RAINFALL RUN-OFF MODELING, FLOOD FREQUENCY ANALYSIS& INUNDATION MAPPING OF KANAKIRIVER BASIN, NEPAL USING HEC HMS & HEC-RAS

Abstract

Nepal is one of the richest countriesin water resource in the world. There are around 6000 rivers within the small geographical boundaries, forming a dense river network. There are four major rivers in Nepal. They are Koshi, Gandaki, Karnali and Mahakali, and all these rivers are originating from the Himalayas. Kankai River is a medium type of river originating from the Mahabharat range. It has low discharge during the winter season, but high discharge has been observed during the monsoon. There are four main tributaries, namely Deo Mai, Puwa, Mai and Jog Mai. Besides this,there isa number of small tributaries. The river flows from the north to south up to the foothill of Mahabharat and changes its direction slightly towards the west traversing the plain fertile land to enter India. Mainachuli, which lies in the Jhapa district, is considered the river's outlet point in this study. The total catchment area up to the outlet station at Kusum is 1106.41 km2. The average annual rainfall over the basin is about 1600mm, 80 percent of which occurs within the four months from June to October. The dissertation involves the study of hydrological and hydrodynamic modeling along with the inundation mapping of the study area using HEC-HMS and HEC-RAS. For establishing the hydrological model, software like Arc GIS 10.2, HEC-Geo-HMS, an extension in Arc GIS, were used for different geospatial processing. The HEC-HMS Model was prepared in HEC Geo-HMS and exported to HEC-HMS as input. The rainfall-runoff modeling was done using the HEC HMS version 4.6. For the calculation of loss, the Initial and constant loss method was used. Similarly, the SCS unit hydrograph method was used for transformation.A constant monthly base flow method was usedto count base flow. Moreover, the Muskingum-Cunge method was used for the routing. The parameters used to check performance were PBIAS, Nash-Sutcliff Efficiency(NSE) and Co-efficient of Determination(R2).The model was calibrated for the periods from 1992 to 1999 and validated for the periods from 2000 to 2008. During the calibration, the values of NSE, R2 and PBIAS for the SCS UH Model were found to be 0.803, 0.823 and 11.33,respectively.The model was validated using parameters optimized during calibration, and the value of NSE,R2 and PBIAS were found to be 0.885, 0.865 and 11.85, respectively. The flood frequency analysis was performed with the flood series data at Mainachuli station( Station No 795) and Sajbote river gauge( Station No 730) from the year 1972 to 2009. The floods with a return period of 25, 50 and 100 years were calculated using different methods, namely Gumbel extreme value distribution, Log Pearson Type-III distribution, Pearson Type-III distribution,Log Normal Distribution and Log distribution.Gumbel distribution showed the best result as NAE, MAE and RMSE was low, so accepted.The high flood discharges with a return period of 25, 50 and 100 iv v

yearsin Mainachuli station were 4460 m3/s, 5214 m3/s and 5963m3/s, respectively, at Mainachuli outlet. Similarly, the same for Sajbote river station were 189.30 m3/s, 215.07 m3/s, 240.66 m3/s, respectively.

The hydrodynamic analysis of the flow of the Kankai River was performed by the use of HEC-RAS. The river length of 35 km was modeled from the foothill of Mahabharat hill up to the boundary of Nepal-India. The model was calibrated for the period of 01 Sep 1997 to 25 Sep 1997 and validated for the period of 01 July 2007 to 21 July 2007 considering the highest flood of each year. The model performance was evaluated by computing the NSE, R2 and PBIAS. The value of NSE, R2, PBIASis 0.73, 0.79,- 0.09 for calibration and 0.72, 0.764, 0.07 for validation illustrated very good performance of the model. The calibrated-validated model was run for the historical peak flood of return period flood of 25, 50, 100 years. Water surface profile, the velocity of the flood wave along the main channel was obtained, and an inundation map was prepared. The inundation area of historical peak flood of return period 25, 50, 100 years return period flood is 37.004 km2, 41.99km2 and 49.99 km2, respectively.