JWSS - Journal of Water and Soil Science

Simulation of the hydraulic behaviour of the river basin boundaries is important for river engineering projects, prediction of flood damages in different conditions and economic feasibility studies, flood control, and other social programs related to the system of the river. In this study, river bed and flood zoning map in the range of over 155 km from the Zayandehrud dam to Nekouabad Diversion Dam were addressed by using the software's ArcGIS, HEC-RAS and HEC-GeoRAS extension. For this purpose, a digital elevation map in 1: 250,000 scale was provided and cross-sectional area was divided in 1085. Manning roughness coefficient was determined by Cowan. Finally, data entered into the software HEC-RAS and was analysed. After determining the area of flood in the return periods of 2, 5, 10, 25, 50, 100 and 200 years in all cross sections, the results were entered into the ArcGIS environment and flood zoning maps were obtained. The results showed that of the 200-year flood lands, 96% of the land flood was related to the return period of 25 years.

Today, the preparation of flood zoning maps is one of the basic and important issues in the study of development projects in the world; it is considered before any investment by the related organizations. In this paper, flood zoning was performed using the two-dimensional model HEC-RAS and GIS in order to assess the risk of the construction of a railway station near the bank of the Iranshahr River, in a range of 2500 meters. Two-dimensional hydraulic application could create a more accurate flow pattern in comparison to the one-dimensional model used in the previous studies, especially in the flood plain areas. In this paper, due to the important role of the topography of the area in ensuring the accuracy of the calculation, a Digital Elevation Model (DEM) was used with very high precision (about 2 meters), as obtained from aerial photos. The results of this study indicated the onrush of flood, depth and flow velocity in different return periods. Based on the comparison of water surface profiles in the floodplains with the return period of 100 and 25 years, the maximum difference between the water levels was 0.5 m, which seemed to be reasonable by considering the low slope of the studied area. The results of this paper, therefore, showed that the location of the railway station was in medium risk and the outskirt of floodplain.

One of the most important problems in flood manegment is the damages induced by this phenomenon. Expected annual damage (EAD) is an important index for basin vulnerability against flood. Prediction of flood damages requires the analysis of spatial and temporal risk and must be calculated by the combination of hydrologic, hydraulic and economic models. In this research, the uncertainty was considered in the flood risk analysis. The probability of flood occurrence was calculated by the parabolistic model. By using the river analysis systems software (HEC-RAS) and the geographic information system (GIS) and utilizing the Google-Earth software, the floodplains of Zayande Roud river in Esfahan province were investigated with the  return period of 25, 50, 100, 200 and 500 years. The Monte Carlo method was also sed to perform the uncertainty analysis in the proposed method. The logarithmic persion type III was selected as the best distribution of flood. The damage-stage relationship was calculated as well. Based on the uncertinity analysis, the river discharge could be regarded as the major parameter in the uncertainty of EAD.

Hydrodynamic models proposed for simulating flow hydraulic in rivers assume the flow in one direction and simulate the hydraulic parameters based on the one-dimensional Saint-Venant equations. In this research, a two-dimensional HEC-RAS model was used to simulate the flow in the Karun River, between Mollasani and Farsiat stations. Geographic information system (GIS) and river cross sections were used to prepare the altitude map using the satellite image of the study area. Modeling results in river bends showed that the maximum velocity occurred in the outer bend, which coincided with the flow mechanism in the bends. Based on the results, grid type and density have little effect on flow depth modeling. However, the characteristics of the mesh used had a great influence on the velocity distribution, so that the regular high-density mesh had the best accuracy in simulating the flow velocity. Statistical analysis showed that the RMSE for the flow discharge and flow depth were 17.95 m³/s and 0.05 m, respectively. In addition, the Nash–Sutcliffe efficiency index was calculated to be above 0.9 for the discharge and flow depth, which could be considered as a desirable value.

The study of annual damage statistics due to floods in Iran and the world shows the extent of flood damage to natural and human resources in different regions. Determining the flood zone of rivers in order to protect national resources and reduce flood damage provides the possibility of protecting the river from encroachment and the construction of any unauthorized facilities in it. Therefore, in the present study, the capability of numerical models in simulating the flood zone of rivers was evaluated in the range of Azarshahr Qushqura river and the two-dimensional hydraulic model HEC-RAS 5.0.7 and one-dimensional HEC-RAS model were compared. Changes in the hydraulic characteristics of the flood flow including depth and velocity of the flow at different cross sections of the models were evaluated. The results showed that the water surface level (flow depth) of the two-dimensional model HEC-RAS compared to the one-dimensional model had the lowest error as compared to other hydraulic parameters of flood flow. The two-dimensional HEC-RAS model showed the highest error rate in the flow velocity parameter in comparison to the one-dimensional model. The results indicated that two-dimensional HEC-RAS model V5.0.7 determined the surface of the flood zone 12.46 % more than the one-dimensional HEC-RAS model. The confirmation of the resulting zones on the current state of the river and comparison with the river aerial photo of 1346 indicated the higher accuracy of the two-dimensional HEC-RAS model in estimating the flood zone of the river.