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Home , Bansagar Dam, Ghaghara, Kosi River, Rihand Dam, Yamuna

Flood Management Strategy for Basin through Storage

by

N. K. Mathur, N. N. Rai, P. N. Singh Central Water Commission Introduction  The Ganga basin covers the eleven States of comprising , , , , West Bengal, , , , , and .

 The occurrence of floods in one part or the other in Ganga River basin is an annual feature during the period. About 24.2 million hectare flood prone area

 Present study has been carried out to understand the flood peak formation phenomenon in river Ganga and to estimate the flood storage requirements in the Ganga basin

 The annual flood peak data of river Ganga and its at different G&D sites of Central Water Commission has been utilised to identify the contribution of different for flood peak formations in main stem of river Ganga. Drainage area map of river Ganga Important tributaries of River Ganga Southern tributaries  (347703 sq.km just before Sangam at ) (141948 sq.km), Betwa (43770 sq.km), Ken (28706 sq.km), Sind (27930 sq.km), Gambhir (25685 sq.km)  Tauns (17523 sq.km)  Sone (67330 sq.km)

Northern Tributaries  Ghaghra (132114 sq.km)  Gandak (41554 sq.km)  Kosi (92538 sq.km including Bagmati)

Total drainage area at Farakka – 931000 sq.km Total drainage area at Patna - 725000 sq.km Total drainage area of Himalayan Ganga and just before Sangam– 93989 sq.km River Slope between Patna and Farakka about 1:20,000 Rainfall patten in Ganga basin Severe rainstorm producing tracks of cyclonic disturbances (2001-06) Flood peak pattern analysis in Ganga river system  Due to significant rainfall variability from west to east and south to north, the influence of different river systems on flood peak formation in main stem of river Ganga is also the mixed one

 The heavy rainfall in Himalayan Ganga is responsible for flooding in Uttarakhand and western part of Uttar Pradesh

 Similarly, , Gandak and systems are responsible for flooding in Eastern Uttar Pradesh and North Bihar.

 The flooding from Allahabad to Patna is quite complex due to very significant contributions from the river Yamuna and its tributaries, Tauns and Sone river systems, which depends upon the severe rainstorm tracks Annual flood peak pattern in river Ganga and its tributaries

Gandhighat Patna Shahjadpur Ganga Pratappur Yamuna Varanasi Ganga Turtipar Ghaghra Sone Koelwar 90000

80000

70000

60000

50000

40000

Discharge (cumec) 30000

20000

10000

0 Annual flood peak pattern in river Yamuna and its tributaries

Pratappur Yamuna Ken Banda Betwa Shahjina Chambal Udi Yamuna Ponta Sind Seondha 50000 45000 40000 35000 30000 25000 20000 15000 Discharge Discharge (cumec) 10000 5000 0 Impact of Tauns, Gandak, Kosi, Gomti and Punpun rivers on Ganga flood peak

 River Tauns joins river Ganga downstream of Allahabad. In majority of floods Tauns contribution was more than 6000 cumec

 Gangdak contribution was more than 10,000 cumec from Patna downstream

 Punpun less than 800 cumec, Gomti less than 2000 cumec

 Flooding in drainage area of Kosi and Bagmati in Bihar is from Kosi and Bagmati flood peaks. The contribution of Kosi and Bagmati in augmenting the flood peak in Ganga is very rare. Flood volume at Gandhighat

Turtipar+Varanasi+Chopan 90000 Gandhighat Ganga 85000 85000 80000 80000

) 75000 75000 70000 70000 ) 65000 60000

cumec 65000 55000

60000 cumec 50000 55000 45000 40000 50000 35000

Discharge ( Discharge 45000 30000 25000 40000 ( Discharge 20000 35000 15000 30000 10000 4 6 8 10 12 14 16 18 20 22 24

Time (day) August 1994 flood at Gandhighat Aug-Sep 2016 Flood

Flood volume above 55000 cumec in August 1994 flood – 10.9 BCM Flood volume above 55000 cumec in Aug-Sep 2016 flood – 12 BCM Flood storage estimate for Ganga Basin Estimated flood storage in Yamuna river system Chambal sub basin – 3 BCM Ken sub basin – 2 BCM Betwa sub basin – 2.5 BCM Sind – 0.3 BCM Estimated flood storage on Tauns – 0.8 BCM Estimated flood storage in Sone river system – 2.5 BCM Estimated flood storage in Ghaghra river system – 3 BCM

Live storage – 16.21 BCM (Karnali-Chisapanai), 1.19 BCM (Namure), 6.038 BCM (Pancheshwar with additional 0.3 BCM between FRL and MWL as flood cushion) Estimated flood storage on Gandak – 2.3 BCM

Estimated flood storage on Kosi – 3.25 BCM (Saptakosi & Sunkosi multipurpose projects on Saptakosi, Bagmati Multipurpose project on river Bagmati and Kamla Dam on Kamla River are proposed storage projects in Kosi river system. total live storage and flood storage of 12180 MCM and 1530 MCM respectively) Moderated flood 90000 80000 Gandhighat -2016 70000 Moderated flood 60000 50000 40000 30000 20000 Discharge (cumec) Discharge 10000 0 29-07-2016 31-07-2016 02-08-2016 04-08-2016 06-08-2016 08-08-2016 10-08-2016 12-08-2016 14-08-2016 16-08-2016 18-08-2016 20-08-2016 22-08-2016 24-08-2016 26-08-2016 28-08-2016 30-08-2016 01-09-2016 03-09-2016 05-09-2016 07-09-2016 09-09-2016 11-09-2016 13-09-2016 15-09-2016 17-09-2016 19-09-2016 21-09-2016

35000 35000 Ghaghra ) 30000 Kosi 30000 25000 25000

Cumec 20000 20000 15000 15000 Flood Flood ( 10000 10000 5000 5000 0 0 0 30 60 90 120 150 0 40 80 120 160 Time (Hours) Time (hours) Year 2016 flood in Bihar

 There was unprecedented flood in Ganga basin during the period 19th to 28th August 2016.

 According to IMD two low pressure systems were active during the period 1st to 10th August

 Chittorgarh: 445 mm (9th Aug), 12th Aug: : 183.0, Daltonganj: 142.8, Chopan: 135.0, Chittorgarh: 73.8

 Meanwhile a slow moving deep depression also formed in Gangetic West Bengal and adjoining on 16th August 2016 and moved very slowly west wards after intensifying from 16th to 21st August 2016

 19th Aug: Rihand Dam: 344.0, : 177.8, Banda: 144.2, Satna: 120, Kaimaha: 113.0, : 70.4, Chopan: 70.4

 Bansagar Dam released a peak of 15,600 cumec for about 21 hours from 2100 hrs of 18th to 18 hrs on 19th August 2016 . Rihand Dam had also released around 5000-6000 cumec which also added to the flow into Sone due to very high rainfall in their drainage area.

 The combined effect of Ganga and Sone was felt at Patna Gandhighat which crossed the HFL of 50.27 m and attained a peak level of 50.52 m on 20th August 2016 by 1800 hrs.

Flow pattern in rivers

Pratappur Yamuna Gandhighat Ganga Chopan Son Varanasi Ganga Shahajadpur Ganga Turtipar Ghaghra 85000 80000 75000 70000 65000 60000 55000 50000 45000 40000 35000 30000 25000 Discharge Discharge (cumec) 20000 15000 10000 5000 0 01-08-2016 03-08-2016 05-08-2016 07-08-2016 09-08-2016 11-08-2016 13-08-2016 15-08-2016 17-08-2016 19-08-2016 21-08-2016 23-08-2016 25-08-2016 27-08-2016 29-08-2016 31-08-2016 02-09-2016 04-09-2016 06-09-2016 08-09-2016 10-09-2016 12-09-2016 14-09-2016 Back water issue of Farakka barrage

 The 100 year flood at Farakka is 76,445 cumec, which can be passed at design HFL of 26.1 m above MSL.The recorded historical flood peaks at Farakka and corresponding flood disposal level is given below:

Observed level above Date Flood peak (cumec) MSL (m)

18-Sep-1987 73154 25.44

01-Sep-1988 67406 25.12

12-Sep-1991 69042 25.29

06-Sep-2013 54007 24.56

29-Aug-2016 50007 24.29 Back water study

76445 cumec flood without barrage

76445 cumec flood with barrage Back water extent for 100 yr flood

Without barrage With barrage 35 34 33 32 31 30 29

Water level (m) level Water 28 27 26 25 0 10000 20000 30000 40000 50000 60000

Chainage (m) upstream of Farakka barrage Velocity profile with and without barrage

Farakka Plan: Plan 01 Legend 2.8 Vel Chnl 100 yr NC 2.6 Vel Chnl 100 yr WB 2.4

2.2 2.0

1.8 Vel Chnl (m/s) Chnl Vel 1.6

1.4 1.2

1.0 0 10000 20000 30000 40000 50000 60000 Main Channel Distance (m ) From water surface and velocity profile it has been estimated that even during the occurrence of 100 yr flood, Farakka back water cannot extend beyond 42 km

Bhagalpur, Begusarai and Patna are located about 170, 270 and 400 km upstream of Farakka barrage and Farakka barrage cannot influence flow dynamics at these locations Possible reasons of sedimentation

 Drop in bed slope of northern tributaries facilitates natural sedimentation

 Encroachment in flood plain

 Ploughing of fields during summer – natural sedimentation in India

 Some role may be of Banana forming as dense banana plants acts as porcupine in some river reaches viz. d/s of Patna and near due to large scale Banana forming at these locations. Possible reasons for slow recede in river stage during the year 2016 flood

 Continuous high discharge contributions from Sone and Yamuna due to heavy rainfall in their drainage area with 1 day maximum as (445 mm in Chambal at Chittorgarh, Pratappur 116 mm, Banda 144 mm (Ken), Satna 172 mm, Rihand 344 mm, Bansagar 178 mm, Chopan 135 mm.

 Intervening catchment contribution – 30000 to 35000 cumec for several days

 An other reason may be greater resistance to flow due to flooding of Banana forms d/s of Patna and at Bhagalpur (Manning’s n about 0.15 to 0.20 for continuous Banana form), brick kiln in flood plains

 Steady contribution of flow from Mokama Taal Possible reason for exceedence of HFL by more than 20 cm at Patna and Bhagalpur

Aggradation of river bed in some reaches and flow congestion

HFLs did not exceed at Munger, Kahalgaon and Sahibganj Banana Form

Banana Form

HFL exceeded by 25 cm

HFL exceeded by 2 cm HFL exceeded by 22 cm

Equivalent Manning’s n has been computed as 0.07 to 0.09. Velocity of flow is inversely proportional to “n” Conclusion  From the pattern analysis of annual flood peaks at different G&D sites in Ganga basin it has been concluded that almost majority of the floods at Patna in past had occurred due to significant contributions from Yamuna, Sone and Ghaghra river systems.

 In order to mitigate the flood peaks at Patna by 20,000 cumec to 25,000 cumec flood storage estimates for different river systems are as under:

 Yamuna river system: 3 BCM in Chambal, 2.5 BCM in Betwa, 2 BCM in Ken, 0.3 BCM in Sind.

 Tauns river system: 0.8 BCM

 Sone river system: 2.5 BCM

 Ghaghra river system: 3 BCM Conclusion contd......

 To ensure the effective flood mitigation strategy, integrated operation of the proposed/existing storage projects is essential.

 Efforts should also be made to ensure some dynamic flood cushion supported with the inflow forecast in existing projects viz Bansagar, Rihand in Sone sub basin, Rajghat, Matatila in Betwa basin.

 Further, about 63% of the drainage area of river Ganga at Patna out of 7,25,000 sq.km is likely to remain untapped due to topographical constraints. Depending upon the rainfall scenario in this area a flood discharge contribution of the order of 30,000 to 40,000 cumec can be expected from this area.

 For mitigating the floods in the drainage area of Gandak and Kosi river system in Bihar, a flood storage of 2.3 BCM in Gandak sub basin and 3.25 BCM in Kosi sub basin has been estimated. The storage projects on these northern tributaries of river Ganga will also help in mitigation of river bed aggradations due to tapping of sediments in the proposed storage reservoirs. Conclusion contd......  From the back water profile study of the Farakka barrage it has been estimated, that the back water effect of Farakka barrage is not expected beyond 42 km upstream even during the rarest flood discharge of 76445 cumec in 100 yr return period. Hence

 During year 2016 the maximum recorded discharge on Farakka barrage was 50007 cumec on 29th August 2016, which was passed at pond level of 24.29 m. In view of above, any influence of Farakka barrage in slow translation of flood water is not possible.

 Due to Farakka barrage velocity gets marginally decreased at barrage site by 4% and becomes exactly same beyond 42 km upstream where afflux is negligible (almost1cm) even in 100 yr flood. The effect of barrage will be limited only within its pond due to constriction and obstruction of water way so far as silting is concerned.

 High sediment load of northern tributaries is main reason for deposition of sediment in main stem of river Ganga in Bihar, hence, measures such as construction of proposed storage project on northern tributaries will provide a effective solution for the aggradations problem of river Ganga in Bihar. Thanks