Determination of the Flooding Zones by using HEC-RAS Model (Case Study: Upstream the Taleghan Dam)

In the hydrology literatures flood is a flowing water with high discharge، which water level arise throughout the viver channel and flooding the land around them that usually not under water. During or after a heavy rain the river discharge increased sharply and the overflow of water from normal Channel and Flows in the floodplain and surrounding areas. If high pour، especially in showery conditions of inequality and vegetation، a large part of rain become runoff and interval shortly after the rainfall the current floods and the damage is usually. Flood zoning maps are widely used in studies flood plain management. Today، this maps is one basic data of studies and construction projects in the world prior to any investment or development projects، review by relevant organizations. Taleghan River watershed is one of the major subbasins of Sefidrood in the southern slopes of Alborz Mountains and is located in north western Tehran. This basin، is between latitude N36ْ 5 ''31 «to 36ْ 23 ''37» and the longitudes of 50ْ 21 ''00 «to 51ْ 01 ''16» East is lolated. In this study flood levels with interval occurrences of 5، 10، 25، 50، 100 and 200 years، in 6/5 km length of the Taleghan river Channel was determined by HEC-RAS model and HEC-geo-RAS rider. In other to، used of digital map 1:400 scale، and provide the TIN، then river and cross sections was determined. Finally this information were transferred to the HEC-RAS software، the geometric characteristics of the river and other required information was added. This information includes: Hydraulic data، Topographic data (longitude is and cross river profiles & river banks). Flood discharges one of the data needed for simulate، is the Manning roughness coefficient، in order to used of cowan respect. Also with the information of petite of sediment in the channels، the Manning coefficient of the field was determined. In addition to the items listed in this section، barriers in the path be entered، they include three bridges on the main route، group of obstacles is also the riverbank or is that residential fence or walls surrounding the houses fence or hedge and gardens have been constructed. The hydrological data، includes، discharge intervals، physical Charactrestics of River channel Related to paper purpose and cross Section is entered to the Software environment. Several factors are involved in a flood in a region that can be shaped increases or are prohibitively. Among these factors is lithology، vegetation، rainfall regime، basin shape، slope and construction of various structures in the path. In terms of lithology، the area studied there are formations related to Precambrian period، Paleozoic، Triassic، Jurassic، Cretaceous، Paleocene، Neogene، Pliocene and Quaternary. The range of choice، Type in riverbank sediments is generally quaternary alluvium that has a high susceptibility to erosion. Precipitation regime in this region based on Glynak weather station is about 500 mm and it is a Mediterranean regime. Month of March and May & April have the highest rainfall. Due to this، Taleghan river floods often occur in spring. There are 18 types of vegetation in the pasture، most land is included pastures. The shape of study area is roughly oblong that rectangular is much further risk of flooding is less in the intense rainfall. To study the basins used of the two methods، the elongation factor was used Gravlyus. In the studied area downstream، the Taleghan dam was constructed، resulting in sediment near the dam thus reducing the gradient to be this helps prevent the occurrence of floods. The results of calculation of water levels at different return periods، the HEC-RAS software through HEC-geo RAS rider was transferred to GIS and flood zones in each period were determined.. Differences in the flood zone، primarily topographic features is from the Valley. Obviously، anywhere the channel width is large، flood level increases. Unlike wherever the valley is narrowed As well as reduced of flood level and depth of flood level increases. The results show that the total area within 200 year flood is about 85/54% risk of flooding by floods 25 years and under. A large part of agricultural land and is part of the settlement this section is located.