Hydrodynamic modelling required to assess vulnerability of coastal
zone
THE Coastal Zone Management Plan (CZMP) gives setback lines of
reservation and restriction based on observed and expected erosion
rates. In the CZMP of 1998, and the revision of 2004 now before the
cabinet, this setback varies from 35m to 300m depending on the stretch
of coast, a release from Lanka Hydraulic Institute said.
The post-tsunami announced buffer zones of 100m on the western coast
and 200m on the eastern coast contravene the setbacks given in the CZMP.
However, setback lines based on erosion and on blanket buffer zones
do not have scientific basis when looking at Coastal vulnerability in
terms of wave propagation (travel) into the coastal hinterland, as made
very clear by the tsunami travel distances varying with location.
The post-tsunami surveys reveal that the tsunami wave has travelled
upto 1km inland on the west coast and 1 to 2km on the east coast and the
erosion of the coast varies 20 to 30m in the both western and eastern
coasts. So are we looking at setback lines/buffer zones objectively with
sound scientific study?
Coastal Vulnerability
The Coastal Zone is vulnerable due to wave action and flooding and
monsoon waves, storm surges and tsunamis.
The distance waves can travel to inland depends on the near-shore
bathymetry (contours/slope of sea bed), existence of rock outcrops,
coral reefs, beach rock, slope of the sea shore, coastline and the
hinterland, sand dunes and other coastal features.
The Lanka Hydraulic Institute (LHI), consultants in coastal
engineering, water resources and urban water, is of the view that in
order to scientifically assess the possible inland propagation, detailed
data must be obtained near-shore and comprehensive near-shore
hydrodynamic modelling should be carried out.
The modelling can be mathematical modelling or physical modelling.
However, with the long coastline involved and considering the current
state-of-the-art in mathematical modelling, it is logical to carry out
mathematical modelling for such an assessment.
Setback lines based on coastal wave propagation vulnerability mapping
Gathering the required data and information and modelling several
scenarios will give us logical vulnerability maps based on
characteristics of the wave propagation (travel) inland.
Vulnerability can then be classified generally as "low", "medium" or
" high" and based on these assessments we can detail setback lines.
However, Sri Lanka has many coastal water bodies - lagoons and
estuarine basins. Therefore, it is important that coastal vulnerability
is assessed together with Hazards propagating from these coastal water
bodies.
This is particularly so in the Eastern Province, with its large
lagoons and estuary basins spread over a large tract of the coastline.
We should not move people away from the coastline to areas that may
actually be more vulnerable to flooding. Hence, we should look at
Coastal Zone Management in combination with river basin water management
including estuaries and lagoons.
Twenty five percent of Sri Lanka's population live along the
coastline, hence any imposition of setback lines and relocation of the
near-coast populace has to be done after a comprehensive but quick
scientific, hydrodynamic modelling study of vulnerability.
A vulnerability study based on wave travel and flooding should be the
first step in the setting up of an all-hazards warning and management
system, the release said.
LHI which maintains a state-of-the-art hydraulics laboratory in
Moratuwa is currently engaged in hydraulic studies of the proposed new
Colombo South Harbour, feasibility study of the Hambantota Sea port and
in the salinity study of the Walawe Ganga.
LHI has capabilities and 20 years of experience in hydrodynamic
modelling of near-shore wave propagation and in river hydraulics, both
in physical modelling and mathematical modelling, the release added. |