Download environmental problems and solution strategies in lowlands

ENVIRONMENTAL PROBLEMS AND
SOLUTION STRATEGIES IN LOWLANDS
Winai Liengcharernsit'
INTRODUCTION
Many large cities in the world are
industrial and agricultural uses. In the coastal
located in lowlands which occurred from
lowland areas where population density is
deposition of river sediment as it drained
high and there exist many human activities,
into the sea. These areas are normally a flat
the rates of groundwater extraction is nor­
plain with fertile soils. so most ancient
mally high which results in lowering of
settlers preferred to situate and developed
piezometric heads in groundwater aquifers,
as agricullural lands. The existing water
and consequently, the problems on land
resources in the areas were mainly used for
subsidence and sallwater intrusion will
farming. Later, as population increased,
occur.
these agricullural communities gradually
Tidal fluctuation in the sea will
developed to urban areas with many human
affect flow pattern and pollutant dispersion
activities. As a resull, the demands of water
in an estuary. Saltwater intrusion into
for various purposes, including
domestic
upstream reaches of the lowland rivers is
consumption, commercial and industrial uses,
one important issue especially during dry
have increased. At present, in many lowland
periods when freshwater discharge is not
areas, shortage of water supply becomes one
adequate to flush the saline water. Deposition
main problem, and allocation of the avail­
of suspended sediment at the river mouths
able water resources for various types of
normally occurs and regular dredging is
water use is one essential work.
needed to enable navigation.
Groundwater is one main source of
Rapid urbanization and industri­
water supply for domestic consumption,
alization in lowlands have caused water
Associate Professer, Department of Civil Engineering, Faculty of Engineering, Kasetsart
University. Bangkok, Thailand & Saga University, Saga, Japan.
pollution problem. Disposal of domestic and
river and its tributaries will have an influ­
industrial wastewaters into the rivers or
ence on the flow characteristics of the
canal networks results in deterioration in
lowland river. In the estuary which drains
water quality and affects aquatic ecosys­
runoff from the river basin into the sea, flow
tems. In some places, water quality is so poor
characteristics are also influenced by tidal
that no fish can survive and use of the river
fluctuation in the sea. Not only does the flow
waters is only for sewage disposal and
pattern vary from time to time, but also water
drainage purposes.
quality characteristics will always change.
the
During the spring tides, sea water intrude the
lowlands is rather poor due to flatness of the
upstream reach of the estuary, while during
areas. As a result, flooding always occurs
the ebb tides, the saline water will be drained
when there is heavy rainfall in the catchment
back into the sea. With the tidal effect,
area of the basin. Flood protection and
dispersion phenomenon in the estuary is
drainage works are very important for
much different from that in the free flowing
lowland development.
fiver.
Usually, drainage capacity
In
In addition, some environmental
The lowland river usually had a
phenomena such as sea level rise due to
very flat slope, so flow velocity is rather low
global warming will cause greater impacts
while the discharge is high because it
on lowland coastal areas than other regions.
receives water from most parts of the basin.
Therefore, it is necessary that the problems
The flow pattern in the downstream reach
in lowland development and management
of the lowland river is mainly governed by
should receive high attention from the
tidal fluctuation which characterizes the
decision makers who are concerned with
back-and-forth flow condition.
water resources and environmental planning
and management.
In some lowland areas there exist
open channel networks which have been
constructed for some purposes including
FLOW CHARACTERISTICS OF
LOWLAND WATERS
age, etc. These open channel networks are
A lowland river or estuary normally
found to be an effective drainage facility in
receives runoff from the whole watershed
the areas which are often subject to flooding
of the river basin. Therefore, distribution
caused by heavy rainfall. They can server as
patterns of rainfall, land use, water use,
retarding reservoirs for flood damage miti­
vegetation cover, soil characteristics,
gation. In some areas, these open channel
topography, hydrogeology, and all water
networks are used as intermediate water
resources development projects in the main
body for receiving wastewater from the
irrigation water supply, navigation, drain­
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.u"::"l1u 2537
communities prior to discharging into the
population density is great and there exist
main river. In order to serve their main
many human activities, the problem of
functions properly, these open channel
saltwater intrusion into groundwater aqui­
networks must be carefully designed, con­
fers usually occurs. This is caused by
structed, and operated. The factors that must
excessive withdrawal of groundwater
be taken into consideration include water
which results in lowering of piezometric
sources, topography, land use, water
head in the groundwater extraction zone.
demand, rainfall and runoff characteristic,
The problem of saltwater intrusion is not
tidal phenomena, etc.
always immediately recognized, but once
,
it is obviously observed it is often evident
GROUNDWATER UTILIZA­
TION
Groundwater is one main source of
that intensive and costly measures are
required to solve the problem (van Dam
1992).
In some regions, a lot of groundwater
water supply either for domestic consump­
wells have been abandoned. In order to
tion, industrial or agricultural use. In the
prevent this phenomenon, the rate of
lowland areas, there usually exist good
groundwater extraction must be reduces, or
groundwater aquifers. Rainfall in the catch­
in some cases, the location of groundwater
ment area may penetrate the surface soil
extraction zone must be changed.
directly to the groundwater aquifers or may
Groundwater plays an important
enter the streams and percolate from these
role in water resources management. In
channels to the aquifers. Groundwater will
many regions it is the only source of water
slowly flow to the lowland areas where
for various uses. The large volume of water
piezometric heads are normally lower and
stored in the aquifers can serve as a buffer
there exists an intensive extraction of
which supplies water during the drought
groundwater for various purposes.
periods. Seasonal fluctuations in ground
In the deltaic plains, the level of
water levels and storage volumes are rather
groundwater table in the upper unconfined
small compared with surface water re­
aquifer is normally high compared with
sources. While regulations of surface water
ground surface. This condition together with
resources requires costly hydraulic struc­
low permeability of clayey soil result in low
tures such as dams, weirs, and storage ponds,
rate of water seepage to the underground
regulations of groundwater resources can be
zone. This will affect drainage capacity of
made by some appropriate management
the areas and also affect the function of
schemes, such as a proper distribution of
seepage pits used in wastewater disposal.
groundwater extraction wells and recharge
In deltaic and coastal areas where
wells (Bear 1979). As regards water quality,
LlJ~1tl'l.l
n~n!l1f'1lJ
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groundwater quality is normally better than
frequencies. sinking of bench marks. and
surface water quality since it is less suscep­
damages to existing sewerage systems and
tible to man-made pollution and due to the
building foundation.
natural purification process - subsurface
In order to decrease the rate of land
filtration. Many groundwater wells provide
subsidence. the rate of groundwater extrac­
clean water which can be directly consumed
tion must be reduced. Natural and artificial
without any treatment.
recharges of groundwater can help slow
Actually. surface water and
down the land subsidence rate.
groundwater are closely interrelated. During
groundwater aquifers through the stream
SALTWATER INTRUSION TO
COASTAL GROUNDWATER
AQUIFERS
banks. In the dry periods. water released
In deltaic and coastal areas where
from groundwater aquifers serves as a base
population density is great and there
flow in the stream. Spring discharge is on
exist many human activities. the problem
example of groundwater emerging at the
of saltwater intrusion into groundwater
ground surface and becoming surface runoff
aquifers usually occurs. This is caused by
(Bear 1979). Thus. the management of
excessive withdrawal of groundwater
regional water resources should consider
which results in lowering of piezometric
both surface water and groundwater simul­
head in the groundwater extraction zone.
taneously.
The
flood periods. some portion of the stream
water will infiltrate into the underlying
interface between fresh and saline
water moves upward and the length of the
LAND SUBSIDENCE
saltwater tongue increases due to inflow
When groundwater is extracted from
of saline water into the aquifers from the
the aquifers. the piezometric head will
seaside. The problem of saltwater intrusion
decrease which results in a decrease in pore
is not always immediately recognized
pressure and an increase in effective stress
because it is in the subsoil. but once it
in soils. and consequently. consolidation
is obviously observed it is often evident
will take place and land subsidence occurs.
that intensive and costly measures
In some deltaic plains with soft clay layers
required to solve the problem (van Dam
and with excessive groundwater extraction.
1992). In some regions. a lot of groundwater
the rate of land subsidence is rather high. in
wells have been abandoned.
the order of
per year or more. Land
Besides excessive extraction of
subsidence can cause some subsequent
groundwater. artificial control of surface
damages such as increasing flood stages and
water in an area or land drainage. particu­
1 0 Col
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LtI:lJY1 22 lJ';::'11lJ 2537
lady in land reclamation projects, can also
for emergencies, etc. The ultimate volume
cause serious saltwater intrusion problem, In
of fresh groundwater depends on the
some land reclamation projects the lowering
mutual location of recharge works and
of water table can be of several meters. It is
extraction works. Artificial recharge
obvious that such great lowering of the water
practiced either from surface water in
table in a large area must have considerable
phreatic aquifers or by means of injection
effect on the regional groundwater flow
wells into shallow and deep aquifers.
system and may cause an unfavorable
3)
IS
Extraction of saline and
distribution of fresh, brackish and saline
brackish groundwater. In some cases
groundwater.
brackish or saline groundwater is extracted
Control of saltwater intrusion into
below fresh groundwater. This water can
the coastal groundwater aquifers can be
be used for cooling purposes or for
made by the following methods (van Dam
desalting. Such extraction causes the volume
1992):
of fresh groundwater to grow and the volume
of
of brackish and saline groundwater to
groundwater extraction and relocation
decrease. Complete control of the interface
extraction wells. In the coastal aquifers. only
is possible by simultaneous extraction of
a fraction of the recharge, whether natural
fresh and saline groundwater, provided that
or artificial, can be extracted safely. The
there is a good monitoring system.
1)
Reducing
the
rate
remaining portion serves for maintaining
it fulfills its function to hold up the
SALTWATER INTRUSION TO
UPSTREAM RIVERS
saltwater intrusion. The fraction of the
Saltwater intrusion to up stream
recharge the can be safely extracted
livers in coastal lowland areas is one
depends on the location of the wells. This
significant issue. This can cause great dam­
implies that the fraction can be optimized
age to agricultural production and restrict
by proper design and operation of the
the use of river water. Since the lowlands are
groundwater wells.
rather flat, saltwater from the sea can intrude
the body of freshwater, while flowing out
Artificial recharge. Artificial
very far to the upstream reaches of the
recharge of groundwater is applied for
rivers, especially during the periods of low
many reasons, such as to increase the safe
freshwater discharges from the upstream
yield, to raise the groundwater table or the
rivers and high tides in the sea.
2)
piezometric head in order to limit or slow
In order to decrease the length of
down the rate of land subsidence, and to
saline water portion in the river, the amount
increase the volume of fresh groundwater
of freshwater discharge must be controlled.
This can be done by constructing a regulat­
while the ground surface is sinking. In­
ing reservoir on the upstream river. The
creased flooding in most areas is related to
reservoir operation will be conducted in such
changes in land use patterns in the upstream
a way that an adequate amount of water will
watersheds as well as in the flooded area
always be released from the reservoir to
itself. Destruction of forest lands results in
flush the intruded saline water. In some
higher rate of surface runoff. Expansion of
areas where construction and operation of
urban areas also increases the runoff coef­
such a regulating reservoir is not possible,
ficient and is one main factor of increased
a storage weir is constructed as a barrier
flooding unless adequate drainage facilities
between the freshwater from the upstream
are provided.
Many mitigation measures have
ri ver and saline water from the sea,
been applied to control flooding so as to
FLOODING, FLOOD CONTROL
AND DRAINAGE
Most lowland areas are down­
stream ponions of the river basins which
collect runoff water from the surrounding
watersheds. In general, drainage capacity in
the lowland is rather poor due to flamess
of the area and its low ground surface
elevation compared with water level in a
minimize the damages. These inel ude
reservoir impoundment to regulate fiver
runoff, construction of levees or flood
walls along the river banks, diversion of
flood water through a bypass channel.
improvement of discharge capacity of
the downstream river to lower flood level,
improvement of drainage systems in the
protected area etc. (Linsley & Franzini
1979).
nearby river or in the sea, In many
lowlands ground surface elevation is even
below thc sea water level and dikes must
be constructed along the river banks and
shoreline. With this topographic feature,
flooding always occurs when there is heavy
rainfall in the watershed area, Sometimes,
flash flood occurs through destruction of
Selection of the most suitable
control measure depends on the cause of
flooding, hydrologic regime, land use
pattern and topographic feature of the area,
as well as the available budget. Combination
of several flood control methods is
normally practiced, especially in the low­
lands where flooding is related to several
factors.
river banks or dikes which can cause serious
Reservoir impoundment can help
damages to lives and propenies. In coastal
reduce the peak runoff and thus decrease
area, flooding can be directly correlated with
flood level. In many reservoir impoundment
land subsidence. The sea water level and the
projects, flood control is considerd as one
storm surge level have gradually increased
main purpose in addition to hydropower and
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LfllJYI 22
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2537
irrigation development. For flood control
overflow often occurs during high discharge
purpose the impounded reservoir should
or high tide period. However. this flood
be located immediatly upstream from the
barrier will obstruct drainage of inland
protected area and is operated to cut off the
storm water to the river or to the most
flood peak. All flow above the safe dis­
widely used method to solve this interior
charge capacity of the downstream river will
drainage problem is to collect water at
be stored in the reservoir, and the stored
some low points and pump over the barrier
water is gradually released to recover
during flood when gravity flow through
storage capacity for next flood (Linsley &
outlet is impossible. The inland water may
Famzini 1979). In practice, however, selection
also be collected in an open channel on the
of the most suitable damsite depends on
land side of the barrier and transport
topographic and geologic features as well as
downstream to a point where gravity dis­
environmental and socio-economic factors.
charge is possible.
Besides the storage reservoir, a
Construction of a diversion chan­
retarding basin can be used to reduce flood
nel or flood way to bypass flood water is
magnitude. This retarding basin is normally
one possible alternative for flood damage
constructed with fixed, ungated outlets
reduction. Opportunities for this alternative
which automatically regulate the outflow in
are usually limited by topography and
accordance with the volume of water in the
availability of low-value land which can be
basin. The outlet can be constructed as a
used as the floodway.
spillway or a sluiceway. The discharge
Improvement of downstream
capacity of the outlet for the retarding basin
channel to increase its hydraulic capacity
at full storage must not be greater than the
can help reduce flood state in the upstream
maximum discharge that downstream river
river. This can be simply done by removing
can pass without causing severe flood
brush and snags, and dredging the deposited
damage. The basin volume must not be less
sediment. Completely lining and straighten­
than the flow volume of the design flood
ing of bends are also undertaken in many
minus the volume of water released during
places.
the flood (Linsley & Franzini
1979).
In order prevent flooding, not only
Construction of a levee or a flood
should the facilities such as dikes, reservoirs,
wall to serve as a barrier for protecting land
or diversion channels be provided. but also
from flood water is one of the oldest and
the drainage system in the protected area
most widely used flood control methods.
must be designed and operated properly. In
This flood barrier is usually constructed
the urban area, storm water is usually col­
along the river banks or shoreline where
lected and transported via buried conduits to
L~l:Y1!J'U
11'l'ln'i'i~.1'i ~n. 81
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a point where it can be discharged into a
will erode surface soils and carry along with
stream or man-made canal. In the lowland
surface runoff to the receiving stream. In
area. an open-challnel
network with a
addition, during the periods of high flow,
number of pumping stations are commonly
erosion of stream banks usually occurs
used for urban drainage. For an agricultural
causing high turbid water in the stream. In
land, the function of the drainage system
the dry season, water stored in natural
includes removal of excess surface water as
storage in the basin including stream banks
well as control of groundwater level below
and groundwater aquifers is gradually re­
the root zone to improve plant growth and
leased. This water is of sediment free which
reduce accumulation of salts in the top soils.
results in clear water in the stream.
Erosion and sedimentation proc­
EROSION AND SEDIMENTA­
TION
esses in the lowland areas have some spe­
Erosion of surface soils in the
ent from other areas. The lowland river is
upstream watersheds is the main source
normally of low slope, thus flow velocity is
of sediment in a stream. Small sediment
rather small which enables deposition of
particles can be transported as suspended
suspended sediment. The tidal effect from
sediment while larger particles are carried
the sea wi II result in an oscillatory flow in the
along the stream bottom as bed load. The
downstream reaches of the river. This will
suspended sediment particles tend to settle
affect sediment transport capacity and as a
to the channel bottom since the specific
result large amount of sediment will be
gravity of these particles is relatively high,
deposited forming deltaic plain at the river
while turbulent flow in the stream will
mouth. Periodic change in water level due to
counteract the gravitational settling. When
tidal fluctuation also affects stability of river
flow velocity in the stream decreases as it
banks. As tidal water propagates back and
drains into the sea or a storage reservoir,
forth in the downstream river erosion of river
sedimentation of the suspended sediment
banks will take place.
cific features which are more or less differ­
will take place. Large particles and most of
Suspended sediments play an im­
the bed load are deposited as a delta at the
portant role in mass transport processes in
river mouth or head of the reservoir, while
estuaries, and they are usually transported
smaller particles are deposited farther and
to the upper region of estuaries by tidal
some may remain in suspension.
dynamics and estuarine circulation. In estu­
Seasonal variations in sediment
aries, the tidal current asymmetry produces
load are normally higher than variations in
sediment transport so that turbidity maxima
streamflow. In the rainy season, rainwater
are formed and travel upstream and down­
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LfUlYI 22 lJ'i::"nlJ 2537
stream according to tidal current (Futawatair
ments, and industries, toxic substances from
& Kusuda 1993).
mineral productions, heavy metals in in­
Protection and rehabilitation of
dustrial wastewater and sludge, pesticides
forest lands in the watersheds can help
from farm lands, poisonous leachate from
reduce the rate of soile reosion. Protection
solid waste landfill sites, etc. Some of these
of river banks either by some vegetation
substances may cause acute poisoning in
covers or complete lining is necessary in
man and animals, but some may accumu­
some places. Periodic dredging of channel
lated in cell tissues and be responsible for
bottom is sometimes needed to enable navi­
chronic poisoning.
gation.
Most lowlands are the downstream
part of a river basin which collects surface
WATER POLLUTION
runoff from the whole catchment area prior
Nowadays, conservation and en­
to discharging into the sea or other receiving
hancement of environmental quality are of
water bodies. Most of the pollutants from
major concern of worldwide public. Among
various sources in the basin will find their
various environmental issues, water pollu­
ways 10 reach to downstream rivers or
tion problem has received a great attention.
estuaries. In addition, most lowlands are of
It is evident that water quality in most natural
dense population areas with several kinds.
streams, rivers, lakes, estuarine and coastal
In addittion, most lowlands are of dense
waters has been deteriorated to some extent
population areas with several kinds of
due to disposal of polluting wastes resulting
human activities are undertaken and large
from human activities, Pollution of surface
amounts of wastes are generated. As a result,
water, coastal water, and groundwater is a
water quality in the rivers or estuaries in
treat to public health, aquatic ecology, aes­
lowlands is usually poorer than in other
thetic value, and various types of water uses.
parts of the regions. Nowadays, water
To conserve and improve the water envi­
pollution problem becomes one of the
ronment' it is necessary to protect the available
major environmental issues in most
water resources from any acts, such as the
lowland areas.
disposal of harmful substances which cause
In the lowland coastal area, not
degradation in water quality and adversely
only does the flow pattern in the estuary be
affect its usefulness.
influenced by tidal ,fluctuation, but also
A variety of polluting substances
estuarine water quality always change from
may enter to a receiving water body or public
time to time. During the spring tides, water
water supply, these include organic materi­
will be more saline due to intrusion of
as
seawater, while during the ebb tides, the
from households, commercial establish­
saline water will be replaced by freshwater
well as the managerial, legislative and
from the upstream river. Fluctuation in
financial structures of the water manage­
estuarine water quality has great influence
ment field.
on the ecosystem and also affects water uses
in the area.
The national policy on water
quality management must be established.
Fluctuation in flow velocity in the
This policy document is a strategic plan
estuary due to tidal phenomena also affects
for solving water pollution problem. The
dispersion of the discharged pollutants.
policy document should set goals for the
During the spring tides, these pollutants
functions of the water systems to be
can propagate to the upstream river for a
achieved in a certain period of time. set
considerable distance, while during the ebb
priorities and standards and initiates
ties, the pollutants will be transported back
changes in the organizational. legislative
to the downstream river and finally drained
and financial structures to facilitate the
out into the sea. Water pollution control and
management.
management in the lowland coastal areas
For the management purpose. some
require clear understanding on the hydro­
water quality parameters must be selected
dynamic and dispersion phenomena of the
to measure status of water quality in the
estuary.
receiving water body and to measure
Lowland water resources nonnally
the strength and harmfulness of the
serve multipurpose of water use including
discharging wastes. Among various water
being the main source of domestic. indus­
quality parameters, biochemical oxygen
trial, and agricultural water supply, trans­
demand (BOD) and chemical oxygen
portation, aquaculture and fishery. recrea­
demand (COD) are widely used as indica­
tion and tourism, etc. Pollution problem in
tors for the organic wastes. Dissolved
the lowland water will cause great impacts
oxygen concentration in water is essential
on the existing communities. Therefore.
for aquatic life and is nonnally used as
water quality management is one essential
an indicator to measure the severity of
element of lowland areas.
pollution problem in surface water. Heavy
The starting point for water quality
metals. pesticides, colifonn bacteria, and
management is a water system analysis.
some other toxic substances
This analysis does not only include the
as pollution indicators.
are also used
physical, chemical and biological qualities
A number of water quality stand­
of a drainage basin up to its watershed, but
ards must be set up. There are many types of
also includes the functional use of water,
standards depending upon the functional
its ecological values and its potentials. as
use of water, these include drinking-water
,
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1J~::'l11J
Li!)J" 22
2537
standards, stream standards, effluent stand­
waters, defined functional uses, and appro­
ards, and water quality standards for some
priate water quality criteria musl be taken
other types of waler use. These standards
into consideration, in order to provide
are considered as the objectives for water
adequate protection of the available waler
quality management which includes
resources, For the purposes of poilulion
water treatment, wastewater treatmenl and
control, enforcement by laws is needed.
disposal, solid waste and hazardous waste
Some regulations and legislalions must
management.
be established with penalty
Public awareness on the conse­
10
those who
violate the laws.
quences of waler pollution and the cosls
of its control should be created in order
10
get the public support and cooperalion
In
pollution control. To create such
REFERENCES
Bear, J.
1979.
Hydraulics of
groundwater. McGraw- Hill, Israel.
awareness, some well-planned public
FUlawatari, T. & T. Kusuda
1993.
education programs should be prepared
Modeling of suspended sediment Iransport
and implemented. the public should be
in a tidal river. In AJ, Mehta (ed.), Nearshore
convinced thaI poor waler quality will
and estuarine cohesive sediment transport.
have direct effect on Iheir health and
Coastal and estuarine studies:
prohibit olher beneficial use of the water
Geophysical Union.
body. Without strong public support, the
waler pollulion problem is often ignored.
The discharge of all wastewater
into the
receiving walers must be
J2.
American
Linsley, R.K. & J.B Franzini
1 979.
Water resources engineering. McGraw- Hill,
Tokyo.
van Dam, J.e.
1992,
Problems
controlled in a rational manner. In
associated with saltwater intrusion into
addition
specific wastewater character­
coaslal aquifers and some solutions. Proc.
iSlics, some other factors such as discharge
ILT seminar on problems of lowland devel­
location, characteristics of the receiving
opment. Saga, Japan.
10