Flood frequency estimation is a challenging task for a researcher and has been associated with confusion and controversies ( Bobee et al. Parameter estimation techniques in flood frequency analysis include the graphical method, frequency factor method, method of moments, and method of probability-weighted moments and L-moments ( Ojha et al. There is no specific rule for the length of data required for the frequency analysis. Flood frequency analysis is used to estimate design floods for sites along a river that uses observed flow discharge data to calculate statistical information, which is utilized to construct frequency distributions. Hydrologic events have random probability distributions for which statistical analysis can be performed, but precise predictions might not be achieved. Design flood estimation is essential for the design of hydraulic structures, flood management and insurance studies, development, and planning. Engineering structures, whose failure may lead to huge loss of lives and properties, are generally designed for floods of large return periods ( Izinyon et al. Designing hydraulic structures for the maximum possible flood for a catchment is very costly. The discharge used for the design of a hydraulic structure is called the “design flood”. Therefore, the main objective of this study is to estimate flood discharges at specific place in ungauged river basins for various return periods, compare estimates and determine the best fit. Rivers that descend from hilly areas of Nepal carry large amounts of sediments, so for the un-gauged rivers, the design of hydraulic structures such as weirs, canals, sluice gates, and dams become more complex ( Sapkota et al.
The availability of discharge data in Nepal is limited. Hydrological stations are not established in all rivers due to economic and geographical limitations, and hence hydrological analysis in such areas is complicated. Researchers are thus challenged to devote more effort to analyzing discharges in the water sources for planning and management.Īdequate discharge data are required for the study, analysis, and quantification of various parameters, including design flood. Increasing temperatures are altering the physical characteristics of catchments by melting snow and glaciers ( Singh et al. Developing such designs becomes more challenging because of the impact of greenhouse gases, which are changing the hydrological cycle, precipitation patterns, and temperature regimes. Over-designing or under-designing of a hydraulic structure may result in the waste of natural resources or may compromise the structural safety ( Reich 1961, 1963). The faulty design of engineering structures will have a serious economic impact due to structural damage. The design and construction of water systems, as well as water resource management, requires in-depth knowledge of different flood events for different return periods ( Tao et al. The analysis and results of this study paved the way for the hydraulic design of water systems in the ungauged study region and demonstrated how the information acquired can be used for water resource management in catchments with similar hydrologic features. This analysis was accomplished using the discharge data of Nayapul station near Jhapre Bagar collected from the Department of Hydrology and Meteorology, Government of Nepal, Kathmandu. The analysis was done based on the annual maxima, peaks above threshold, and widely used regional empirical methods. From each probability density function or regional empirical method, we predict the multi-year return periods for floods, information that is generally required to design the hydraulic structures. The various methods are evaluated by comparing the goodness of fit of an array of hydrologic distribution functions. This study illustrates a variety of approaches that can be used to perform flood frequency analysis of typical ungauged mountainous rivers, where discharge data are available from hydrologically similar catchments. In mountainous countries like Nepal, the design of hydraulic structures in these steeply sloped rivers is of prime importance for flood control, as well as for electricity generation where hydraulic head is gained over short, steep reaches. Predicting flood discharges in the rivers of an ungauged basin is tedious because essential hydrological data is lacking.
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