Determination of flood prone areas with topographic wetness indices in the river basins

Human being has settled down main stream lines and locations near by the water resources to start an established civilized life from the begining of the civilization due to its conveniences and that helped to provide water for many reasons including agriculture. Such convenience come with the price as the flood in small and large scales. Flood has created many serious threads to human from the begining of the civilization. Just because of miss estimations, wrong approaches and other simple calculation errors, floods have caused many disasters concluded with death and economic losses. In Turkey, flood are the main disasters and reasons of severe losses after earthquakes. In this consequences, to reduce and even to prevent the life and economical losses, flood codes and standards have been developed by the authority. Turkey, as a candidate country to European Union, has to follow the all requirements and also integrate the codes and standards its own codes and standards in terms of established requirements. In European Union, to prevent the flood hazards, "European Union Flood Direction" as standard has been prepared to prevent the effects of flood on environment, human health, cultural heritage, economy and other civil attractions. This standard has been developed in five different stages as preperation, prevention, protection, emergency cases, normalization and investigation. Detailed investigation in flood and flood areas requires a detailed analysis of hydrological, hydraulic and topographical components in time and field dimensions and functions. Recently, determination of flood areas in computer modelling has been used to answer accurately the where and when questions for floods. With this method, flood estimation and flood areas have been determined more accurately and faster. For these kinds of studies, hydrological models and geographic information systems (GIS) give an ideal support to the investigators and researchers. Implementation of GIS technologies in flood analyses in recent years have been increasing significantly due to its convienece, reliability and fast response. Since with GIS is a technology, results of hydrological models can be evaluated in detail and more accurately comparing to other methods, it is effectively used as a main component of flood analysis and any type of analysis components. When flood effects the daily life in civilized areas, it becomes critical to investigate the flood areas besides their hydrological definitions. With the current technology it becomes possible with the implementation of existing technological tools. GIS can be considered as a main tool in such technological tools. In the present thesis, determining the areas with flood risk in river basins and creating wetness index maps in topographical base have been aimed. In this study, six different basins from Tekirdag located in North-West of Turkey and Samsun located in Mid-North of Black Sea region of Turkey have been investigated. In this basins, areas under flood risks have been determined with soil wetness index models. For the study, GIS based softwares have been used. In the analyses, geographic data of the region and Digital elevation model (DEM) have been used. In the thesis, for aforementioned six basins from Tekirdag and Samsun, contour maps have been provided from General Command of Mapping and watershed basin maps have been provided from Ministry of Forestry and Water Affairs. With these provided maps, detailed flood analyses have been carried out. First, maps have been adopted to GIS based softwares. For adoption of the maps, a pre-investigation has been carried out. In this pre-investigation process, after DEM maps have been created, with model analyses, two different index analyses have been carried out and as a result of these analyses, flood maps have been created. Flood analyses have been carried out with topographic maps and in the analyses, a model has been used based on topographic wetness index (TWI) and SAGA wetnees index (SAGA WI). Model analysis results flow directions, catchment area, catchment slope, stream power index besides flood maps. Wetness indices, mentioned in the present thesis, are the parameters used to define water saturation, in other words, water content capacity. These indices are used effectively and widely to define and detect the areas with higher water contaminations with their locations and dimensions. In the thesis, Quantum GIS (QGIS) which is GIS based software has been used. This software makes possible DEM maps obtained with SAGA GIS which is also GIS based software to analyze with designed models as the data. Hence, areas with flood risk in basins have been determined in the developed maps. When analysing with QGIS, accurate determination of locations of Tekirdag and Samsun in the World scale is very important for accuracy. Analyses have been carried out with WGS84 (World Geodetic System 1984). In UTM (Universal Transverse Mercator) projection, Tekirdağ is defined at UTM35N and Samsun is UTM36N reference regions. Therefore, for Tekirdag, WGS84/UTM35N reference coordination system and for Samsun, WGS84/UTM36N reference coordination system have been selected. This information is important as a screen shot to define accurate lattidue and longitute informations. Here, UTM projection is the projection for World's cylindirical tangent definition for the Ecvator. Based on such tangent definition, World can be divided into 60 zones with initial lattidude 6° incrementals and reference lattitude is Equator. Each zone lattitude length is 84° north, 80° south. As a result of the thesis, flood maps created with topographical wetness index and SAGA wetness index have been investigated and compared. Each index is similar to the other and gives accompanying results with basin topogrophy. SAGA wetness index gives a wider area range as bigger areas with flood risk. In the basins, at higher upstream regions, these two indices are giving similar results. However, with the smaller slopes, at the downstream regions, and at the river basins, SAGA wetness index give wider areas with flood risk comparing to topographical wetness index. Therefore, SAGA wetness index can be consider the one giving safer results and it is important to prevent and to reduce the effects of hazards due to floods, since SAGA wetness index gives wider range as bigger areas with flood risks. However, on the other hand, this index can be over protective and cause unneccesary warnings and calls for flood risks. As a result, instead of maps with 1/25000 scales, using sensitive maps with higher resolutions, using work stations instead of personal computers, and most importantly getting hydrometeorological data such as precipitation, evaporation, temperature and discharge and intersecting these data with cross sectional data of river is very important for an accurate hydrological model and that gives more accurate and reliable results for determining areas with flood risk.