Flood Management Strategy for Ganga Basin through Storage

by

N. K. Mathur, N. N. Rai, P. N. SinghCentral Water Commission

Introduction The Ganga River basin covers the eleven States of India

comprising Bihar, Jharkhand, Uttar Pradesh, Uttarakhand, WestBengal, Haryana, Rajasthan, Madhya Pradesh, Chhattisgarh,Himachal Pradesh and Delhi.

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

Present study has been carried out to understand the floodpeak formation phenomenon in river Ganga and to estimatethe flood storage requirements in the Ganga basin

The annual flood peak data of river Ganga and its tributariesat different G&D sites of Central Water Commission has beenutilised to identify the contribution of different rivers for floodpeak formations in main stem of river Ganga.

Drainage area map of river Ganga

Important tributaries of River GangaSouthern tributaries Yamuna (347703 sq.km just before Sangam at Allahabad)

Chambal (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.kmTotal drainage area at Patna - 725000 sq.kmTotal drainage area of Himalayan Ganga and Ramganga just before Sangam– 93989 sq.kmRiver 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 andsouth to north, the influence of different river systems onflood peak formation in main stem of river Ganga is alsothe mixed one

The heavy rainfall in Himalayan Ganga is responsible forflooding in Uttarakhand and western part of Uttar Pradesh

Similarly, Ghaghara, Gandak and Kosi river systems areresponsible for flooding in Eastern Uttar Pradesh andNorth Bihar.

The flooding from Allahabad to Patna is quite complex dueto very significant contributions from the river Yamuna andits tributaries, Tauns and Sone river systems, which dependsupon the severe rainstorm tracks

Annual flood peak pattern in river Ganga and its tributaries

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Gandhighat Patna Shahjadpur Ganga Pratappur Yamuna

Varanasi Ganga Turtipar Ghaghra Sone Koelwar

Annual flood peak pattern in river Yamuna and its tributaries

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Pratappur Yamuna Ken Banda Betwa Shahjina

Chambal Udi Yamuna Ponta Sind Seondha

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 fromKosi and Bagmati flood peaks. The contribution of Kosi andBagmati in augmenting the flood peak in Ganga is very rare.

Flood volume at Gandhighat

August 1994 flood at Gandhighat

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Flood volume above 55000 cumec in August 1994 flood – 10.9 BCMFlood volume above 55000 cumec in Aug-Sep 2016 flood – 12 BCM

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Turtipar+Varanasi+ChopanGandhighat Ganga

Aug-Sep 2016 Flood

Flood storage estimate for Ganga Basin

Estimated flood storage in Yamuna river systemChambal sub basin – 3 BCMKen sub basin – 2 BCMBetwa sub basin – 2.5 BCMSind – 0.3 BCMEstimated flood storage on Tauns – 0.8 BCMEstimated flood storage in Sone river system – 2.5 BCMEstimated 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 BCMEstimated flood storage on Kosi – 3.25 BCM (Saptakosi & Sunkosi

multipurpose projects on Saptakosi, Bagmati Multipurpose project on river Bagmati and KamlaDam on Kamla River are proposed storage projects in Kosi river system. total live storage andflood storage of 12180 MCM and 1530 MCM respectively)

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Gandhighat -2016Moderated flood

Moderated flood

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: Rihand Dam: 183.0, Daltonganj: 142.8,Chopan: 135.0, Chittorgarh: 73.8

Meanwhile a slow moving deep depression also formed in Gangetic West Bengal andadjoining Bangladesh on 16th August 2016 and moved very slowly west wards afterintensifying from 16th to 21st August 2016

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

Bansagar Dam released a peak of 15,600 cumec for about 21 hours from 2100 hrs of18th to 18 hrs on 19th August 2016 . Rihand Dam had also released around 5000-6000cumec which also added to the flow into Sone due to very high rainfall in their drainagearea.

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

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Pratappur Yamuna Gandhighat Ganga Chopan Son

Varanasi Ganga Shahajadpur Ganga Turtipar Ghaghra

Back water issue of Farakka barrage

Date Flood peak (cumec)

Observed level above

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

The 100 year flood at Farakka is 76,445 cumec, whichcan be passed at design HFL of 26.1 m above MSL.Therecorded historical flood peaks at Farakka andcorresponding flood disposal level is given below:

Back water study76445 cumec flood without barrage

76445 cumec flood with barrage

Back water extent for 100 yr flood

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Velocity profile with and without barrage

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Farakka Plan: Plan 01

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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 sedimentationin India

Some role may be of Banana forming as dense bananaplants acts as porcupine in some river reaches viz. d/s ofPatna and near Bhagalpur due to large scale Bananaforming at these locations.

Possible reasons for slow recede in river stage during the year 2016 flood

Continuous high discharge contributions from Sone and Yamunadue to heavy rainfall in their drainage area with 1 day maximumas (445 mm in Chambal at Chittorgarh, Pratappur 116 mm,Banda 144 mm (Ken), Satna 172 mm, Rihand 344 mm, Bansagar178 mm, Chopan 135 mm.

Intervening catchment contribution – 30000 to 35000 cumecfor several days

An other reason may be greater resistance to flow due toflooding 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

HFL exceeded by 25 cm

Banana Form

Banana Form

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”

Aggradation of river bed in some reaches and flow congestion

HFLs did not exceed at Munger, Kahalgaon and Sahibganj

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 floodsat Patna in past had occurred due to significant contributions fromYamuna, Sone and Ghaghra river systems.

In order to mitigate the flood peaks at Patna by 20,000 cumec to25,000 cumec flood storage estimates for different river systems are asunder:

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 cushionsupported 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 of7,25,000 sq.km is likely to remain untapped due to topographicalconstraints. Depending upon the rainfall scenario in this area a flooddischarge contribution of the order of 30,000 to 40,000 cumec can beexpected from this area.

For mitigating the floods in the drainage area of Gandak and Kosiriver system in Bihar, a flood storage of 2.3 BCM in Gandak sub basinand 3.25 BCM in Kosi sub basin has been estimated. The storageprojects on these northern tributaries of river Ganga will also help inmitigation of river bed aggradations due to tapping of sediments inthe 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 expectedbeyond 42 km upstream even during the rarest flood discharge of 76445cumec in 100 yr return period. Hence

During year 2016 the maximum recorded discharge on Farakka barrage was50007 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 floodwater is not possible.

Due to Farakka barrage velocity gets marginally decreased at barrage site by4% and becomes exactly same beyond 42 km upstream where afflux isnegligible (almost1cm) even in 100 yr flood. The effect of barrage will belimited only within its pond due to constriction and obstruction of water wayso far as silting is concerned.

High sediment load of northern tributaries is main reason for deposition ofsediment in main stem of river Ganga in Bihar, hence, measures such asconstruction of proposed storage project on northern tributaries will providea effective solution for the aggradations problem of river Ganga in Bihar.

Thanks