Abstract:
Floods are globally recognised as a foremost and expensive hazard, cause extensive damage
worldwide, affecting lives, property, and crops. Low lying terrain in most of the river basins of
Sri Lanka and heavy rainfall in central highlands magnify the susceptibility to these
catastrophic events. The study area is lower Kelani basin which is the second largest river in
Sri Lanka. It originates from the Central Highlands and flows westwards to the country's
western coast. The lower basin of the Kelani River, including the Colombo metropolitan, the
business capital of Sri Lanka with a high population, frequently experiences severe inundation
due to high discharges of the Kelani River and localised heavy rainfall events.
HEC-RAS 2D model was used in this study to analyse water levels in the Kelani River and
assess flood extents with depths and velocities, enabling reliable future predictions despite
limited data availability. The model was calibrated and validated by comparing observed and
simulated water levels during the May 2008 and June 2016 flood events, demonstrating
satisfactory performance with a Root Mean Square Error of 0.134 m, coefficient of correlation
of 0.951, Nash-Sutcliffe efficiency of 0.914, and a goodness-of-fit index of 70.1% for
inundation extents. Additionally, an ArcGIS model was employed to generate a flood
susceptibility map for the lower basin of the Kelani River.
This study involves several steps, including collecting data, setting up the HEC-RAS 2D
model, analysing model parameter sensitivity, selecting parameters, calibrating and validating
model simulations, generating flood inundation maps, and exploring the effectiveness of the
flood bunds during extreme flood events. The study included identifying flood-prone areas in
Colombo suburbs, assessing the flood risk through simulation of hypothetical flood bund
breaches, estimating the flood extent and related maximum flood depths and maximum
velocities. Evaluating the efficiency of existing flood bunds for floods with different return
periods and assessing the vulnerability of areas protected by flood bunds on the left bank of
Kelani River during 50 and 100-year Kelani floods and a 50-year rainfall in Colombo
metropolitan area were the main tasks of the study.
The bund breaching simulation reveals that the total inundated area covers 108 km2, with 35%
of that area experiencing an inundation depth exceeding 2 m. During a 50-year flood, the total
overtopping length at the right bank is 300m, while it increases to 650m during a 100-year
flood. This analysis provides valuable insights into the potential advantages of investing in
infrastructure improvements to mitigate flood risk in the Colombo Suburbs. The generated
flood inundation maps due to the Kelani River flood and the localised rain and flood
susceptibility map will serve as a valuable tool for future flood risk planning, formulating
implementation strategies aimed at minimising flood damage in Colombo suburbs.