The World Bank

Flood Risk Assessment for the Ganges Basin in South Asia

Hazard Report

June 2015

Submitted by

RMSI A-8, Sector 16 Noida 201301, INDIA Tel: +91-120-251-1102, 2101 Fax: +91-120-251-1109, 0963 www.rmsi.com Contact: Dr. MVRL Murthy Vice – President email - [email protected] The World Bank Exposure and Hazard Flood Risk Assessment for the Ganges Basin in South Asia

For the attention of: Mr. Bill Young Program Leader, South Asia Water Initiatives The World Bank, The Hindustan Times House (Press Block) 18-20, Kasturba Gandhi Marg New Delhi - 110001 Email: [email protected]

Company Information: Name RMSI Private Limited CIN U74899DL1992PTC047149 Registered Office Address SEATING 3, UNIT NO. 119, FIRST FLOOR, VARDHMAN STAR CITI MALL, SECTOR-7, DWARKA NEW DELHI Delhi-110075 INDIA Corporate Office Address A-8, Sector-16 NOIDA, 201 301 India Tel:+91 120 251 1102, 251 2101 Fax:+91 120 251 1109, 251 0963 E-mail: [email protected]

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Acknowledgements

We are thankful for the support and suggestions provided by Dr. Satya Priya, Senior Water Resource Management Specialist, World Bank, New Delhi, India. RMSI is also thankful to Dr. Bill Young, Program Leader, South Asia Water Initiative, World Bank, New Delhi, India for the continuous support provided throughout the project.

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Table of Contents

Acknowledgements ...... 3 Table of Contents ...... 4 List of Figures ...... 6 List of Tables ...... 9 Abbreviations Used ...... 10 1 Introduction ...... 11 1.1 Background...... 11 1.2 Topography...... 14 1.3 Climate ...... 14 1.4 Meteorological causes of heavy rainfall over Ganges River Basin ...... 15 1.5 Rainfall Pattern ...... 16 1.6 Objectives of the study ...... 16 1.7 Scope of the study ...... 16 1.8 About this Report ...... 16 2 Flood Hazard Assessment ...... 18 2.1 Methodology Overview ...... 18 2.2 Data Availability ...... 19 2.2.1 Meteorological Data ...... 19 2.2.2 Hydrological Data ...... 20 2.2.3 Topographical Information ...... 22 2.2.4 Land Use Land Cover (LULC) ...... 23 2.2.5 Soil Map ...... 24 2.2.6 Flood History...... 25 2.3 Hydrological Modeling ...... 32 2.3.1 Basin Delineation Using HEC Geo-HMS ...... 33 2.3.2 Model Development ...... 35 2.3.2.1 Rainfall Loss and Infiltration: SCS Curve Number ...... 35 2.3.2.2 Rainfall-Runoff Transformation: SCS Unit Hydrograph ...... 36 2.3.2.3 Stream Flow Routing: Muskingum Method ...... 36 2.3.2.4 Meteorological Model: Gauge Weights ...... 37 2.3.3 Model Calibration ...... 37 2.3.3.1 Upper Ganges ...... 40 2.3.3.2 Ghagra ...... 42 2.3.3.3 Kosi ...... 46 2.3.3.4 Chambal ...... 49

Hazard Report Confidential Page 4 of 86 The World Bank Exposure and Hazard Flood Risk Assessment for the Ganges Basin in South Asia 2.3.3.5 Yamuna ...... 51 2.3.3.6 Betwa and Ken ...... 53 2.3.3.7 Ramganga and Middle Ganges ...... 55 2.3.3.8 Sone ...... 58 2.3.3.9 Lower Ganges ...... 60 2.3.4 Model Validation ...... 63 2.3.5 Return period flows ...... 71 2.4 Hydraulic Modeling ...... 72 2.4.1 Model Set Up ...... 72 2.4.2 Model calibration ...... 73 2.4.3 Flood hazard mapping for return period flows ...... 77 References ...... 84

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List of Figures

Figure 1-1: Ganges Basin in India, Nepal, Bangladesh and China ...... 13 Figure 2-1: Flood hazard assessment framework ...... 18 Figure 2-2: Location of rainfall stations used in the study ...... 20 Figure 2-3: Flow gauge station locations ...... 22 Figure 2-4: Elevation map of the study area ...... 23 Figure 2-5: LULC map of Ganges River basin ...... 24 Figure 2-6: Soil map for the study area ...... 25 Figure 2-7: Flood Footprint: 2007 flood ...... 30 Figure 2-8: Flowchart for hydrological modeling ...... 33 Figure 2-9: Delineated catchments and major sub basins of Ganges River Basin ...... 34 Figure 2-10: HEC-HMS model setup for Ganges Basin ...... 37 Figure 2-11: Location of the discharge gauges used to calibrate the model at sub-basin level ...... 40 Figure 2-12: Location of the discharge gauge used in calibration for Upper Ganges sub basin ...... 41 Figure 2-13: Comparison of observed and simulated hydrographs at discharge gauges of Rudraprayag in Upper Ganges sub basin ...... 42 Figure 2-14: Locations of the discharge gauges used in calibration of Ghagra sub basin .... 43 Figure 2-15: Comparison of observed and simulated hydrographs at the discharge gauge of Chispani in Ghagra sub basin ...... 44 Figure 2-16: Comparison of observed and simulated hydrographs at the discharge gauge of Rudauli-Faizabad in Ghagra sub basin ...... 45 Figure 2-17: Comparison of observed and simulated hydrographs at the discharge gauge of Gosaniganj in Ghagra sub basin...... 46 Figure 2-18: Locations of discharge gauges used in calibration for Kosi sub basin ...... 47 Figure 2-19: Comparison of observed and simulated hydrographs at the discharge gauge of Kampughat in Kosi sub basin ...... 48 Figure 2-20: Comparison of observed and simulated hydrographs at the discharge gauge of Saharsa in Kosi sub basin ...... 49 Figure 2-21: Location of the discharge gauge used in calibration for Chambal sub basin .... 50 Figure 2-22: Comparison of observed and simulated hydrographs at Baranwada discharge gauge in Chambal sub basin ...... 51 Figure 2-23: Location of the discharge gauge used in calibration for Yamuna sub basin ..... 52 Figure 2-24: Comparison of observed and simulated hydrographs at Agra discharge gauge in Yamuna sub basin ...... 53 Figure 2-25: Location of the discharge gauge used in calibration for Betwa and Ken sub basins ...... 54

Hazard Report Confidential Page 6 of 86 The World Bank Exposure and Hazard Flood Risk Assessment for the Ganges Basin in South Asia Figure 2-26: Comparison of observed and simulated hydrographs at Chillaghat discharge gauge for Betwa and Ken sub basins ...... 55 Figure 2-27: Location of the discharge gauges used in calibration for Ramganga and Middle Ganges sub basins ...... 56 Figure 2-28: Comparison of observed and simulated hydrographs at Ankinghat discharge gauge of Ramganga and Middle Ganges sub basins ...... 57 Figure 2-29: Comparison of observed and simulated hydrographs at Varanasi discharge gauge of Ramganga and Middle Ganges sub basins ...... 58 Figure 2-30: Location of the discharge gauge used in calibration for Sone sub basin ...... 59 Figure 2-31: Comparison of observed and simulated hydrographs at Dehri discharge gauge in Sone sub basin ...... 60 Figure 2-32: Locations of the discharge gauges used in calibration for Lower Ganges sub basin ...... 61 Figure 2-33: Comparison of observed and simulated hydrographs at Sahibganj discharge gauge of Lower Ganges sub basin ...... 62 Figure 2-34: Comparison of observed and simulated hydrographs at Jangipur discharge gauge of Lower Ganges sub basin ...... 63 Figure 2-35: Comparison of observed and simulated hydrographs for flow gauge station of Upper Ganges sub basin for a validation event ...... 65 Figure 2-36: Comparison of observed and simulated hydrographs for flow gauge stations of Ghagra sub basin for various validation events ...... 66 Figure 2-37: Comparison of observed and simulated hydrographs for flow gauge stations of Kosi sub basin for various validation events ...... 67 Figure 2-38: Comparison of observed and simulated hydrographs for flow gauge station of Chambal sub basin for a validation event ...... 67 Figure 2-39: Comparison of observed and simulated hydrographs for flow gauge station of Yamuna sub basin for a validation event ...... 68 Figure 2-40: Comparison of observed and simulated hydrographs for flow gauge station of Betwa and Ken sub basins for a validation event ...... 68 Figure 2-41: Comparison of observed and simulated hydrographs for flow gauge stations of Ramganga and Middle Ganges sub basins for various validation events ...... 69 Figure 2-42: Comparison of observed and simulated hydrographs for flow gauge station of Sone sub basin for a validation event ...... 69 Figure 2-43: Comparison of observed and simulated hydrographs for flow gauge stations of Lower Ganges sub basin for various validation events ...... 70 Figure 2-44: L moment ratio diagram ...... 71 Figure 2-45: HEC RAS model set up for the study area ...... 73 Figure 2-46: Flood Extent Map of September, 2001 flood event (Source: DFO) ...... 74 Figure 2-47: Flood Extent Map of September, 2005 flood event (Source: DFO) ...... 75 Figure 2-48: Comparison between modeled and observed flood extents of September, 2001 event ...... 76 Figure 2-49: Comparison between modeled and observed flood extents of September, 2005 event ...... 76

Hazard Report Confidential Page 7 of 86 The World Bank Exposure and Hazard Flood Risk Assessment for the Ganges Basin in South Asia Figure 2-50: Flood hazard map for 2-year return period for Ganges basin ...... 78 Figure 2-51: Flood hazard map for 5-year return period for Ganges basin ...... 79 Figure 2-52: Flood hazard map for 10-year return period for Ganges basin ...... 80 Figure 2-53: Flood hazard map for 25-year return period for Ganges basin ...... 81 Figure 2-54: Flood hazard map for 50-year return period for Ganges basin ...... 82 Figure 2-55: Flood hazard map for 100-year return period for Ganges basin ...... 83

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List of Tables

Table 1-1: Country wise composition of Ganges Basin ...... 12 Table 1-2: Areas Occupied by the basin in the four countries ...... 13 Table 1-3: Historical flood events – Ganges basin (Source: Dartmouth Flood Observatory, USA) ...... 14 Table 2-1: Station wise rainfall data records ...... 19 Table 2-2: Availability of Discharge Data ...... 20 Table 2-3: Additional flow gauges used in study ...... 21 Table 2-4: Flood loss summary: Bihar, 1979 to 2006 ...... 26 Table 2-5: Flood loss summary: Uttar Pradesh, 1973 flood ...... 27 Table 2-6: Flood loss summary: Bihar, 2007 flood ...... 31 Table 2-7: Major sub basins considered in study ...... 34 Table 2-8: Curve numbers used for analysis ...... 35 Table 2-9: Details of the gauges, flow data and events used for calibration simulation of hydrological model at various sub basins ...... 39 Table 2-10: Comparison of simulated and observed flows (cumec) for discharge gauge station of Upper Ganges sub basin for various calibration events ...... 41 Table 2-11: Comparison of simulated and observed flows (cumec) for discharge gauge stations of Ghagra sub basin for various calibration events ...... 43 Table 2-12: Comparison of simulated and observed flows (cumec) for discharge gauge stations of Kosi sub basin for various calibration events ...... 47 Table 2-13: Comparison of simulated and observed flows (cumec) for discharge gauge stations of Chambal sub basin for various calibration events ...... 50 Table 2-14: Comparison of simulated and observed flows (cumec) for discharge gauge station of Yamuna sub basin for various calibration events...... 52 Table 2-15: Comparison of simulated and observed flows (cumec) for discharge gauge station of Betwa and Ken sub basins for various calibration events ...... 54 Table 2-16: Comparison of simulated and observed flows (cumec) for discharge gauge stations of Ramganga and Middle Ganges sub basins for various calibration events ...... 56 Table 2-17: Comparison of simulated and observed flows (cumec) for discharge gauge stations of Sone sub basin for various calibration events ...... 59 Table 2-18: Comparison of simulated and observed flows (cumec) for discharge gauge stations of Lower Ganges sub basin for various calibration events ...... 61 Table 2-19: Details of the validation events for different discharge gauges of the sub basins ...... 64 Table 2-20: Simulated and observed peak flows (cumec) for sub basins at various gauge stations for validation events ...... 65 Table 2-21: Simulated and observed water level for the two historical events of August, 2001 and August, 2005 ...... 77

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Abbreviations Used

Abbreviation/Acronym Expanded Form DEM Digital Elevation Model DHM Department of Hydrology and Meteorology ft feet GIS Geographic Information System GPS Global Positioning System Ha Hectares HEC Hydrologic Engineering Center HEC-RAS Hydrologic Engineering Centre’s River Analysis System HVRA Hazard Vulnerability Risk Assessment INR Indian Rupees IT Information Technology km kilometer m meter MDR Mean Damage Ratio mm millimeter MS Microsoft PML Probable Maximum Loss RAS River Analysis System SRTM Shuttle Radar Topographic Mission UP Uttar Pradesh WRD Water Resources Department

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1 Introduction

1.1 Background The Ganges basin is one of the largest river basins. It lies in China, Nepal, India and Bangladesh and drains an area of about 10,00,000 sq. km. In India, its catchment lies in the states of Uttar Pradesh, Uttarakhand, Madhya Pradesh, Chhattisgarh, Bihar, Jharkhand Rajasthan, West Bengal, Haryana, Himachal Pradesh and Delhi. It lies between latitude 22.450 N and 31.470 N and longitude 73.370 E and 89.310 E. The Ganges River originates as Bhagirathi from the Gangotri Glaciers in the Himalayas at an elevation of about 7,000 m above mean sea level, in the Uttarkashi district of Uttarakhand. The Bhagirathi is joined by the Alaknanda at Deoprayag and the combined stream under the name Ganges flowing through the mountain region debouches into the plains at Rishikesh. It is joined by a large number of tributaries on both the banks in the course of its total run of about 2,500 km before its outfall into the Bay of Bengal. The important tributaries are the Yamuna, the Ramaganga, the Gomti, the Ghagra, the Son, the Gandak, the Kosi and the Mahananda. At Farakka in West Bengal, the river divides into two arms namely the Padma which flows to Bangladesh and the Bhagirathi which flows through West Bengal. The Yamuna River is the biggest tributary of the Ganges River. It originates from Yamunotri Glacier near Banderpoonch peaks in the Mussourie range of the lower Himalayas at an elevation of about 6,300 m above the mean sea level in the district of Uttarkashi. The Himalayas exercise a dominating influence on climate in the northern region of the Upper Yamuna catchment. In this region, winters are very cold, while summers are moderate. The average annual rainfall varies between 400 mm to 1,500 mm. The entire catchment comes under the influence of the southwest monsoon and a major part of the rainfall is received between June and September. Winter rainfall is scanty and occurs between December and February. In the lower part of the Yamuna basin, temperatures are relatively moderate. In summer, temperatures frequently exceed 400 C. The Chambal River is the largest of the rivers flowing through Rajasthan State. This tributary of the Yamuna is about 900 km long. The total area drained by the Chambal up to its confluence with the Yamuna is about 1,43,000 sq. km out of which about 76,000 sq. km lies in Madhya Pradesh state, about 65,000 sq. km in Rajasthan state and about 1,100 sq. km in Uttar Pradesh. The Ramganga is the one of the major tributaries joining the Ganges River. It rises at an altitude of about 3,100 m in the lower Himalayas near the Lohba village in the Garhwal district of Uttaranchal. The length of the Ramganga River, from the source to the confluence with the Ganges, is about 600 km. During its course, the river flows through a mountainous terrain and has a number of falls and rapids. The river flows entirely in the states of Uttarakhand and Uttar Pradesh. The catchment area of the Ramganga is about 30,000 sq. km. The Gomti River originates near Mainkot, about 3 km east of Pilibhit town in Uttar Pradesh, at an elevation of 200 m. The river drains the area between Ramganga and Ghaghra systems. The total length of the river is about 900 km and it flows entirely in the state of Uttar Pradesh. The total drainage area of the river is 30,000 sq. km. The Gaghra River originates at an elevation of 4,800 m near Mansarover Lake. The river is also known as Manchu and Karnali in Nepal. After flowing for about 70 km in a southeasterly direction, the river enters Nepal. Gaghra enters into India at Kotia Ghat near Royal Bardia National Park, Nepal Ganj, where it is known as the river Girwa for about 25 km. The total catchment area of the Gaghra River is about 132,000 sq. km, out of which 45% falls in India.

Hazard Report Confidential Page 11 of 86 The World Bank Exposure and Hazard Flood Risk Assessment for the Ganges Basin in South Asia The total length of the Gaghra River before its confluence with the Ganges River is about 1,000 km. The Son River is an important right bank tributary of the Ganges River. The river originates at an elevation of 600 m at Sonbhadra in the Maikala range in Madhya Pradesh. The catchment area of the basin is about 68,000 sq. km. Total length of the river is about 800 km, out of which about 500 km lies in Madhya Pradesh, about 80 km in Uttar Pradesh and the remaining in Jharkhand and Bihar. The river meets the Ganges River about 16 km upstream of Dinapur in the Patna district of Bihar. The Gandak River originates near the Nepal-Tibet border at an altitude of about 7,600 m to the northeast of Dhaulagiri and flows about 100 km in a southeasterly direction in Nepal. After that it debauches into the plains of the Champarann district of Bihar at Trivani. The total length of the river from its source to outfall into the Ganges is about 600 km of which about 380 km lie in Nepal and Tibet. The total drainage area of the river is about 46,300 sq. km of which about 7,600 sq. km is in India. The Kosi River is a major tributary of the Ganges River, which originates at an altitude of 7,000 m in the Himalayas. The total catchment area of the Kosi River is 70,000 sq. km out of which 20,376 sq. km lie in India. Kamla Balan and Bagamati are two major tributaries on the right side of the river in Bihar. The delta of the Ganges is said to begin at the Farakka barrage. About 40 km downstream of Farakka, the river splits into two arms. The right arm, known as the Bhagirathi River, flows towards the south and enters the Bay of Bengal about 150 km downstream of Kolkata. The left arm, known as Padma, turns towards the east and enters Bangladesh. While flowing in Bangladesh, Padma meets the Brahmaputra River at Goalundo. The combined flow, still known as Padma, is joined by Meghna, at Chandpur, about 100 km downstream of Goalundo. Further down, the river flows into the Bay of Bengal. The country wise spread of the basin across eleven states in India, ten districts in Bangladesh, five Provinces in Nepal, and seven Counties in China is depicted in Table 1-1.

Table 1-1: Country wise composition of Ganges Basin

Country State Country Provinces Country Districts Country Counties Uttar Central Dinajpur Dinggye Pradesh Madhya Eastern Kustia Nyalam Pradesh Far Rajasthan Meherpur Tingri Western Mid Bihar Naogaon Burang Western India Nepal Bangladesh China West Bengal Natore Gamba Uttarakhand Nawabgunj Gyirong Jharkhand Pabna Haryana Panchagarh Western Chhattisgarh Rajshahi Sa'gya Himachal Pradesh Thakurgaon Delhi The major part of the geographical area of the Ganges basin lies in India and it is the biggest river basin in the country draining an area of 790,223 sq.km, which is slightly more than one fourth (26.3 %) of the total geographical area of the country.

Hazard Report Confidential Page 12 of 86 The World Bank Exposure and Hazard Flood Risk Assessment for the Ganges Basin in South Asia The following table shows the share of geographical area of the Ganges Basin in the four countries.

Table 1-2: Areas Occupied by the basin in the four countries

Country Area occupied by Ganges Basin in km2 Bangladesh 7,014 China 39,133 India 790,223 Nepal 147,706 Total 984,076

The following figure shows the Ganges basin in the four countries.

Figure 1-1: Ganges Basin in India, Nepal, Bangladesh and China

Hazard Report Confidential Page 13 of 86 The World Bank Exposure and Hazard Flood Risk Assessment for the Ganges Basin in South Asia Ganges basin is frequently affected by floods due to high discharges in the Ganges river system. Table 1-3 shows some of the more recent devastating flood events that affected various districts in India (Source: Dartmouth Flood Observatory, USA).

Table 1-3: Historical flood events – Ganges basin (Source: Dartmouth Flood Observatory, USA)

S. No. Date of event Affected cities/ location Lakhimpur, Bahraich, Gonda, Ayodhya, Tanda, Uttar 1 September 28, 2008 Pradesh Gorakhpur, Siwan, Chhapra, Samastipur, Madhupura, 2 August 24, 2008 Sahibganj, Bihar 3. August 3, 2007 Bahraich, Ayodhya, Siwan, Uttar Pradesh Motihari, Madhubani, Sitamarhi, Samastipur, Monghyr, 4. July 19, 2007 Bihar The main causes of floods are widespread and heavy rainfall in the catchment areas and the inadequate capacity of the river channels to contain the flood flows within the banks of the rivers. India and Bangladesh suffer heavily from floods. For example, the 2007 floods were Bihar’s worst in 20 years, affecting more than 24 million people, killing nearly 1,000 people, and destroying over 700,000 homes. The breach in the Kosi embankment in Nepal in 2008 also caused problems in areas not normally flood-prone, rendering about 3 million homeless, as the Kosi flowed down an older channel to the Ganges. In Bangladesh about 26,000 km2, (around 18%) of the country is flooded each year, killing over 5,000 people and destroying more than 7 million homes. During severe floods, the affected area may exceed 75% of the country, as was seen in 1998. The floods have caused devastation in Bangladesh throughout history, especially during the years 1966, 1987, 1988, and 1998. 1.2 Topography The Ganges basin is bounded on the north by the Himalayas, on the west by the Aravallis as well as the ridge separating it from Indus basin, on the south by the Vindhyas and Chhotanagpur Plateau and on the east by the Brahmaputra ridge. The main physical sub- divisions of the Ganges basin are the northern mountains, the Gangetic Plains and the central highlands. The northern mountains comprise of the Himalayan ranges including their foothills. The Gangetic plains, situated between the Himalayas and the Deccan plateau, constitute most of the sub-basin ideally suited for intensive cultivation. The central highlands lying to the south of the great plains consist of mountains, hills and plateaus intersected by valleys and river plains. They are largely covered by forests. Aravali uplands, Bundelkhand upland, Malwa plateau, Vindhyan ranges and Narmada valley lie in this region. 1.3 Climate The Ganges Basin has a tropical climate. The annual average rainfall in the basin varies between 390 mm to 2,000 mm, with an average of 1,000 mm. The climate in the Ganges basin is characterized by a distinct wet season during the period of southwest monsoon (June to October). The southwest monsoon makes landfall at the mouth of the Ganges around the first week of June and advances upstream. By the end of July, the monsoon reaches the western end of the Ganges basin. In the majority of the basin, the rainy season spreads over three months (July, August and September) and usually 80% of the total annual rainfall occurs during this period. In the eastern part of the basin, such as in West Bengal and Bihar, the wet season is longer, usually starting in June and continuing until the end of September or early October. The lowest precipitation in the Gangetic plains occur in Haryana (less than 500 mm per year), with the rainfall increasing downstream until reaching lower Bengal, where nearly 1,600 mm of rainfall occurs. Heavier rainfall continues in the upper Himalayan region, such

Hazard Report Confidential Page 14 of 86 The World Bank Exposure and Hazard Flood Risk Assessment for the Ganges Basin in South Asia as in Dehra Dun, where the rainfall is as high as 2,209 mm per year. Snow is also a significant part of precipitation in the higher reaches of the basin. The winter precipitation that occurs in the form of snow in hilly areas accumulates until summer. During summer, the melting of snow contributes to considerable runoff. The average temperature in the basin ranges between 9°C to 40°C. 1.4 Meteorological causes of heavy rainfall over Ganges River Basin The weather in the Ganges basin is characterized by a distinct wet season during the period of southwest monsoon (June to October). The air temperature starts falling with onset of the monsoon from June onwards, making the weather more humid. The southwest monsoon makes landfall at the mouth of the Ganges around the first week of June and advances upstream. By the end of July the monsoon reaches the western end of the Ganges basin. In the majority of the basin, the rainy season spreads over three months (July, August and September) and usually 70 to 80% of the total annual rainfall occurs during this period. In the eastern part of the basin, such as in West Bengal and Bihar, the wet season is longer, usually starting in June and continuing until the end of September or early October. Lowest rainfall occurs in Haryana (less than 500 mm per annum) with the rainfall increasing downstream until reaching lower Bengal, where nearly 1,600 mm of rainfall occurs. Heavier rainfall continues in the upper Himalayan region, such as in Dehra Dun, where the rainfall is as high as 2,209 mm per annum. As mentioned above, most of the rainfall over the Ganges River Basin occurs during the southwest monsoon season. During this season, cyclonic disturbances are important synoptic systems that cause heavy spells of rainfall in the Ganges River Basin. On an average seven cyclonic disturbances (mainly depressions) form in the Bay of Bengal during the 4 months from June to September. These disturbances generally move in a west- northwest direction after their formation at the head of the Bay of Bengal up to the central parts of the country before weakening. It is well known that heavy rainfall occurs in the southwestern sector of the monsoon depressions due to strong convergence in that sector. The part of Ganges River Basin comes in the southwest of monsoon depressions tracks and as such heavy to very heavy rainfall occurs over different parts of the Ganges River Basin. The topography of the basin including Himalayas also plays an important role in causing heavy rainfall in the parts of the basin during the southwest monsoon season. When monsoon depressions are formed in the Bay of Bengal, the Arabian Sea currents are strengthened and cause heavy rainfall over the parts of the basin. The main synoptic situations of the southwest monsoon system that produce heavy rainfall over the Ganges River Basin are formation and subsequent movement of monsoon depressions, low- pressure systems from the head Bay of Bengal and well marked seasonal trough. On average, five to seven western disturbances move over the northwestern region of India during the winter months. An equal number of induced lows form to their south and move east and northeastward, giving heavy to very heavy rains over the plains of Punjab, Haryana, and Uttar Pradesh. Normally, the southwest monsoon starts withdrawing from the northwest and adjoining regions of India from 15 September; it gets revived again in association with Bay of Bengal/Arabian Sea depressions, which move toward this region and interact with western disturbances, causing exceptionally heavy rainfall over the region, especially in J&K, sub-Himalayan regions of Himachal Pradesh, Punjab, and Uttarakhand Himalayas. The tracks of monsoon depressions or low-pressure areas from the Bay of Bengal sometimes re-curve in a northerly to northeasterly direction when they travel over the central parts of the country. This is mostly due to the movement of strong westerlies (western disturbances) over the extreme northwest to northeast Himalayas. The synchronization of movement of westerly waves in the extreme north with the passage of monsoon

Hazard Report Confidential Page 15 of 86 The World Bank Exposure and Hazard Flood Risk Assessment for the Ganges Basin in South Asia disturbances in the lower latitudes cause heavy to very heavy rainfall along the foothills of the Himalayas (Nandargi and Dhar, 2012). 1.5 Rainfall Pattern The mean annual rainfall of the Ganges River Basin is about 1,019 mm with a CV of 89%. About 91% of the annual rainfall is received during the southwest monsoon season of June to October. The bulk of the remaining 9% occurs mostly in the winter, pre-monsoon and post-monsoon periods. However, mean annual rainfall over the individual sub-basins varies widely due to orographic influences and preferential occurrence of rain producing systems in certain parts of the basin. The mean annual rainfall of a catchment is as low as 575 mm in the western part of the basin. Rainfall gradually increases towards the east. 1.6 Objectives of the study In view of the regular recurrence of losses due to floods in different parts of the basin, a comprehensive study for the entire Ganges system was considered essential to understand flood risks in the region. Keeping this need in mind, the World Bank has initiated a River Basin Approach to develop a shared knowledge base and analytical framework for flood risk assessment and has appointed RMSI as consultant to complete this assignment. The main objectives of the present study are to understand the geographical impacts of floods on various sectors exposed in the areas the Ganges Basin passes through the four countries and thereby enhance the knowledge base for better understanding the socio-economic impacts of flooding in the basin. 1.7 Scope of the study Key activities include: 1. Probabilistic hazard assessment  Collection and compilation of existing data pertaining to hydro-meteorological parameters from state run meteorological bureaus and water resources departments for river flow data from respective countries available in public domain  Development of hydrological and hydraulic models for the entire Ganges basin up to its confluence with the Brahmaputra River in Bangladesh for probabilistic flood hazard assessment 2. Exposure and vulnerability development and estimation of direct losses to derive various risk metrics (i.e. average annual loss, loss exceedence curves, and loss costs)  Development of exposure database for various classes such as buildings, infrastructure, critical and essential facilities, demography, and agriculture  Development of vulnerability (damage function) curves for each of the vulnerability classes  Probabilistic assessment of direct economic losses, by asset classes and by block (sub district) 3. Development of an open-source web GIS based Flood Risk Atlas to view the results of this study over the web 1.8 About this Report The second deliverable of the Flood Risk Assessment for the Ganges Basin in South Asia is presented in two reports, viz., Exposure Report and Hazard Report. The reports include:

Exposure data compilation Initial results of hazard assessment Model development results, assumptions and challenges

Hazard Report Confidential Page 16 of 86 The World Bank Exposure and Hazard Flood Risk Assessment for the Ganges Basin in South Asia The chapter wise synopsis of each of the reports is given below:

Exposure Report: Chapter 1: The present chapter provides the reader with an overview of the project, its scope and its objectives. It also provides the reader with an understanding of key concepts related to the project. Chapter 2: The next chapter describes exposure data collection and management, including the following:  The data sources and the methodology for exposure data management  Exposure data development for demographic data, general building stocks, essential facilities, transportation systems and discusses their valuation. Hazard Report:

Chapter 1: The present chapter provides the reader with an overview of the project, its scope and its objectives. It also provides the reader with an understanding of key concepts related to the project. Chapter 2: The chapter provides details of the Flood Hazard Assessment, including the following:  Describes description of the data availability for flood assessment  Provides initial results of the hydrological model, development, calibration, and validation  Provides initial results of the hydraulic model, development, calibration, and validation  Flood Hazard Mapping for the probabilistic events of 2, 5, 10, 25, 50, and 100 years.

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2 Flood Hazard Assessment

2.1 Methodology Overview Flood hazard assessment identifies and demarcates those parts of the study area, which are exposed to floods. It provides information on the extent and depth of flooding throughout flood prone areas for a range of flood magnitudes. The flood hazard model development framework adopted for this study is given in Figure 2-1, which comprises of the following: Collection and compilation of relevant hydro meteorological and biophysical data. These data include terrain, soil, land use land cover, runoff/river discharge, and flood protection measures to form the input for the model. Probabilistic analysis of runoff to simulate various return period events (2, 5, 10, 25, 50, and 100 years) Hydraulic modeling to estimate flood levels throughout the basins for various flows generated for key return period events. Flood hazard mapping to show flood extent and flood depth for different return periods.

Figure 2-1: Flood hazard assessment framework

Hazard Report Confidential Page 18 of 86 The World Bank Exposure and Hazard Flood Risk Assessment for the Ganges Basin in South Asia 2.2 Data Availability This section describes the availability status of meteorological, hydrological, topographical, soil, LULC and other data required as inputs at the various steps of modeling. 2.2.1 METEOROLOGICAL DATA Daily rainfall data were obtained from IMD and DHM Nepal. The following were available: Half degree gridded rainfall data from 1971 to 2005 from IMD, India. Daily station rainfall from 1960 to 2014 from DHM Nepal Half-degree gridded rainfall data from IMD and daily rainfall data from DHM Nepal have been used for the hydrological modeling of the basin. The locations and the spatial distribution of the rain gauges are shown in Figure 2-2. Table 2-1 shows the list of the rainfall stations along with the duration of the data for the respective stations used in the study.

Table 2-1: Station wise rainfall data records

Sr. Station ID/Name Country Duration Source No.

India Half Degree Gridded Data (283 India and 1 1971-2005 IMD, India Stations) Bangladesh Nepal 2 206- Asara Ghat Nepal 1963-2013 DHM, Nepal 3 218- Dipayal (Doti) Nepal 1982-2013 DHM, Nepal 4 311- Simikot Nepal 1978-2006 DHM, Nepal 5 403- Jamu (Tikuwa Kuna) Nepal 1963-2013 DHM, Nepal 6 606- Tatopani Nepal 1969-2013 DHM, Nepal 7 927- Bharatpur Nepal 2001-2014 DHM, Nepal 8 1002- Aru Ghat D. Bazar Nepal 1960-2013 DHM, Nepal 9 1004- Nuwakot Nepal 1960-2013 DHM, Nepal 10 1115- Nepalthok Nepal 1960-2013 DHM, Nepal 11 1210- Kurule Ghat Nepal 1960-2013 DHM, Nepal 12 1316- Chatara Nepal 1960-2013 DHM, Nepal 13 1317- Chepuwa Nepal 1960-2013 DHM, Nepal

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Figure 2-2: Location of rainfall stations used in the study

2.2.2 HYDROLOGICAL DATA The flow data was available for limited duration/dates only and most of the data was re- analyzed flow data, which had some quality issues. The location and the spatial distribution of the flow gauges are shown in Figure 2-3. Table 2-2 shows the details of the stations along with the duration of available flow data. In Table 2-2, eleven stations are in India and five are in Nepal. Stations in India include one CWC gauge station at Agra and ten stations from Dartmouth Flood Observatory (DFO). For Nepal, the data for all five stations was available from (Department of Hydrology and Meteorology) DHM Nepal.

Table 2-2: Availability of Discharge Data

Sr. No. Discharge Data Country Duration Source 1 Agra India 1950-2009 CWC, India 2 200- Baehraich India 1998-2014 DFO 3 199- Rudauli-Faizabad India 1998-2014 DFO 4 203- Gosaniganj India 1998-2014 DFO 5 198- Bettiah India 1998-2014 DFO 6 205- Gopiganj India 1998-2014 DFO 7 2015- Dehri India 1998-2014 DFO 8 195- Begusarai India 1998-2014 DFO

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Sr. No. Discharge Data Country Duration Source 9 52- Saharsa India 1998-2014 DFO 10 51- English Bajar India 1998-2014 DFO 11 193- Jangipur India 1998-2014 DFO 12 420- Kotagaon Shringe Nepal 2002-2006 DHM, Nepal 13 449.91- Kali Khola Nepal 2002-2006 DHM, Nepal 14 695- Chatara-Kothu Nepal 2002-2007 DHM, Nepal 15 280- Chispani Nepal 2002-2006 DHM, Nepal 16 450- Narayan Ghat Nepal 2002-2006 DHM, Nepal

*DFO-Dartmouth Flood Observatory, *DHM -Department of Hydrology and Meteorology, Nepal Additional data was acquired during this project to improve the quality of calibration and validation. This is provided in Table 2-3. The average duration of the flow data for these stations is from 1975 to 2005.

Table 2-3: Additional flow gauges used in study

Sr. No. Names Latitude Longitude 1 Delhi Rly Bridge 28.51 77.42 2 Rudraprayag 30.30 79.02 3 Ankinghat 26.74 80.13 4 Tonk 26.18 75.73 5 Bigod 25.20 74.95 6 Baranwada 25.87 76.62 7 Etawah 26.61 79.09 8 Chillaghat 25.76 80.51 9 Paliakalan 28.25 80.71 10 Ayodhya 26.81 82.23 11 Mejja Road 25.27 82.09 12 Japla 24.52 83.87 13 Varanasi 25.49 83.16 14 Koelwar 25.70 84.91 15 Hathidah 25.32 86.06 16 Lalbegia ghat 26.50 85.03 17 Sahibganj 25.10 87.82 18 Samaijighat 28.52 81.66 19 Jamu 28.76 81.35 20 Chisapani 28.64 81.29 21 Jalkundi 27.95 82.23 22 Borlangpul 27.97 83.57 23 Kotagaun 27.75 84.35 24 Kampughat 26.87 86.82

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Figure 2-3: Flow gauge station locations

2.2.3 TOPOGRAPHICAL INFORMATION Topographical data is required for the delineation of catchment areas and for generating the river network. The same topographical data is required to estimate the elevation information for cross-sections in hydraulic modeling. SRTM DEM having 90-m resolution was used for this purpose. Figure 2-4 shows the elevation map of the study area.

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Figure 2-4: Elevation map of the study area

The Ganges basin lies between elevations of 0 to 8,800 m. Most of the Gangetic plains lie at an elevation of less than 400 m with some southern and southwestern parts having an elevation range of 400 m to 800 m. Uttarakhand in India and Nepal lie at an elevation of more than 1,000 m. 2.2.4 LAND USE LAND COVER (LULC) Land-use land-cover (LULC) classes have been identified from the Food and Agriculture Organization’s (FAO) site. According to this data, approximately 7% of the study area is forest, 24% area is open/grassland, and the remaining 62% area is used as cropland. Figure 2-5 shows the LULC map of Ganges River basin.

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Figure 2-5: LULC map of Ganges River basin

2.2.5 SOIL MAP Soil data is an essential parameter used to estimate the hydrological response characteristics of a river basin. The project team reviewed available data sets from project documents and in-house datasets. Since a single soil database is not available for the entire basin, soil data included in the Food and Agriculture Organization’s (FAO) Harmonized World Soil Database was used. This soil data is provided as a set of land units, each with a unique ID number. This unique ID number is used to match the textural properties and other parameters of soils. Based on the soil textural class, a hydrological soil group was assigned to each land unit within the basin. The soil map for the study area is shown in Figure 2-6. The soils are mainly clay (53%) and loamy sand (23%).

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Figure 2-6: Soil map for the study area

2.2.6 FLOOD HISTORY The RMSI team has obtained some information regarding historical flooding in the form of reports, maps, and photographs. Detailed information for the whole basin for a single event is sparse but historical loss information for major flood events that occurred during 1979- 2006 has been obtained from various publicly available sources at the state level. This loss data, available for area and production losses in agriculture, population, and houses in Bihar is provided in Table 2-4. Similarly, detailed damage information caused by the flood event of 1973 for the state of Uttar Pradesh has been obtained and the same has been reproduced in Table 2-5.

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Table 2-4: Flood loss summary: Bihar, 1979 to 2006

Total Public Area Cropped Crop Human House Property Year District Blocks Panchayat Village (in area (in Damaged (in (in Lakh) Affected Damaged (in Lakh Lakh ha) Lakh INR) Lakh INR) ha)

2006 14 63 375 959 10.89 1.81 0.87 706.63 18,637 8,456.17 2005 12 81 562 1,464 21.04 4.6 1.35 1,164.50 5,538 305.00 2004 20 211 2,788 9,346 212.99 27 13.99 52,205.64 9,29,773 1,03,049.60 2003 24 172 1,496 5,077 76.02 15.08 6.10 6,266.13 45,262 1,035.16 2002 25 6 2,504 8,318 160.18 19.69 9.40 51,149.61 419,014 40,892.19 2001 22 194 1,992 6,405 90.91 11.95 6.50 26,721.79 222,074 18,353.78 2000 33 213 2,327 12,351 90.18 8.05 4.43 8,303.70 343,091 3,780.66 1999 24 150 1,604 5,057 65.66 8.45 3.04 24,203.88 91,813 5,409.99 1998 28 260 2,739 8,347 134.70 25.12 12.84 36,696.68 199,611 9,284.04 1997 26 169 1,902 7,043 69.65 14.71 6.55 5,737.66 174,379 2,038.09 1996 29 195 2,049 6,417 67.33 11.89 7.34 7,169.29 116,194 1,035.70 1995 26 177 1,901 8,233 66.29 9.26 4.24 19,514.32 297,765 2,183.57 1994 21 112 1,045 2,755 40.12 6.32 3.50 5,616.33 33,876 151.66 1993 18 124 1,263 3,422 53.52 15.64 11.35 13,950.17 219,826 3,040.86 1992 8 19 170 414 5.56 0.76 0.25 58.09 1,281 0.75 1991 24 137 1,336 4,096 48.23 9.8 4.05 2,361.03 27,324 139.93 1990 24 162 1,259 4,178 39.57 8.73 3.21 1,818.88 11,009 182.27 1989 16 74 652 1,821 18.79 4.71 1.65 704.88 7,746 83.7 1988 23 181 1,616 5,687 62.34 10.52 3.95 4,986.32 14,759 150.64 1987 30 382 6,112 24,518 286.62 47.5 25.7 67,881.00 1,704,999 680.86 1986 23 189 1,828 6,509 75.80 19.18 7.97 10,513.51 136,774 3,201.99 1985 20 162 1,245 5,315 53.09 7.94 4.38 3,129.52 103,279 204.64

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Total Public Area Cropped Crop Human House Property Year District Blocks Panchayat Village (in area (in Damaged (in (in Lakh) Affected Damaged (in Lakh Lakh ha) Lakh INR) Lakh INR) ha)

1984 23 239 3,209 11,154 135.00 30.50 15.87 18,543.85 310,405 2,717.72 1983 22 138 1,224 4,060 42.41 18.13 5.78 2,629.25 38,679 258.14 1982 15 110 1,112 3,708 46.81 9.32 3.23 9,700.00 68,242 955.33 1981 21 201 2,138 7,367 69.47 12.61 7.71 7,213.19 75,776 1980 21 193 1,869 7,010 74.45 17.86 9.43 7,608.43 118,507 1979 13 110 37.38 8.06 2.74 1,901.52 27,816

Table 2-5: Flood loss summary: Uttar Pradesh, 1973 flood

Number of Total Area Number of Total Area (in Sr. Affected Total Affected Human Area Name Affected Affected Houses Lakh No. Population Crop Area (ha.) loss villages (ha.) Damaged Hectare) 1 Muzaffarnagar - 8,913 54,543 92,113 2,784 - 10.76 2 Meerut 158 45,039 99,764 42,443 1,053 2 14.87 3 Buland Shahar 64 22,857 21,628 8,298 40 - 12.08 4 Aligarh 8 140 170 166 25 - - 5 Mathura 110 39,093 23,883 14,332 668 - 9.38 6 Agra 35 2,714 6,000 741 13 - 11.90 7 Etah 127 96,191 91,207 38,445 523 - 10.97 8 Farrukhabad 407 116,392 206,338 71,205 3,625 4 11.52 9 Etawah 116 9,992 26,728 12,057 - - 10.07 10 Bijnor 444 119,641 151,461 61,509 7,463 13 11.54 11 Bareilly 934 231,103 175,880 84,999 909 - 10.17 12 Pilibhit 126 6,065 128,000 34,408 39 - 8.64 13 Shahjahanpur 672 270,106 122,864 78,666 2,164 2 11.28 14 Muradabad 735 201,758 334,032 174,444 19,069 1 14.62

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Number of Total Area Number of Total Area (in Sr. Affected Total Affected Human Area Name Affected Affected Houses Lakh No. Population Crop Area (ha.) loss villages (ha.) Damaged Hectare) 15 Badaun 685 274,892 361,646 147,981 3,681 2 12.78 16 Rampur 561 238,165 162,414 144,836 7,350 - 5.73 17 Varanasi 358 48,147 26,593 14,218 16 - 12.57 18 Mirjapur 344 138,225 121,800 38,540 73 - 27.96 19 Jaunpur 495 194,952 125,000 41,155 713 4 8.87 20 Gazipur 534 312,733 125,620 71,971 15 2 8.35 21 Balia 1,564 695,577 304,232 182,463 17,226 12 7.56 22 Gorakhpur 1,478 750,483 392,171 222,256 29,910 17 15.56 23 Deoria 3,509 2,600,000 919,097 722,470 50,882 35 13.35 24 Basti 3,832 970,016 498,628 403,707 23,787 8 18.05 25 Azamgarh 1,043 675,753 228,565 409,530 2,103 14 14.21 26 Gonda - - 28,700 98,693 1,200 - 18.10 27 Behraich 242 50,000 63,000 54,208 489 - 16.76 28 Faizabad 721 148,400 23,360 64,122 825 15 10.90 29 Sultanpur 213 174,995 26,284 26,284 175 - 10.96 30 Prataphgarh 980 712,084 228,658 186,536 288 - 9.33 31 Allahabad 537 205,922 152,391 58,197 2,981 2 17.92 32 Kanpur 157 13,481 26,958 7,289 390 - 15.08 33 Fatehpur 124 25,351 33,858 82,224 15 - 10.39 34 Kheri 137 36,605 33,207 26,860 255 4 19.02 35 Sitapur 188 105,000 50,000 13,000 - -1 14.31 36 Hardoi 448 290,398 242,755 96,212 2,477 2 14.80 37 Lucknow 70 30,257 5,540 3,212 130 1 6.20 38 Unnao 476 350,000 73,612 73,612 2,595 1 11.35 39 Raibariely 1,283 874,099 831,593 270,988 7,198 6 11.25 40 Barabanki 242 92,823 101,291 52,242 1,600 1 11.96 41 Jalaun 83 23,691 44,892 20,123 - - 11.28

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Number of Total Area Number of Total Area (in Sr. Affected Total Affected Human Area Name Affected Affected Houses Lakh No. Population Crop Area (ha.) loss villages (ha.) Damaged Hectare) 42 Hamirpur 112 23,537 71,053 38,340 100 - 17.76 43 Banda 265 118,230 190,103 77,110 465 - 18.86

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RMSI has also obtained historical flood footprint maps for various flood events that have occurred at various places from year 2001 to 2008. These have been collected from various public sources such as DFO. These maps have been processed, geo-referenced, and digitized to validate the hydraulic model output. These flood footprints will also be used in the proposed Web Risk Atlas to assess the historically damaged areas of the basin. One such flood footprint of the 2007 flood event has been shown in Figure 2-7.The reported damages due to this event in Bihar have also been obtained and are shown in the Table 2-6.

Figure 2-7: Flood Footprint: 2007 flood

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Table 2-6: Flood loss summary: Bihar, 2007 flood

Estimated Public No. of Cropped Estd. Crop No. of house Lives lost Sr. Value of house Properties District Blocks Affected Area (Lakh damage (Rs. damaged (Nos) No. damage In (Rs. damage (Rs. village ha.) Lac) fully/partially Human Lac) Lac) 1 Muzaffarpur 15 1,704 1.24 12,663.00 65,550 11,073.00 24,951.00 104 2 Sitamarhi 17 806 0.51 7,803.94 103,193 16,084.85 63,618.25 33 3 Saharsa 6 184 0.26 985.82 16,412 935.25 140.38 35 4 E.Champaran 27 1,159 1.58 15,400.00 52,840 8,278.31 96 5 Supaul 6 94 0.25 574.84 15,000 300.00 17.75 1 6 Darbhanga 18 2,104 1.75 6,606.10 83,127 13,106.14 18,271.02 140 7 Madhubani 20 836 1.39 7,936.23 96,362 9,116.45 25,733.68 49 8 Samastipur 19 842 1.25 16,710.07 29,391 775.00 17,896.46 157 9 Sheohar 5 150 0.25 693.00 50,728 6,477.10 105.00 4 10 Khagaria 7 203 0.5 8,507.33 32,500 8,507.33 372.00 101 11 Begusarai 11 346 0.94 16,057.84 40,740 11,773.10 444.00 54 Total 151 8,428 9.92 93,938.17 585,843 86,426.53 151,549.54 774

Hazard Report Confidential Page 31 of 86 The World Bank Exposure and Hazard Flood Risk Assessment for the Ganges Basin in South Asia 2.3 Hydrological Modeling A hydrological analysis was conducted for the study area to identify the basin and sub-basin characteristics and determine the flow in each sub-basin for given rainfall. A hydrological model establishes the flow behavior of the watershed or basin by converting the rainfall into runoff. They often represent the spatial variability of the atmosphere and land surface characteristics that control the rainfall-runoff process. In usual hydrologic practices, hydrological models are regularly applied worldwide. The hydrological model, prior to being employed into flow simulation, needs to be developed, calibrated, and validated for the study area. Hydrological response to the flood events is simulated using the Hydrologic Engineering Center’s Hydrological Modeling System (HEC-HMS) model. The HEC-HMS model, an open source software, is developed by the US Army Corps of Engineers. This model is widely used for hydrologic modeling and is publicly available from the USACE. The HEC-HMS model is a generalized modeling system capable of representing many different watersheds. HEC-HMS is designed to simulate the precipitation-runoff processes of dendritic watershed systems (USACE. 2009) It is applicable across a wide range of geographic areas for addressing a variety of project goals. Applications include large river basin water supply and flood hydrology, as well as supporting small urban or natural watershed runoff modeling. To apply the model for a specific purpose and location, a model of the watershed is constructed by dividing the hydrologic cycle into manageable pieces, by constructing boundaries around the watershed of interest, and establishing appropriate geographic and other parameters in the model. The model provides a completely integrated work environment, including a database, data entry utilities, computation engine, and reporting tools, with a graphical user interface. Additional information on the model is available at: http://www.hec.usace.army.mil/software/hec-hms/ The core elements of the HEC-HMS model are the basin model, meteorological model, control specifications, and time-series data manager. To develop the model for a particular use and location, the following steps are generally implemented: a. Basin Delineation (using HEC–GeoHMS) b. Model Development i. Creation of Basin Model (including all elements such as sub-basins, channels and reservoirs) and estimation of Physical Loss, Routing and Transformation Parameters (for each sub-basin element) ii. Addition of Time-Series Data (for various meteorological parameters) and Setting Control Specifications (for running the model) c. Calibration and Validation d. Return Period Flows Estimation HEC-HMS allows the user to select from a number of methods to represent catchment characteristics for Rainfall Loss and Infiltration, Rainfall-Runoff Transformation, Stream Flow Routing, Base flow Methods and input of meteorological data (USACE, 2009). The flowchart in Figure 2-8 explains the step-by-step approach adopted for the hydrological modeling of Ganges River Basin.

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Figure 2-8: Flowchart for hydrological modeling

2.3.1 BASIN DELINEATION USING HEC GEO-HMS A number of hydrologic modeling inputs were developed using the Geospatial Hydrologic Modeling Extension (HEC- GeoHMS) tool (USACE 2009). HEC-GeoHMS works within a Geographic Information System (GIS) interface. HEC- GeoHMS transforms digital terrain information like drainage paths and watershed boundaries into a hydrologic data structure that represents the watershed response to precipitation. The following sections detail the steps taken to develop the HEC-HMS model. Using HEC Geo-HMS, the river network and sub basins were delineated using a systematic approach. The approach creates raster grids for catchment delineation using DEM as input. Activities to complete the model include filling sinks, creating flow directions and flow accumulation grids, processing catchment grid, and processing drainage line. Filling sinks is the process of numerically correcting the DEM, where large sinks (abnormal depressions) or voids are present. Physical representation of the basin incorporates various hydrologic elements (sub basins, river reaches, junctions, and reservoirs), which are connected in a dendritic network to simulate the rainfall-runoff process. Figure 2-9 shows the map of delineated sub basins for the Ganges River basin. Various small catchments were merged together based on the major river they are contributing to. A major sub basin file was created to calibrate and validate the model and aggregate exposure, hazard, and damage. The list of major sub basins is provided in Table 2-7.

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Table 2-7: Major sub basins considered in study

Name of Basin Names of Major Sub basins Bagmati Betwa Chambal Gandak Ghagra Gomti Kamla-Balan Ken Kosi Ganges River Basin Lower Ganges Mahananda Middle Ganges Ramganga Sind Sone Tons Upper Ganges Yamuna

Figure 2-9: Delineated catchments and major sub basins of Ganges River Basin

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2.3.2 MODEL DEVELOPMENT The river profile was checked and used to merge several sub basins at locations with smooth slopes. Physical characteristics of streams and sub-basins were extracted by calculating River Length, River Slope, Basin Slope, Longest Flow Path, Basin Centroid, and Centroidal Flow Path. Hydrologic parameters such as SCS (Soil Conservation Service) curve number (CN), time of concentration (Tc), channel routing coefficients, etc. were developed based on the physical properties of the sub-basins. Hydrologic elements such as node, links, and junctions were created using HEC-GeoHMS to define the hydrologic model that was exported into HEC-HMS. The basin model comprises of hydrologic elements and their connectivity to characterize the movement of water through the drainage system. For each sub-basin, the overland flow length (L), average basin slope (S), and overland flow Manning’s roughness value (n) were determined. This information was used to calculate the initial Tc for each sub-basin, based on the TR-55 methodology developed by the United States Department of Agriculture (USDA). 2.3.2.1 Rainfall Loss and Infiltration: SCS Curve Number The Curve Number (CN) method of the U.S. Dept. of Agriculture, Natural Resources Conservation Service (NRCS) (formerly Soil Conservation Service, SCS) known as SCS CN was used to predict the runoff properties of the surface based on the hydrologic soil group and ground cover (US SCS, 1986). The SCS curve numbers were assigned based on the USDA soil classification and land use data for the basin. The soil and land use information was merged using Arc GIS Spatial Analyst and HEC-GeoHMS. A Curve Number (CN) grid that includes soil and land use information for the basin was developed. The CN is an index that combines hydrologic soil group and land use factors to estimate the amount of rainfall that becomes runoff. Table 2-8 shows the CN used for the present analysis.

Table 2-8: Curve numbers used for anal ysis

Available Land use class USDA classification A B C D

Water Water 100 100 100 100 Settlement Medium Residential 57 72 81 86 Coconut Forest Forest 30 58 71 78 Secondary Forest Banana Cassava Coconut Crops Cultivated Land Agricultural 67 77 83 87 Open Land / Grassland Rice Unknown Crops Hydrologic Soil Group A is sand, loamy sand or sandy loam types of soils. It has low runoff potential and high infiltration rates even when thoroughly wetted. It consists chiefly of deep, well to excessively drained sands or gravels and has a high rate of water transmission.

Hazard Report Confidential Page 35 of 86 The World Bank Exposure and Hazard Flood Risk Assessment for the Ganges Basin in South Asia Group B is silt loam or loam. It has a moderate infiltration rate when thoroughly wetted and consists chiefly or moderately deep to deep, moderately well to well drained soils with moderately fine to moderately coarse textures. Group C soils are sandy clay loam. It has low infiltration rates when thoroughly wetted and consist chiefly of soils with a layer that impedes downward movement of water and soils with moderately fine to fine structure. Group D soils are clay loam, silty clay loam, sandy clay, silty clay or clay. This HSG has the highest runoff potential. They have very low infiltration rates when thoroughly wetted and consist chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a claypan or clay layer at or near the surface and shallow soils over nearly impervious material. (http://directives.sc.egov.usda.gov/OpenNonWebContent.aspx?content=17757.wba) 2.3.2.2 Rainfall-Runoff Transformation: SCS Unit Hydrograph The transformation of excess precipitation into surface run-off is accomplished using the U.S. Soil Conservation Service (SCS) Unit Hydrograph Transform Method. This method requires lag time as an input. The lag time, was taken to be 60% of the time of concentration (USACE, 2005). The time of concentration represents the time required for a drop of water to travel from the most hydrologically remote point in the sub catchment to the outlet. The time of concentration (Tc) is calculated using the Kirpich formula (Kirpich, 1940), which requires the maximum length of the flow path and the average slope of the watershed as inputs: Tc (min) = 0.0078 L0.77 S-0.385 ------(1) Where L = flowpath length (m) and S = average slope (m/m). 2.3.2.3 Stream Flow Routing: Muskingum Method The Muskingum Method (McCarthy, 1938) has been widely adopted for hydrologic flow routing and was used for flow routing through the streams. This method uses a simple conservation of mass approach to route flow through each stream. This method represents a river reach as a linear time-invariant system with its inflow I, outflow Q, and storage w, related as: w = K [ X I + ( I – X ) Q ] ------(2) Where K and X are parameters. The Muskingum parameters K and X were calculated based on stream length and upstream and downstream elevations for routing of flows in streams. Upstream and downstream elevations and stream lengths were based on the SRTM data. These parameters were later refined and adjusted during the calibration of the model. The Ganges River Basin hydrologic model includes more than 400 sub basins and the same number of reaches. The model’s delineated area for the Ganges Basin is approximately 950,000 km2. The HEC-HMS model setup for the whole Ganges basin is shown in Figure 2-10.

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Figure 2-10: HEC-HMS model setup for Ganges Basin 2.3.2.4 Meteorological Model: Gauge Weights The Gauge Weight Method (also known as Thiessen Polygon Method) has been used for the areal distribution of rainfall from rain gauge stations across a range of different sub basins. Thiessen Polygons are straight-edged areas whose boundaries define the area that is closest to a specified point (in this case, a rain gauge) relative to all other points (other rain gauges). This polygon method subdivides a drainage basin into multiple polygons, each containing a rain gauge. First, the rain gauges are plotted on a base map. These rain gauge points are then connected by drawing straight lines between them. The lines are bisected by perpendiculars, which meet to form the polygons. The areas of the polygons are then calculated and expressed as fractions or weights of the total area of each sub basin. The daily level precipitation data for each station is given in the time series data, which is stored in DSS file format. A weighted approach has been used to calculate the rainfall in a basin. This is achieved by multiplying each fraction of the area by the precipitation recorded by the rain gauge in that polygon and then summing up the weighted precipitations to represent the total precipitation over the sub basin (catchment area). 2.3.3 MODEL CALIBRATION In the HEC-HMS model calibration phase, simulations were carried out for different historical events for various sub basins at the places for which both rainfall and stream flow records were available. It is to be emphasized here that the Ganges Basin has a vast drainage area and it is not necessary that a single event will cover the whole basin at the same time. Thus, different events have been taken for different sub basins. To calibrate the model, historical and modeled flows were compared. Model parameters, as those discussed

Hazard Report Confidential Page 37 of 86 The World Bank Exposure and Hazard Flood Risk Assessment for the Ganges Basin in South Asia above, were reviewed to obtain reasonable agreement between the observed and modeled flow hydrographs. In flood risk studies, an emphasis is placed on emulating the peak flow of water contained in the hydrograph. Ideally, a number of flood events can be fitted adequately with only small parameter variations. The calibration process is usually manual, using engineering judgment to iteratively adjust hydrologic parameters and evaluate the fit between the computed and observed hydrographs. The calibration process of the model was initiated at the most upstream flow gauge. It next considered the flow gauges in the center of the basin and finally at the end, the most downstream flow gauges. Various sub basin parameters such as lag time, initial abstraction, and SCS curve number were adjusted to improve the match between observed and modeled flows during the calibration process. The streams, parameters such as K and X were also adjusted to match observed and modeled flows. As discussed in the previous section (Section 2.2.2), stream flow data for a number of discharge gauges was collected. Historical stream flow data of some of these gauges were used to calibrate the model while the rest of the data was used to validate the model. Out of the all the discharge gauges stations, 14 gauges were used to calibrate the hydrological model for various sub-basins of the Ganges Basin. The remaining discharge gauges could not be used due to data quality issues. Any unusual or very high value, which was not consistent with the long-term average of a discharge gauge from reported values in literature or in the public domain, were also discarded while carrying out the calibration and validation process. These discharge gauges cover a significant length of the river network of the basin. The downstream area of main Ganges River is more prone to frequent floods and is also adequately covered by these discharge gauges. Therefore, a higher number of discharge gauges have been taken as we move downstream on the river to have better calibration results. Table 2-9 shows the list of discharge gauges used in the various sub basins along with the simulation durations of the flow data used in the model calibration. Figure 2-11 shows the locations of these discharge gauges used for the model calibration at various sub-basins.

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Table 2-9: Details of the gauges, flow data and events used for calibration simulation of hydrological model at various sub basins

Event/Duration Sr. No. Sub Basin Discharge Gauge Simulation 1 Simulation 2 1 Upper Ganges Rudraprayag 01 Jun 1996 - 01 Oct 1996 01 Apr 1999 - 01 Dec 1999 2 Chisapani 01 Jun 1992 - 01 Feb 1993 01 Jun 1994 - 01 Oct 1994 3 Ghagra Rudauli-Faizabad 01 Jun 2000 - 31 Dec 2000 01 Jan 2003 - 31 Dec 2003 4 Gosaniganj 15 Jun 2000 - 31 Oct 2000 15 Apr 2003 - 31 Dec 2003 5 Kampughat 01 Jan 1990 - 31 Dec 1990 01 Jan 2000 - 31 Dec 2000 Kosi 6 Saharsa 20 Jun 2000 - 31 Oct 2000 01 Jun 2003 - 31 Oct 2003 7 Chambal Baranwada 01 Jan 1992 - 31 Dec 1994 01 Jan 1996 - 31 Dec 1996 8 Yamuna Agra 01 Jan 1995 - 31 Dec 1995 01 Jan 1998 - 31 Dec 1998 9 Betwa and Ken Chillaghat 01 Jun 1990 - 31 Dec 1995 01 Jun 1996 - 31 Dec 1999 10 Ramganga and Middle Ankinghat 01 Jan 1990 - 31 Dec 1995 01 Jan 1996 - 31 Dec 2000 11 Ganges Varanasi 01 Dec 1992 - 31 Dec 1994 01 Jan 1996 - 31 Dec 1999 12 Sone Dehri 01 Jun 1998 - 31 Oct 1998 01 Jan 2001 - 31 Dec 2001 13 Sahibganj 01 May 1992 - 31 Dec 1992 15 Apr 2002 - 31 Dec 2002 Lower Ganges 14 Jangipur 01 Jan 1998 - 21 Dec 1998 01 Jun 1999 - 31 Dec 1999

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Figure 2-11: Location of the discharge gauges used to calibrate the model at sub - basin level

As described in the Table 2-7, the entire Ganges Basin has been divided into 18 sub-basins taking into consideration the major river segments and all nearby tributaries contributing into each of those sub-basins. The RMSI team has tried to calibrate the hydrological model at the various discharge gauges discussed above, lying in the sub-basin and/or at a junction of one or more sub-basins to get better calibration results. The detailed calibration results for various sub-basins are described in the following sections.

2.3.3.1 Upper Ganges The Upper Ganges sub basin of Ganges Basin has a drainage area of around 1,00,000 sq. km. This sub basin comprises of the main Ganges River along with smaller tributaries joining it. The river originates from the mountainous region of Gangotri in Uttarakhand and after covering some part of the state of Uttar Pradesh, drains out near the border of Kannauj and Hardoi districts where it meets with the River Ramganga. The Hydrological Model calibration for the Upper Ganges sub-basin has been carried out at Rudraprayag discharge gauge for continuous simulation of two historical events, namely, 01 Jun 1996 to 01 Oct 1996 and 01 Apr 1999 to 01 Dec 1999. Rainfall and stream flow records were available for both the events. Figure 2-12 shows the Upper Ganges sub basin and its tributaries along with the location of the discharge gauge used for calibration.

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Figure 2-12: Location of the discharge gauge used in calibration for Upper Ganges sub basin

Comparison between observed and simulated hydrographs for flow at Rudraprayag discharge gauge station of Upper Ganges sub basin for continuous simulation for the two historical events are shown in Figure 2-13. The duration of calibration simulation, comparison between observed and simulated peak flows and year of occurrence of the peak flow is given in Table 2-10. From the plots, it can be observed that the simulated flows are in close agreement with observed flows. The results show that the hydrological model of Upper Ganges sub basin is adequately calibrated.

Table 2-10: Comparison of simulated and observed flows (cumec) for discharge gauge station of Upper Ganges sub basin for various calibration events

Sub Basin: Upper Ganges

Peak Flow Discharge Gauge Station Simulation Event/Duration Year Simulated Observed

01 Jun 1996 - 01 Oct 1996 1996 997 1022 Rudraprayag 01 Apr 1999 - 01 Dec 1999 1999 1684 1784

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Figure 2-13: Comparison of observed and simulated hydrographs at discharge gauges of Rudraprayag in Upper Ganges sub basin

2.3.3.2 Ghagra The Ghagra sub basin of Ganges Basin has a drainage area of around 1,32,000 sq. km. This sub basin comprises of the Ghagra River along with small tributaries joining it. The river originates from the mountainous region of Nepal and after covering the Far Western and Mid Western Provinces of Nepal and some parts of Uttarakhand and Uttar Pradesh States in India drains out in district Saran where it meets with the main Ganges River. The calibration of the Hydrological Model for Ghagra sub basin has been carried out at Chisapani discharge gauge in Nepal for continuous simulation of two historical events, namely, 01 Jun 1992 to 01 Feb 1993 and 01 Jun 1994 to 01 Oct 1994. Similarly, calibration has been done at Rudauli- Faizabad and Gosaniganj discharge gauges in India for continuous simulation for two historical events each, namely, 01 Jun 2000 to 31 Dec 2000 and 01 Jan 2003 to 31 Dec 2003, and 15 Jun 2000 to 31 Oct 2000, 15 Apr 2003 to 31 Dec 2003 respectively for which both rainfall and stream flow records were available. Figure 2-14 shows the Ghagra sub basin and its tributaries along with the locations of discharge gauges used for calibration.

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Figure 2-14: Location s of the discharge gauges used in calibration of Ghagra sub basin

The hydrological model for the sub basin has been calibrated starting from the upstream gauge of Chisapani, then at the Rudauli-Faizabad gauge and then at the most downstream gauge of Gosaniganj. Comparison between observed and simulated hydrographs for flow at these three discharge gauge stations for continuous simulation for two historical events each are shown in Figure 2-15 to Figure 2-17 respectively. The duration of calibration simulation, comparison between observed and simulated peak flows, and year of occurrence of peak flows is given in Table 2-11. From the plots, it can be observed that the simulated flows are in close agreement with observed flows. The results show that the hydrological model of the Ghagra sub basin is adequately calibrated.

Table 2-11: Comparison of simulated and observed flows (cumec) for discharge gauge stations of Ghagra sub basin for various calibration events

Sub Basin: Ghagra Peak Flow Discharge Gauge Station Simulation Event/Duration Year Simulated Observed

01 Jun 1992 - 01 Feb 1993 1992 3172 3487 Chisapani 01 Jun 1994 - 01 Oct 1994 1994 3165 3466 01 Jun 2000 - 31 Dec 2000 2000 9043 9078 Rudauli-Faizabad 01 Jan 2003 - 31 Dec 2003 2003 7360 8230 15 Jun 2000 - 31 Oct 2000 2000 8441 8587 Gosaniganj 15 Apr 2003 - 31 Dec 2003 2003 7038 6510

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Figure 2-15: Comparison of observed and simulated hydrographs at the discharge gauge of Chispani in Ghagra sub basin

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Figure 2-16: Comparison of observed and simulated hydrographs at the discharge gauge of Rudauli-Faizabad in Ghagra sub basin

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Figure 2-17: Comparison of observed and simulated hydrographs at the discharge gauge of Gosaniganj in Ghagra sub basin

2.3.3.3 Kosi The Kosi sub basin of Ganges Basin has a drainage area of around 70,000 sq. km. This sub basin comprises of the Kosi River along with small tributaries joining it. The river originates from the Himalayan region of Tibet in China and after covering the areas of Central and Eastern Provinces of Nepal and some parts of Bihar State in India, drains out in district Katihar where it meets with the main Ganges River. The calibration of the Hydrological Model for Kosi sub basin has been carried out at Kampughat discharge gauge in Nepal for continuous simulation of two historical events, namely, 01 Jan 1990 to 31 Dec 1990 and 01 Jan 2000 to 31 Dec 2000; and Saharsa discharge gauge in India for continuous simulation of two historical events, namely, 20 Jun 2000 to 31 Oct 2000 and 01 Jun 2003 to 31 Oct 2003 as rainfall and stream flow records were available for both sets of events. Figure 2-18 shows the Kosi sub basin and its tributaries along with the location of discharge gauges used for calibration.

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Figure 2-18: Location s of discharge gauges used in calibration for Kosi sub basin

The hydrological model for the sub basin has been calibrated starting from the upstream gauge of Kampughat and then at the downstream gauge of Saharsa. Comparison between observed and simulated hydrographs for flows at these two discharge gauge stations for the two sets of historical events are shown in Figure 2-19 and Figure 2-20. The duration of calibration simulation, comparison between observed and simulated peak flows, and year of occurrence of the peak flows is given in Table 2-12. From the plots, it can be observed that the simulated flows are in close agreement with the observed flows. The results show that the hydrological model of Kosi sub basin is adequately calibrated.

Table 2-12: Comparison of simulated and observed flows (cumec) for discharge gauge stations of Kosi sub basin for various calibration events

Sub Basin: Kosi Peak Flow Discharge Gauge Station Simulation Event/Duration Year Simulated Observed 01 Jan 1990 - 31 Dec 1990 1990 2893 2807 Kampughat 01 Jan 2000 - 31 Dec 2000 2000 3568 3550 20 Jun 2000 - 31 Oct 2000 2000 6305 6968 Saharsa 01 Jun 2003 - 31 Oct 2003 2003 6052 6405

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Figure 2-19: Comparison of observed and simulated hydrographs at the discharge gauge of Kampughat in Kosi sub basin

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Figure 2-20: Comparison of observed and simulated hydrographs at the discharge gauge of Saharsa in Kosi sub basin

2.3.3.4 Chambal The Chambal sub basin of the Ganges Basin has a drainage area of around 1,43,000 sq. km. This sub basin comprises of the Chambal River along with other tributaries like Banas, etc. joining it along its course. The river flows north-northeast through Madhya Pradesh, running for a time through Rajasthan, then forming the boundary between Rajasthan and Madhya Pradesh before turning southeast to join the Yamuna at the border of Bhind and Etawah districts of Uttar Pradesh State. The calibration of the Hydrological Model for the Chambal sub basin has been carried out at Baranwada discharge gauge for continuous simulation of two historical events, namely, 01 Jan 1992 to 31 Dec 1994 and 01 Jan 1996 to 31 Dec 1996. Rainfall and stream flow records were available for both the events. Figure 2-21 shows the Chambal sub basin and its tributaries along with the location of the discharge gauge used for calibration.

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Figure 2-21: Location of the discharge gauge used in calibration for Chambal sub basin

Comparison between observed and simulated hydrographs for flows at the Baranwada discharge gauge station of Chambal sub basin for continuous simulation of two historical events are shown in Figure 2-22. The duration of calibration simulation, comparison between observed and simulated peak flows, and year of occurrence of the peak flows is given in Table 2-13. From the plots, it can be observed that the simulated flows are in close agreement with the observed flows. The results show that the hydrological model of Chambal sub basin is adequately calibrated.

Table 2-13: Comparison of simulated and observed flows (cumec) for discharge gauge stations of Chambal sub basin for various calibration events

Sub Basin: Chambal Peak Flow Discharge Gauge Station Simulation Event/Duration Year Simulated Observed

01 Jan 1992 - 31 Dec 1994 1994 8059 8040 Baranwada 01 Jan 1996 - 31 Dec 1996 1996 10301 10420

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Figure 2-22: Comparison of observed and simulated hydrographs at Baranwada discharge gauge in Chambal sub basin

2.3.3.5 Yamuna The Yamuna sub basin of Ganges Basin has a drainage area of around 1,00,000 sq. km. alone whereas it has a total cumulative drainage area, including the drainage areas of all the rivers (Chambal, Sind, Betwa, Ken, etc.) draining into it, of around 3,45,000 sq. km. This sub basin comprises of the Yamuna River along with the other tributaries joining it on its course. The river originates from the Yamunotri glacier on the southwestern slopes of the Banderpooch peaks in the uppermost region of the Lower Himalayas in Uttarakhand. It crosses several states, Uttarakhand, Haryana, Uttar Pradesh, and Delhi and meets its tributaries on the way, including Tons, its longest tributary in Uttarakhand, Chambal, followed by Sindh, the Betwa, and Ken. It finally merges with the Ganges at Triveni Sangam, Allahabad. The calibration of the Hydrological Model for the Yamuna sub basin has been carried out at Agra discharge gauge for continuous simulation of two historical events, namely, 01 Jan 1995 to 31 Dec 1995 and 01 Jan 1998 to 31 Dec 1998. Rainfall and stream flow records were available for both. Figure 2-23 shows the Yamuna sub basin and its tributaries along with the location of the discharge gauge used for calibration.

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Figure 2-23: Location of the discharge gauge used in calibration for Yamuna sub basin

Comparison between observed and simulated hydrographs for flow at the Agra discharge gauge station of Yamuna sub basin for continuous simulation of two historical events are shown in Figure 2-24. The duration of calibration simulation, comparison between observed and simulated peak flows, and year of occurrence of the peak flows is given in Table 2-14. From the plots, it can be observed that the simulated flows are in close agreement with observed flows. The results show that the hydrological model of Yamuna sub basin is adequately calibrated.

Table 2-14: Comparison of simulated and observed flows (cumec) for discharge gauge station of Yamuna sub basin for various calibration events

Sub Basin: Yamuna Peak Flow Discharge Gauge Station Simulation Event/Duration Year Simulated Observed

01 Jan 1995 - 31 Dec 1995 1995 5622 5868 Agra 01 Jan 1998 - 31 Dec 1998 1998 4120 4315

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Figure 2-24: Comparison of observed and simulated hydrographs at Agra discharge gauge in Yamuna sub basin

2.3.3.6 Betwa and Ken The Betwa and the Ken sub basins of the Ganges Basin have a drainage area of around 44,000 sq. km. and 28,600 sq. km. respectively. The Betwa River originates in the Vindhya Range just north of Hoshangabad in Madhya Pradesh and flows northeast through Madhya Pradesh and Orchha to Uttar Pradesh. The confluence of the Betwa and the Yamuna is Hamirpur district in Uttar Pradesh. The Ken River originates near village Ahirgawan on the northwest slopes of Kaimur Range in Jabalpur district in Madhya Pradesh. It travels through some parts of Madhya Pradesh and Uttar Pradesh before merging with the Yamuna River at Chilla village near Fatehpur district in Uttar Pradesh. The calibration of the Hydrological Model for the Betwa and the Ken sub basins has been carried out at Chillaghat discharge gauge, which lies on River Yamuna. As the whole upstream area of Yamuna, Chambal, Sind, Betwa, and Ken sub basins contribute to flows at this discharge gauge, the calibrated model of the River Yamuna has been used to calibrate it further at Chillaghat gauge for continuous simulation of two historical durations, namely, 01 Jun 1990 to 31 Dec 1995 and 01 Jun 1996 to 31 Dec 1999. Figure 2-25 shows the Betwa and the Ken sub basins and their tributaries along with the location of discharge gauge used for calibration.

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Figure 2-25: Location of the discharge gauge used in calibration for Betwa and Ken sub basins

Comparison between observed and simulated hydrographs for flow at the Chillaghat discharge gauge station of Betwa and Ken sub basins for continuous simulation of two historical events are shown in Figure 2-26. The duration of calibration simulation, comparison between observed and simulated peak flows, and year of occurrence of the peak flows is given in Table 2-15. From the plots, it can be observed that the simulated flows are in close agreement with observed flows. The results show that the hydrological model of Betwa and Ken sub basins and the complete upstream drainage area up to the discharge gauge is adequately calibrated.

Table 2-15: Comparison of simulated and observed flows (cumec) for discharge gauge station of Betwa and Ken sub basins for various calibration events

Sub Basin: Betwa and Ken Peak Flow Discharge Gauge Station Simulation Event/Duration Year Simulated Observed

01 Jun 1990 - 31 Dec 1995 1995 28301 29470 Chillaghat 01 Jun 1996 - 31 Dec 1999 1999 28907 28580

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Figure 2-26: Comparison of observed and simulated hydrographs at Chillaghat discharge gauge for Betwa and Ken sub basins

2.3.3.7 Ramganga and Middle Ganges The Ramganga sub basin has a total drainage area of around 29,400 sq. km. whereas the Middle Ganges sub basin has a drainage area of around 47,000 sq. km from its confluence with Ramganga River till its confluence with River Sone. Ramganga River originates from the Doodhatoli ranges in the Garhwal and Almora districts of Uttarakhand state. It flows southwest from Kumaun Himalaya and covers some parts of Uttarakhand and Uttar Pradesh states before draining out near the border of Kannauj and Hardoi districts where it merges with the River Ganges. The calibration of the Hydrological Model for the Ramganga and Middle Ganges sub basins has been carried out at Ankinghat and Varanasi discharge gauge stations, both of which lie on the main Ganges River. As the whole upstream area of the Ramganga and Upper Ganges sub basins contribute to flows at these discharge gauges, the calibrated model of Upper Ganges sub basin has been used to calibrate it further at Ankinghat and Varanasi. The model has been calibrated for continuous simulation of two sets of historical events, namely, 01 Jan 1990 to 31 Dec 1995 and 01 Jan 1996 to 31 Dec 2000 at Ankinghat gauge and 01 Dec 1992 to 31 Dec 1994 and 01 Jan 1996 to 31 Dec 1999 at Varanasi gauge. Figure 2-27 shows the Ramganga and the Middle Ganges sub basins and their tributaries along with the locations of discharge gauges used for calibration.

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Figure 2-27: Location of the discharge gauges used in calibration for Ramganga and Middle Ganges sub basins

Comparison between observed and simulated hydrographs for flows at Ankinghat and Varanasi discharge gauge stations for continuous simulation of two sets of historical events are shown in Figure 2-28 and Figure 2-29. The duration of calibration simulation, comparison between observed and simulated peak flows, and year of occurrence of the peak flows is given in Table 2-16. From the plots, it can be observed that the simulated flows are in close agreement with observed flows. The results show that the hydrological model of Ramganga and the complete upstream drainage area of Ganges up to the location of discharge gauges in Middle Ganges is adequately calibrated.

Table 2-16: Comparison of simulated and observed flows (cumec) for discharge gauge stations of Ramganga and Middle Ganges sub basins for various calibration events

Sub Basin: Ramganga and Middle Ganges Peak Flow Discharge Gauge Station Simulation Event/Duration Year Simulated Observed

01 Jan 1990 - 31 Dec 1995 1995 13050 13000 Ankinghat 01 Jan 1996 - 31 Dec 2000 1997 8885 8623 01 Dec 1992 - 31 Dec 1994 1993 41815 41840 Varanasi 01 Jan 1996 - 31 Dec 1999 1997 43295 43680

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Figure 2-28: Comparison of observed and simulated hydrographs at Ankinghat discharge gauge of Ramganga and Middle Ganges sub basins

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Figure 2-29: Comparison of observed and simulated h ydrographs at Varanasi discharge gauge of Ramganga and Middle Ganges sub basins

2.3.3.8 Sone The Sone sub basin of the Ganges Basin has a drainage area of around 67,400 sq. km. This sub basin comprises of the Sone River along with small tributaries joining it over the course. The Sone River originates near Amarkantak in Madhya Pradesh, just east of the headwater of the Narmada River, and flows north-northwest through Madhya Pradesh state before turning sharply eastward. It parallels the Kaimur hills, flowing east-northeast through Uttar Pradesh, Jharkhand and Bihar states to join the Ganges just above Patna district in Bihar. The calibration of the Hydrological Model for Sone sub basin has been carried out at Dehri discharge gauge for continuous simulation of two historical events, namely, 01 Jun 1998 to 31 Oct 1998 and 01 Jan 2001 to 31 Dec 2001. Rainfall and stream flow records were available for both. Figure 2-30 shows the Sone sub basin and its tributaries along with the location of the discharge gauge used for calibration.

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Figure 2-30: Location of the discharge gauge used in calibration for Sone sub basin

Comparison between observed and simulated hydrographs for flow at the Dehri discharge gauge station of Sone sub basin for continuous simulation of the two historical events are shown in Figure 2-31. The duration of calibration simulation, comparison between observed and simulated peak flows, and year of occurrence of the peak flows is given in Table 2-17. From the plots, it can be observed that the simulated flows are in close agreement with observed flows. The results show that the hydrological model of Sone sub basin is adequately calibrated.

Table 2-17: Comparison of simulated and observed flows (cumec) for discharge gauge stations of Sone sub basin for various calibration events

Sub Basin: Sone Peak Flow Discharge Gauge Station Simulation Event/Duration Year Simulated Observed

01 Jun 1998 - 31 Oct 1998 1998 5493 5658 Dehri 01 Jan 2001 - 31 Dec 2001 2001 4967 5304

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Figure 2-31: Comparison of observed and simulated hydrographs at Dehri discharge gauge in Sone sub basin

2.3.3.9 Lower Ganges The Lower Ganges sub basin of the Ganges Basin has a drainage area of around 50,500 sq. km. This sub basin comprises of the main Ganges River along with small tributaries joining the main river. The drainage area of this particular sub-basin starts from its confluence with River Sone to the outlet of the Ganges Basin near the border of the Kushita and Pabna districts in Bangladesh. The sub basin covers some parts of Bihar, Jharkhand, and West Bengal States in India and some parts of Bangladesh. The calibration of the Hydrological Model for the Lower Ganges sub basin has been carried out at Sahibganj and Jangipur discharge gauges, both of which lie on the main Ganges River. As the whole upstream area of the basin contributes to flows at these discharge gauges, the calibrated results of all the previous models have been used in the Lower Ganges sub basin’s hydrological model and then used to calibrate it further at Sahibganj and Jangipur discharge gauges. The model has been calibrated for continuous simulation of two sets of historical events, namely, 01 May 1992 to 31 Dec 1992 and 15 Apr 2002 to 31 Dec 2002 at Sahibganj gauge, and 01 Jan 1998 to 21 Dec 1998 and 01 Jun 1999 to 31 Dec 1999 at Jangipur gauge. Figure 2-32 shows the Lower Ganges sub basin and its tributaries along with the locations of discharge gauges used for calibration.

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Figure 2-32: Location s of the discharge gauges used in calibration for Lower Ganges sub basin

The hydrological model for the sub basin has been calibrated starting from the upstream gauge of Sahibganj gauge and then at the downstream gauge of Jangipur, which lies very close to the outlet of basin. Comparison between observed and simulated hydrographs for flows at the Sahibganj discharge gauge station for continuous simulation of the two historical events are shown in Figure 2-33. Similar hydrographs for Jangipur gauge are shown in Figure 2-34. The duration of calibration simulation, comparison between observed and simulated peak flows, and year of occurrence of the peak flows is given in Table 2-18. From the plots, it can be observed that the simulated flows are in close agreement with observed flows. The results show that the hydrological model of Lower Ganges and the complete upstream drainage area of Ganges up to the location of these discharge gauges is adequately calibrated.

Table 2-18: Comparison of simulated and observed flows (cumec) for discharge gauge stations of Lower Ganges sub basin for various calibration events

Sub Basin: Lower Ganges Peak Flow Discharge Gauge Station Simulation Event/Duration Year Simulated Observed

01 May 1992 - 31 Dec 1992 1992 68879 73490 Sahibganj 15 Apr 2002 - 31 Dec 2002 2002 68493 70230 01 Jan 1998 - 21 Dec 1998 1998 66990 68623 Jangipur 01 Jun 1999 - 31 Dec 1999 1999 75059 64895

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Figure 2-33: Comparison of observed and simulated hydrographs at Sahibganj discharge gauge of Lower Ganges sub basin

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Figure 2-34: Comparison of observed and simulated hydrographs at Jangipur discharge gauge of Lower Ganges sub basin

For all the above sub basins, the observed and the simulated flow hydrographs show a good agreement at most of the flow gauge stations. Therefore, it can be concluded that hydrological processes are modeled realistically and that this model can be used further in probabilistic simulation of flows after validation. 2.3.4 MODEL VALIDATION The validation process intends to ensure that the model parameters are well set to reflect the physical nature of each basin. Validation runs have been made with the selected “best fit” parameters without further parameter changes. A good fit in this case indicates a robust model, which can be used with reasonable confidence. A poor fit, on the other hand, indicates low confidence. The validation process uses events that were not included in the calibration to evaluate the reliability of the model for other historical events. The model parameters remain unchanged during the model validation process. Table 2-19 gives the list of events used for validation runs for the sub basins described in the calibration section. In validation runs, the average values from the two calibration events for each model parameter were given as inputs for sub basins and streams above the respective flow gauge stations.

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Table 2-19: Details of the validation events for different discharge gauges of the sub basins

Sr. Simulation Event/Duration Sub Basin Discharge Gauge No. for Validation 1 Upper Ganges Rudraprayag 01 Jul 2002 - 01 Dec 2002 2 Chisapani 01 Jun 2002 - 01 Oct 2002 3 Ghagra Rudauli-Faizabad 05 Apr 2005 - 31 Dec 2005 4 Gosaniganj 01 Jun 2005 - 31 Dec 2005 5 Kampughat 01 Jan 2005 - 31 Dec 2005 Kosi 6 Saharsa 01 Jun 2004 - 31 Dec 2004

7 Chambal Baranwada 01 Jan 2004 - 31 Dec 2004

8 Yamuna Agra 01 Jan 2002 - 31 Dec 2002 9 Betwa and Ken Chillaghat 01 Jun 2001 - 31 Dec 2005 10 Ankinghat 01 Jan 2001 - 31 Dec 2004 Ramganga and Middle Ganges 11 Varanasi 01 Jan 2001 - 31 Dec 2005 12 Sone Dehri 01 Jan 2002 - 31 Dec 2002 13 Sahibganj 15 Apr 2005 - 31 Dec 2005 Lower Ganges 14 Jangipur 01 Jun 2000 - 31 Dec 2000 The comparison between observed and modeled flow hydrographs for all the above sub basins at the various discharge gauges are shown in Figure 2-35 to Figure 2-43. The comparison between observed and simulated peak flows and years of occurrence of the peak flows for validation events are given in Table 2-20. From the plots, it can be concluded that the simulated flows are in close agreement with observed flows. The results show that the model is adequately calibrated and validated at all the discharge gauges and that this model can be used further in stochastic simulation of flows for the various return periods of 2, 5, 10, 25, 50, and 100 years.

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Table 2-20: Simulated and observed peak flows (cume c) for sub basins at various gauge stations for validation events

Peak Flows Discharge Gauge Station Simulation Event/Duration Year Simulated Observed

Sub Basin: Upper Ganges

Rudraprayag 01 Jul 2002 - 01 Dec 2002 2002 1467 1461 Sub Basin: Ghagra Chisapani 01 Jun 2002 - 01 Oct 2002 2002 4003 4051 Rudauli-Faizabad 05 Apr 2005 - 31 Dec 2005 2005 7311 7333 Gosaniganj 01 Jun 2005 - 31 Dec 2005 2005 6982 6996 Sub Basin: Kosi

Kampughat 01 Jan 2005 - 31 Dec 2005 2005 2963 3162

Saharsa 01 Jun 2004 - 31 Dec 2004 2004 5734 5739 Sub Basin: Chambal Baranwada 01 Jan 2001 - 31 Dec 2005 1994 9208 8871 Sub Basin: Yamuna Agra 01 Jan 2002 - 31 Dec 2002 2002 4419 4565 Sub Basin: Betwa and Ken Chillaghat 01 Jun 2001 - 31 Dec 2005 2002 25142 25340 Sub Basin: Ramganga and Middle Ganges Ankinghat 01 Jan 2001 - 31 Dec 2004 2003 7925 8129 Varanasi 01 Jan 2001 - 31 Dec 2005 2003 40757 42150 Sub Basin: Sone Dehri 01 Jan 2002 - 31 Dec 2002 2002 5215 5171 Sub Basin: Lower Ganges Sahibganj 15 Apr 2005 - 31 Dec 2005 2005 63607 66930 Jangipur 01 Jun 2000 - 31 Dec 2000 2000 74507 75314

Figure 2-35: Comparison of observed and simulated hydrographs for flow gauge station of Upper Ganges sub basin for a validation event

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Figure 2-36: Comparison of observed and simulated hydrographs for flow gauge stations of Ghagra sub basin for various validation events

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Figure 2-37: Comparison of observed and simulated hydrographs for flow gauge stations of Kosi sub basin for various validation events

Figure 2-38: Comparison of observed and simulated hydrographs for flow g auge station of Chambal sub basin for a validation event

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Figure 2-39: Comparison of observed and simulated hydrographs for flow gauge station of Yamuna sub basin for a validation event

Figure 2-40: Comparison of observed and simulated hydrographs for flow gauge station of Betwa and Ken sub basins for a validation event

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Figure 2-41: Comparison of observed and simulated hydrographs for flow gauge stations of Ramganga and Middle Ganges sub basins for various validation events

Figure 2-42: Comparison of observed and simulated hydrographs for flow gauge station of Sone sub basin for a validation event

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Figure 2-43: Comparison of observed and simulated hydrographs for flow gauge stations of Lower Ganges sub basin for various validation events

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2.3.5 RETURN PERIOD FLOWS The validated hydrological models presented in the previous section were used to estimate runoffs from the river basins for all return period events. The outcome of these model runs were the flows for each event for all hydrological elements. It is implicit in this modeling approach that the rainfall of a given event rate or probability will result in runoff of the same rate of probability. This is true only if all other factors, such as temporal and spatial distributions and antecedent moisture conditions, are at their median values and are “probability neutral”. The median AMC is represented by average daily rainfall values (from all the historically available data) for the most flood-producing month. There should be at least 20 years of continuous historical rainfall data to perform return period rainfall calculations. All the rainfall stations used in the study have more than 20 years of data. Therefore, all these station were considered for the analysis. These station locations have already been shown in the previous section of the report. The historical rainfall has been used to extract annual maximum rainfall for each station. This annual maximum rainfall has been further processed to derive return period rainfall for 2, 5, 10, 25, 50, and 100 years. Return period rainfalls have been estimated using two different distributions, namely, the Gumbel distribution and the Generalized Extreme Value (GEV) distribution. After comparing the results of the model using both the rainfall in the model with the available literature for Ganges basin, Gumbel distribution was further used for the estimation of the return period event generation. The L moments method was used for the estimation of the GEV distribution parameters. Figure 2-44 shows the L-moment ratio diagram for stations used in the return period rainfall estimation.

Figure 2-44: L moment ratio diagram

These calculated return period rainfalls were used as the input to the validated hydrological models. In order to provide an independent check on the results of the runoff generated from the return period rainfall simulation methodology, a comparison of flows from observed

Hazard Report Confidential Page 71 of 86 The World Bank Exposure and Hazard Flood Risk Assessment for the Ganges Basin in South Asia annual peak flow and stochastically simulated peak flows from this methodology was undertaken. 2.4 Hydraulic Modeling Flood flows estimated in hydrological model are provided as the input to the hydraulic model. A detailed hydraulic analysis of the major streams of the study area was conducted to develop water surface elevations, floodplain boundaries, and depth of flooding in streams for selected frequency storm events. The hydraulic analysis was performed using HEC-RAS and HEC-GeoRAS. The steps included in the hydraulic analysis are: Model set up Model calibration Flood hazard mapping 2.4.1 MODEL SET UP The hydraulic model calculates flood elevations along streams and rivers for flood flows of various historical and return periods ranging from the most frequent to rare events. Flood elevations are then used to delineate the aerial extent of flooding adjacent to the streams and rivers. This technical effort serves to identify areas of flood inundation within the floodplain that are at risk and subject to flood damage. Derivation of flood extent and flood depths is determined using 1D hydraulic modeling through the river system for all historical and probabilistic events. Detailed hydraulic modeling requires discharge information, cross-sections of streams and rivers, and elevation information. HEC-RAS Model developed by the United States Army Corps of Engineer’s Hydrologic Engineering Centre (USACE 2010), was used for performing hydraulic calculations for the river stretches. HEC-RAS is an integrated system of software that contains one-dimensional hydraulic analysis components for both steady and unsteady, gradually varied flow simulation for a full network of natural and constructed channels. The basic computational procedure is based on the solution of the one-dimensional energy equation. Energy losses are evaluated by friction (Manning’s equation) as also expansion and contraction losses. The momentum equation is utilized in situations where the water surface profile is rapidly varied. The situations include a mixed flow regime (USACE 2010). Basin geometric data consist of the river system connecting all segments, cross-section data, reach lengths, energy loss coefficients, and stream junction information. The river system schematic defines how the various river reaches are connected as well as establishes the naming conventions for referencing all the other data. The connecting river reaches are important for the model to understand how the computations should proceed from one reach to the next. The river system schematic is performed using HEC-Geo-RAS (an Arcview extension for pre and post processing of RAS) in GIS environment using ESRI’s Arcview. HEC-GeoRAS was used to create a HEC-RAS import file containing geometric attribute data from a Digital Elevation Model (DEM). The HEC-RAS models were setup for all segments of the river basins under the study area. Steady flow simulation was adopted for this study. Using the rivers delineated in the basin delineation process of hydrological modeling, the cross sectional geometry was derived for all the rivers at approximate intervals of 100 m. The elevation information was extracted using the SRTM DEM. Hydraulic roughness was estimated initially using the land use map and visual representation from Google Earth. Initial roughness values of 0.025 were adopted for the river channels and 0.035 for the floodplains. These were subsequently varied during the model calibration process. Estimated runoff is thus routed through the river system using the above one-dimensional hydraulic analysis to delineate flood extents and depth at all the river segments. Figure 2-45 shows the HEC-RAS set up with a plan view of the modeled reaches, longitudinal riverbed

Hazard Report Confidential Page 72 of 86 The World Bank Exposure and Hazard Flood Risk Assessment for the Ganges Basin in South Asia profile of a reach, and a typical cross-section for a reach for the river network in the study area.

Figure 2-45: HEC RAS model set up for the study area

2.4.2 MODEL CALIBRATION The development of hydraulic models across a large floodplain requires a rigorous calibration process to ensure the hydraulic model accurately reproduces the observed flooding behavior. The calibration process consists of systematically comparing observed flooding behavior within the study area against the hydraulic model’s reproduction of that behavior. This process generally incorporates comparisons between simulated flood levels and observed flood levels. This can also be done by comparing the areas of inundation from historical event with simulated flood extent from the model. (http://www.wcma.vic.gov.au/index2.php?option=com_docman&task=doc_view&gid=385&Ite mid=50). The first approach requires detailed data about the flood levels over time (temporal distribution) at discrete points of interest within and along the river at various locations such as important bridges, levees, and embankments. The second approach requires flood extent and/or depth measurements (spatial distribution) for particular events. The global mapping agency such as Dartmouth Flood Observatory (DFO) records the behavior of historical flood events and provides footprints of recent floods. On the other hand, government agencies also map the areas under inundation. RMSI has tried to calibrate the hydraulic model wherever historical flood information is available. Two such historical flood footprints have been used for this purpose. The flood extent maps of the September 2001 and September 2005 events were used for the

Hazard Report Confidential Page 73 of 86 The World Bank Exposure and Hazard Flood Risk Assessment for the Ganges Basin in South Asia calibration of the flood extents as shown in Figure 2-46 and Figure 2-47 respectively. During the September 2001 flood event, flood plains of Bagmati and Lower Ganges sub basins were severely affected as shown in Figure 2-46. The dark blue areas in the map represent the flood-inundated areas. After looking at the flow data of this event for various flow gauge elements of the model in the basin, it was observed that this event occurred during 4th to 18th September, 2001. Similarly, during the September 2005 flood event, flood plains of Ramganga, Upper Ganges, and Ghagra sub basins were severely affected as shown in Figure 2-47. The red areas in the map represent the flood-inundated areas. After looking at the flow data of this event for various flow gauge elements in the basin, it was observed that this event was occurred during 19th to 30th September, 2005.The reported flooded areas in these imageries are not very accurate, as it has not included flood extents of all the nearby tributaries. These areas were compared with simulated flood extent.

Figure 2-46: Flood Extent Map of September, 2001 flood event (Source: DFO)

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Figure 2-47: Flood Extent Map of September, 2005 flood event (Source: DFO)

Flood extent and flood depth maps have been generated by post-processing the simulated results of HEC-RAS in Arcview environment with HEC-GeoRAS extension. Flood plain boundaries and inundation depth data sets were generated from exported cross-sectional water surface elevations. For generating the flood extent map of this event, the HEC RAS model set up (Figure 2-45) was used and peak flow values during the period of these events (September,2001 and September, 2005) were given as input. The Manning’s roughness coefficients were varied during the trials to match the simulated flood extents with observed flood extents. A comparison of simulated and observed flood extent for the September 2001 event has been made in Figure 2-48. Similarly, comparison of simulated and observed flood extent for the September 2005 event has been made in Figure 2-49. Both indicate that the simulated flood extent is in good agreement with the observed flood extents.

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Figure 2-48: Comparison between modeled and observed flood extents of September, 2001 event

Figure 2-49: Comparison between modeled and observed flood extents of September, 2005 event

Hazard Report Confidential Page 76 of 86 The World Bank Exposure and Hazard Flood Risk Assessment for the Ganges Basin in South Asia RMSI has also tried to calibrate the modeled water level with the available historical water level. The historical water level for some of the gauges were available from various Government and public domain websites like the Department of Disaster Management, DFO, etc. Historical water level information for two such events of August 2001 and August 2005 were used for this purpose. The hydraulic model was run for these two events and the comparison between modeled and observed water levels for these two events have been shown in Table 2-21. The above comparisons of flood extents and water levels for simulated and historical data for various historical events, indicate that the hydraulic model has been adequately calibrated. Therefore, this set up was then used to derive the flood extent maps for design events of 2, 5, 10, 25, 50, and 100 years.

Table 2-21: Simulated and observed water level for the two historical events of August, 2001 and August, 2005

Event: August, 2001 Event: August, 2005 Sr. No. Place Water Level, m Water Level, m Simulated Observed Simulated Observed 1 Varanasi 73.52 69.26 74.37 70.12 2 Buxar 61.77 59.88 62.06 59.7 3 Patna (Dighaghat) 52.78 50.17 52.92 49.99 4 Hathidah 41.84 41.73 41.59 41.58 5 Farakka 23.75 23.67 23.75 23.43 6 Chopan 168.81 171.71 168.89 170.67 7 Daltonganj 209.2 209.08 208.53 208.46 8 Baltara 34.53 34.51 34.85 34.98

2.4.3 FLOOD HAZARD MAPPING FOR RETURN PERIOD FLOWS Flood hazard maps depicting flood extents and flood depths were derived by performing one-dimensional hydraulic routing through the river system for 2, 5, 10, 25, 50, and 100 year return period discharges. The derived flood extents can be used to determine the various exposures at risk. The flood hazard maps for 2, 5, 10, 25, 50, and 100 years return periods for the whole basin are shown in Figure 2-50 to Figure 2-55.

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Figure 2-50: Flood hazard map for 2- year return period for Ganges basin

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Figure 2-51: Flood hazard map for 5- year return period for Ganges basin

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Figure 2-52: Flood hazard map for 10 - year return period for Ganges basin

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Figure 2-53: Flood hazard map for 25- year return period for Ganges basin

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Figure 2-54: Flood hazard map for 50- year return period for Ganges basin

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Figure 2-55: Flood hazard map for 10 0- year return period for Ganges basin

These flood hazard grids for each return period will be used to estimate the damage/loss for each type of exposure described in the exposure report.

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