Evaluating the Impacts of an Improperly Designed Bridge across the Batticaloa Lagoon

Evaluating the Impacts of an Improperly Designed
Bridge across the Batticaloa Lagoon
S. Santharooban
Department of Zoology, Eastern University, Sri Lanka, Batticaloa, Sri Lanka
And

M. Manobavan
North East Costal Community Development Project, Batticaloa, Sri Lanka

Abstract
This research paper mainly focuses on the negative repercussions, resulting from
the construction of a New Bridge (Puthupalam), on the Batticaloa lagoon.
Construction of a New Bridge across Batticaloa lagoon is an adverse modification,
which will change the spatial structure of the Batticaloa lagoon while
simultaneously destruct the biodiversity of the lagoon. In this research, simulation
modeling approach is used to support the hypothesis, which assumes that this New
Bridge negatively impacts on the lagoon ecosystem by reducing the depth which
will change the spatial system of the Batticaloa lagoon. In this respect,
commercially available modeling software STELLA® (Version 7.0.3) was used to
conceptually develop a simulation model, called N-BIOL (New Bridge Impact on
Lagoon) based on the idea of literature and the measurements of lagoon depth and
water velocity, which showed the differences on either side of the New Bridge.
According to the simulated outputs, it is obvious that this New Bridge gradually
reduces the depth over time, which leads to the change of spatial structure of the
Batticaloa lagoon.

Keywords: Ecosystem, Simulation model, Spatial structure, Sediments

Introduction
Lagoons are an important part of the cultural landscape of the Batticaloa district.
The Batticaloa lagoon (7o 24’- 7o 46’ N and 81o 35’- 81o 49’ E) is the largest of
three lagoons (i.e. Batticaloa lagoon, Vakarai lagoon and Valaichchenai lagoon) in
the Batticaloa district and it occupies an area of 168 square kilometers and is 56 km
long (Shanmugaratnam, 1995). The maximum depth of the lagoon is about four
meters (Scot, 1989). More than 90% of the lagoon is located in Batticaloa district,
but the southern end near Kalmunai is located in the Ampara district. The deep
broad lagoon leads into the sea by three Bar mouths, namely Paalameenmadu Bar
mouth in Batticaloa, Periyakallar Bar mouth and Koddaikallar Bar mouth. The
flow of the water is towards the bar mouth when the Bar mouth is remains open
(mostly during the rainy season) and vice versa. This lagoon supports
economically and esthetically to those, who are living closer to it, as it comprises a

Water Professionals’ Day Symposium – October, 2005

Evaluating the Impacts of an Improperly Designed Bridge across the Batticaloa Lagoon

rich variety of aquatic flora (mainly mangroves) and fauna. In addition to this,
Batticaloa lagoon and its peripheral systems act as a form of natural rainwater
harvesting mechanism and can be considered as an important groundwater
recharging zone (Manobavan and Jeyakumar, 2004). The Batticaloa lagoon also
acts as a shock absorber of the Tsunami impact (within its limit) via its peripheral
wetlands and interconnected channels (Manobavan, 2005). The Batticaloa lagoon
comprises several islands within it, namely Puliyantheevu, Mantheevu,
Erumaitheevu (Buffalo Island), Sirayatheevu, Elubutheevu (Bone Island). In these
islands, the Puliyantheevu is important island as it holds central city of the
Batticaloa. Population is mainly concentrated in this island and this created a need
to have bridges across the Batticaloa lagoon to connect the island with the main
land of the Batticaloa district. As a result, three bridges, namely Koddamunai
Bridge, Puthur Bridge and New Bridge (Puthupalam1) were constructed across the
Batticaloa lagoon (Figure 01). The objective of this research paper is to evaluate
the negative environmental impacts, generated by the construction of a New Bridge
in the Batticaloa lagoon.

Figure 01: Location of Bridges across the Batticaloa lagoon (L and R represent the
Left and Right sides, respectively of the New Bridge, and the Bridge is 9.0 m
wide).

1
‘Tamil’ word for New Bridge and this was constructed in the year of 1985.

Water Resources Research in Sri Lanka

S. Santharooban And M. Manobavan

Outline of the Problem
Even though this New Bridge and contiguous road is extended for less than quarter
kilometer over the Batticaloa lagoon, it allows only a small opening for the
provision of the natural water flow. Hence, this bridge and associated road acts as
partial impoundments or as a quasi-dam across the lagoon. This partial
impoundment of this road is suspected to be creating lots of problems on lagoon
system dynamics and its spatial structure. And, it is also suspected to be changing
the biodiversity of the lagoon. This bridge may alter the flow regime by reducing
the water velocity of the lagoon due to its insufficient openings. According to the
results obtained from sampling2, the velocity variation curve was plotted (Figure
02c). According to Figure 02c, when water flows from left to right across the
bridge, the current velocity is decreased in right side or vice versa. This reduction
in velocity is resulted from partial impoundments and it is also obvious from the
graph (Figure 02c) that the velocity under the bridge is high. This can be explained
by using Equation 1.

AV = k [01]

Where; k – Flow rate
A – The cross sectional area of the stream
V- Fluid velocity

This equation can be applied to incompressible and non-viscous fluid undergoing
steady flow on a streamline (Roger, 1981). In applying this equation (Eq. 1) to this
lagoon system, when the water velocity is increased as the cross sectional area
decreases i.e. when width of the lagoon decreases, the water velocity is increased to
maintain an equal flow rate in all points. It is important to note here that when the
velocity under the bridge increases, the soil erosion is accelerated. This is
supported from the results of sampling as shown in Figure 02a, where the P is the
proximal point to the bridge (at the edge) and has high depth and S is the distal
point to bridge (120 m from either side) and has low depth and Q and R are points
in between (The distance between each two point is 40 km). This indicates the
erosion of lagoon bed under the bridge. This eroded bed load will accumulate in
one side leading to the further depth reduction.

Allan (1995) stated that extensive phytoplankton blooms often develop as a
consequence of slowed passage of water. In supporting this comment, it was
obvious that there was heavy accumulation of waste, aquatic macrophytes, algal

2
Sampling was done periodically across the New Bridge from 6th December 2004
to 11th April 2005 to measure the water current velocity and lagoon depth in either
side of the New Bridge (Figure 01)



bloom3 (Chladophora sp dominates the algal bloom in the left side of the bridge)
the in left side of the lagoon due to the New Bridge and contiguous road. When all
these accumulates decompose, the suspended solid particles increase, which in
turns increasing the turbidity of the water (Figure 02b). According to Figure 02b, it
is obvious that the turbidity is high in many points in the left side than in the right
side. The results further show that turbidity is significant difference between two
sides of the bridge (t= 2.871, p= 0.014 for two tail two sample student t-test).
Hence, these increased loads of suspended solid particles undergo sedimentation by
the influence of reduced current velocity. This increased sedimentation will result
the reduction of depth in this area. This can be supported by the results, which
showed a statistically significant difference in two sides (t= -8.077, p= 0.00 for the
two tail two sample student t-test) and this can be shown by the graph (Figure 02d),
which clearly indicates that the average depth of the center of lagoon is high in
right side (R) than in left.

As the depth is reduced then again velocity of the water is reduced according to the
following Manning equation for velocity of flow in a channel (in metric units)

1 2 / 3 1/ 2
U = R S [2]
n
Where; R is the hydraulic radius about equal to mean depth for most channels, S is
the energy gradient; approximately the slope of the water surface and n is the
Manning resistance coefficient (Barns, 1967).

Apart from this New Bridge (Puthupalam), there is another bridge, called Puthur
Bridge, which also lacks the proper natural water flow. As a result, a pond like
portion, enclosed by two bridges was created in left side (L) of the New Bridge
(Figure 01). Hence, as a combined effect of these two bridges, there will be a
heavy depth reduction in the area of the lagoon, enclosed by the New Bridge and
Puthur Bridge. This in future makes the change in the lagoon spatial system. In
this research, the modeling approach is used to predict the temporal changes of the
lagoon depth.

Hypothesis:
Batticaloa lagoon has been divided in to two half by the construction of the New
Bridge (Puthupalam), which negatively impacts on lagoon system by creating the
depth reduction in lagoon and this depth reduction will change the spatial process
of the Batticaloa lagoon overtime.



250 50 L R

Turbidity (FAU)
200 L R 40
Depth (cm)

150 30
100 20
50
10
0
0
P Q R S
1 2 3 4 5 6 7 8 9 10111213
Location from Bridge
Week s
P is proximal point to New Bridge and S is
the distal point to New Bridge
(a) (b)
(a)
L
Under Bridge
0.6 R 200 L R
0 180
0.4 160
0
140
Depth (cm)

0.2 120
0
100
Velocity (M/S)

0 80
1 2 3 4 5 6 7 8 9 10 11 12 13 0
60
-0.2
- 40
20
-0.4
- 0
1 2 3 4 5 6 7 8 9 10111213
-0.6 -

Weeks
We eks
-0.8 -

Water flow direction from L to R is assumed as
positive and from R to L is assumed as negative

(c) (d)

Figure 02: Graphical results of sampling carried out across the New Bridge in
lagoon; (a) Depth of lagoon in different localities from New Bridge; (b) Turbidity
variation on either side of New Bridge; (c) Variation in water velocity in either side
of Bridge; (d) Variation in lagoon depth in either side of the Bridge.



Development of a Conceptual Model
As already mentioned, a modeling approach is used to predict the changes in
future. A model is a formulation that mimics real world phenomena and by means
of which predictions can be made (Odum, 1971). We can continually study the
behavior of systems by creating mental or conceptual models of them. The
conceptual model uses the conceptual idea of the modeler. In this research, a
conceptual model was developed using commeciallly available modeling software
STELLA® Version 7.0.3. The STELLA software provides a simple, icon-based
language that enables ones to construct their understanding of any dynamic
phenomenon, occurring in the world. A conceptual model was developed as shown
in Figure 03, based on the idea of literature evidences and sampling data, presented
in section 2.0. Based on the conceptual model (Figure 03), another conceptual
model, called N-BIOL (New Bridge Impacts on Lagoon) was developed using
STELLA modeling platform. This is shown in Figure 04. This STELLA
conceptual model was simulated for ten years to obtain the simulation output.

Length of New Bridge Road
across the lagoon is increased

Velocity of water in
Water flowing width either side of road is Waste, phytoplankton and
of lagoon is reduced aquatic macrophytes are
decreased
accumulated

Sedimentation is
increased

Velocity under the
bridge increases
Changes in spatial
Depth of lagoon is structure of
decreased lagoon
Erosion of lagoon
bed is accelerated

Temperature of water
increases

Figure 03: Conceptual model developed based on literature evidence and results of
sampling by authors.



water in lagoon water out flow

water inflow
stagnation of accumulates
concentration of accumulates ~
~ Turbidity
inflow of accumulates accumulates
Water velocity
out flow of accumulates

Depth of water
inflow of solid particles sedimentation

amount of suspended
solid particles out flow of solid paticles
water temperature

Figure 04: The model developed using the STELLA modeling software.

Simulation Results and Discussion
The simulation output shown in following Figure 5a, indicates that the amount of
suspended solid particles increases while accumulate fluctuate with time. Because
of the reduced out flow of accumulates by the crossing structure, they retains in the
lagoon and at the same time they undergo decomposition, thereby accumulates
fluctuate with time. However, suspended solid particles are recruited with time as
accumulates undergo decomposition, so that, the amount of suspended solid
particles increases with time. However, it is expected that there will be seasonal
variations as accumulates have high rate of settlement in the non-rainy seasons, and
could undergo a state of disturbance in the wet season. Hence, the simulation
shows seasonal lows and highs, though the general trend as a whole is that of an
increasing one for the amount of suspended particles in the lagoon bed.



(a)

1: Depth of water 2: Amount of suspended solid particles 3: Sedimentation

(b)

Figure 05: Simulation outputs of Model N-BIOL (a) Temporal variation of
amount of accumulates and amount of suspended solid particles; (b) Temporal



variation of depth of lagoon, amount of suspended solid particles and
sedimentation.

The simulation output, shown in Figure 05b, indicates that the sedimentation
process increases overtime as the suspended solid particles increases.
Sedimentation can also be increased due to the reduction of current velocity.
However, it is not shown in the computer simulated model. And this output also
indicates the depth variation overtime, which is our key concern. According to this
output, depth of the lagoon will gradually decrease overtime. This reduction will
be possible in the portion of lagoon, where the accumulations are high. Hence, this
depth reduction now was observed in left side of the New Bridge after twenty years
of time period. Moreover, it should be noted that as shown in Figure 05b, the
seasonal fluctuation in accumulates influences the seasonal fluctuation of the depth
as well. Therefore, in the Batticaloa lagoon especially in left side of the New
Bridge (Figure 01), which is enclosed by the presence of two bridges, namely New
Bridge (Puthupalam) and Puthur Bridge, the lagoon depth will be reduced
gradually with time. As a result, the lagoon bed will be raised (since its depth is
reduced) and in the long run it will evolve into a semi-dry land, which will be dried
out in dry seasons and be flooded in wet seasons. This will lead to disturb the
biodiversity even further. This is supported by the modeling outputs (Figure 05),
and the lagoon depth will be reduced further in future affecting the aquatic
biodiversity.

It should be noted that since the simulations outputs are the result of a conceptual
modeling exercise, hence the validation of the model output results against the field
data gathered becomes impossible. Whilst this could be taken as a weaker element
in this exercise, considering the timeframe and the resources needed, such a
validation becomes an activity that is beyond the scope of this project.

Conclusions
The New Bridge functions as a partial impoundment across the Batticaloa Lagoon
and increases the sedimentation over space and time in the area of the lagoon,
enclosed by the New Bridge and Puthur Bridge. Increased levels of sedimentation
could be due to the following reasons:

• During the rainy season, lagoon receives floodwater which carries huge
amount of debris and sand.

• When the floodwater drain from the lagoon through the New Bridge, only
the surface water drains, while debris and sand accumulate on the bottom.

• Algal blooms, waste accumulation, and aquatic macrophyets also increase
the sedimentation when they undergo decomposition.



• The reduced velocity, resulted from partial stagnation of water also
increases the sedimentation.

• Therefore, depth of lagoon, especially the area, enclosed by the New
Bridge and Puthur Bridge will gradually decrease overtime.

• Furthermore, increased algal bloom will facilitate the plant succession,
which leads to the formation of dry vegetation in that particular locality.

Therefore, as hypothesized in section 2.0, this New Bridge and contiguous road
structure negatively impacts on the Batticaloa lagoon by creating the depth
reduction and hence, it will create the change in the spatial structure of the
Batticaloa lagoon and a reduction of the biodiversity consequently.

References
Allan, J.D. (1995). Stream ecology, structure and function of running waters.
Chapman and Hall publication, pp: 310-323.

Barns, H.H. (1967). Roughness characteristics of natural channel. U.S. Geological
Survey, Water supply paper, 1849. pp 213

Manobavan, M., and Jeyakumar, P. (2004). Utilization of natural peripheral water
bodies adjoining the Batticaloa lagoon for fresh water harvesting and/or
recharging: Reflection on the Keeriodai experience. Lanka Rainwater Harvesting
Forum, The Eight Symposium.

Manobavan, M. (2005). Some hypothesis on the behavior of the Batticaloa lagoon
and peripheral wetland systems to the tsunami wave: An earth systems perspective.
available at
http://www.eusl.info/inPerspective/DrManobavan/Some%20hypotheses%20
(Revised).pdf

Odum, P.E. (1971). Fundamentals of ecology. Saunders College Publishing (3rd
ed).

Roger, M. (1981). A-Level physics (4th edn), Stanley Thorne publisher Ltd, pp:
197.

Scot, A.D. (1989). A directory of Asian wetlands. IUCN, The world conservation
union, Cambridge, pp: 605-606.

Shanmugaratnam, N. (1995). The need for and steps towards a master plan for
suitable utilization of the Batticaloa lagoon. Report to NORAD, pp: 1.


Evaluating the Impacts of an Improperly Designed Bridge across the Batticaloa Lagoon

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