periods, clearly shows the expansion of flood zones in the urban part of Kermanshah province, which is located on the main route of the Qarasou River.
Results
Calculating topographic information is one of the most important components of mathematical models before performing any flood zoning. This information includes longitudinal and transverse profiles of the river and its tributaries. As mentioned, the preparation of TIN is in fact to more accurately and easily identify rivers and their shores, which is essential in drawing the cross sections of the route under study. It should be noted that the cross sections are selected and harvested in a way that represents the general shape of the river and according to it, the main flow path is introduced to the model. After preparing the TIN, through the Hec-GeoRas extension, first determine the main course of the river and then, according to the TIN map, the areas along the river that are considered as beaches can be easily identified by changing the color. are; Also based on matching the TIN map with the Google Earth map of the coast
Left and right of the river were introduced to the software.
In the next step, parallel to the shore lines and the main river route, in the direction of the river flow, the flood flow path should be drawn. The flood route must be within the TIN range. The distance between the lines for the flood flow should be drawn in proportion to the shore lines; For this purpose, in this study, the distance between the shore lines and the main river was considered 5 meters. At this stage, there is no need to redraw the main axis of the river and automatically, the program detects the main axis and draws the flood flow path on both sides of the river banks based on the shore lines. Following this trend, through the HecGeoRas extension, we determine the direction of flood flows as well as the direction of the shores. After drawing the direction of the flood flow, it is time to draw the cross sections. Cross sections should be drawn from the left and perpendicular to the axis of the river flow. Transverse sections are drawn according to the schematic feature of the river and its flow. If the river of the basin under study has a lot of twists and turns, the distances of short cross-sections should be drawn; if the river is in a straight line and does not have many twists and turns, the distances of long cross-sections should be drawn; Also, the width of the cross-sections should cross the shoreline and the direction of the flood flow so that the entire area is completely covered when flooding in Hec-Ras software. Here, given that the river is long and the winding of the river is medium; The distance between sections was 10 to 14 meters and the average width of sections was 26 meters. Add the cross-section output to the Hec-Ras software environment, and after ensuring the accuracy of the cross-sections, it is time to enter the flow data and flow conditions. At this stage, for the drawn river, a flow (Q) must enter. In order to simulate the river flow, the discharges of the return periods, 5, 7, 15, 25, 50 and 100, the maximum daily discharge statistics of the old bridge station were used; According to the results of studies, the longer the return periods, the higher the flow rate. To calculate the return period, CumFreq software was used in which the maximum daily flow rate of the stations was presented to the software. Then, according to the software calculations, different return periods were calculated. In the next step, according to the characteristics of the area, the flow velocity and the depth of the river; The Manning coefficient (n) was calculated using the following formula (Cowan, 1956): N = (NB + N1 + N2 + N3 + N4) * M

- Conclusion
The floods are causing heavy damage to roads, infrastructure, bridges, residential houses, cultural heritage and agricultural farms in various provinces, killing many people. Based on the obtained results, we have studied and studied the existing practical solutions to prevent this catastrophe in the future. Because urban infrastructure is not compatible with the drainage requirements of the land on which it is built, urban sprawl has increased the likelihood of flooding. Flooding is a natural disaster that can occur at any time and place and can not be completely prevented, but there are always ways to minimize damage and manage this crisis in the best possible way. One of the main and important goals is to prepare flood zoning maps that help to predict risk areas and reduce financial and human losses for efficient planning and management. In this research, using the maximum daily flow rate of Pul-e-Kohneh station, vegetation map, soil hydrological map, calculation of return periods; Flood zoning was prepared for return periods of 15, 25, 50 and 100 years. The obtained zoning maps along with the study of the longitudinal profile diagram of the river show that the longer the flood return period, the more areas of urban and residential lands in the watershed are affected by the flood; Matching the obtained flood zones with GOOGLE - EARTH satellite images; Indicates the accuracy of the results, which indicates the possibility of flooding in the Qarahsoo River area in Kermanshah province is very high.