FLOOD ESTIMATE FROM POSSIBLE DAM BREAK SCENARIO OF SUN KOSI LAND SLIDE DAM, NEPAL. O. P. GUPTA Director Central Water Commission

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1 FLOOD ESTIMATE FROM POSSIBLE DAM BREAK SCENARIO OF SUN KOSI LAND SLIDE DAM, NEPAL by O. P. GUPTA Director Central Water Commission

2 Introduction Landslide dams are natural phenomenon that poses a major threat of flash flood in d/s area due breaching of such dams. In the past number of flash flood events (Parechu, Kurichhu etc occured. Information in respect of possible rise of WL, time of flood peak is required in shortest possible time for an effective disaster management. In majority of cases, it may not be possible to get requisite input data for simulation. Case study of Sun Kosi landslide, Nepal

3 Event Brief On the at 2:35 AM a major landslide took place in the upperreachesofsunkosiriverin Sindhupalchowk district in Nepal at a location having latitude: N, longitude: E LSD The landslide location is around 1.2 km u/s of existing Sun Kosi Hydro Power Dam and around 11.3 km u/s of confluence of Sun Kosi and Bhute Kosi As per the initial information the depth of water column was approximately 50 m to 60 m. The reservoir thus formed submerged the area upto around 4 to 5 km u/s of the landslide location.

4 Data constraints Data available Latitude and Longitude of land slide dam photograph of LSD (LSD)location and Approximate height of LSD and very rough estimate of reservoir volume Prevailing discharge at Kosi barrage located about 280 km downstream of land slide dam location, which was about 3540 cumec Data required Proper estimate of storage volume behind LSD Cross section of river from LSD location up to Kosi barrage Catchment area of the river at LSD and Kosi barrage

5 Major Challenge To estimate the possible rise in Kosi water level in the event of LSD breach To achieve the target better estimate of storage volume behind LSD, catchment details and river cross sections from LSD upto Kosi barrage were essential

6 How did we proceed Location of Land Slide dam marked on Google Earth ASTER DEM of 30m x 30 m resolution having grid size 1 degree x 1 degree has been downloaded and a mosaic has been prepared to get the coverage of entire catchment of Kosi river up to Kosi barrage Catchment area and hypsometry of the river catchment at LSD and Kosi barrage has been estimated by GIS processing of the DEM using HEC GeoHMS extention of HEC. The catchment area of major streams between LSD and Kosi barrage has also been estimated for distribution of prevailing initial condition flow in Kosi river

7 Drainage area of Kosi river at Kosi Barrage LSD Location

8 ASTER DEM of Kosi Cross section of river upstream of LSD has been extracted from ASTER DEM for estimating the storage volume behind LSD Cross section of river from LSD upto Kosi barrage has been extracted for hydrodynamic routing of dam break flood to get flood peak, its travel time and possible additional rise in river water level at different locations

9 Estimate of storage volume behind LSD 8.6 MCM for 49 m depth 31 MCM for 80 m depth

10 Estimated volume for 80 m depth of water Reach River Sta Profile Top Min Ch El W.S. Elev Flow Area Width Volume (m) (m) (m2) (m) (1000 m3) LSDUS 8000 PF LSDUS 7000 PF LSDUS 6000 PF LSDUS 5000 PF LSDUS 4000 PF LSDUS 3000 PF LSDUS 2000 PF Volume Plan: 840Plan Elevation (m) River LSDUS Main Channel Distance (m) Legend WS PF 1 Ground

11 Cross sections of Sun Kosi and Kosi rivers

12 MIKE11 model set up for dam break simulation

13 Bed profile of river from land slide dam up to kosi barrage [meter] :00: Lowest point in cross section (marker 2) KOSI [m]

14 Distribution of Initial condition flow Breach width 80 m Breach depth 40 m Breach development time 2 hours Initial condition flow 3540 cumec at Kosi barrage, distributed in catchment area proportion and impinged at confluence points of tributaries

15 Flood peak and its travel time [m^3/s] Time Series Discharge Discharge KOSI KOSI KOSI KOSI KOSI LAKE :00: :00:00 08:00:00 12:00:00 16:00:00 20:00:00 00:00: :00:00

16 Flood peak and additional rise in water level Locatiom (m) downstream of land slide dam Initial flood in river (cumec) Initial flood + land slide dam breach flood (cumec) Flood peak due to land slide dam (cumec) Possible additional rise in water level due to land slide dam breach (m) Flood peak occurrence time (date- hr:min Land slide dam /8/2014 2:15 KOSI /8/2014 2:19 KOSI /8/2014 2:30 KOSI /8/2014 2:34 KOSI /8/2014 2:55 KOSI /8/2014 3:19 KOSI /8/2014 7:24 KOSI /8/ :04 KOSI /8/ :34 KOSI (Near Kosi barrage) /8/ :20

17 Limitations Flood peak due to LSD breach and its travel time obtained using the river cross sections from ASTER DEM, may differ in comparison to surveyed river cross sections. A very high rainfall in the drainage area of the rivers may influence the attenuation pattern of the flood peak, resulting some higher discharge in comparison to estimated one. It is not possible to exactly predict the dam breach parameters, hence a major variation in dam breach parameter may result in variation of estimated flood peak

18 Thank You