Journal of Spatial Analysis Environmental Hazards

fa شبیه‌سازی پهنه‌‌های سیل‌گیر در حوضه‌های آبریز کلان شهر تهران (حوضه کن) Simulation of floodplain zones in Tehran's metropolitan watershed (case study: Kaan basin) تخصصي Special پژوهشي Research دشتهای سیلابی و مناطق مجاور رودخانهها به دلیل شرایط خاص خود همواره در معرض خطرات ناشی از وقوع سیلابها قرار دارند. در حوضه رودخانه کن از بسیاری از اراضی مسکونی و زراعی در فاصله بسیار کمی از بستر رودخانه قرار دارند. با توجه به اینکه پاییندست حوضه با رشد سریع ساخت و ساز مواجه است، فعالیتهای انسانی و تغییر کاربری اراضی در این منطقه، سیکل هیدرولوژیکی و تولید رواناب را به شدت متاثر ساخته است. در این تحقیق ، سطوح سیل گیر در دوره بازگشت های 2 تا20 ساله در بازهای به طول20 کیلومتر از بستر رودخانه کن با استفاده از مدل HEC-RAS و الحاقیه HEC-geoRAS تعیین گردیده است. به این منظور از نقشههای رقومی 1:25000 و DEM 10 متری، آمار دبی ایستگاه سولقان، ویژگیهای مورفولوژی بستر و مقاطع عرضی استفاده شده است. در نهایت داده ها به نرم افزارHEC-RAS وارد و مورد تجزیه و تحلیل قرار گرفته است. پس از تعیین حدود سیلگیری در دوره بازگشتهای گوناگون در هر مقطع عرضی، نتایج به محیط Arc GIS وارد و از این طریق خروجی نقشههای پهنهبندی سیلاب به دست آمده است. نقشه زمینشناسی و مشاهدات میدانی نشان داده است که عامل اصلی تفاوت عرض دره در محدوده مورد مطالعه مربوط به جنس سنگها میباشد. نتایج حاصل از خروجی مدل هیدرودینامیکی نشان میدهد که در بالادست رودخانه افزایش دبی به شکل افزایش ارتفاع سطح آب بوده و گسترش عرضی کمتری در سطوح سیلابگیر به چشم میخورد. اما در بخشهای میانی و کم شیب پایین دست رودخانه به علت کاهش ارتفاع سطح آب، رودخانه دارای گسترش جانبی بیشتری بوده و پهنههای سیلگیر در این بخشها نسبت به بالادست رودخانه دارای وسعت بیشتری میباشند. همچنین به ازای دوره بازگشتهای طولانیتر میزان دبی و ارتفاع سطح آب افزایش یافته و پهنه سیلابی نیز از وسعت بیشتری برخوردار بوده است. Simulation of floodplain zones in Tehran's metropolitan watershed (case study: Kaan basin) Ezaatollah Ghanavati, Associate prof. Geographical science faculty, Kharzmi University Ali Ahmmadabadi. Assistance prof. Geographical science faculty, Kharzmi University Negar Gholami, MA in Geomorphology, Geographical science faculty, Kharzmi University 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.   Key words: Environmental hazards, Flood, Flood areas, Kaan River, HEC-RAS       . مخاطرات محیطی, سیلاب, مناطق سیل گیر, رودخانه کن,HEC-RAS Environmental hazards, Flood, Flood areas, Kaan River, HEC-RAS 95 108 http://jsaeh.khu.ac.ir/browse.php?a_code=A-10-770-1&slc_lang=fa&sid=1 Masoumeh Gholami معصومه غلامی nnegar0@gmail.com 10031947532846007168 10031947532846007168 No Tehran,Kharazmi,University دانشگاه خوارزمی تهران Ezzatollah Ganavati عزت اله قنواتی ezghanavati@yahoo.com 10031947532846007169 10031947532846007169 Yes Tehran,Kharazmi,University دانشگاه خوارزمی تهران Ali Ahmadabadi علی احمدآبادی a_ahmadabadi@yahoo.com 10031947532846007170 10031947532846007170 No Tehran,Kharazmi,University دانشگاه خوارزمی تهران