Implementation of a methodological framework for creating flood risk maps in Sarantapotamos river basin

Floods are a natural phenomenon that is often characterized as extreme, due to the serious material damage and human losses. A flood can simply be defined as the temporary covering by water of land not normally covered by water. Floods are normally caused by climatic processes, while their evolution depends mainly on geomorphologic factors, such as soil stability and permeability, vegetation cover, as well as the geometrical characteristics of the river basin. In this postgraduate thesis, entitled as, “Implementation of a methodological framework for mapping flood inundation in Sarantapotamos river basin”, the process of hydrological river basin simulation, the hydraulic simulation of a stream section and finally the export of piezometric flood depth maps and flood depth polygon maps are examined. The study was accomplished through the use of the software developed by the Hydrologic Engineering Center of the U.S. Army Corps of Engineers, specifically the hydrological model HEC-HMS and hydraulic model HEC-RAS. The first, applied to simulate rain-runoff processes in river basins, while the second allows the onedimensional analysis and simulation of natural water courses or artificial systems. The preprocessing of the data obtained with the geographical information system ArcGis and specifically with ArcGis extensions, HEC-GeoHMS and HEC-GeoRAS. The case study of the developed methodology is a part of Sarantapotamos river basin, in Eleysina, Greece, covering an area of 200 km2. For this region, the collected raw data consisted of the digital elevation model, as well as the land use and soil type maps. Due to lack of rainfall data, for the hydrologic inputs, we initially used reliable rainfall data in the estimation of rainfall intensity curves, as adapted to data from adjacent basins. The hydraulic simulation is run on 20 km along Sarantapotamos river in steady flow analysis. The hydrologic simulation was designed and run through the use of HEC-HMS, with the collaboration of HEC-GeoHMS, which is a hydrologic add-in tool in the ArcGis platform. The input files required, are the basin model file, which is constructed in HECGeoHMS software, and the meteorologic model file. Initially, we selected the methods for the computation of rainfall losses and direct runoff. Specifically, the SCS method was applied for the computation of rainfall losses and the user specified unit hydrograph (a variation of the British Hydrological Institute method) was applied for the calculation of the direct runoff. The channel flow routing is used for specific cases by lag time and the baseflow component is omitted on purpose, due to lack of data. The meteorologic model file consists of rainfall events which derived from the estimated rainfall intensity curves, corresponding to the return periods T=20 yrs, T=100 yrs, T=1000 yrs, applied in each subbasin of the case study. The simulation was run for this design storms as well as, for the observed rainfall event. The hydraulic simulation of 20 km along Sarantapotamos river was designed and run for the selected return periods through the use of HEC-RAS, with the collaboration of HEC GeoRAS, an add-in to ArcGis. Firstly, a geometric file is created in HEC GeoRAS, which includes the basic layers of the geometry of river, i.e. stream centreline, banks, cross sections etc. After the accomplishment of the first step, the geometric file is imported in HEC-RAS program and the hydrologic file is created. This file includes the peak discharges as they are computed in HEC-HMS. The river was simulated in steady flow conditions and the relevant boundary conditions, as set up according to the data. The whole computational procedure is based on the resolution of the energy balance equation between successive river cross-sections. Particularly, the calculations result in the determination of the piezometric depth, the mean kinetic energy and the energy gradient for every cross-section. In the following chart is presented a river cross section to a downstream position for a 20 year flood, 100 year flood and 1000 year flood. The results of hydrologic and hydraulic simulation are imported in HEC GeoRAS and they include all the necessary information for the creation of flood maps. The software compares the grid cell values of the digital elevation model (i.e. topographical elevations) with the corresponding values of piezometric surface (i.e. water surface elevations). Consequently, the software constructs a three dimensional model of water surface profiles from the river cross section and the piezometric depth on each cross section. In points where the elevation of the piezometric surface is higher than the ground elevation, the difference between two heights is calculated, which represents the corresponding water depth at the position. The output product is a new grid surface, which depicts the water depths in the whole area of the river basin. A floodplain inundation map for a 100 year flood event is presented in the end of this document. The comparison between the inundated areas between the three different flood events leads to the conclusion that there is a little correlation between the increase in peak discharge flows with the corresponding increase in the floodplain area. Listed in a comparative chart, are the areas of floodplain for these cases.