Climate Change and Water Resources in South Asia - Chapter 3 ppt

These increases have largely been attributed to worsening flood eventsincreased river discharge and spatial extent in the GBM basins in India and Bangladesh M.. Therefore, this paper exa

Are Floods Getting Worse in the Ganges,

Brahmaputra and Meghna Basins?

The Ganges, Brahmaputra and Meghna/Barak (GBM) river systems occupy about

175 million hectares (mha) of South Asia (Fig 3.1) and supports more than 500 millionpeople (Verghese and Iyer, 1993) They are unique in the world with respect to water andsediment supplies, channel processes, and instability While the Brahmaputra ranks fourthamong the largest rivers of the world with regard to mean annual discharge, the Gangesranks thirteenth (Mirza, 1997) The estimated annual sediment yield of the Brahmaputra is1,028 tons/km2, the highest among the world’s largest rivers On the other hand, thesediment yield of the Ganges is only 502 tons/km2 although its basin area is two times that

of the Brahmaputra (Barua, 1994) The swinging and avulsion of the courses of the Gangesand Brahmaputra Rivers in recent history have significant influence on the morphology oftheir alluvial floodplains (Rahman, 1993; Brammer, 1996) They are characterized by highflows during the monsoon and low flows during the dry season For example, the ratio ofmonsoon flow to dry season flow of the Ganges River at Hardinge Bridge in Bangladesh is6:1 (Mirza and Dixit, 1997) The high flows often cause floods in many parts of these vastriver basins

Sitting at the confluence of the three major rivers, Bangladesh (area 148,000 sq km)

is considered to be the most flood-affected country in the world followed by India Everyyear, slightly over one-fifth of its land area becomes flooded and in extreme cases, morethan two-thirds of the country is affected In upstream India (area 3,280,000 sq km),floods annually inundate an area larger than half of Bangladesh

Available information shows that in recent years, flood damage in Nepal, India andBangladesh is increasing Substantial increases in flood damage in Nepal during the 1980swere reported by the Asia Development Bank (ADB, 1991) For India, the Center for

Science and Environment (CSE, 1992) reported that the annual flood damage had

increased 40 times from the 1950s to the 1980s (Fig 3.2) According to Mirza (1991a),compared to the 1960s and 1970s, flood damage in Bangladesh was the greatest in the1980s (Fig 3.3) These increases have largely been attributed to worsening flood events(increased river discharge and spatial extent) in the GBM basins in India and Bangladesh

M MONIRUL QADER MIRZA

(CSE, 1992; BBJTO, 1989; RBA, 1980; and Ives, 1991) These claims, do not, however,appear to be based on systematic analyses of relevant data Therefore, this paper examineswhether floods in the GBM basins are getting worse by applying statistical tests to:1) the peak discharge data of the three rivers recorded at various stations in India, Nepaland Bangladesh; and 2) the flooded area data The latter were used to determine changes

in spatial severity of flooding in India and Bangladesh Peak discharge recording stationsand period of records are shown in Table 3.1 It is possible that the reported increases inflood problems are due to increased human activities in flood-prone areas But thatelement of flood hazard is not the subject of this paper

Fig 3.1 The Ganges, Brahmaputra and Meghna basins Location of some discharge measurement stations have also been shown.

Fig 3.2 Flood damage in India during 1953-1999 (Source: CWC, 1989; ADRC, 2000a).

Fig 3.3 Flood damage in Bangladesh during 1954-1998 (Source: Mirza, 1991a; ADRC, 2000a).3.2 HYDRO-METEOROLOGY OF THE GBM BASINS

Of the three river basins, the Ganges is the largest Its 109.5 mha basin area is distributed

over China, Nepal, India and Bangladesh The Ganges River rises South of the mainHimalayan divide near Gangotri at a height of 4,500 m in the Uttar Pradesh (UP), India InNepal, India and Bangladesh, mean annual precipitation in the basin is 1,860 mm, 908 mmand 1,568 mm, respectively Mean annual runoff of the Ganges River at Farakka, India andHardinge Bridge, Bangladesh, is estimated to be 415 x 103 million cubic meters (mcm)and 352 x 103 mcm, respectively (Mirza, 1997) The highest annual peak discharge(80,230 m3sec-1) was recorded at Hardinge Bridge in 1998 (See Fig 3.1 for the location ofsome of the stations referred to in this paper.)

The Brahmaputra basin area is 58 mha It is regarded as one of the world’s largestbraided river systems in terms of discharge, sediment transport, and channel processes(JMBA, 1989) The river originates at an elevation of 5,150 m in a large glacier mass in theKailash range of the Himalayas, very close to Manassarovar Lake Mean annualprecipitation in the basin area in India and Bangladesh is 2,500 mm and 2,400 mm,respectively Mean annual runoff of the Brahmaputra at Pandu, India and Bahadurabad,Bangladesh is estimated to be 511 x 103 mcm and 643 x 103 mcm, respectively The highestpeak discharge was 98,600 m3sec-1 recorded at Bahadurabad in 1988

The Meghna/Barak basin is the smallest of the three basins, with an area of 8 mha Theheadstream of the river in India is known as Barak and originates on the Southern slope ofthe mountain range to the North of Manipur, India In Bangladesh, the river is known asMeghna and flows Southwest to meet the Padma (combined flow of the Ganges andBrahmaputra Rivers) at Chandpur Mean annual precipitation of the basin in India andBangladesh is 2,640 mm and 3,574 mm, respectively Mean annual runoff of the Barak inIndia is 41 x 103 mcm (measured at Badarpurghat) (Kothyari and Garde, 1991) At BhairabBazaar in Bangladesh, the mean annual runoff is estimated to be 151 x 103 mcm Thehighest peak discharge at Bhairab Bazaar was 19,900 m3sec-1 recorded in 1993

Table 3.1 Statistical properties of the peak discharge and flooded area data

* non-random.

River Station Period of

Record

Latitude (deg N)

Longitude (deg E)

Mean (m3sec-1)

Coefficient of Variation (CV)

Lag-1 Autocorrelation Coefficient Hardwar 1885-1971 29.58 78.10 6,639.00 0.47 - 0.42*

The Ganges Farakka 1949-1980 25.00 87.91 56,516.00 0.17 + 0.05

Hardinge

Bridge

1934-2000 23.06 89.03 51,184.00 0.18 + 0.22 The Kosi Barahkshetra 1948-1978 - - 10,190.00 0.44 - 0.18

The Brahmaputra Pandu 1955-1974 26.20 91.50 50,524.00 0.20 + 0.20

Bahadurabad 1956-1999 25.15 89.66 67,389.00 0.18 +0.03 The Meghna Bhairab Bazaar 1964-1998 24.03 59.98 14,072.00 0.19 - 0.23

The Surma-Meghna Kanairghat 1969-1993 - - 2,224.00 0.16 + 0.35

Flooded Area (mha)

India

Bangladesh

1953-1997 1954-1999

7.28 3.03

0.47 0.68

+0.17 +0.16

3.3 THE FLOOD PROBLEM

Flooding of catastrophic proportions often occurs in the GBM river basins Extremeprecipitation in the monsoon, together with the physical settings of the river basins hascaused many severe floods in the last few decades Causes and characteristics of floodsvary between the highlands in Nepal, the middle ground in India, and the flat deltaic terrain

in Bangladesh

In Nepal, the flood problem is mainly restricted to the Terai region along the border itshares with India Rivers in the Terai region are very unpredictable and cause heavy flooddamage as a result of intensive downpours on the Southern slopes of the Siwalik Himalaya(SAARC, 1992) The high Himalayan Mountains of Nepal are affected by Glacier LakeOutburst Floods (GLOF) These floods do not cause much damage to human settlementsbecause the upper mountainous areas are sparsely populated In contrast, floods in theTerai occur regularly and cause considerable damage to densely populated floodplains

In India, floods in the Ganges region are caused by the following factors either singly

or in combination: excessive precipitation, inadequate river channel capacity, obstruction

in streams, inadequate waterways at confluences, human encroachments and lack ofadequate drainage, failure of flood control embankments and deforestation (Rangachari,1993; Chowdhury, 1989; Dhar and Nandargi, 1998) Similar factors cause floods in theBrahmaputra region, but they are compounded by local physiographic features Theregion is interspersed with a large number of streams, flooding from which inundatesthe intervening narrow valleys The riverbeds in some cases are higher than the surround-ing valley land Therefore, any breach or spilling causes deep flooding in the valleys TheBrahmaputra region in India is highly prone to earthquakes and this often causes

landslides These seriously disturb the drainage system The Barak region lies between the

Khasi and Jaintia Hills in the North and Mizo Hills in the South The river often overflowsits banks inundating low areas on either side There is a series of bowl-shaped depressions,locally known as “Haors”, which fill with floodwater The gradient of the river is extremelyflat and the outfall at the border with Bangladesh is congested

In recent years, the role of deforestation in the upstream areas in causing flooding inthe downstream areas of the GBM basins has triggered interesting debates (BWDB, 1987;Carson, 1985; Thompson and Warburton, 1985; Hamilton, 1987; Hofer, 1998; Ives and

Messerli, 1989; Messerli and Hofer, 1995; Rogers et al., 1989) BWDB (1987) indicated

that deforestation in the upstream contributed significantly to the increased rates ofsediment supply and accretion However, the existing publications do not report anysignificant recent increase in the sediment load of the larger rivers and their tributaries, or

in the magnitude of annual flooding and levels of river discharge (Ives and Messerli, 1989).Thompson and Warburton (1985) questioned the linkage between massive floods in theplains and land-use activities upstream in the Himalayas However, they noted that therewas some technical uncertainty encountered when analyzing the human components oferosion, flooding and shifting hydrological patterns Hofer (1998) concluded that land-usechanges in the Himalayas were not responsible for the floods far downstream in Indiaand Bangladesh In the aftermath of the devastating floods in Bangladesh in 1988,

Rogers et al (1989) remarked that there were no grounds for considering deforestation in

the Himalayas as a significant cause of the flooding in the delta of the river system Carson(1985:36) mentioned, “…Flooding and sedimentation problems in India and Bangladeshare a result of the geomorphic character of the rivers and man’s attempts to contain therivers Deforestation likely plays a minor, if any, role in the major monsoon flood events onthe lower Ganges.” The role of deforestation in the sedimentation and flooding processes

in South Asia is a highly contentious issue and it needs adequate scientific research andattention.

Of the three countries affected by flooding in the GBM river basins, Bangladesh is themost vulnerable because of its geographic location, high monsoon cross-border flow, andthe physiographic characteristics of its deltaic floodplains Half of the country is under12.5 m above the mean sea level (CBJET, 1991) Because of its flatness, floodwaterscannot drain quickly The three rivers together may generate as much as 200,000 m3sec-1 ofpeak discharge The problem becomes more complicated when the peak flow of each ofthe three rivers synchronises In such a case, the flooded area may exceed 60% of thecountry (as occurred in 1988 and 1998), about three times the normally flooded area(Fig 3.4) Although the river levels fall rapidly from September through November, waterlevels on adjoining floodplains fall more slowly because of low gradients, congesteddrainage, and substantial depression areas The latter may stay submerged until December

to January and some throughout the whole dry season (November to May)

Floods cause considerable damage in the GBM basins and four main economicsectors - agriculture, housing, industry and transportation infrastructure are particularlyvulnerable Flood related damage puts considerable strain on the economies of thecountries that share the GBM basins This is particularly true in terms of diversion ofresources for recovery activities and the loss in growth of Gross Domestic Products (GDP).For example, during the 1998/1999 fiscal year in Bangladesh, GDP growth declined to4.6% from 5.2% of the previous year due to the devastating floods of 1998 Industrysector growth, however, decreased by 3.4% during the same period as a result offlood-induced disruptions in the manufacturing subsector (ADB, 2000)

020

Fig 3.4 Flooded area in Bangladesh during 1954-1999 (Source: BWDB, 2000b).

In Nepal, government statistics show an increasing trend in damage to public andprivate property from floods and landslides in recent years The estimated damage toproperty increased from US$ 1.0 million in 1983 to US$ 100.0 million in 1989 (ADB,1991)

In India, out of the 34 mha of “flood-prone” area, some 23 mha are in the GBMbasins Fifteen Indian states and the union territory of Delhi lie in the basin However, fourstates alone account for over two-thirds of the flood-prone area: Uttar Pradesh, Bihar,West Bengal and Assam (Rangachari, 1993) Data compiled by India’s Central WaterCommission (CWC) show that during 1953-1987, the average area affected by floods was

7.66 mha (22.5% of the flood-prone area), of which 3.51 mha were cropped (CWC, 1989).Recent data published by the Ministry of Water Resources, Government of India showsthat during 1953-1997 annual average flood affected area has declined to 7.42 mha (MWR,2000) from that of the 1953-1987 average (Fig 3.5) This is due to a decline in flooded area

in the period 1987-1997 Available evidence indicates increasing flood damage in recentyears in India State governments estimated that flood damage in 1987 and 1988 was US$1.5 billion and US$ 2.5 billion, respectively (Rangachari, 1993) In 1953 it was 524 millionrupees and remained around that level until the middle of the 1960’s when damage trackedupward (Fig 3.2) (CWC, 1989)

Fig 3.5 Flooded area in India during 1953-1997 (Source: MWR, 2000).

In Bangladesh, the area prone to floods in the GBM basin is 6.14 mha This is 42% ofthe country’s geographical area On an annual average, 20.5% of Bangladesh (3.03 mha)becomes inundated The loss caused by floods in Bangladesh in a normal year is aboutUS$ 175 million; but in extreme cases, the damage may exceed two billion dollars The

1998 flood damage was the worst in history, totaling in the range of US$ 2 billion to2.8 billion (ADRC, 2000a; MOFA, 1998) The industry and infrastructure sectors wereworst hit, followed by agriculture (MDMR, 1998) The flood damage in Bangladesh for

the period 1954-1998 is shown in Figure 3.3

Flood damage estimation methods in Nepal, India and Bangladesh only take intoaccount the direct damage Death, trauma, accidents, post-flood health and nutritionproblems are not considered direct damage as their monetary valuations are unaccountedfor Almost every year, a significant number of people die due to floods During1953-1987, the annual average loss of human lives in India due to floods was 1,439(CSE, 1992) In West Bengal, India 1,262 people had died and another 117 were reportedmissing during the devastating monsoon floods of 2000 (UNICEF, 2000) In Nepal, duringthe period of 1981-1999, a total of 5,453 people lost their lives with the highest, 2,307people, in 1993 (ADRC, 2000a) In the catastrophic floods of 1998 in Bangladesh, thenumber of reported deaths was 1,050 (ADRC, 2000a) The number of deaths caused byfloods in India, Bangladesh and Nepal is summarized in Table 3.2

Flood damage is an indicator of flood hazard, which in turn, is a function of potential flood events in relation to human use of flood-prone land This includes activities aimed at

alleviating the flood problem, such as embanking river channels and elevating floor levels

of buildings Thus, flood hazard effects (registered as property damage, social disruption,

and human injury) rise or fall with changes in the parameters of the flood event

0 25 50 75 100

(e.g., discharge and areal extent) or human use of flood-prone land (e.g., type and density),

or both

In the wake of increasing flood damages in the GBM basins, special emphasis hasbeen given to flood problems in both India and Bangladesh The Government of Indiacreated the Rashtriya Barh Ayog (National Commission on Floods) in 1976 Devastatingfloods in India in 1987 led to the setting up of two committees to look into the problem(Rangachari, 1993; Mirza, 1991a) Bangladesh formulated a Water Master Plan in 1964that recommended 59 flood control projects as a result of the consecutive floods of 1954

and 1955 (Mirza, 1991b) However, from the mid-seventies to the late-eighties, flood

control received little attention in Bangladesh In response to the devastating flood of

1988, Bangladesh carried out 28 studies under the “Flood Action Plan (FAP)” during theperiod 1989-1995 All of these efforts were based on the claim by government (as well asnon-government agencies) that floods in the GBM basin areas were getting worse(CSE, 1992; BBJTO, 1989; RBA, 1980; Ives, 1991)

As floods are generally accompanied by over bank spilling, assessment of peakdischarges in the major rivers is the best way to determine whether changes in flood eventshave occurred or not In order to detect changes, Cumulative Deviation, WorsleyLikelihood Ratio, Kruskal-Wallis and Mann-Whitney U tests (Annex 3.1) were applied tothe peak discharge series of the three rivers Similar tests were also applied to the floodedareas in India and Bangladesh to see if there were associated changes in the spatial (areal)extent of flooding

Table 3.2 Number of deaths due to floods in India, Bangladesh and Nepal during the period 1953-2000

Two observations were missing in the peak discharge data series for the Ganges River.One observation was missing for each of the Farakka (1969) and the Hardinge Bridge(1971) sites These were filled by determining the correlation coefficient and then applying

the method of Salinger (1980).1 Similarly, one observation was also found missing foreach of the Pandu (1964) and the Bahadurabad (1971) sites for the Brahmaputra River.These were also filled by the method applied for the missing data of the Ganges River Forthe Meghna River at Bhairab Bazaar, four observations (1977-1980) were missing Thesemissing observations were filled by applying the precipitation-peak discharge regressionmodel (Mirza, 1997).2 Errors involved with the discharge measurement, processing andstorage of data are not generally reported and documented Standard equipment, methodsand specifications are being used in discharge and water level measurements in Nepal,India and Bangladesh However, in Bangladesh, due to changing bed forms, velocitymeasurements from non-anchored boats and inaccurate measurement of depths for currentmeter may cause ≤10% and ≤15%-20% uncertainty in discharge and water levelmeasurements, respectively (Sir William Halcrow and Partners, 1991; FAP 24, 1993) Themagnitudes of errors in the measurement of discharge and water levels in India and Nepalare not known But, due to similar characteristics of river channels, they are assumed to bethe same as those of Bangladesh.

Statistical properties of the peak discharge data are shown in Table 3.1 Annual peakdischarge of the Ganges River at Hardwar is found to be highly variable, followed by theKosi River at Barahkshetra The almost equal coefficients of variation of the Brahmaputra,Meghna and Surma-Meghna Rivers indicate that they drain the catchment areas withsimilar characteristics Lag-1 autocorrelation coefficient was determined using theequation shown below This coefficient is used to determine the presence of “persistence”

in the data A negative value of r

1 is indicative of marked high frequency (i.e short-period)oscillations On the other hand, positive values indicate Markov linear type persistence

(Mirza et al., 1998) The presence of this type of persistence in a peak discharge series

means that a large (or small) peak discharge for one year is more likely to be followed by

a large (or small) for the next year:

where X i is annual peak discharge at year i, n is the sample size, and is mean peak

discharge

The randomness of the series can be tested to identify presence of trend or cycle using

the one-tail 95% confidence limit of the Gaussian distribution (Mitchell et al., 1966) The

1 The missing observation for one year was calculated using the ratio of the mean peak discharge of the two stations with a missing record to the adjacent data-possessing station multiplied by the peak discharge of that year.

2 The regression model for estimating annual peak discharge is Qp = -10531 + 3.41*P1 + 5.69* P2 (R 2 = 87%) Where P1 is average precipitation in the North Assam meteorological sub-division and P2 is the average precipitation in the South Assam meteorological sub-division and Bangladesh part

of the basin.

test value ( )r1 t is computed from: