THE METHOD OF DETERMINING FLOOD INDEX BASED ON THE FUNCTION OF PEAK DISCHARGE HYDROGRAPH INFLOW, THE AREA OF INUNDATION, DEPTH OF INUNDATION AND TIME OF INUNDATION (A CASE STUDY ON THE UPPER CITARUM WATERSHED)

• Main Author: SURYADI (NIM 35002004), YADI
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/14550
• Institution: Institut Teknologi Bandung

External factors which contribute to the occurrence of floods are global climate change that causes alteration in the level of rainfall at watersheds, topographic condition with an elevation area lower than the river water surface, changes in land use that leads to higher surface runoff coefficient, land subsidence due to the exploitation of ground water or the characteristic of geologic formation as well as human behavior in utilizing river and drainage infrastructure. On the other hand, the internal factor refers to the physical condition of the river affected by the its cross section capacity that is incapable of accommodating the discharge load of the river. <br /> <br /> In Indonesia as well as other countries including developed nations, flood occurs almost every year especially during the rainy season when high level rainfall occurs within a short period as a consequence of discharge inflow that exceeds the capacity of the river.Widespread floods will result in extensive damage to the inundated area that will not only devastate infrastructure but will also have an adverse social impact, thus will eventually debilitate the economy of the area. <br /> <br /> To alleviate the degree of damages caused by floods, developed countries have managed to predict the occurrence of flood through an early warning system that normally forecasts the possibility of rainfall in a specified time. The system is developed through the support of numerical model in predicting floods. However, there has yet to be an indicator that can describe on the correlation between the causes of flood especially rainfall, hydrograph inflow with hydraulic physical parameters such as the area, depth and time of inundation. <br /> <br /> The objective of this research is to develop an indicator model related to floods and thus known as the flood index. This index enables the formulation of the relationship between discharge inflow, inundation area, inundation depth and inundation time. In relation to this, a field research has been conducted on the upper Citarum watershed in West Java. <br /> <br /> By applying the MIKE software, hydrograph inflow can be calculated based on hourly rainfall data, hourly evaporation data, watershed area, and the estimation of ground water flow parameter. The single dimension program with full dynamic wave available in the MIKE 11 software, then calculated the hydrograph inflow into the river system by converting it into river flow fluctuations. Flood will occur when the flow of discharge load exceeds the capacity of the cross section river and thus water surface elevation will surpass the peak of the embankment. The spilling over of water that exceeds the embankment’s peak is considered as water flowing through the side spillway construction. Therefore, with the support of the MIKE 21 and MIKE FLOOD program, water will flow into an area designed according to the spatial form of the Digital Elevation Model that functions as the flood or inundation area. <br /> <br /> The flood index model developed in this research is an index model that involves the effect of hydrology and hydraulic related to floodings. The effect of hydrology is rainfall and its derivative i.e. hydrograph inflow, while the impact is the area , depth and time of inundation. Flood index consists of four index components, i.e., peak discharge index, inundation area index, inundation depth index, and inundation time index. The formula of each index component is the price ratio of the difference between occurrence with minimum price in comparison to the difference between the maximum and minimum price. <br /> <br /> With the application of the statistical model of Partial Least Square (PLS) from the Structural Equation Modeling (SEM), the correlation value between index component can be calculated and therefore the flood index formula can be obtained. The correlation indicates that the relationship between flood index and discharge index component is relatively insignificant compared to other index components. This is therefore, an indication that the flood index of the upper Citarum watershed is predominantly influenced by physical hydraulic factors such as the area, depth and time of inundation. A well-established drainage system will relatively reduce the flood index even though the discharge remains at a high level. <br /> <br /> Flood index can also be linked to maximum rainfall that occurs in the area with satisfactory results. It will then be easy to obtain the flood index value merely by knowing the cumulative maximum rainfall at the watershed / sub watershed and subsequently, it will be relatively easy to estimate flood parameters by using the correlation chart between index with the discharge inflow (Qp), the correlation between flood index with inundation area (Ag), correlation between flood index with inundation depth (Hg) and the correlation between flood index with inundation time (Tg). <br />