The Simulation of Mereg river floods using HEC_RAS hydraulic model (Case study: the distance between Dar Amroud Olya and Chalabeh Sofla villages, Mahidasht catchment, Kermanshah Province)

Introduction
Increase in precipitation, urbanization and topographic changes have led to a sharp rise in the occurrence of natural hazards. In addition, agricultural and urban activities affect river systems. Increasing pressure on river systems has increased flood events and damage to life and property, so this has become a global concern. Also, Iran is one of the several countries in the world experiencing severe flooding in urban and rural areas. The determination of the hazard extent is an essential preliminary step for all strategies that aim at controlling and reducing flood risk consequences through appropriate tools. In this research HSE-RAS one-dimensional hydraulic modeling was applied to simulate flood in river Mereg located in Mahidasht catchment of Kermanshah province.
Materials and methods
HEC-RAS is widely used in management operations, and is accepted as an efficient program for developing flood models and inundation maps. 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 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 Consulting Engineering 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.). The return periods of 25, 50 and 100 years for the catchment area were considered. Also, 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 for different return periods were calculated.
Results and discussion
In this study, in order to identify the flood zone according to the regional conditions, the hydraulic model has been implemented as a steady state flow. To implement the one-dimensional HEC_RAS model, topographic data for cross-sections, Manning’s roughness values and discharge with different return periods were provided. The study reach (40 km) was divided into 4 reaches.
Reach 1: All the meander bolts of the Mereg River have been flooded in this section and the use has been agricultural inside all the bolts of the Mereg River shore. This can cause a lot of damage to the residents along the river.
Reach 2 & 3: The flood zone has expanded by an average of 20 to 205 meters above the cross-section during different return periods. The flood zone has expanded more than the previous reach.
Reach 4: The spread of flood is less than other reaches and in addition, due to human activities and dredging of the canal, the depth of the canal has been more than other reaches and due to the increase in flow, the flood zone has expanded less.
Conclusion
The results of this study show the flood zone of 3.2 (km2) in the 25-year return period, 3.4 (km2) in the 50-year return period and 3.5 (km2) in the 100-year return period along the Mereg river. Increase in the curvature coefficient and decrease in the slope of the flood zone in the third period reflect the high possibility of the largest rural area being risked by floods.