Elaboration of flood inundation maps in Rafina basin

Floods can simply be defined as the physical phenomena, during which an initially dry land area is covered by water. Floods are normally caused by climatic changes, while their evolution depends mainly on geomorphologic factors, such as soil stability, vegetation cover, as well as the geometrical characteristics of the river basin. The aim of this thesis, entitled as «Elaboration of flood inundation maps in Rafina basin», is the development of an integrated methodology, which combines the fields of hydrology, hydraulics, geomatics and cartography for the creation of several maps for observed runoff events. The software, which was used for the accomplishment of this thesis, is HEC-HMS 3.5 and HEC-RAS 4.0 programs, ArcGIS 9.3 and especially Hec-GeoHMS and Hec- GeoRAS toolbars. The case study of the developed methodology is part of the basin of Rafina in Athens covering an area of 126 km2 and it is divided into 12 sub-basins. For this area the collected raw data, consisted of the digital elevation data, as well as the land use and soil type maps. Concerning the hydrologic data, there were rainfall measures of the HOA (Hydrologic Observatory of Athenes) stations that were installed in this area. The 2 rainfall events, which were collected, are the 04/02/2011 event and 24/02/2011 event. The hydrologic simulation was completed through the use of HEC-HMS combined with Hec-GeoHMS, which is an add-in toolbox in the platform of ArcGIS. The first step of this procedure is the creation of the basin model through the use of Hec- GeoHMS working in the environment of ArcMap. The representation of watershed hydrologic processes was based in the formulation of models of the computation of rainfall losses, direct runoff and channel flow routing. Baseflow components were omitted on purpose, as a result of their minor role in the final target of the thesis. There are many methods to compute the different models. The selection of the methods was determined by the shortage of the reliable data of the area. So, the SCS method was applied for the computation of rainfall losses, lag and muskingun methods for the computation of flow routing and a change of the unit hydrograph of the British hydrological institute was used for the estimation of the direct runoff. The hydrologic and hydraulic simulation provides all the necessary data for the creation of floodplain maps, which is implemented by the use of Hec-GeoRAS. The software compares the grid values of digital elevation model (topographical elevations) with the corresponding values of piezometric surface (water surface elevations). If the latter is greater than the initial elevation, then the corresponding cell gets the value of the subtraction «water surface elevation – topographical elevations». The output product is a new grid surface, which depicts the water depths in the whole area of the river basin.