جستجوی مقالات مرتبط با کلیدواژه « ضریب زبری مانینگ » در نشریات گروه « علوم انسانی »

The main effect of vegetation in the rivers course is on the flow velocity. The velocity is affected by different vegetation type such as shrub, bush and grass-like in the bed, river side and floodplain. The water velocity at a river channel cross-section tends to decrease, due to the effects of vegetation stems and leaves by the flow resistance. As the flow velocity decreased, the water depths increase, which results the backwater and flooding of the floodplains. The regular and irregular (typical of many natural channels) geometric shape are different cross section types within the reach, which affect the flow characteristics especially during floods. The Manning’s n is a coefficient which characterizes the roughness of the river channel cross section and longitudinal variations. Manning’s n-values are often defined from tables, but can be estimated from field measurements. The selection of a Manning’s roughness coefficient value is depending on the expert judgment and can strongly affect computational results of the river flow velocity and discharge. In the present study, the effect of vegetation cover and the affecting factors of Manning roughness coefficient and flow velocity estimation were investigated. The aim of this work is to study the variation of Manning roughness coefficient (n) with the distance and flow river geometry along a river flowing in a low-lying plain.

In order to conduct this research, the characteristics of the vegetation and water flow velocity were recorded through field survey in 24 cross sections. The study area was a reach of the Qarehsou River, (from Anzab to Poleh Samian locations in a 16.70 km river length). Field measurements of flow velocity and evaluation of Qharahsoo riverbed were performed in May 2016. It should be noted that in the mentioned season, the river flow has a moderate flow that is fed through tributaries that originate from the forest area of Fandolo, Abi Begloo and upstream of Namin. The type and density of vegetation in the studied periods is different and is a combination of shrubs (turmeric), herbaceous plants (grasses, weeds) and in some cases natural trees (willow) or plantation (poplar). In the present study, the effect of vegetation on flow velocity has been evaluated through the instructions in the standard table for estimating vegetation conditions in calculating the roughness coefficient. Then the manning coefficient were estimated using the Cowan method for each river cross-section. Also, the relationship between vegetation related and flow depended variables inter-relationship were tested using Pearson correlation method in the SPSS software.

The values of Manning’s roughness coefficient, n found in this study is in the range of 0.0275 to 0.0831. According to the results, the flow depth had a significantly inverse relationship (r=-0.889, r=-0.934, p<0.01) with the velocity and the channel width. While, a negative significant correlation (r=-0.375, p<0.05) exists between the flow depth and manning roughness coefficient. Also, stream width had a negative correlated with Manning coefficients (r =-0.387, p<0.05), and had a direct relationship with flow velocity (r =0.941 and p<0.01). The value of manning roughness coefficient decrease, the amount of flow velocity, discharge, hydraulic radius, and flow width increases (r=-0.347, r=-0.474, r= -0.412, r=-0.387, with p<0.05 significance level). The variations of n manning roughness is due to changes of the river cross-sections, the flow depth and velocity, and the irregularities of the bed forms which affect the estimated flow velocity.

Also, the results showed that the estimated and measured velocity had an acceptable agreement, which is proved using the Pearson correlation coefficient. The accuracy of the velocity estimation indicates the ability of the Cowan method in manning roughness coefficient estimation and flow velocity. River channel geometry and stream flow characteristics are inter-related in natural channels with irregular condition. Variations in the geometry and vegetation of the river channel can impact stream velocity and resulted discharge estimation. In general, based on the results of the range of computational values, the minimum and maximum Manning roughness coefficients of 0.0275 and 0.0836 show that the values presented in the range are close to the roughness coefficient of natural rivers. It should be noted that the studied Qharahsoor River reaches passes through the agricultural lands of Ardabil plain, so the impact of agricultural land use and human interventions in the destruction of vegetation is evident and is one of the cases that has changed the dynamics of flow and erosion. In many cases, agricultural lands have been plowed along the river, and the removal of vegetation has caused sloping shores and intensified erosion. The relationship between discharge variables and the rate of erosion along the river bank and the bed is one of the items that can be studied in future research. According to the results, Manning's coefficient variations is influenced by the river morphometry and the vegetation characteristics in the cross section of the river. The manning equation in the studied meander river provide reliable results for flow velocity estimation, flood routing and inundation simulation studies.

The flood phenomenon as a hydro-climatic event and one of the most important natural hazards in Iran, the preparation of maps of areas vulnerable to this danger in urban areas is one of the most important concerns of environmental crisis planners. To achieve this goal, researchers must use appropriate data and hydrology models with the best efficiency. In fact, one of the most important issues affecting the results of various hydrological models that determine the risk areas of floods is the role of digital elevation model and roughness factor. Roughness factor coefficient is one of the important characteristics in studying the characteristics of river flow, and determining the appropriate roughness coefficient that represents the real conditions of the river is of special importance. In this study, the HEC-RAS model was used in WMS software, which used the most practical hydrological models. In this model, the effect of digital elevation model with different resolution and sensitivity of the model to determine the Manning roughness coefficient in the exact area of flooded areas (flooding levels) in the model output has been investigated. The study area of this study is the Kashkan River watershed with an area of 9276 km, part of which has been studied. The results of this study show that increasing the resolution of the digital elevation model in determining the vulnerable areas of flooding is very effective in the accuracy of this model, reducing the accuracy of the range is very low so that it does not correspond to environmental reality. The HEC-RAS-WMS model was also sensitive to changes in the Manning roughness factor and in order to determine the flood zones in this model, the changes in the roughness factor must be carefully adjusted. the importance of this issue, researchers in this field should be careful enough to use the data used in such topics to have the results of appropriate application in urban and regional planning.

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.