فهرست مطالب seyed meysam moshashaie

The U. S. Environmental Protection Agency lists sediment as the most common pollutant in rivers, streams, lakes, and reservoirs defined bank erosion as a natural geomorphic process or disturbance that occurs during or soon after floods. By producing sediment load, it causes pollution and reduces the quality of drinking water. In addition to that channel enlargement, bank instability, degradation of physical habitat and numerous other geomorphic responses accelerate the process of bank erosion Bank degradation is the result of a process that combines the erosive power of water and the effect of gravity. Bank erosion is one component of the natural disturbance regime of river systems and is integral to long-term geomorphic evolution of fluvial systems and ecological sustainability. Bank erosion is, therefore, a desirable attribute of rivers. In this study, Near bank stress (NBS) Rosgen shore for a part of Mereg Mahidasht river has been investigated. Mereg River Due to its location at high altitudes, high length and variable width, its meandering pattern and flood nature have been prominent features of this river. To suffer from general and localized erosion during floods and to reduce possible damages in the future by examining the erodible points

In order to conduct erosion studies, first a digital model of the altitude of the region, a 1: 1000 map of the region that covers the river and part of the flood plain has been used. The first stage was preparation of input data in ArcGIS using the HECGeoRAS extension. HEC-GeoRAS helps in creation of the data needed for the HEC-RAS model and the transfer of data between ArcGIS and HEC-RAS. The next stage was done within HEC-RAS (5.0.3) using the river geometry prepared in the previous stage. The final stage consists of analyzing the results from the HEC-RAS model within ArcMap. Three input parameters must be specified: stream geometry, flow data, and the model plan to create the flood and inundation maps of the Mereg river in HEC-RAS. In order to create the river geometry for HEC-RAS, elevation data were needed. High resolution digital elevation model was obtained from 1:1000 topographic map that was prepared by Navandish Water Processors Consulting Engineers Company Company. The HEC-GeoRAS extension was used to set up the necessary features that would be needed for the HEC-RAS model (i.e., stream centerline, bank lines, cross sections, etc.). In addition to elevations, Manning’s roughness coefficient values were applied to each cross sections using Cowan method. 44 sections of different river sections were selected and measurements were performed in HEC-RAS environment. And then , for bank stability from the Near bank stress (NBS) Rosgen NBS method is used. Two Rosgen methods have been used: (ii) Ratio of radius of curvature to bank-full width & (iii) Ratio of near-bank maximum depth to bank-full mean depth

According to the morphology and meandering pattern of the Mereg River, the total reach is 28 km and It is divided into four reach.
First reach: To study method of Near bank stress. In the first reach, 12 cross sections have been selected. In the first part, the river has a radius of curvature of 1.39. Bank Erosion of the Mereg River was low to severe in the first period.
Second reach: the average slope of the Merege River channel was 0.34%. The value of the radius of curvature is 1.64, which is considered as a meandering river. In this reach, 12 cross section have been selected to measure the degree of erodibility. Most arches were highly erodible.
Third reach: The slope of the canal in this part is 0.2%. The Mereg River has a radius of curvature of 1.7 and has a meandering pattern. And the rate of erosion of the shore in this section is estimated to medium or less.
Fourth reach: The number of cross section studied in the fourth reach is 5 and this reach of the river has a radius of curvature of 1.2 and has a straight pattern. The rate of bank erosion has been low and medium. 

Comparison of the two methods used to analyze the erosion of the Mereg River shows that the risk of erosion in all arches in the range of the river has been high erosion. And the reason is the intensification of hydraulic stress in these reaches. In the second and third reaches, the average bank erosion is calculated as medium to high. The reason for this is the reduction of the slope, which according to the longitudinal profile of the Mereg River, the average slope was 0.23%. In addition, there is a decrease in vegetation, an increase in the pattern of river winding and change of use and conversion of lands around the river to agricultural use in these periods.
Comparison of the results obtained from different methods of estimating the risk of erosion with field observations showed that Ratio of near-bank maximum depth to bank-full mean depth in the river Mereg Mahidasht was closer to reality and It is suitable for checking the degree of erodibility of the bank.

Dynamic analysis of flood risk using HEC-RAS hydraulic model (Case study: Shahinshahr River, Isfahan Province)AbstractFlooding is a natural phenomenon and the risk of it occurring in urban areas is a global issue. Rapid urbanization and climate change have increased the risk of urban flooding, causing massive infrastructure and human losses. The aim of this study is geomorphological zoning of Shahinshahr River flood risk. Therefore, the HEC_RAS numerical model was used to simulate the flood and The geometric data was processed in the GIS by the HEC–GEORAS extension. The return periods of 25, 50 and 100 years of catchment area as well as its physiographic characteristics including area, length of main stream, CN curve number, concentration time, latency in the watershed were entered into the HEC-HMS software. Accordingly, the output results of maximum flood discharge with different return periods were calculated whereby river pattern reflect three reaches. Finally, manning's roughness coefficient was calculated for each reach that the Manning roughness coefficient was calculated using the Coon method. The results of studies show that the flood zone in the 25-year return period of 0.948 (km2) in the 50-year return period of 1.13 and in the 100-year return period of 1.34 (km2) of lands along the Shahin river. In the last reaches, due to the reduction of the slope and the reduction of the flow velocity, the flood zone has become wider than other periods.