The hazard and vulnerability against floods has continuously increased due to the change of climate and river conditions as well as socio-economic changes. City developments progressed along with the encroachment of natural waterlogged ground such as wetlands and natural flood plains have increased the exposure to floods and potential damage, and this resultant climate change has increased flood risks, increasing the flood potential and its strength itself.
A host of flood management measures have been prepared in order to properly respond to possible life damage and property loss due to this increasing flood risk, and among these countermeasures, flood risk mapping has been used as an excellent instrument to establish an emergency action plan, town development plan, and basic plan for nation safety management because it can forecast the range of flooding and flood depth in advance when a flood actually occurs.
This dissertation attempts to present a flood risk map that can express actual flood risks considering the calculated flood vulnerability within the flood inundation extent based on the concept of the existing Flood Hazard Maps that can express such factors as the range of flooding and flood depth in case of an actual flood. A flood risk map reflected flood velocity and travel time of maximum flood wave as well as flood depth that is considered as those factors for preparing for the flood hazard map when preparing for the existing domestic flood risk maps. A flood was classified into two concepts such as watershed runoff and river flood to prepare for the Flood Hazard Maps.
Furthermore, I prepared for watershed runoff Flood Hazard Maps considering direct inundation damages by surface flow water due to localized heavy rain as well as by flash floods due to the failure or breach of hydraulic structures such as dams and levees. Then, I analyzed the mechanism of runoff; classified the surface flow process into runoff production, transport and accumulation; and then selected runoff production factors and runoff accumulation factors based on the mechanism of the runoff production and accumulation process. In addition, I drew up runoff production maps and runoff accumulation maps, and utilized the drawn-up runoff accumulation maps as Flood Hazard Maps for basin runoff.
Flood Hazard Maps for river floods were prepared by considering the inland inundation within protected lowlands occurring by river inundation due to the breach of hydraulic structures such as dams, levees, etc., and by manhole overflow due to the disorganization of city drainage system. And then, I classified various causes of floods into dam breach, levee breach, overtopping, and manhole overflow in downtown areas, making Flood Hazard Maps by each cause. Furthermore, I suggested an integrative Flood Hazard Map for river floods connecting inland and river waters by linking the river inundation by dams and levees with the inland inundation from manhole overflow.
In order to make Flood Hazard Maps considering levee breach and inland inundation, I selected the whole area of Gimcheon City within Gimcheon River Valley as an analysis target area, and took into account the mapping of Typhoon Rusa in 2002 and that of Typhoon Sanba in 2012 for the purpose of testing and correcting the model applied to this analysis. Then, I conducted a test and correction of a one-dimensional dynamic hydraulic model using water level data in various level-monitoring points in order to calculate the rate of inflow into protected lowlands by dam breach or overflow, and calculated the flux of overflow through manholes in city drain system built up for urban outflow interpretation. In addition to this, I made a submergence analysis for the protected lowlands that had connected inland waters and outland ones, using the rate of inflow through levees and flux of overflow through manholes, and completed the test and correction of the two-dimensional inundation interpreting model by comparing inundation interpreting results and actual inundation ranges.
Based on the data tested and corrected in this analysis, I conducted a two-dimensional inundation interpretation by dam breach, levee breach and overflow, and manhole overflow for the sake of preparing for Flood Hazard Maps in Gimcheon Area for river floods. I took account of the breach of Buhang dam in Buhang River, a tributary of the upper Gimcheon, to make Flood Hazard Maps for dam breach, and also considered the areas where the levees had been analyzed as black spots by the one-dimensional hydraulic analysis and the spots where dams had been breached in the vicinity of Gimcheon City in the past in order to draw up Flood Hazard Maps by levee breach. In addition, I selected Sineum and Pyungwha drainage area in Gimcheon City Area as areas for urban runoff analysis to make Flood Hazard Maps for inland inundation.
Next, I organized various scenarios of direct dam breach runoff hydrographs, inflow rate conditions through levees and flux conditions for manhole overflow to take into account the uncertainty of flooding, and drew up two-dimensional inundation interpreting results. To utilize the results of Flood Hazard Maps as two-dimensional inundation interpretations, I considered the maximum flood depth, maximum flow speed, and maximum travel time of the maximum flood wave as the flood indicators, and composed index values as interpreting results. I considered each grid of fuzzy layers, designated as the triangle fuzzy membership function, as an alternative in Multi Criteria Decision Making(MCDM). I calculated the priority of relative risks among the grids as alternatives by using each flood index as criteria based on the TOPSIS Technique which is one of the MCDM techniques. I also considered the Closeness Coefficient calculated by the Fuzzy TOPSIS, which is an evaluation value for appraising the extent of Flood Hazards, and classified risks into five grades using the Z-score method.
Moreover, I made individual Flood Hazard Map based on dam breach, levee breach and manhole overflow using the Fuzzy TOPSIS Technique, and drew up an integrative Flood Hazard Map for river floods, considering the criteria by each cause of floods at the same time. I also made Flood Risk Maps considering the Flood Vulnerability using the information of population and assets within the range of Flood Hazards. Besides, I confirmed the areas where the highest grade of hazard was recorded through the drawn-up integrative Flood Risk Map, and then compared them with those indicated in the existing flood risk maps. Flood Risk Map made in this dissertation was drawn up through an objective evaluation procedure considering the relative closeness between ideal and non-ideal solution using factors such as maximum flood depth, maximum flow speed, and travel time of flood wave within inundation areas and the information about population and assets within the area where inundations actually occurred as well as the range of inundation occurrence. In addition, the Flood Risk Map made in this analysis indicated some leveled hazard areas, showing potential benefits where it can reflect the uncertainty of the factors that have various results for each scenario by Fuzzy logic. Finally, I expect that if I can apply the Flood Risk Map methodology suggested in this dissertation even to manufacturing the current flood risk maps, I will be able to make a new Flood Risk Map to even consider the priorities for hazard areas, including more varied, important, and critical information than ever before. Furthermore, I look forward to utilizing it as a new methodology to provide a good contribution in decision-making procedures of an integrative flood disaster management and policies in the future.