FLOOD PLAIN ANALYSIS AND RISK
ASSESSMENT OF LOTHAR KHOLA
BIKRAM MANANDHAR

Abstract
Flood occurs repeatedly in Nepal and cause tremendous losses in terms of property and life, particularly in the lowland areas. The majority of flood disasters’ victims are poor people living in floodplain. Therefore, the study was carried out to perform floodplain analysis and risk assessment of Lothar Khola. The study describes the technical approach of probable flood risk, vulnerability and hazard analysis. Lothar Khola with a catchment area of 170 sq. km., lies between Chitwan and Makawanpur District, with average discharge (June – September) 10.17m3/s. The average annual rainfall in the area is 1944 mm and the maximum rainfall of 24 hrs is 130.17 mm. Flood frequency analysis for 2, 10, 50, 100 and 200-years return period was done by Gumbel, Log Pearson Type III, and Log Normal method based on Maximum Instantaneous flow recorded at Lothar Khola station and also, by WECS/DHM method. One dimensional hydraulic model HEC-RAS with HECGeoRAS interface in co-ordination with ArcView was applied for the analysis. 

Triangulated Irregular Network (TIN) was prepared from contour and spot elevations in ArcView GIS. Required data sets as stream centerline, banks, flow paths and cross sections were prepared in HEC‐-eoRAS thus, creating import file and imported in HEC-RAS. In HEC-RAS, boundary conditions for upstream and downstream were defined. Similarly, flood discharges for different return periods were also inputted. Steady flow analysis was done for the results. Approach developed by Gilard (1996) was used for flood risk assessment.

The results of flood frequency analyzed by Log Pearson Type III method showed discharges of 286, 647, 990, 1347 and 1284 m3/s for 2, 10, 50, 100 and 200-years return period floods, which were higher among the results obtained using different methods and used in modeling. Area inundated by 2, 10, 50, 100 and 200-years return period floods were 230, 239, 246, 249 and 252 ha respectively. The classification of flood depth area showed most of the flooding area had water depth greater than 3m. The assessment of the flood inundated area showed that large percentage (> 40 %) of vulnerable area lied in sand area followed by forest and cultivation area, which were 23% and 18% respectively. Also, flood area increased with flood intensity. Higher flood depth increased and lower flood depth decreased with increase in flood intensity. Flooding of cultivation land indicated potential damages in food production and negative effects on the livelihoods.

Thus, finding of the study may help in planning and management of flood plain area of Lothar Khola to mitigate future probable disaster through technical approach.

Keywords: Flood, Lothar Khola, Flood frequency, Hazard, Vulnerability, Risk