Determination of Flood Danger Level at Flood Forecasting Stations of Karnali River at Chisapani and Rapti River at Rajaiya

Donor: GoN.

Flooding is a major natural disaster in Nepal causing loss of life and property. It is seen that the number of lives lost due to extreme events is decreasing but at the same time the number of families affected, agricultural areas impacted and monetary damages is steadily increasing. Flood forecasting and warning is an effective non-structural method of flood management. With this background, the objective of this study is to determine Flood Danger Level and mark it at Flood Forecasting Station of mainly two rivers: Karnali Riverat Chisapani and Rapti River at Rajaiya.

The river originates from the south of Mansarovar and Rakas lakes located in China (Tibet) and enters Nepal as Humla Karnali near Khojarnath. It is a trans-boundary, snow fed and perennial river, which has basin area in Tibet (China), Nepal and India. The total area of the basin at the Nepal-India border is 43,679 Km2. The drainage area of the river in China is approximately 2500 km2. Originating from the Himalayan range, the river flows south through one of the most remote and least explored areas of Nepal as the Karnali River. The river network of the basin The major tributaries of Karnali are West Seti, Bheri, Humla Karnali, Mugu Karnali, Sinja, Tila, Lohare and Thuli Gad. The 202 km Seti River drains the western part of the catchment and joins the Karnali River in Doti District north of Dundras hill. Another tributary the 264 km long Bheri rises in the western part of Dhaulagiri Himalaya and drains the eastern part of the catchment, meeting the Karnali near Kuineghat in Surkhet.

The catchment area in Nepal lies in eighteen districts, namely Humla, Dolpa, Jumla, Kalikot, Mugu, Bhajhang, Bajura, Dailekh, Jajarkot, Rukum, Salyan, Surkhet, Achham, Baitadi, Dedeldhura, Doti, Bardiaya and Kailali. After crossing the Mahabharat range, the river flows to the plains of Terai and enters India. Cutting southward across the Siwalik Hills, it splits into two branches near Chisapani. The left branch is called the Geruwa and the right branch is called Karyala. The two branches rejoin south of the Indian border. The Karnali joins the Sarda River (Mahakali River) at Brahmaghat in India. The river is called Ghaghra River in India, which is a major left bank tributary of the Ganges. With a length of 507 km, it is one of the largest rivers in Nepal and one of the largest tributaries of the Ganges.

The Karnali River basin has 1,361 glaciers and 907 lakes, with glaciers covering an area of 1,740.22 km2 and an estimated ice reserve of 127.72 km3.

• Prepare DEM for the area from SRTM data and other point elevation data available at the Department of Survey.

• Take cross-section survey at upstream boundary (Forecasting station), downstream boundary (Nepal-India border) and at least two more sections in between where necessary (i.e. the land is vary flat). Take cross section survey upto 15 m above the highest flood level. Take longitudinal survey along river stretches of 100 to 500 m either upstream or downstream to find out river bed (water surface) slope.

• Compute Manning's and establish stage-discharge relationships from measured discharges and cross-section survey.

• Set up and run hydrodynamic model (e.g. HEC-RAS, GeoSFM or similar) for the area and generate different scenarios of flooding.

• Run the model for embankment breach analysis and other obstruction for flood forecasting.

• Prepare a GIS-based hazard map for the area for different water level at upstream boundary (forecasting station).

• Validate the map using field information.

• Find out danger level (threshold water level) above which there would be significant flooding and damage to life and properties.

• Mark that warning level and danger level at the forecasting station with different colour by erecting flood pillar.

• Take a photograph of the survey work and stations showing location, staff gauges, gauge house, cable way, river bed and other features.

• Prepare reports including all above mentioned things.