Estimation of Probable Maximum Precipitation and Probable Maximum Floods Using Synder Unit Hydrograph and HEC- HMS: A Case Study in Kaligandaki River

Abstract

Highlyvariedingeologicalconditionswithintheshortrangeof reachi.e.,about200kminwidthwise creates a wide range of climatic conditions in Nepal that means high variation in precipitation and flow. The precipitation is highly concentrated in monsoon season where nearly 80 percent of total precipitation occurs. Due to the combination of high relief, steep mountainous landscapes, deep and narrow river valleys and regular mass wasting processes rises the country’s susceptibility to natural hazards producing substantial annual damage to property and loss of human lives. Therefore, there is necessity of hydraulic structures to regulate and safely passes the maximum flood happening without damaging. The maximum flood that may be expected is probable maximum flood (PMF) which is developed from the combined effect of extreme metrological and hydrological conditions. PMF is computed by using the probable maximum precipitation (PMP), has physical meanings to provide an engineer for operating and designing the hydraulic structure as upper limit of the interval. Nowadays, PMF is generally taken as the standard value for safety design of hydraulic structures.ThisresearchaimstofocusestoestimatePMP/PMFattheoutletAnsingof Kaligandaki River by two different methods and analyze the PMFs estimates. Hydro-meteorological method and Statistical method are two methods used to determine the PMPson the basin of outlet. Estimation of PMPs covers the creation of watershed, dividing it into sub basins, finding characteristics of each sub basins from the extension of HEC GeoHMS in ArcGIS 10.2. In hydro-meteorological, basin average PMP is estimated from 1-day maximum PMP contour map and DAD curve whereas annual maximum daily rainfalls are used for PMP estimation in Hershfield method. Snyder unit hydrograph is used as rainfall runoff model to generate the unit hydrograph (UH) at outlets of each sub basin. This UH is convoluted with basin average PMPs to develop the direct runoff hydrograph (DRH). DRH of the flow from every sub basin undergoes routing by HEC-HMS model to estimate PMF at outlet of watershed. HEC HMS model is calibrated and validated at outlets by optimizing its parameters for only routing purpose. Since, the reliability of the PMF relies on the quantity of available data, quality and depth of analysis of PMP estimation, it is essential to analyze, compare and harmonize PMP results from multiple perspectives. Estimated PMFs by statistical is 17.24% more than the estimated PMFs by hydro meteorological method.