پيش بيني سيلاب هاي تاريخي رودخانه كشكان با استفاده از مدل هيدرولوژيكي HEC-HMS

Prediction of Historical Flood Using HEC-HMS Hydrological Model (Case Study: Kashkan River, Lorestan Province)

در مديريت يكپارچه حوضه هاي آبريز، پيش بيني، مهار و كاهش خسارت هاي ناشي از سيل اهميت ويژه اي دارد. با اقدامات مناسب مي توان شدت خسارات ناشي از سيل در حوضه را كاهش داده و در برنامه ريزي، طراحي و پياده سازي سيستم هشدار سيل استفاده كرد. حوضه آبريز رودخانه كشكان واقع در استان لرستان داراي پتانسيل بالاي سيل خيزي مي باشد. در اين پژوهش سعي شده است با كمك مدل هيدرولوژيكي HEC-HMS، ميزان رواناب حاصل از بارش در اين حوضه را شبيه سازي نمود. در اين راستا، ابتدا مدل HEC-HMS توسط داده هاي ثبت شده بارش-رواناب روزانه طي دوره ترسالي، واسنجي و پارامترهاي مورد نياز بهينه شدند. سپس كارآيي اين مدل در پيش بيني سيلاب هاي تاريخي (با دوره بازگشت هاي 2 تا 1000 ساله) با وارد نمودن بارش با دوره بازگشت هاي مختلف به عنوان ورودي مدل، مورد بررسي قرار گرفت. براي مقايسه نتايج مدل، تحليل فراواني سيلاب داده هاي حداكثر دبي لحظه اي ثبت شده در ايستگاه هيدرومتري نيز انجام گرفت. نتايج به دست آمده نشان داد كه مدل HEC-HMS داراي كارآيي بالايي در شبيه-سازي رواناب روزانه طي دوره ترسالي و همچنين در حداكثر دبي لحظه اي سيلاب به ازاي دوره بازگشت هاي كمتر از 300 سال را دارد. لذا به خوبي مي توان از اين مدل هيدرولوژيكي در شبيه سازي رواناب روزانه و حداكثر دبي لحظه اي سيلاب به ازاي دوره بازگشت هاي كوچك در حوضه مورد مطالعه استفاده نمود. اما به ازاي دوره بازگشت هاي بالاتر از 300 سال، فاصله بين دبي هاي سيلاب پيش بيني شده توسط مدل با مقادير به دست آمده از تحليل فراواني بيشتر مي شود. بنابراين براي پيش بيني سيلاب هاي تاريخي بزرگ توسط مدل HEC-HMS، بايد ضريب اطمينان لازم را منظور و يا با توجه به عوارض ژيومورفولوژيك كه شواهد عيني شرايط هيدرولوژيك حاكم بر رودخانه طي دوران متمادي مي باشند، ضريب اصلاحي لازم را براي افزايش دقت مدل اعمال نمود.

Introduction Prediction, control and reduction of flood damage has main role in integrated watershed management. Exploration of floodplain and flood-affected areas in a watershed is essential for planning, design and operation of flood warning system. Kashkan watershed in Lorestan’s province has great potential for hazard flood production. In this watershed during the recent 56 years (since 1959), 16 floods have been occur with peak flow greater than 1000 m3/s. The aim of this study is simulation of rainfall-runoff process with emphasizing on the flood with high return-period using the HEC-HMS model and comparing the results of this model with those obtained by frequency analysis. Methodology Kashkan watershed with area of 9300 km2 is located in southwest of Iran in longitude of 48° 18ʹ E and latitude of 33° 48ʹ N. Annual mean discharge of this watershed at outlet point is 33 m3/s with specific discharge of 9 lit/s.km2. Annual mean precipiotation of this watershed is 550 mm. The maximum precipitation in the watershed is occur in February, whereas the maximum discharge of this river has occur by two months lags in April. This denotes to dominance of snowmelt runoff in river discharge. From viewpoints of geomorphologic indices, the streams of this watershed has low potential of erosivity and low techtonic activity. In this study, 24-years data (from 1968 to 1992) of daily rainfall and rrunoff for hydrometric and meteorological stations respectively in the oulet and center of watershed was collected from Regional Water Organization of Lorestan. Then, the hydrological model of HEC-HMS (US Army products) was calibrated using recorded daily rainfall-runoff data during wet period and the incorporated parameters were optimized. Then, the efficieny of the calibrated model has been evaluated for prediction of high return-period floods (from 2-1000 years). Five goodness of fit criteria including determination coefficient ( ), scattering index (SI), percentage error in flood volume prediction (%V), percentage error in peak flow prediction ( ) and percentage error in time to peak prediction ( ) were considered for evaluating the efficiency of HEC-HMS model. In the next step, the efficiency of HEC-HMS model for prediction of historical floods (with return periods of 2-1000 years) were compared with those obtained by the frequency analysis of recorded peak flow in the watershed outlet. Results and Disscussion Results of the HEC-HMS model during the calibration period indicated that the parameters of time of concentration, lag time, curve nimber, and initial loss were optimized equal to 37.2 hr, 22.0 hr, 48, and 40 mm respectively. Using these optimized parameters, the efficiency of the model was assessed for two simulation periods. Results of goodness of fit criteria for simulation modes were calculated equal to and 2.5 days. Frequency analysis of maximum daily precipitation and peak flow data recorded in the watershed outlet was carried out by the Easyfit software and the results indicates that the Gumble and Pearson type IV are the best fitted ones. The HEC-HMs model with calibrated parameters were run repeatedly for precipitation data with different return periods (2-1000 years) and the corresponding simulated floods were compared by those achieved through the frequency analysis. Good agreement was observed between the results of HEC-HMS and frequency analysis for floods with return periods less than 300 years, whereas for return periods greater than 300 years, more discrepancy was observed. Conclusion The obtained results indicated that the HEC-HMS model has good performance for prediction of daily runoff and as well of flood with return periods less than 300 years. But, increasing the return period up to the 300 years, the efficiency of the HEC-HMS model reduces to predict the peak flood. Therefore, using the HEC-HMS model for prediction of great floods needs to considering the appropriate safety factor. Nevertheless, attention to the geomorphologic landforms of river as eyewitness of hydrological conditions occur in the watershed can help to increasing the accuracy of the model results.