Different parts of Sri Lanka are regularly subjected to a wide range of natural disasters, of which floods are most common. When severe, flooding can decrease the economic and social development of the country and the Government of Sri Lanka has to spend huge amounts of money each year to address such problems. Floods occur mostly because of heavy rainfall combined with human-induced factors in the catchment areas. In this project, tidal level variation is considered as a factor for floods in the river basins. The tidal level changes periodically due to the gravitational attraction from the sun and the moon and the centrifugal force of the earth’s rotation. This project studied the relationship between changes in tide and river water level in the mouth of the Kelani River. Tidal data was collected from the Colombo Harbor, and water level data and river flow data was obtained from the Nagalagam Street gauge and Hanwella gauge. It was found that there is a direct relationship between tidal level and flood level in the river mouth area. Therefore, it is proposed that tidal level variations be considered in order to make accurate flood predictions in the river mouth areas.Length: pp.119-126River basinsFlooding

Gunathilaka, M. D. E. K.

Perera, D. N. D.

Wikramanayake, W. A. L.

English

Research Papers in Economics

119
Identifying the Impact of Tidal Level Variation on River 
Basin Flooding 
M. D. E. K. Gunathilaka, W. A. L. Wikramanayake and D. N. D. Perera 
Department of Surveying and Geodesy, Faculty of Geomatics, Sabaragamuwa University of 
Sri Lanka, Belihuloya, Sri Lanka
Abstract
Different parts of Sri Lanka are regularly subjected to a wide range of natural disasters, of 
which ﬂoods are most common.  When severe, ﬂooding can decrease the economic and social 
development of the country and the Government of Sri Lanka has to spend huge amounts of 
money each year to address such problems. Floods occur mostly because of heavy rainfall 
combined with human-induced factors in the catchment areas. In this project, tidal level variation 
is considered as a factor for ﬂoods in the river basins. The tidal level changes periodically due 
to the gravitational attraction from the sun and the moon and the centrifugal force of the earth’s 
rotation. This project studied the relationship between changes in tide and river water level in 
the mouth of the Kelani River.  Tidal data was collected from the Colombo Harbor, and water 
level data and river ﬂow data was obtained from the Nagalagam Street gauge and Hanwella 
gauge. It was found that there is a direct relationship between tidal level and ﬂood level in the 
river mouth area. Therefore, it is proposed that tidal level variations be considered in order to 
make accurate ﬂood predictions in the river mouth areas.
Introduction
Flood is deﬁned as a body of water that rises and overﬂows into land settlements that are not 
normally under water (Kulathilaka 2007). It is known that with the population increases since national 
independence, land areas have become scarce and as a result, low-income families have occupied 
the ﬂood plains of rivers, which have been demarcated as reservations. Putting up structures and 
using land without considering ﬂood risks has often invited ﬂood situations (NDRSC 2008). 
Due to the heavy rain resulting from the South-West Monsoon, some districts of the 
Western, Southern and Sabaragamuwa Provinces of Sri Lanka experience frequent ﬂood 
situations. The most affected districts are Colombo, Gampaha, Kalutara, Rathnapura, Kegalle, 
Galle and Matara.  Riverine ﬂoods develop slowly when there is rainfall for a continuous period 
or may come in the form of ﬂash ﬂoods caused by excessive and intense rainfall over a short 
duration (Kulathilaka 2007). This usually happens when a tropical cyclone or a depression 
strikes. The ﬂood effect can be in the form of a local impact in the neighborhood or community 
or it can affect a very large area.120
M. D. E. K. Gunathilaka, W. A. L. Wikramanayake and D. N. D. Perera
Floods cause huge damage to physical infrastructure, disrupt economic and social 
activities, and cause loss of life, and human suffering. In 2007, some 438,880 people were 
affected by ﬂoods and 20 deaths were reported by district authorities due to ﬂoods in Sri Lanka 
(Table 1; NDRSC 2008).  
Table 1.  Disasters compared in terms of the events of 2007. 
Event  Number of affected   Deaths  Injured    Houses damaged 
 people      Fully    Partially
Flood 438,880  20  17  2,568  7,237
Land Slide    15,199  14  11     437  1,371
Drought    57,435  -  -  -  -
Strong Wind      4,914    3  51    1,532
Fire      1,193    4  24     282        6
Lightning            9    7    2  -  -
Total 517,630  48  105  3,287  10,146
Source: NDRSC, 2008
According to these ﬁgures, ﬂoods are the biggest natural disaster related problem in Sri 
Lanka and the Government of Sri Lanka spent Rs.160 million on relief, rehabilitation and 
reconstruction programs in 2007 for ﬂood affected districts (Figure 1).
Figure 1.  Costs of relief, rehabilitation and reconstruction of Sri Lanka in 2007.
Source: NDRSC 2008121
Identifying the Impact of Tidal Level Variation on River Basin Flooding 
Tides
The surface of all ocean waters rise and fall periodically as a result of the gravitational attraction 
of the moon and the sun on the earth and the centrifugal force of the earth’s rotation (Abbott 
and Ingham 1992). Far out at sea, tidal changes go unnoticed, but along the shores and beaches, 
the tides govern many of our water-related activities, both commercial and recreational but are 
also recognized as a hazard by ship navigators (NOAA 2009). 
With the relative movements of the moon and the sun with respect to the earth’s location, 
the tidal level varies. At the same time, the tidal level of a particular locality is affected by the 
bathymetry of the sea ﬂoor, resonance of the basin, friction of the bottom, currents, freshwater 
inﬂow and seasonal variations (UTM 2007). Therefore, tides must be observed in the relevant 
location for accurate applications. Figure 2 shows the tidal variations in the Colombo Harbor 
in the month of April 2008.
Figure 2.  Tidal height changes in April 2008 at the Colombo Harbor tide gauge.
Aim
River ﬂooding is a natural phenomena based on many different factors. This project expected 
to identify a relationship between tide level changes and river basin ﬂooding near river-mouth 
areas. It raises the question as to how river ﬂood risk will change under the inﬂuence of tidal 
level variations. 
Methodology
Both sides of the Kelani River from Hanwella towards the sea are identiﬁed as ﬂood-prone 
areas. Therefore, this area was selected for the study and data on the river water level and the 
tidal level at the river mouth were needed.  Since there was no tide gauge located at the mouth 
of the Kelani River, the tide data was obtained from the Colombo Harbor tide gauge, which 122
M. D. E. K. Gunathilaka, W. A. L. Wikramanayake and D. N. D. Perera
is situated close to the site (Figure 3). It is maintained by the National Aquatic Resources 
Research and Development Agency (NARA) and the data was obtained with the help of the 
National Hydrographic Ofﬁce (NHO). 
Figure 3.  Tide gauge and the river water level gauge locations (1 inch sheet - Colombo).
Excessive rain into the upper catchment brings lots of water into the river and increases 
the river ﬂow. This is the main cause for river ﬂooding. This can be detected with the rising of 
the water level gauge at the river. For this study, two water level gauges in the Kelani River 
were used; one at Nagalagam Street and the other one at Hanwella. They are maintained by 
the Irrigation Department of Sri Lanka. The Nagalagam Street gauge is situated closer to the 
river mouth (Figure 3) and was compared with the tidal data to identify a pattern. The Hanwella 
gauge is situated quite far from the river mouth and is used as a measure of river ﬂow and 
indicator of ﬂooding. 
In this study, both tidal and water level data were obtained on an hourly basis on each 
day from April to June 2008 and analyzed for a pattern to identify a relationship between them. 
During this period, several ﬂood occasions were reported at the site.  From the data obtained, 
a graphical analysis was carried out for the following cases: 
•  The water level pattern of the river-mouth with the tidal pattern during the non-ﬂood 
periods (with low river ﬂow).
•  The water level pattern of the river-mouth with the tidal pattern during the ﬂood periods 
(with high river ﬂow).123
Identifying the Impact of Tidal Level Variation on River Basin Flooding 
Results and Analysis
The water level of the Nagalagam Street gauge changed directly with the Colombo Harbor tide 
gauge (Figure 4). When there was high tide, the water level at Nagalagam Street was also high; 
when it was low tide, the water level was also low. In the mean time, the Hanwella gauge shows 
minimum correlation with the tide gauge (during low water levels and non-ﬂood) because it 
is situated away from the river-mouth towards the upstream.  It gives information about how 
the water ﬂows in the up-stream-river during the period.
Figure 4.  Variation of the tide and river water level during a non-ﬂood period. 
 
Low Tide  High Tide 
From 01:00 hours on 19/07/2008 to 02:00 hours on 21/07/2008, and 11:00 hours on 
23/07/2008 to 14:00 hours on 24/07/2008, the water level of the Hanwella gauge rose, and these 
two periods were identiﬁed as ﬂood periods in the Kelani River (Figure 5). During the same period, 
the water level of Nagalagam Street also increased, but this increment is not as obvious as in the 
Hanwella gauge. Here, the water level of Nagalagam Street has not behaved in a manner that 
corresponds with the tides as clearly as during the non-ﬂood periods as shown above. Therefore, 
the graphs for these durations are magniﬁed and analyzed as follows (Figure 6).124
M. D. E. K. Gunathilaka, W. A. L. Wikramanayake and D. N. D. Perera
Figure 5.  Variation of the tide and river water level during the ﬂood period.
 
Case -01 
Case -02 
Figure 6.  Comparison of the river water level and the tidal level during the ﬂood period.
(a) Case-01 (19-20/07/2008) (b) Case-2 (23-24/07/2008)125
Identifying the Impact of Tidal Level Variation on River Basin Flooding 
At about 13:00 hours (19/07/2008) the tide gauge read a high tide (Figure 6a). At the same 
time the river water level at Hanwella was getting close to a high level. With this situation, the 
Nagalagam Street gauge had the highest reading. When it reached 22:00 hours the river water 
level at Hanwella was at its peak but the Nagalagam Street gauge read a lower water level than 
at 13:00 hours because the tide level was lower than that at 13:00 hours (low tide). Again the 
Nagalagam Street gauge reading rose at about 05:00 h the next day, even though the Hanwella 
gauge was less than at 22:00 h, because it was another high tide time.
The same pattern can be observed in the second case (Figure 6b). At 17:00 hours on 
23/07/2008 and 05:00 hours on 24/07/2008, Nagalagam Street gauge read a high water reading 
because at the same time the tide gauge was giving a high tide and the Hanwella gauge read 
a high reading. Even though the Hanwella gauge gave the highest reading at 23:00 hours on 
22/07/2008, the Nagalagam Street gauge reading was lower than that of 17:00 hours and 05:00 
hours, since it was low tide. 
Conclusion
The water level of the river-mouth is directly correlated with the tidal variation of the sea. The 
collective result of the river ﬂow and the tidal level will determine the water level of the lower 
river areas. When there is high water ﬂow and a high tide, the water level of the river-mouth 
area is increasing. If the river ﬂow is in a ﬂood risk situation, then the high tide will cause the 
water level of the river-mouth to increase and it will increase ﬂood risk. With the start of the 
low tide, the ﬂood risk will decrease. Therefore, when predicting ﬂoods in lower river basins, 
one has to consider the tidal factor, especially in areas where there is a large tidal variation.
Recommendation
The tide gauge that was being used for this work was not situated at the river mouth, therefore, 
in future studies it is recommended to have a tide gauge at the river mouth. 
In this study, we could not collect the river ﬂow data at the river mouth area (Nagalagam 
Street) along with the water level data, therefore, we used the Hanwella gauge water level data 
to get an idea about the river ﬂow, but for more accurate work, one has to use the river ﬂow 
data along with the water level data at the same place.
Usually at river mouths, sand bars are formed due to the tidal currents and other water 
movements. These will block the natural ﬂow of the fresh water of the river to the sea. Therefore, 
it is recommended to carryout a bathymetric proﬁle survey at the river-mouth to verify any 
sand movements during the time.
Acknowledgements
We must express our sincere thanks to Ms. P. P. G. Dais, Deputy Director-Hydrology of the 
Irrigation Department of Sri Lanka and her staff for providing the water level data and to   
Mr. R. K. A. Ariyrathne, Surveyor of the National Hydrographic Ofﬁce NARA for providing 126
M. D. E. K. Gunathilaka, W. A. L. Wikramanayake and D. N. D. Perera
the tidal data to carryout this work. Without their valuable contribution, this project could not 
have been a success. 
References
Abbott V. J.; Ingham, A. E. 1992. ‘Hydrography for the Surveyor and Engineer’, 3rd Edition. Oxford, UK: 
WileyBackwell.
Atlantic Kayak Tours Experts Center. 2008. ‘Navigation Skills: Tides and Currents’, Atlantic Kayak Tours 
Inc. 
Kulatilaka, P. H. K. 2007. ‘Flood and Obsolete Warning System’, Sunday Observer (September 29, 2007).
National Disaster Relief Services Center. 2008. Online: www.ndmc.gov.lk . Accessed 15-11-2008.
National Oceanic and Atmospheric Administration, USA (NOAA). Tide and Currents. Online: http://co-ops.
nos.noaa.gov/restles4.html. Accessed: January 25, 2009.
Part, C. C.  2001. ‘The Environment: Principles and Applications’, 2nd Edition. Routledge.
Sverdrup, K. A.; Duxbury, A. C.; Duxbury, A. B.  2002. ‘An Introduction to the World’s Ocean’, 7th Edition. 
McGraw-Hill College.
UTM. 2007. ‘Lecture Notes on Tides’, Department of Geomatics, Faculty of Geoinformation Sciences and 
Engineering, University of Technology, Malaysia.

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