شبيه‌سازي پهنه‌‌هاي سيل‌گير در حوضه‌هاي آبريز كلان شهر تهران (حوضه كن)

(Simulation of floodplain zones in Tehran's metropolitan watershed (case study: Kaan basin

دشت­هاي سيلابي و مناطق مجاور رودخانه­ ها به دليل شرايط خاص خود همواره در معرض خطرات ناشي از وقوع سيلاب­ها قرار دارند. در حوضه رودخانه كن از بسياري از اراضي مسكوني و زراعي در فاصله بسيار كمي از بستر رودخانه قرار دارند. با توجه به اينكه پايين­دست حوضه با رشد سريع ساخت ­و ساز مواجه است، فعاليت­هاي انساني و تغيير كاربري اراضي در اين منطقه، سيكل هيدرولوژيكي و توليد رواناب را به شدت متاثر ساخته است. در اين تحقيق ، سطوح سيل گير در دوره بازگشت هاي 2 تا20 ساله در بازه­اي به طول20 كيلومتر از بستر رودخانه كن با استفاده از مدل HEC-RAS و الحاقيه HEC-geoRAS تعيين گرديده است. به اين منظور از نقشه ­هاي رقومي 1:25000 و DEM 10 متري، آمار دبي ايستگاه سولقان، ويژگي­هاي مورفولوژي بستر و مقاطع عرضي استفاده شده است. در نهايت داده ها به نرم افزارHEC-RAS وارد و مورد تجزيه و تحليل قرار گرفته است. پس از تعيين حدود سيل­گيري در دوره بازگشت­هاي گوناگون در هر مقطع عرضي، نتايج به محيط Arc GIS وارد و از اين طريق خروجي نقشه­ هاي پهنه ­بندي سيلاب به دست آمده است. نقشه زمين­شناسي و مشاهدات ميداني نشان داده است كه عامل اصلي تفاوت عرض دره در محدوده مورد مطالعه مربوط به جنس سنگ­ها مي­ باشد. نتايج حاصل از خروجي مدل هيدروديناميكي نشان مي­دهد كه در بالادست رودخانه افزايش دبي به شكل افزايش ارتفاع سطح آب بوده و گسترش عرضي كمتري در سطوح سيلاب گير به چشم مي ­خورد. اما در بخش­هاي مياني و كم شيب پايين دست رودخانه به علت كاهش ارتفاع سطح آب، رودخانه داراي گسترش جانبي بيشتري بوده و پهنه­ هاي سيل ­گير در اين بخش­ها نسبت به بالادست رودخانه داراي وسعت بيشتري مي­ باشند. همچنين به ازاي دوره بازگشت­هاي طولاني ­تر ميزان دبي و ارتفاع سطح آب افزايش يافته و پهنه سيلابي نيز از وسعت بيشتري برخوردار بوده است.

Extended abstract Floodplains and adjacent rivers are always at risk from flood events due to their specific circumstances. Flood prone area identification in the watersheds is one of the basic solutions for destructive flood control and mitigation. Flood mapping is one of the best methods for flood prone area planning and identifying. Considering the importance of flood hazard, it is important to understand the role of uncertainty and incorporate that information in flood hazard maps. The hydrodynamic modeling approach is suitable for accounting various uncertainties, and thus lends itself to creating probabilistic floodplain maps. For this purpose, flow boundary conditions, peak instantaneous discharge with different return periods, cross sections and their distance and roughness coefficients for each cross section were entered to HEC-RAS hydraulic model in Kaan watershed located in the Tehran province, Iran, and this model was then run and flood water surface profile at different return periods were estimated. In the Kaan Basin, most residential and agricultural lands are located in a very small distance from the river bed. The rapid growth of construction, human activities and land use change in the downstream of the basin have caused a change in the hydrological cycle and runoff production. Floodplain mapping using hydrodynamic models is difficult in data scarce regions. Additionally, using hydrodynamic models to map floodplain over large stream network can be computationally challenging. Some of these limitations of floodplain mapping using hydrodynamic modeling can be overcome by developing computationally efficient statistical methods to identify floodplains in large and ungauged watersheds using publicly. The aim of this study is to determine flood areas within 20 kilometers of the Kaan River by using the HEC-RAS model and Arc GIS software to identify flood lands in different return periods. The Kaan basin is located in the central Alborz Mountains. This basin is limited to south, north, east and the west respectively to Tehran, Jajrood Basin, Darakeh Basin and Karaj River Basin. The most important River in the area is the Kaan River and originated from high mountains. Most commonly, the hydrodynamic modeling approach is used to create flood hazard maps corresponding to a rare high flood magnitude of 100-year return period or higher. Although this approach can provide very accurate floodplain maps, it is computationally demanding. As a result, the modeling approach to flood hazard mapping works well for individual streams, but its efficiency drops significantly when used to map floodplains over a large stream network. In this research, floodplain areas in the Kaan basin in return periods of 2 to 20 years are determined using the HEC-RAS model and the HEC-geoRAS extension. For this purpose, digital maps 1: 25000, DEM (10m), discharge values of Sulaghan Station, morphological characteristics of the river bed and cross sections have been used. Digital Elevation Models (DEMs) play a critical role in flood inundation mapping by providing floodplain topography as input to hydrodynamic models, and then enabling the mapping of the floodplain by using the resulting water surface elevations. Finally, the data is entered into the HEC-RAS software and analyzed. After determining the flood ranges in the various return periods at each cross-section, enter the results to the Arc GIS software and the flood zoning maps were obtained. In this research roughness coefficients (Maning,s coefficients) for each cross section were obtain be the n= (nb+n1+n2+n3+n4) m (Eq.1) Geological map and field observations have shown that the main difference between the widths of the valley in the study area is related to the type of rock. The results of the hydrodynamic model show that in the river upstream, the increase in discharge had led to the water level increase and expansion in the floodplain surfaces. But in the middle and low slopes in the downstream of the river, due to the reduced discharge, the river has a larger lateral extension and the flood areas are larger than the upstream of the river. Also, for a longer period of return, the discharge rate and the water level increase and the flood plain was more extensive. The results show that in the downstream of the basin due to instability the bed, existence of wide and eroded chanels, high ability in sedimentation, erosion of the channel bed, and low impact of vegetation, this section They can be restored and regenerated and constantly changing. Due to the location the Tehran-North high way from the Kaan basin, had the construction of roads and structures, the flood plain areas of the river should be fully observed or retrofitted.