Hydro-Morphometric Modeling for Flood Hazard Vulnerability Assessment of Old Brahmaputra River Basin in Bangladesh - Juniper Publishers
Juniper Publishers - Open Access Journal of Engineering
Technology
Abstract
The present research introduces hydro-morphometric
modeling to assess flood hazard vulnerability in old Brahmaputra River
Basin in Bangladesh. Hydro-morphometric modeling is a spatial analysis
in a GIS environment that has been applied for the estimation of flood
hazard vulnerability. In this study the developed methodology processes
information of seven parameters namely basin, flow accumulation, flow
direction, stream order, stream length, stream density and drainage
density. By applying Horton's and Strahler's formula of stream order,
stream density and drainage density of old Brahmaputra River Basin are
calculated. According to Horton's stream density and drainage density
reflects the runoff and discharge status of a basin which also revealed
that basin having high stream density and drainage density is most
vulnerable to flooding. The old Brahmaputra River Basin subdivided into
five sub-Basin named as sub-basin A, B, C, D and E and their stream
density are 1.318km-2, 1.113 km-2, 1.2009km-2, 1.028km-2, 0.998km-2 and their drainage density are 0.713 km-1, 0.690km-1, 0.689km-1, 0.703 km-1 and 0.703km-
1, respectively. Among the five sub-basins of this river basin,
sub-basin A contains high stream and drainage density, which means this
basin is most vulnerable to flooding, whereas basin C and D are least
vulnerable because of its low stream and drainage density. Among the
different floods, severe river flooding affects the area of 33.66
hectors agriculture land, 855.27 hectors fallow land, and 548.55 hectors
vegetation cover, 275.22 hectors settlements and 177.3 hectors water
bodies in this river basin. This study also revealed that agriculture
land and resources, and life defence of marginal community of the Old
Brahmaputra river basin are vulnerable to flood hazard.
Keywords:
Hydro-morphometric modeling; Flood hazard vulnerability assessment; Old
brahmaputra river basin; High level of poverty; Neighbourhood;
Flooding; Destructive floods; Crop production; Livelihoods; Monsoon;
Stream density; Stream order
Abbreviations:
LFF: Low Flash Flooding; SFF: Severe Flash Flooding; LRF: Low River
Flooding; MRF: Moderate River Flooding; SRF: Severe River Flooding;
USGS: United States Geological Survey
Introduction
Bangladesh is one of the most vulnerable countries to
the impacts of flood hazard in south Asia. This country is extremely
vulnerable to flood because of its geographical location, high
population density, high level of poverty, and the reliance of many
livelihoods on flood hazard sensitive sectors [1].
Moreover, the climate of the country is strongly influenced by monsoon
rainfall which creates flood in three major river basin named as Ganges-
Padma, Brahmaputra-Jamuna, and Meghna River Basin. Though Bangladesh
occupies only 7% of the combined catchment area of the Ganges-Padma,
Brahmaputra-Jamuna and Meghana river basin, the country has to drain out
92% of the flow into the Bay of Bengal [2].
Too much water in the monsoon period affects different sectors along
with livelihoods and food security. There is six major destructive
floods occurred in 1984, 1987, 1988, 2004 and 2007 since after
liberation war of Bangladesh [3].
In 1984 flood about 50,000sq.km area of land was flooding. In 1987
flood about 2,055 people died, flood affected area was 50,000sq.km and
losses 1 billon US $. Among the six floods, more destructive flood was
1988. In the historical memory of Bangladesh most destructive flood was
occurred in 1988 and where 61 percent of the country was inundated,
2,000-6,500 people died, about 45 million people become homeless and
caused 1.2 million US $. In 1998, flood inundates nearly 100,000 sq. km,
damages 500,000 homes and rendered 30 million people homeless, death
counted 1,100 people and estimated loss was 2.8 million US $. In last
2007 flood about 32, 000sq.km area was flooding and 649 people losses
their lives and property losses is about 1 billion
US $ [4].
Flood visits every year. In 2017, flood visits Bangladesh for two
times. Early Flash flood at the Haor region of Bangladesh caused heavy
loss of crop production and other life defence. Second flood of 2017
visits northern, north-west and north-east parts of Bangladesh at the
time of monsoon season; damaged agricultural production and created
human sufferings.
Moreover, the inhabitants of Old Brahmaputra river
basin are exposed to flood risk mainly due to geographic location,
geological and geomorphological formation, and heavy rainfall during the
monsoon and river siltation. In this basin, people understand the risks
and still living in the neighbourhood for decades. People of the basin
developed some indigenous approaches of their own to adapt against the
hazards. Government and non-government organization are also working for
the welfare of flood area people.
Therefore, flood risk management needs to overcome national borders, geographic location and socio-economic limitations [5]. Flood risk management usually includes flood risk assessment and flood risk mitigation [6].
This distinction takes into account apart from the hazard also its
impact, since the total elimination of risk is neither possible nor
efficient. Indisputably, strategies against floods' impact at a region
scale require the identification of prone areas [7]. To provide early warning, facilitate quick response and decrease the impact of possible flood events [8]
hydro-morphometric modeling, flood hazard zoning and risk for people's
resource are better option in flood hazard risk flood Risk management in
old Brahmaputra River Basin.
Study Area
The studied Old Brahmaputra River Basin comprised of
nine districts of Bangladesh, namely Sherpur, Jamalpur, Mymensingh,
Tangail, Netrokona, Kishoreganj, Brahmanbaria, Narsingdi and Gazipur.
Total area of the studied river basin is 6851.58km2. The topographic
elevation of this river basin extract from DAM satellite image which
range -25m to 53m.
The main river of this Basin is Old Brahmaputra which
is one of the main distributaries of the Jamuna (Brahmaputra) that
distributes part of Jamuna discharge over a large area of north central
region of Bangladesh. The earliest study of the Brahmaputra was made by
Rennell who mapped the Brahmaputra as a braided river flowing into the
Meghna River and following a course now occupied by a much smaller river
known as the Old Brahmaputra. Since then the river has changed course
and avulsed into its present course known as the Jamuna River. Gradual
tilting caused the Old Brahmaputra River to become antecedent in places,
necessitating river scour into slowly or periodically rising,
comparatively resistant Pleistocene sediments [9].
Faulting was probably the major cause of the recent shift of the
Brahmaputra River from its course east of the Madhupur tract to its
present position [10].
The Old Brahmaputra River is at present reduced to a left bank spill
channel of the
Brahmaputra River and only active during the high stage of the
Brahmaputra River. The discharge and sediment transport through the
river is dependent on opening of the off take with the Brahmaputra River
[11].
During the recent years, the river basin is more vulnerable to flood
due to lower elevation, river siltation and monsoon rainfall. Moreover,
Monsoon rainfall overtop the banks of this river causes serious
destruction of the inhabitants settlement, Agricultural crops and
domestics animals (Figure 1 & 2).
Methods and Materials
In this research for identification of stream
channels to delineate the extent of regular inundation in Old
Brahmaputra River basin satellite Image (DEM) is collected from United
States Geological Survey (USGS). By applying Strahler [12],
stream ordering method in GIS environment Old Brahmaputra River Basin
stream are ordered. For exploration of flood depth grids using spatial
analyst tools (hydrological model) and prepare a comprehensive flood
zone map which is integrated with flood zoning data of Bangladesh Water
Development Board. Predict and determine the potential flood
vulnerability and list of vulnerable sectors, elements and resources for
the community of old Brahmaputra River Basin Satellite Image (Landsat
TM) is also collected from United States Geological Survey (USGS) for
Land Use and Land Cover classification using supervised classification
of RS tools and techniques for mapping potential flood risk for the
community.
Stream Channel Extraction
The stream network of the study area is extracted from a series of geo processing tools. According to Strahler [12],
the output of this technique creates a stream network grid with stream
classification. Strahler's system of classification designates a segment
with no tributaries as a first-order stream. Where two first-order
stream segments join, they form a second- order stream segment and so
on. The highest stream order in the study area was computed as third.
Manual corrections are made by merging the streams of same order with
separated nodes.
Stream Order
To analyze drainage basin, it is needed to determine
stream orders. In this study, the channel segment of the drainage basin
has been ranked with the lens of Strahler's stream ordering system.
According to Strahler [12],
the smallest fingertip tributaries are designated as order 1. Where two
first order channels join, a channel segment of order 2 is formed;
Where two of order 2 joins, a segment of order 3 is formed.
Stream Length
In this study, Horton law has been applied to measure
stream length (Lu) of Old Brahmaputra River basin. Stream Length
reflects surface runoff of a river basin which is one of the essential
hydro-morphometric features for assessing flood hazard vulnerability.
According to Horton [13],
Stream length of a river basin is higher in first order and gradually
decreases in next orders. GIS spatial analysis in hydrological
environment is used to compute number of stream in a stream order and
their stream length.
Drainage and Stream Density
Drainage and Stream density are important
morphometric parameters to analyze flood risk. The concept of drainage
and stream density model developed by Horton [14]
has been used in this study to analyze spatial vulnerability of flood
in Old Bramhaputra River basin. The core ideas of these models are that a
basin having high stream and drainage density is most vulnerable to
flooding [13].
With the use of GIS tools and techniques firstly measured stream length
and then divided it by total area of the basin to calculate drainage
density. Furthermore, stream density computed by counting number of
stream and then divided it by area of basin.
Drainage Density (Dd) = Lu / A (l)
Stream Density (Sd ) = Ns / A (2)
Where, Dd is drainage density, Lu is the total stream
length of all orders, Sd is stream density, Ns is the total stream of
all orders and A is the Basin area (km2).
Results and Discussion
Flood vulnerability of old brahmaputra river basin from hydro-morphometric modeling
According to Horton's stream density and drainage
density reflects the runoff and discharge status of a basin which also
revealed that basin having high stream density and drainage density is
most vulnerable to flooding. The old Brahmaputra River Basin subdivided
into five sub-Basin named as sub-basin A, B, C, D and E and their stream
density are 1.318km-2, 1.113km2, 1.2009km-2, 1.028km-2, 0.998km-2 and their drainage density are 0.713km-1, 0.690km-1, 0.689km-1, 0.703km-1 and 0.703km-1, respectively (Figure 3) and (Table 1).
Among the five sub-basins of this river basin, sub-basin A contains
high stream and drainage density, which means this basin is most
vulnerable to flooding, whereas basin C is least vulnerable because of
its low stream and drainage density.
Community flood vulnerability of brahmaputra river basin
The flooding area of this river basin is 2956.43km2, including 0.056139km2 in Brahmanbaria, 12.527km2 in Gazipur, 46.0295km2 in Jamalpur, 114.664km2 in Kishoreganj, 1926.51km2 in Mymensingh, 267.303km2 in Narsingdi district, 533.258km2
in Netrokona, 55.4984km2 Sherpur and 0.583869km2 in Tangail of 24% of total land; Vegetation covers the area of 153752 district (Figure 4) and (Table 2).
Among the studied districts hectors which is the 14% of total land;
water bodies covers of this river basin, 65% of area of Mymensingh is
vulnerable to the area of 147410 hectors which is the 14 % of total land
and flood hazard.
Flood risk for resources of old brahmaputra river basin
In Brahmaputra River Basin land use and land cover classification (Figure 5)
revealed that Agricultural Land covers the maximum area of 397444
hectors which is 37% of total land; fallow land covers the area of
250696 hectors which consists settlement covers area of 119729 hectors
which is 11% hectors of the research area (Table 3).
On the basis of severity flood in Brahmaputra River Basin are
classified into 6 such as low flash flood, severe flash flood, low river
flood, moderate river flood and severe river flood and storm tidal
surge (Figure 4).
Low flash flooding affects area of 2719.62 hectors agriculture land,
200.07 hectors fallow land, 336.96 hectors vegetation cover, 396.27
hectors settlements and 84.33 hectors water bodies in this river basin.
Severe flash flooding affects area of 157.86 hectors agriculture land,
4.95 hectors fallow land, and 2.7 hectors vegetation cover, 18.54
hectors settlements and 0.63 hectors water bodies in this river basin.
Low River Flooding affects area of 128218 hectors agriculture land,
57509.2 hectors fallow land, and 392 37.9 hectors vegetation cover,
55319.6 hectors settlements and 5666.4 hectors water bodies in this
river basin.
Moderate river flooding affects area of 1910.52
hectors agriculture land, 1704.42 hectors fallow land, and 1333.26
hectors vegetation cover, 527.58 hectors settlements and 154.62 hectors
water bodies in this river basin. Severe river flooding affects area of
33.66 hectors agriculture land, 855.27 hectors fallow land, and
548.55hectors vegetation cover, 275.22hectors settlements and 177.3
hectors water bodies in this river basin (Table 4) and (Figure 6).
(Note: LFF=Low Flash Flooding, SFF= Severe Flash
Flooding, LRF: Low River Flooding, MRF= Moderate River Flooding and SRF:
Severe River Flooding)
From this analysis it can be said that agriculture
land and resource of this river basin are more vulnerable and which is
life defence of the community of Brahmaputra River basin.
Conclusion
The drainage network of Old Brahmaputra River Basin
reveals that the basin is vulnerable to flooding due to its geotectonic
formation and low elevation where over rain often causes flash flood and
seasonal flood visit every single year. The flood zoning showed in the
study that comparatively north eastern part of the basin area is at more
risk that southern part due to elevation and number of streams of high
order that carries huge volume of water. Community people of Old
Brahmaputra River Basin are more at risk than the actual figures show in
the research. As land use classes for assessing resources vulnerability
reflect that vegetation and agriculture are proportionally much higher
in account of area than settlements that do not mean people are safer,
people living in the vicinity live densely due to lack of higher grounds
that stays above water. Though people know that they are vulnerable and
exposed to disasters the sense of love for the place and locality makes
them stay and a few don't even have the ability to move somewhere to
settle. Keeping all the risks aside they developed some indigenous
knowledge by living there for years where they anticipate hazards and
tries to protect and mitigate in their own local capabilities.
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