A GIS-BASED FLASH FLOOD RUNOFF MODEL USING HIGH RESOLUTION DEM AND METEOROLOGICAL DATA

Natural hazards are historically a substantial threat to the progress and development of human communities. Floods hold a dominant position among these specific phenomena due to their frequent occurrence as well as their large spatial spread. Certainly, the aforementioned facts become more visible under the light of the assessment of the dramatic effects brought about by their occurrence. Consequently, the need to deal with the impact of floods on human communities with an effective way leads to a systematic involvement of the international scientific community on the subject of "Management of Natural Hazards". The present study describes an attempt to model surface runoff in a typical ungauged basin, which is directly related to catastrophic flood events, by creating a system based on GIS technology. The main object was to construct a direct unit hydrograph for an excess rainfall by estimating the stream flow response at the outlet of a watershed. Specifically, the methodology was based on the creation of a spatial database in GIS environment and on data editing. Moreover, rainfall timeseries data came from Hellenic National Meteorological Service and they were processed in order to calculate flow time and the runoff volume. Apart from the meteorological data, background data such as topography, drainage network, land cover and geological data were also collected. A high resolution DEM was of great importance in order to achieve the final result. The study area is the sub-basin of Archaia Olympia (Kladeos sub-basin) in Greece, and the examined event occurred on February 5th, 2012. INTRODUCTION Floods are one of the most common types of natural disasters that can be caused by many different naturally-occurring events such as thunderstorms, hurricanes, tidal waves and melting ice or snow. Floods can have several positive and negative effects on the environment. One of the negative effects is the great extent of damage that can be cause to man-made structures. The occurrence of this catastrophic phenomenon is directly related to population pressures which is the climate change and the environmental impact of human activity. In Greece, flood events occur mostly in small to medium-sized catchments drained by ephemeral water courses. Usually, disasters, in these flash flood prone basins, are mainly caused by highintensity rainfall falling over a short period of time. Several regions in Greece suffer from frequent and extreme flood that is a phenomenon generally caused by intense rainstorms (1). The majority of drainage basins in Greece are relatively small with steep slopes, configured by a torrent with braided main channel morphology. These systems become particularly active during extreme flood events and this may be a source of significant damage to human infrastructure. Despite the importance of these floods, the hydrological analysis of catchments in Greece has been especially difficult due to the lack of precipitation and discharge gauges. Generally, floods in the Mediterranean area are linked to storming events, but there are additional factors that can intensify flooding such as the pattern of the drainage network, the morphology of the catchment and the human interventions (2). It has been shown that a catchment’s morphometric variables control its hydrologic response. Understanding a basin’s response to extreme rainfall based on geomorphological indices can be valuable when studying flood hazard in catchments (1). DOI: 10.12760/01-2013-1-04 EARSeL eProceedings 12, 1/2013 34 One of the most prevalent ways to assess this runoff that is generated by rainfall is the unit hydrograph introduced by Sherman in 1932. This theory has played a prominent role in runoff routing computation for several decades and assumes that the basin response to rainfall input is linear and time invariant. Moreover, the unit hydrograph combined with the excess rainfall, can give the discharge at the basin’s outlet. In the recent years, the use of Geographic Information System (GIS) facilitates the estimation of the runoff from watersheds, and this is the reason why it has gained increasing attention. Development of GIS software allowed rapid and accurate calculation of geometric basin parameters and improved results in hydrograph derivation methods that required spatial analysis (3). Melesse and Graham (2004) (4), unlike previous approaches (5,6,7) proposed a routing model that is related with the travel time. This model can develop a direct hydrograph for each spatially distributed rainfall event without relying on developing a spatially lumped unit hydrograph. The sum of travel times of cells along a flow path is the travel time from each grid cell to the watershed outlet. The direct runoff flow was defined by the sum of the volumetric flow rates from all contributing cells at each respective travel time (4). The model is based on raster data structures. Grids such as elevation, land use, soil type, are used to describe spatially distributed soil parameters. Moreover, hydrologic features of each grid, like slope, flow accumulation, flow direction and flow length, can be calculated using standard function included in GIS (4). Several previous studies have tried to establish methodologies that have been developed for instantaneous unit hydrographs derivation based on morphometric parameters (among others 1,3, 5,6,7,8). The object of the study is to present the impact of a severe flood event occurred on February 5th, 2012, in Ilias prefecture and to model surface runoff by creating a system based on GIS technology. Particularly, a unit hydrograph is constructed for the excess rainfall by estimating the stream flow response at the outlet of the Olympia sub-basin (Kladeos sub-basin), which is included in Alfios River basin, located in western Peloponnese. METHODS Study Area The Alfios River is the longest river in the Peloponnese and the ninth longest river in Greece. It drains an area of almost 2,575 km and the drainage basin is elongated along an almost S-W trending axis. The main channel follows an S-W flow direction for about 110 km in Western Peloponnese. It discharges at Kiparissiakos Gulf in Ionian Sea and its source is near the village Dorizas. Its catchment encompasses different types of terrain, including steep mountain slopes, narrow valleys and bedrock canyons. The main channel (seventh order branch by Strahler) traverses a wide, flat valley (9). This study focuses to Olympia’s sub-basin, which is included in Alfios basin area (Figure 1). The selected sub-basin as it was proven from the assessment of this flood disaster was affected the most. Olympia’s sub-basin is elongated for about 103.7 km. It drains an area of 32.28 km and its maximum height reaches 618 metres in the northern part. Moreover, the area consists of Neogene sediments. More specifically, the bulk consists of Pleistocene deposits, whereas in areas with the densest flow alluvial deposits are found. The majority of the study area is rural except for some central parts with forests and the southern part in which the city of Archaia Olympia is located and thus it can be characterized as urban. The greatest percentage of the sub-basin’s area presents an altitude of 30 to 200 metres (47%). The 32% of sub-basin’s area has an altitude of 200 to 400 metres and the 21% area presents an altitude over 400 metres. Most steep slopes can be found in the northern region where the highest altitudes are located. The geology of the selected sub-basin follows the deposition of the Neogene sediments. The entire area was subject to intense uplift, and further movement along existing faults. During this period alluvial erosion was concentrated along tectonic boundaries and deeply incised the Neogene sediments, creating the deep and wide river valleys (9,10). The less resistant Neogene sediments EARSeL eProceedings 12, 1/2013 35 are prone to mass movement, including landslides and creep. These processes tend to normalize the relief created by the deep incision of the earlier periods by lowering the higher points and smoothing the steepness of the cliffs and can provide important sediment sources for subsequent fluvial transport. They also cause major problems for the stability of settlements in combination with the high seismicity of the area (9,10,11). This process tends to reinforce catastrophic flood phenomena. Figure 1: Study Area (coordinates in Greek grid). Northwest Peloponnesus experiences a typical Mediterranean climate. The average annual rainfall over the area is around 821.9 mm, and it is very close to the average annual rainfall over the country, which is 821.3 mm. Rainfall, is distributed relatively unevenly with about 75% of it occurring between the months of October and March. The average temperature is around 17oC but during summer, it ranges from 35.8 oC to 45.8 oC. The average relative annual humidity is near 68.7%, which is considered normal (9,2,12) EARSeL eProceedings 12, 1/2013 36 The Event On the 5th of February, 2012, the Western Peloponnese was struck by one of the most extreme storms that had ever occurred in the meteorological history of the prefecture of Ilias. Specifically, on Saturday 4th in February 2012, a low barometer moved rapidly from Italy to the east, causing heavy storms in the northern Ionian and Epirus at midday and the western Central mainland as well as the central and southern Ionian Sea in the afternoon. At night, it affected the northwestern prefecture of Ilia where it had rapid transit (Figure 2). On Saturday midnight, it started to affect the cities of Pyrgos and Archaia Olympia, where it literally "stuck" for more than eight hours. Figure 2: Weather forecast map (3-4 February 2012). In the study area, 151.4 mm of water were recorded in about eight hours from 00:30 to 8:20 from which 106,5 mm (70% of total precipitation) were recorded in the first 3.5 hours. The long storm, accompanied with hail caused a lot of problems in the munic