HYDROLOGICAL AND HYDRODYNAMIC ASSESSMENT OF KEDARNATH FLOOD. N.N.Rai Director, Central Water Commission, India

Transcription

1 HYDROLOGICAL AND HYDRODYNAMIC ASSESSMENT OF KEDARNATH FLOOD by N.N.Rai Director, Central Water Commission, India

2 Location Map

3 INTRODUCTION Kedarnath Shrine is situated at Lat N and Long E in Garhwal region of Himalyas at an altitude of about 355o masl. On 16 and 17 th June 2013 heavy rains together with Chorabari glacial lake failure caused flooding in Saraswati and Mandakini rivers in Rudraprayag district of Uttarakhand, India. The observatory on Chorabari Glacier Camp (3820 masl) recorded 210 mm rainfall in 12 hours during 15 th June 2013 (5:00 PM) and 16 th June 2013 (5:00 AM). On 16 June 2013 from 5:00 AMto 5:00PM 115 mm rainfall was recorded, total 325 mm rain in 24 hours.

4 Spatial distribution of Rainfall Rainfall (mm) The India Meteorological Department (IMD) linked heavy to very heavy rain-fall on the higher Uttarakhand, Himachal and Nepal Himalaya to the convergence of the Southwest Monsoon trough and westerly disturbances, which led to the formation of dense cloud over the Uttarakhand Himalaya Rainfall (mm)

5 Terrain around Kedarnath shrine

6 Event 1 On 16 June 2013 at 5:15PM, the torrential rains flooded the Saraswati River and Dudh Ganga catchment area, resulting in excessive flow across all the channels. The voluminous water studded with debris from the surrounding regions and glacial moraines moved towards Kedarnath town, washing off upper part of the city (Sankaracharya samadhi, Jalnigam guest house, Bharat Seva Sangh Ashram,etc.) and leading to the biggest ever devastation we have seen in the region. Due to heavy downpour, the town of Rambara was completely washed away on 16 June evening.

7 Event 2 The second event occurred on 17 June2013 at 6:45a.m., after overflow and collapse of the moraine dammed Chorabari Lake which released large volume of water that caused another flash flood in the Kedarnath town leading to heavy devastation in Kedarnath and downstream viz Rambara, Gaurikund, Sonprayag, Phata, etc.

8 Event-1: Flood Computations for June 2013 Rainfall Total 24 hr rainfall 325 mm Time Cum % of 24 hr rainfall Time Cum % of 24 hr rainfall

9 Cum % of 24 hr rainfall Time (hour)

10 Kinematic Wave model for Flood Computation Rainfed CA at Rambara-50 sq.km, SF-14 sq.km

11 Peak discharge cumec, Effective 24 hr rainfall 301 mm

12 Peak discharge cumec, Effective 24 hr rainfall 301 mm

13 Peak discharge cumec, Effective 24 hr rainfall 301 mm

14 KD Plan: River = Riverm and3 Reach = 3 RS = 1800 River cross sections from Aster DEM KD Plan: River = Rivermand3 Reach = 3 RS = Ked Plan: Plan 01 River = Mandakini Reach = 1 RS = 5000 Elevation (m) Station (m) Legend Ground Bank Sta Elevation (m) Station (m) Legend Ground Bank Sta Elevation (m) Station (m) Ked Plan: Plan 01 River = Mandakini Reach = 1 RS = 4000 Legend Ground Bank Sta Elevation (m) Legend Ground Bank Sta Station (m) Ked Plan: Plan 01 River = Mandakini Reach = 1 RS = 2000 Elevation (m ) Station (m) Legend Ground Bank Sta

15 Bed Profile of Mandakini river from Kedarnath complex to Rambara

16 Cross section of Mandakini river near Rambara [meter] MANDAKINI /15/2013 5:00:00 PM [meter]

17 Time series of Water level at Rambara Rise in Water level at Rambara about 4.5 m, Flood peak velocity 8.5 m/s [meter] Time Series Water Level Water Level MANDAKINI :00: :00: :00:00 12:00:00 18:00:00 00:00: :00:00 12:00:00

18 Time series of Water level at Rambara Rise in Water level at Rambara about 4.5 m, Flood peak velocity 8.5 m/s [meter] Time Series Water Level Water Level MANDAKINI :00: :00: :00:00 12:00:00 18:00:00 00:00: :00:00 12:00:00

19 Time series of Velocity at Rambara [m/s] Time Series Flow velocity Flow velocity MANDAKINI Velocity :00: :00:00 00:00: :00:00

20 Breach Location of Chorabari Lake Chorabari Glacier Emptied Chorabari Lake Breach Location Source: Bhuvan, NRSC, India

21 MIKE11 mathematical model setup for Glacial lake outburst flood

22 Bed Profile of river from Chorabari lake to Kedarnath Bed profile

23 Chorabari lake outburst flood just d/s of lake location Breach depth 20 m, Breach width (bottom) 40 m, side slope 0.75H:1V Breach development time : 10 minutes [m^3/s] Time Series Discharge Discharge LAKE :00: :00:00 07:00:00 08:00:00

24 Lake outburst flood near Kedarnath complex [m^3/s] Time Series Discharge Discharge :00: :00:00 07:00:00 08:00:00 MANDAKINI

25 Rise in water level near Kedarnath complex due to lake outburst 4.5 m [meter] Time Series Water Level Water Level MANDAKINI :00: :00:00 07:00:00 08:00:00 As per reports water level rise at Kedarnath was about 5 m

26 Lake outburst flood velocity near Kedarnath complex [m/s] Time Series Flow velocity Flow velocity MANDAKINI Velocity :00: :00:00 07:00:00 08:00:00

27 Lake outburst flood hydrograph near Rambara [m^3/s] Time Series Discharge Discharge MANDAKINI :00: :00:00 07:00:00 08:00:00

28 Rise in water level near Rambara due to lake outburst [meter] Time Series Water Level Water Level MANDAKINI :00: :00:00 07:00:00 08:00:00

29 Lake outburst flood velocity near Rambara [m/s] Time Series Flow velocity Flow velocity MANDAKINI Velocity :00: :00:00 07:00:00 08:00:00

30 Conclusion Due to saturated terrain condition because of continuous rain, only two hours consecutive rainfall of intensity about 40 mm/hr was sufficient to create flash flood of the order of 600 cumec. The average slope of the river from Kedarnath up to Rambara is about 160 m/km, which created a shooting flood wave of 4.5 m deep travelling with velocity of about 8.5 m/s, causing devastation of Rambara and some of the portions of Kedarnath on evening of 16 th June The slope of hills near Kedarnath area is about 30 degree from vertical resulting time of concentration of about 1 hronly The Chorabari lake outburst flood was about 1200 cumec, which caused sudden rise is water level by about 5 m near Kedarnath and 6 m near Rambara. This flood wave translated with a shooting velocity of about 10 m/s, resulting complete devastation of Kedarnath, Rambara and other locations.

31 Flood forecast constraints Every year lakhs of people visit Holy Kedarnath shrine during Charodham yatra of Uttarkhand in India This Yatra occurs when monsoon is also active in this part of India, resulting flash floods in steep Himalayan terrain For issuing advanced flood warning in steep hilly terrains of Himalayas, a reliable hourly/three hourly rainfall forecast at least few hours in advance is essential, but the same is a major constraints at present.

32 Thank You