Detailed Island Risk Assessment in Maldives

Transcrição

Detailed Island Risk Assessment in Maldives
Volume III: Detailed Island Reports
Dh. Kudahuvadhoo – Part 1
DIRAM team
Disaster Risk Management Programme
UNDP Maldives
December 2007
Table of contents
1. Geographic background
1.1 Location
1.2 Physical Environment
2. Natural hazards
2.1 Historic events
2.2 Major hazards
2.3 Event Scenarios
2.4 Hazard zones
2.5 Recommendation for future study
3. Environment Vulnerabilities and Impacts
3.1 General environmental conditions
3.2 Environmental mitigation against historical hazard events
3.3 Environmental vulnerabilities to natural hazards
3.4 Environmental assets to hazard mitigation
3.5 Predicted environmental impacts from natural hazards
3.6 Findings and recommendations for safe island development
3.7 Recommendations for further study
4. Structural vulnerability and impacts
4.1 House vulnerability
4.2 Houses at risk
4.3 Critical facilities at risk
4.4 Functioning impacts
4.5 Recommendations for risk reduction
1. Geographic background
1.1 Location
Kudahuvadhoo is located on the southern end of South Nilandhe Atoll (Dhaalu Atoll),
next to the Kudahuvadhoo Kanduolhi (reef pass) at approximately 73° 39' 3"E and 4° 22'
28" N (Figure 1.1). It is located at about 181 km from the nation’s capital Male’ and about
113 km from the nearest Airport Laamu Atoll, Kadhoo. Kudahuvadhoo is the Atoll Capital
of Dhaalu Atoll, amongst a group of 7 inhabited islands. It’s nearest inhabited islands are
Maaenboodhoo (8 km), and Vaanee (13 km). Dhaalu atoll is located along the western
line of atolls.
3° 00' N
Meedhoo
Ban'didhoo
Rin'budhoo
Hulhudheli
South Nilandhe Atoll
(Dhaalu Atoll)
2° 45' N
N
Vaanee
Kudahuvadhoo
73° 00' E
Maaen'boodhoo
Location Map
of Kudahuvadhoo
0
5
10
kilometers
Figure 1.1 Location map of Kudahuvadhoo.
1.2 Physical Environment
Kudahuvadhoo is a fairly large island with a length of 1040m and a width of 880 m at its
widest point. The total surface area of the island is 69.7 Ha (0.69 km2). The reef of
Kudahuvadhoo is one of largest in Maldives with a surface area of 5601 Ha (56.01 km2).
The reef also hosts 3 other uninhabited islands.
Kudahuvadhoo is located at the southern tip of the reef system, approximately 450m
from the southern wave break zone and 320m from the eastern reef edge. The depth of
the reef flat is quite shallow averaging less than -1m MSL. The island has an elevation
ranging from +0.45 to +1.5m MSL along the island topographic profile survey line. The
island could be describes as located on an east west orientation and appears to be
growing towards west.
Kudahuvadhoo has a natural harbour due the extensive lagoon on the western side of
the island. The lagoon extends to about 12km within the reef. The growth of the island
towards west has meant that sand is constantly deposited within the deep lagoon
creating a steep underwater slope on its western end and allowing vessels to approach
close to the shoreline.
The island had large areas of undeveloped land allowing the presence of strong
vegetation cover on its southern side. Much of this new land is now being developed for
Tsunami related resettlement schemes, agriculture, recreation and is predicted to be
depleted in the near future.
The existing natural environment of the island has been modified, especially in the
northern side, although the extent of coastal modifications is small compared to most
other inhabited islands. Major modifications include development of a harbour, land
reclamation from the excess dredge material during harbour development and clearing
of vegetation.
2. Natural hazards
This section provides the assessment of natural hazard exposure in Feydhoo Island. A
severe event history is reconstructed and the main natural hazards are discussed in
detail. The final two sections provide the hazard scenarios and hazard zone maps which
are used by the other components of this study as a major input.
2.1 Historic events
The island of Kudahuvadhoo has been exposed to multiple hazards in the past although
its exposure has been limited. A natural hazard event history was reconstructed for the
island based on known historical events. As highlighted in methodology section, this was
achieved using field interviews and historical records review. Table 2.1 below lists the
known events and a summary of their impacts on the island.
Table 2.1 Known historic hazard events of Kudahuvadhoo.
Metrological
hazard
Dates
of
the Impacts
recorded events
Flooding caused •
by Heavy rainfall
Frequent events
commonly
occurring during
SW monsoon.
Flooding limited to a few topographic low
areas on the island. The magnitudes and
impacts of these floods are small with
water levels barely exceeding 0.3m.
However, disruptions to socio-economic
activities have occurred in the past with
schools and shops being closed for over
24 hours.
Flooding caused No records of major by swells or incidents
surges
Flooding caused
by
monsoonal
wind waves or
Udha
Windstorms
•
•
Annually
Impact limited to 10 m from shoreline. In
intensity is very low and rarely effects
property or human well-being
24 June 19871
18-20 Dec 19922
Windstorms are common on the island.
These events affect housing structures,
1
All dates in italics are adopted from MANIKU, H. A. (1990) Changes in the Topography of Maldives,
Male', Forum of Writers on Environment of Maldives. and news paper reports.
2
Unnamed tropical depression passing over Dhaalu atoll between 18-20 December 1992. Source: UNISYS
& JTWC (2004) Tropical Cyclone Best Track Data (1945-2004).
http://www.pdc.org/geodata/world/stormtracks.zip, Accessed 15 April 2005, Unisys Corporation and Joint
Typhoon Warning Center.
•
5 other major vegetation and crops. The island is
events
since generally protected from thick coastal
1980’s
(dates vegetation but even before the settlements
expanded, there were reports of windstorm
unknown).
related damages. The effect of wind storm
is spread across the entire island.
Droughts
No major event have been reported
Earthquake
No major event have been reported
Tsunami
26th Dec 2004
The effect of tsunami 2004 was limited on
this island. Flooding occurred on the
southern and parts of eastern side. There
were no major structures in this area and
as a result lost was very little. The flood
itself came only about 150m on the
southern side and up to a 300m on the
eastern side. All these areas were
uninhabited. Tsunami floods destroyed a
number of agricultural fields, however. One
farmer reported a loss of MRF 30,000
worth of crops. The repercussions for the
damage were far reaching. He supplied
vegetables to a number of islands within
the atoll and employs 12 staff seasonally.
The staff and the farmer therefore endured
loss of income. Apart from the agricultural
fields; the only notable damage to the
island was to its Harbour. The outer
harbour walls collapsed and part of the
quay wall was damaged beyond repair. It
is still in use but appears unstable.
The historic hazardous events for Kudahuvadhoo showed that the island faced the
following hazards: 1) windstorms, 2) flooding caused by heavy rainfall, and 3) tsunami.
Impacts and frequency of these events vary significantly. Windstorms are the most
commonly occurring events followed by flooding due to heavy rainfall. The occurrence of
windstorms is usually limited to SW monsoon although occasional localised ‘freak
storms’ due to low pressure systems forming above Maldives effects the island during
NE monsoon. It is noteworthy that in spite of the exposure to southern Indian Ocean
swell waves, flooding from swell waves or storm surges are almost non-existent in the
historical records. The absence of historical records on swell related flood events in
nearby islands with similar geographic settings such as Nilandhoo and Magoodhoo,
further enhances this finding. The tsunami of 2004 is by far the most significant hazard
event in Kudahuvadhoo with substantial economic damage. However, despite its
location in a high tsunami hazard zone, the scale of impacts in Kudahuvadhoo is small
compared to the other nearby islands such as Vilufushi, Kolhufushi, and Vaanee. The
reasons behind the lack of impacts will be explored in latter sections.
2.2 Major hazards
Based on the historical records, meteorological records, field assessment and Risk
Assessment Report of Maldives (UNDP, 2006) the following meteorological, oceanic and
geological hazards have been identified for Kudahuvadhoo.
•
Windstorms
•
Heavy rainfall (flooding)
•
Tsunami
•
Long distance swell waves and local wind waves
•
Earthquakes
•
Climate Change
2.2.1 Swell Waves and Wind Waves
Long distance swell waves
Kudahuvadhoo Island is exposed to effects of swell waves approaching from the
Southern Indian Ocean. No site specific wave studies have been undertaken for
Kudahuvadhoo
but
studies
undertaken
around
the
country
reports
a
predominantly southwest to a southerly direction for swell waves (Kench et. al
(2006), Young (1999), DHI(1999) and Binnie Black & Veatch (2000)). As a result
the island is directly in the path of these long distance waves which occasionally
reaches abnormal levels capable of flooding (see Figure 2.1).
Meemu Atoll
Dhaalu Atoll
NE Monsoon
Wind waves
Kudahuvadhoo
SW monsoon
Wind waves
s
eeeeeesss
aaaavvvvvv
w
w
w
w
w
w
llllllllll
weeeeee
w
w
w
w
ssssw
E
E
E
SSSE
aaallllll S
maaa
m
m
r
r
r
m
m
m
r
r
r
noooooo
oooooonnn
Abbbbbb
AA
AA
A
Thaa Atoll
SW
SS
S
SS
W
W
W
Swww
SS
S
SS
wwweee
eeellllll
lllW
W
Waaa
W
aaavvvveee
eeessssss
Laamu Atoll
Estimated wave propogation
patterns around Kudahuvadhoo
0
25
50
kilometers
Figure
2.1
Estimated
(predominant)
wave
propagation
patterns
around
Kudahuvadhoo.
The shape of the reef and island, and location of the island within the reef system
appears to control the flow of wave energy reaching its shoreline (Figure 2.2).
Hence, unlike islands located on the western rim of the island (for example G.Dh.
Thinadhoo), Kudahuvadhoo enjoys more protection due to its location on the
southern rim. Based on an estimation of wave behaviour around Kudahuvadhoo,
it is highly likely that waves approach the reef system at an angle and refract
along the reef slopes before they dissipate their energy on the reef edge and reef
flat. The wider reef flat is also likely to play a major role in energy dissipation
before they reach the shoreline. Kench and Brander (2006) reported a relationship
between wave energy propagation across a reef flat and, reef width and depth. Using
their proposed Reef Energy Window Index, the percentage of occurrence of gravity
wave energy on Kudahuvadhoo reef flat is approximately 30%.
May 2007 Flood Extent
Wave defraction
on reef system edge
Kudahvadhoo
Reef Flat
Reduced wave rays
on reef flat
Wave Refraction and
breaking on reef slopes and edge
Wa
ll
we
S
f
no
ti o
c
e
Dir ves
Wa
0
ve
Cr
es
ts
500
1,000
meters
Figure 2.2 Estimated behaviour of swell waves around Kudahuvadhoo.
Udha
Flooding is also known to be caused in Kudahuvadhoo by a gravity wave phenomenon
known as Udha. These events are common throughout Maldives and especially the
southern atolls of Maldives during the SW monsoon.
The intensity and impacts of udha waves are usually very low with flooding occurring
within 10m of coastline at less than 0.3m height above the ground. It is not expected to
be a major hazard in the short-term.
The origins of the udha waves as yet remain scientifically untested. No specific research
has been published on the phenomenon and has locally been accepted as resulting from
local wind waves generated during the onset of southwest monsoon season. The
relationship has probably been derived due to the annual occurrence of the events
during the months of May or June. It is highly probable that waves originate as swell
waves from the Southern Indian Ocean and is further fuelled by the onset of southwest
monsoon during May. The timing of these events coincides as May marks the beginning
of southern winter and the onset of southwest monsoon. The concurrent existence of
these two forms of gravity waves during the southwest monsoon is confirmed by Kench
et. al (2006) and DHI(1999). It is also questionable whether the southwest monsoon
winds waves alone could cause flooding in islands since the peak tide levels on average
are low during May, June and July. Furthermore the strongest mean wind speeds in
Male’ has been observed for November and is more consistent during October to
November than during May and June period (Naseer, 2003). This issue needs to be
further explored based on long term wave and climatological data of the Indian Ocean
before any specific conclusions can be made. However if the relationship does exists,
this phenomena could prove to be a major hazard in the face of climate change since
the intensity of southern Indian Ocean winter storms is expected to increase.
Storm Surges
The Disaster Risk Assessment report of 2006 (UNDP, 2006), reported that
Kudahuvadhoo was located in a moderate storm surge hazard zone with
probable maximum event reaching 0.6m above MSL or 1.53m with a storm tide.
The combined historical records of nearby islands in Meemu, Thaa and Laamu
Atoll does not show any flooding caused by a storm surge. The occurrence of
any abnormal swell waves or surge on Kudahuvadhoo reef flat is dependent on a
number of factors such as the wave height, location of the original storm event
within the Indian Ocean, tide levels and reef geometry.
Future swell event prediction
Due to its location, swell related flooding should be considered a serious hazard for
Kudahuvadhoo. The island is expected to be exposed to storm waves mainly from south
and west south west as shown in the map (Figure 2.3). Events beyond this arch may not
influence the island due to the protection offered by Laamu and Thaa Atoll.
Possible range of
swell wave direction
in Dh.Kudahuvadhoo:
SE to S
Historic storm events 1945 - 2007
Figure 2.3 Historical storm tracks (1945-2007) and possible direction of swell
waves for Kudahuvadhoo Island.
At present, it is very difficult to forecast the exact probability of swell hazard event and
their intensities due to the unpredictability of swell events and lack of research into their
impacts on Maldives. Assessment in Kudahuvadhoo is further limited by the lack of
historical events. However, since the hazard exposure scenario is critical for this study a
tentative exposure scenario has been estimated for the island.
There is a probability of major swell events occurring every 20 years with probable water
heights above 0.5 m and every 5 years with probable water heights of 0.2-0.5 m. Events
with water heights less than 0.2 m are likely to occur annually especially as Udha.
The timing of swell events is expected to be predominantly between November and
June, based on historic events and storm event patterns (see Table 2.2).
Table 2.2 Variation of Severe storm events in South Indian Ocean between
1999 & 2003 (source: (Buckley and Leslie (2004)).
Longitude band
30 °E to 39 °E
40 °E to 49 °E
50 °E to 59 °E
60 °E to 69 °E
70 °E to 79 °E
80 °E to 89 °E
90 °E to 99 °E
100 °E to 109 °E
110 °E to 119 °E
120 °E to 130 °E
Severe wind event
variation
Winter
Summer
12.5
7.5
7.5
6
6
12
12
8
15
13.5
17
10
26
14
6
6
8
3
7
2
The reclamation plans for Kudahuvadhoo were incomplete at the time of this study. The
existing drafts show land reclamation for an airstrip on the southern half of the island.
After this development the reef flat width will be reduced to approximately 250m. This
reduction in the reef flat width will increase the percentage of occurrence of gravity wave
energy on this reef flat to approximately 40% and therefore increasing the probability of
flooding caused by surges by 20%. Similarly the impact of flooding will increase relative
to encroachment of settlement to coastal areas, even if the probability of flood events
remains constant. Potential increase in frequency and intensity of flood events are also
probable with climate change and is addressed in a latter section.
2.2.2 Heavy Rainfall
The rainfall pattern in the Maldives is largely controlled by the Indian Ocean monsoons.
Generally the NE monsoon is dryer than the SW monsoon. Rainfall data from the three
main meteorological stations, HDh Hanimaadhoo, K. Hulhule and S Gan shows an
increasing average rainfall from the northern regions to the southern regions of the
country (Figure 2.4). The average rainfall at S Gan is approximately 481mm more than
that at HDh Hanimadhoo.
Mean annual rainfall (mm)
3500
3000
2500
2000
1500
1000
500
0
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
2003
Year
Gan
Hulhule
Hanimadhoo
Figure 2.4 Map showing the mean annual rainfall across the Maldives archipelago.
The closest meteorological station to Kudahuvadhoo is Kadhoo airport which became
operational in 1986. Unfortunately this study does not have access to Kadhoo data.
Moreover, Kadhoo data may be limited for long term trend observation due smaller
number of detailed observation years. Hence, to resolve the issue, data from Hulhule’
has been used. It is recommended that further assessment be made once Kadhoo data
becomes available.
The mean annual rainfall of Hulhule’ is 1991.5mm with a Standard Deviation of 316.4mm
and the mean monthly rainfall is 191.6mm. Rainfall varies throughout the year with mean
highest rainfall during October, December and May and lowest between February and
April (See Figure 2.5).
Figure 2.5 Mean Monthly Rainfall in Hulhule’(1975-2004).
Historic records of rainfall related flooding on the island of Kudahuvadhoo indicates that
this island is often flooded and its intensity is high in certain areas. Records for all
incidents have not been kept but interviews with locals and research into newspaper
reports show that localised levels of flooding within sections of the island. These areas
usually correspond to topographic lows within the island. Moreover, substantial
topographic variations exist within the island, as is common on larger islands of
Maldives. Settlement expansion into and along the edges of these low lying areas have
exposed them to flood impact. Heavy rainfall related flooding has been reported to reach
up to 0.3 m above the ground level in central areas of the island.
The impacts of flooding so far reported has not been disastrous, but has had continued
impacts on the community such as disruptions to socio-economic functions such as
temporary school and business closures, occasional damage to personal property and
crops.
It would be possible to identify threshold levels for heavy rainfall for a single day that
could cause flooding in Kudahuvadhoo, through observation of historic daily rainfall data.
Unfortunately, the nearest weather station, Kadhoo, is 113 km south of the island
reducing its applicability to local level analysis. Moreover, we were unable to acquire
daily historical data of Kadhoo itself. Available limited severe weather reports shows that
Kadhoo received a maximum precipitation of 110.8mm for a 24 hour period on 21th
November 2004 and the island of Meemu Atoll Muli, 82 km NE of Kudahuvadhoo,
received a maximum of 193mm on 15 November 2003 (DoM, 2005). Based on
interviews with locals, the 2004 event did not have an impact on the island but the 2003
event caused moderate levels of flooding for 2 days. The interviewees were unable to
recall a single event with significant impact, suggesting a low intensity of flood events.
The probable maximum precipitations predicted for Hulhule’ and S.Gan by UNDP (2006)
are as follows as shown in Table 2.3:
Table 2.3 Probable Maximum Precipitation for various Return periods in
Hulhule’ and Gan.
Station
Return Period
50 year 100 year 200 year 500 year
Hulhule’ 187.4
203.6
219.8
241.1
Gan
218.1
238.1
258.1
284.4
Given the high variations in rainfall in Kadhoo, these figures may vary. Based on the field
observations and correlations with severe weather reports from Department of
Meteorology ((DoM, 2005) the following threshold levels were identified for flooding
(Table 2.4). These figures must be revised once historical daily rainfall data becomes
available.
Table 2.4 Threshold levels for rainfall related flooding in Kudahuvadhoo.
Threshold level Impact
(daily rainfall)
50mm
Puddles on road, flooding in low houses,
occasional minor damage to household
goods in most vulnerable locations, disruption
to businesses and primary school in low
areas.
100mm
Moderate flooding in low houses; all low lying
roads flooded; minor damage to household
items, temporary (minor to Moderate)
disruptions to socio-economic functions for
less than 24 hours
150mm
Widespread flooding on roads and low lying
areas. Moderate damage to household
goods,
disruptions
to
socio-economic
functions for more than 24 hours.
200mm
Widespread flooding on roads, low areas and
houses. Moderate damage to household
goods, sewerage network, backyard crops,
disruption to socio economic functions for
more than 24 hours, gullies created along
shoreline, possible damage to road
infrastructure.
230+mm
Widespread flooding around the island. Major
damages to household goods and housing
structure, socio economic functions disrupted
for more than 48 hours, businesses closed,
damage to crops, damage to road
infrastructure, sewerage network and quay
wall.
Quite often heavy rainfall is associated with multiple hazards especially strong winds and
possible swell waves. It is therefore likely that a major rainfall event could inflict far more
damages those identified in the table.
2.2.3 Wind storms and cyclones
Maldives being located within the equatorial region of the Indian Ocean is generally free
from cyclonic activity. There have only been a few cyclonic strength depressions that
have tracked through the Maldives, all which occurred in the northern and north central
regions. According to the hazard risk assessment report (UNDP, 2006), Kudahuvadhoo
falls within the second least hazardous zone for cyclone related hazards and has a
maximum predicted cyclonic wind speeds of 56 Kts (see Figure 2.6). There are no such
records for the southern region, although a number of gale force winds have been
recorded due to low depressions in the region. Winds exceeding 35 knots (gale to strong
gale winds) were reported as individual events in Kadhoo weather station annually
between 2002 and 2006, all caused by known low pressure systems near Maldives
rather than the monsoon (DoM, 2005). The maximum wind speed in Kadhoo during this
period was approximately 46 kts.
Kulhudhufushi
Fonadhoo
Thulusdhoo
Kudahuvadhoo
Vilufushi
Gan
probable maximum
cyclone wind speed (kts)
Hazard Zones
Villingili
Thinadhoo
Hithadhoo
Feydhoo
5
96.8
4
84.2
3
69.6
2
55.9
1
0.0
Figure 2.6 Cyclone hazard zones of the Maldives as defined by UNDP (2006).
Interviews with the locals have indicated that the island has been affected by numerous
wind storms. Unfortunately records have not been kept for these events, especially their
dates or its impacts. However two events have been identified that had moderate impact
on the island: June 1987 and December 1992 events. The event of June 1987 affected a
number of islands across of Maldives causing damage to crops, vegetation and housing
structures. The December 1992 event reached wind speeds over 25 knots and caused
moderate damage to crops and vegetation. Damage to properties was mostly caused by
falling trees, especially breadfruit trees (Artocarpus altilis).
Hence, wind speeds close to near gale winds (see Table 2.5) have caused moderate
damage to property and trees on the island. Kudahuvadhoo does have lush vegetation
on the southern side dominated by larger trees species, which acts to minimise the
direct exposure of properties when winds approach from a south to south westerly
direction. Much of this vegetation is now being cleared for development activities,
especially for the new stadium and tsunami resettlement housing.
In order to perform a probability analysis of strong wind and threshold levels for damage,
daily wind data is crucial. However, such data was unavailable for this study.
Table 2.5 Beaufort scale and the categorisation of wind speeds.
Beau- fort No
Description
Cyclone
category
Average wind
Average wind
speed
speed (Knots) (kilometres per
hour)
Specifications for estimating speed over land
0
Calm
Less than 1
less than 1
Calm, smoke rises vertically.
Direction of wind shown by smoke drift, but not by wind
vanes.
Wind felt on face; leaves rustle; ordinary wind vane moved
by wind.
Leaves and small twigs in constant motion; wind extends
light flag.
1
Light Air
1 -3
1-5
2
Light breeze
4-6
6 - 11
3
7 - 10
12 - 19
4
Gentle breeze
Moderate
breeze
11 - 16
20 - 28
5
Fresh breeze
17 -21
29 - 38
6
Strong breeze
22 - 27
39 - 49
7
Near gale
28 - 33
50 - 61
8
Gale
Category 1
34 - 40
62 - 74
9
Strong gale
Category 1
41 - 47
75 - 88
10
Storm
Category 2
48 - 55
89 - 102
11
Violent storm
Category 2
56 - 63
103 - 117
Breaks twigs off trees; generally impedes progress.
Slight structural damage occurs (chimney pots and slates
removed).
Seldom experienced inland; trees uprooted; considerable
structural damage occurs.
Very rarely experienced; accompanied by widespread
damage.
12
Hurricane
Category 3,4,5
64 and over
118 and over
Severe and extensive damage.
Raises dust and loose paper; small branches moved.
Small trees in leaf begin to sway; crested wavelets form on
inland waters.
Large branches in motion; whistling heard in telegraph
wires; umbrellas used with difficulty.
Whole trees in motion; inconvenience felt when walking
against the wind.
The threshold levels for damage are predicted based on interviews with locals and
housing structural assessments provided by risk assessment report (UNDP, 2006), as
shown in Table 2.6.
Table 2.6 Threshold levels for wind damage based on interviews with locals
and available meteorological data.
Wind speeds
1-10 knots
11 – 16 knots
17 – 21 knots
22 – 28 knots
28 – 33 knots
34 - 40 knots
40+ Knots
Impact
No Damage
No Damage
Light damage to trees and crops
Breaking branches and minor damage to
open crops, some weak roofs damaged
Minor damage to open crops, minor to
moderate damage to vegetation, probability
of damage to property due to falling trees.
Minor to Moderate to major damage to
houses, crops and trees
Moderate to Major damage to houses, trees
falling, crops damaged
2.2.4 Tsunami
UNDP (2006) reported the region where Kudahuvadhoo is geographically located to be
a very high tsunami hazard zone.
Kudahuvadhoo Island was considered ‘lucky’ to
escape without substantial damage to properties or infrastructure. This is perhaps due to
the fact that other nearby islands such as Kohufushi and Vilufushi were almost
completely devastated. Island within Dhaalu Atoll itself, especially the neighbouring
Vaanee also experienced extensive damage.
According to official reports 20% of the island was flooded. Field surveys and aerial
photographs immediately after the event revealed that approximately 30% of the island
was flooded. Flooding occurred on the southern and parts of eastern coastline. There
were no major structures in this area and as a result impact was minimal. The flood itself
came only about 150-200 m on the southern side and up to a 300 m on the eastern side.
All these areas were uninhabited. The main damage occurred to the agricultural fields,
however. One farmer reported a loss of MRF 30,000 worth of crops. Apart from the
agricultural fields; the only notable damage to the island was to its Harbour. The outer
harbour walls collapsed and part of the quay wall was damaged beyond repair.
The tsunami run-up height at the eastern shoreline of the island was reported to be
approximately 0.8m above MSL reducing to 0.1 m, 300 m inland. Run-up height on the
southern shoreline was estimated at 0.9m. It is difficult to identify the local tsunami
induced tide level due to the absence of a nearby tide station. Tsunami induced tide
level within the lagoon predicted using the tide data from the nearest tide station at
Hulhule’ shows that the island experienced water heights higher than 0.2 above the
average northern coastline (Figure 2.7). The small levels of flooding from the northern
side, most likely reflects this rise.
Figure 2.7 Water level recordings from the tide gauge at Hulhule’ indicating
the wave height of tsunami 2004 (source: University of Hawai’i SeaLevel
Centre, http://ilikai.soest.hawaii.edu/uhslc/iot1d/male1.html)
Comparatively higher exposure of the southern half of Dhaalu Atoll may be partially due
to the refraction of the wave caused by the Indian Ocean bathymetry as it travelled
westwards Maldives (Ali, 2005). The Indian Ocean bathymetry (Figure 2.8) shows
shallower water depths extended far offshore at around the central region of the
Maldives (at around the atolls of Laamu – Meemu). This shallower area caused the
wave to bend away from the southern atolls and became focused towards the central
region of the country. It is likely that a similar pattern may persist in any future event if
the waves originate from the northern Sundra trench.
Fig 2.8 Submarine topography around Maldives archipelago and modelled
wave refraction for the December 2004 tsunami (source: Ali (2005)).
The predicted probable maximum tsunami wave height for the area where
Kudahuvadhoo is located is 2.5 – 3.2 m (UNDP, 2006). Examination of the flooding that
will be caused by a wave run-up of 4.5m for the island of Kudahuvadhoo indicates that
such a magnitude wave will flood much of the island. The first 150-200m from the
shoreline will be a severely destructive zone (Figure 2.9). The theoretical tsunami flood
decay curve was plotted for a wave that is applied only for the direct wave from the
south eastern oceanward side of the island. It’s also is well understood that the tsunami
wave will also travel into the atoll lagoon which will cause the water level in the atoll
lagoon to rise. This could cause flooding of the island from the lagoonward side of the
island, if the water level rises above the height of the island. The maximum tsunami
wave induced water level height predicted for the atoll lagoon near Kudahuvadhoo is
1.8m. This could flood the island from the lagoonward side.
5.0
4.0
Height rel MSL (m)
3.0
Threshold level of flooding for severe structural damage
2.0
1.0
0.0
-1.0
0
100
200
300
400
500
600
700
800
900
1000
-2.0
-3.0
Extent of most
destructive zone
-4.0
Distance from oceanward shoreline (m)
Figure 2.9 Tsunami related flooding predicted for Kudahuvadhoo based
upon theoretical flood decay curve and the maximum probable tsunami
wave height at Kudahuvadhoo.
Despite the prediction of a 2.5-3.2 m tsunami in Kudahuvadhoo (UNDP, 2006), it is
important to note the geophysical settings of Kudahuvadhoo which could make it
comparatively less exposed to severe intensity of such events. Characteristics such as
island orientation, reef orientation, vegetation cover and presence of multiple coastal
ridges could help controlling the intensity and even height of water run-up on the island.
Furthermore, the lack of intensity may have been attributed to the lack of development
on the south eastern and eastern half of the island. Unlike most islands devastated in
the region, Kudahuvadhoo had a 380 m vegetated buffer zone, between the settlement
and the coastline. In effect, only a handful of people witnessed flood waters on the
southern side while majority of the population witnessed rising waters near the harbour.
This tends to suggest the ‘remoteness’ of the southern areas. With increasing
development in the area, especially the tsunami resettlement, an event of similar
magnitude will cause more significant damage today.
2.2.5 Earthquakes
There hasn’t been any major earthquake related incident recorded in the history of
Kudahuvadhoo or even Maldives. However, there have been a number of anecdotally
reported tremors around the country.
The Disaster Risk Assessment Report (UNDP 2006) highlighted that Thaa Atoll is
geographically located in the lowest seismic hazard zone of the entire country.
According to the report the rate of decay of peak ground acceleration (PGA) for the zone
1 in which Kudahuvadhoo is located has a value less than 0.04 for a 475 years return
period (see Table 2.7). PGA values provided in the report have been converted to
Modified Mercalli Intensity (MMI) scale (see column ‘MMI’ in Table 2.7). The MMI is a
measure of the local damage potential of the earthquake. See Table 2.8 for the range of
damages for specific MMI values. Limited studies have been performed to determine the
correlation between structural damage and ground motion in the region. The conversion
used here is based on United States Geological Survey findings. No attempt has been
made to individually model the exposure of Kudahuvadhoo Island as time was limited for
such a detailed assessment. Instead, the findings of UNDP (2006) were used.
Table 2.7 Probable maximum PGA values in each seismic hazard zone of
Maldives (modified from UNDP, 2006).
Seismic
hazard zone
1
2
3
4
5
PGA values for
475yrs return period
< 0.04
0.04 – 0.05
0.05 – 0.07
0.07 – 0.18
0.18 – 0.32
MMI3
I
I
I
I-II
II-III
Table 2.8 Modified Mercalli Intensity description (Richter, 1958).
MMI
Value
I
3
Shaking
Severity
Low
II
Low
III
Low
IV
Low
Description of Damage
Not felt. Marginal and long period effects of large
earthquakes.
Felt by persons at rest, on upper floors, or favourably
placed.
Felt indoors. Hanging objects swing. Vibration like
passing of light trucks. Duration estimated. May not be
recognized as an earthquake.
Hanging objects swing. Vibration like passing of heavy
trucks; or sensation of a jolt like a heavy ball striking the
walls. Standing motor cars rock. Windows, dishes,
doors rattle. Glasses clink. Crockery clashes. In the
Based on KATZFEY, J. J. & MCINNES, K. L. (1996) GCM simulation of eastern Australian cutoff lows.
Journal of Climate, 2337-2355.
V
VI-XII
Low
Light Catastrophe
upper range of IV, wooden walls and frame creak.
Felt outdoors; direction estimated. Sleepers wakened.
Liquids disturbed, some spilled. Small unstable objects
displaced or upset. Doors swing, close, open. Shutters,
pictures move. Pendulum clocks stop, start, change
rate.
Light to total destruction
According to these findings it is unlikely that Kudahuvadhoo will receive an earthquake
capable of causing destruction. It should however be noted that the actual damage may
be different in Maldives since the masonry and structural stability factors have not been
considered at local level for the MMI values presented here. Usually such adjustments
can only be accurately made using historical events, which is almost nonexistent in
Maldives.
2.2.6 Climate Change
The debate on climate change, especially Sea Level Rise (SLR) is far from complete.
Questions have been raised about SLR itself (Morner et al., 2004, Morner, 2004) and
the potential for coral island environments to naturally adapt (Kench et al., 2005,
Woodroffe, 1993). However the majority view of the scientific community is that climate
is changing and that these changes are more likely to have far reaching consequences
for Maldives. For a country like Maldives, who are most at risk from any climate change
impacts, it is important to consider a cautious approach in planning by considering worst
case scenarios. The findings presented in this section are based on existing literature.
No attempt has been made to undertake detailed modelling of climate change impacts
specifically on the island due to time limitations. Hence, the projection could change with
new findings and should be constantly reviewed.
The most critical driver for future hazard exposure in Maldives is the predicted sea level
rise and Sea Surface Temperature (SST) rise. Khan et al. (2002, Woodroffe, 1993)
analysis of tidal data for Gan, Addu Atoll shows the overall trend of Mean Tidal Level
(MTL) is increasing in the southern atolls of Maldives.
Their analysis shows an
increasing annual MTL at Gan of 3.9 mm/year. These findings have also been backed
by a slightly higher increase reported for Diego Garcia south of Addu Atoll (Sheppard,
2002). These calculations are higher than the average annual rate of 5.0 mm forecasted
by IPCC (2001), but IPCC does predict a likely acceleration as time passes. Hence, this
indicates that the MTL at Gan by 2100 will be nearly 0.4m above the present day MTL.
Similarly, Khan et al. (2002) reported air temperature at Addu Atoll is expected to rise at
a rate of 0.4C per year, while the rate of rise in SST is 0.3C. Although no specific studies
have been done for Thaa Atoll, the findings from Addu Atoll could be used as a guide to
predicted changes.
Predicted changes in extreme wind gusts related to climate change assumes that
maximum wind gusts will increase by 2.5, 5 and 10 per cent per degree of global
warming (Hay, 2006). Application of the rate of rise of SST to the best case assumption
indicates a 15% increase in the maximum wind gusts by the year 2010 in southern
Atolls.
The global circulation models predict an enhanced hydrological cycle and an increase in
the mean rainfall over most of the Asia. It is therefore evident that the probability of
occurrence and intensity of rainfall related flood hazards for the island of Gan will be
increased in the future. It has also been reported that a warmer future climate as
predicted by the climate change scenarios will cause a greater variability in the Indian
monsoon, thus increasing the chances of extreme dry and wet monsoon seasons (Giorgi
and Francisco, 2000). Global circulation models have predicted average precipitation in
tropical south Asia, where the Maldives archipelago lies, to increase at a rate of 0.14%
Increase of precipitation (%)
per year (Figure 2.10).
12
10
8
6
Rate of increase = 0.135% per year
4
2
0
2010
2020
2030
2040
2050
Year
2060
2070
2080
2090
Figure 2.10 Graph showing the rate of increase of averaged annual mean
precipitation in tropical south Asia (Adger et al., 2004).
There are no conclusive agreements over the increase in frequency and intensity of
Southern Indian Ocean Storms. However, some researchers have reported a possible
increase in intensity and even a northward migration of the southern hemisphere storm
belt (Kitoh et al., 1997) due rise in Sea Surface Temperatures (SST) and Sea Level
Rise. If this is to happen in the Southern Indian Ocean, the frequency of and intensity of
storms reaching Kudahuvadhoo Island coastline will increase and thereby exposing the
island more frequent damages from swell waves. The increase in sea level rise will also
cause the storms to be more intense with higher flood heights.
The above discussed predicted climate changes for Kudahuvadhoo and surrounding
region is summarised in Table 2.9. It should be cautioned that the values are estimates
based on most recent available literature on Maldives which themselves have a number
of uncertainties and possible errors. Hence, the values should only be taken as guide as
it existed in 2006 and should be constantly reviewed. The first three elements are based
climate change drivers while the bottom three is climatological consequences.
Table 2.9 Summary of climate change related parameters for various hazards.
Element
Predicted Predicted change (overall rise)
Possible impacts on
rate
of
Best Case
Worst Case
Hazards in Gan
change
SLR
Air Temp
SST
Rainfall
3.9-5.0mm
/yr
0.4°C
decade
0.3°C
decade
Yr
+0.2m
2050: Yr 2050: +0.4m
Yr
+0.4m
2100:
Yr 2100: +0.88m
/ Yr
+1.72°
2050:
Yr
+3.72°
2100:
/ Yr
+1.29°
2050:
Yr
+2.79°
2100:
+0.14% / Yr
2050:
yr
(or +1384mm
+32mm/yr)
Yr
2100:
Tidal flooding, increase
in swell wave flooding,
reef drowning
Increase
in
storm
surges and swell wave
related flooding, Coral
bleaching & reduction
in coral defences
Increased
flooding,
Could effect coral reef
growth
+2993mm
Wind gusts
5%
and Yr 2050: +3.8 Yr
2050: Increased windstorms,
10%
/ Knots
+7.7Knots
Increase in swell wave
degree of
related flooding.
Yr 2100: +8.3 Yr 2100: +16.7
warming
Knots
Knots
Swell
Waves
Frequency
expected
to change.
Increase in swell wave
related flooding.
Wave
height in
reef
expected
to be high
3.3 Event Scenarios
Based on the discussion provided in section 2.2 above, the following event scenarios
have been estimated for Kudahuvadhoo Island (Tables 2.10, 2.11, and 2.12).
Table 2.10 Rapid onset flooding hazards
Hazard
Max
Impact thresholds
Probability of Occurrence
Prediction
Low
Swell Waves
Moderat
e
Sever
e
Low
Moderate
Severe
Impact
Impact
Impact
NA
< 2.3m
> 2.3m
> 3.0m
High
Moderate
Low
3.7m
< 2.3m
> 2.3m
> 3.0m
Modera
te
Low
Very
low
SW monsoon 1.5m
high seas
< 2.3m
> 2.3m
> 3.0m
High
Very low
Unlikely
Heavy Rainfall
<60m
m
> 60mm
>175m
m
High
Moderate
Low
(wave heights
on reef flat –
Average Island
ridge
height
+1.9m
above
reef flat)
Tsunami
(wave heights
on reef flat)
(For a 24 hour
period)
241mm
1.0.1.
Table 2.11 Slow onset flooding hazards (medium term scenario – year 2050).
Hazard
Impact thresholds
Low
Moderate Severe
Low
SLR:
Tidal < 2.3m
Flooding
> 2.3m
> 3.0m
Moderate Very Low
Very
Low
SLR:
Waves
Swell < 2.3m
> 2.3m
> 3.0m
Very high Moderate
Low
SLR:
Heavy <60mm
Rainfall
>60mm
>175mm
Very
High
Low
Table 2.12 Other rapid onset events.
Hazard
Max
Impact thresholds
Moderate
Moderate
Severe
Prediction
Low
Moderate Severe
Low
Moderate Severe
Wind storm
NA
<30
knts
> 30 knts
>
45Knts
Very
High
High
Moderate
Earthquake
I
< IV
> IV
> VI
Very
Low
Unlikely
none
(MMI
value4)
2.4 Hazard zones
Hazard zones have been developed using a hazard intensity index. The index is based
on a number of variables, namely historical records, topography, reef geomorphology,
vegetation characteristics, existing mitigation measures (such as breakwaters) and
hazard impact threshold levels. The index ranges from 0 to 5 where 0 is considered as
no impact and 5 is considered as very severe. In order to standardise the hazard zone
for use in other components of this study only events above the severe threshold were
considered. Hence, the hazard zones should be interpreted with reference to the hazard
scenarios identified above.
2.4.1 Swell waves and SW monsoon high Waves
The intensity of swell waves and SW monsoon udha is predicted to be highest 50m from
the coastline on the ocean ward side (see Figure 2.11) and 30 m from the lagoonward
side. Swell waves higher than 3.0 m on reef flat are predicted to penetrate inner island
4
Refer to earthquake section above
up to or beyond 200m from the coastline. The longest run-up would be from the
oceanward coastline where it could penetrate 250 m inland. The run-up on the island is
controlled by topography as the flood waters decay below 0.4 m. The presence of
multiple ridges on the oceanward side helps prevent flood run-up beyond 250 m.
The lagoonward side is relatively safe form swell related flooding due to the protection
provided by the atoll rim. However, waves could refract around the reef system and the
island causing flooding close to the shoreline. Such impacts are predicted to be limited
to 10-30 m from the lagoonward coastline and their intensity is expected to remain low.
SW monsoon high waves (udha) are not expected to have an impact beyond 50m of the
coastline and are more likely to influence the coastline right around the island.
Hazrad Zoning Map
Swell Waves, Udha
& Storm Surges
Intensity Index
Low
1
2
3
4
5
High
Contour lines represent intensity
index based on a severe event
scenarios
0
150
300
metres
Figure 2.11 Hazard zoning map for swell wave, storm surges and southwest
monsoon high seas.
2.4.2 Tsunamis
When a severe threshold of tsunami hazard (>3.0 m on reef flat) is considered, 70% of
the island is expected to be directly affected (Figure 2.12). If the waves reach beyond
4.0 m MSL 90% of the island is likely to be flooded due the prevalent tide levels. High
intensity waves will flush through the island from the eastern and southern side while
tide related surges will occur within the atoll, flooding the northern coastline. The
intensity of flood waters will be highest 200-250 m from the shoreline. The water run-up
is expected to be controlled on the southern and south eastern side by the presence of
second relic ridge 250 m from the coastline. Similar high areas exist around the island
which could control the flood waters as the wave decays.
Wave height around the island will vary based on the original tsunami wave height, but
the areas marked as low intensity is predicted to have proportionally lower heights
compared to the coastline.
Hazrad Zoning Map
Tsunami
Intensity Index
Low
1
2
3
4
5 High
scenario
(wave heights +3.0m MSL)
0
150
metres
300
Figure 2.12 Hazard zoning map for tsunami flooding.
2.4.3 Heavy Rainfall
Heavy rainfall above the severe threshold is expected to flood parts of the settlement
(Figure 2.14). The areas predicted for severe intensity are the topographic lows in the
southern and central parts of the island. These areas act as drainage basins for the
surrounding higher areas and due the large size of the island the ‘catchments area’ is
considerable for surface runoff during heavy rainfall.
The intensity is generally expected to be low in most locations. The hazard zone
presented in the map below is based on the topographic surveys done on the island.
Due to the large size of the island it was impossible to assess the topographic variation
across the entire island during this project. Hence the hazard zones shown below should
be considered as the most prominent zones only. More detailed assessment is required
once high resolution topographic data becomes available.
Hazrad Zoning Map
Heavy Rainfall
Intensity Index
Low
1
2
3
4
5
High
scenario
(+175mm in 24 hours)
Note: White areas
represent areas with
no data
0
150
metres
Figure 2.14 Hazard zoning map for heavy rainfall related flooding.
300
2.4.4 Strong Wind
The intensity of the strong wind across the island is expected to remain fairly constant.
Smaller variations may exist between the west and east side where by the west side
receives higher intensity due to the predominant westerly direction of abnormally strong
winds. The entire island has been assigned an intensity index of 4 for strong winds
during a severe event.
2.4.5 Earthquakes
The entire island is a hazard zone with equal intensity. An intensity index of 1 has been
assigned.
2.4.6 Climate Change
Establishing hazard zones specifically for climate change is impractical at this stage due
to the lack of topographic and bathymetric data. However, the predicted impact patterns
and hazard zones described above are expected to be prevalent with climate change as
well, although the intensity is likely to slightly increase.
2.4.6 Composite Hazard Zones
A composite hazard zone map was produced using a GIS based on the above hazard
zoning and intensity index (Figure 2.15). The coastal zone approximately 200m from the
oceanward coastline and the topographically low areas within the island are predicted to
be the most intense regions for multiple hazards. The eastern side is particularly
identified as a hazard zone due to the exposure to swell waves, storm surges, udha and
tsunamis. The overall composite increase in intensity for hazard zones have resulted
from high exposure to intense windstorms across the island.
Hazard Zoning Map
Multiple Hazards
Intensity Index
Low
1
2
3
4
5 High
index based on severe event
scenarios
0
150
metres
Figure 2.15 Composite hazard zone map.
300
2.5 Limitations and recommendation for future study
The main limitation for this study is the incompleteness of the historic data for different
hazardous events. The island authorities do not collect and record the impacts and
dates of these events in a systematic manner. There is no systematic and consistent
format for keeping the records. In addition to the lack of complete historic records there
is no monitoring of coastal and environmental changes caused by anthropogenic
activities such as road maintenance, beach replenishment, causeway building and
reclamation works.
It was noted that the island offices do not have the technical
capacity to carryout such monitoring and record keeping exercises. It is therefore evident
that there is an urgent need to increase the capacity of the island offices to collect and
maintain records of hazardous events in a systematic manner.
The second major limitation was the inaccessibility to long-term meteorological data from
the region. Historical meteorological datasets atleast as daily records are critical in
predicting trends and calculating the return periods of events specific to the site. The
inaccessibility was caused by lack of resources to access them after the Department of
Meteorology levied a substantial charge for acquiring the data. The lack of data has
been compensated by borrowing data from alternate internet based resources such as
University of Hawaii Tidal data. A more comprehensive assessment is thus
recommended especially for wind storms and heavy rainfall once high resolution
meteorological data is available.
The future development plans for the island are not finalised. Furthermore the existing
drafts do not have proper documentations explaining the rationale and design criteria’s
and prevailing environmental factors based on which the plan should have been drawn
up. It was hence, impractical to access the future hazard exposure of the island based
on a draft concept plan. It is recommended that this study be extended to include the
impacts of new developments, especially land reclamations, once the plans are finalised.
The meteorological records in Maldives are based on 5 major stations and not at atoll
level or island level. Hence all hazard predictions for Gan are based on regional data
rather than localised data. Often the datasets available are short for accurate long term
prediction. Hence, it should be noted that there would be a high degree of estimation
and the actual hazard events could vary from what is described in this report. However,
the findings are the closest approximation possible based on available data and time,
and does represent a detailed although not a comprehensive picture of hazard exposure
in Gan.
References
ALI, S. (2005) December 26 2004 Tsunami Impact Assessment and a Tsunami
Risk Assessment of the Maldives. School of Civil Engineering and the
Envrionment. Southampton, United Kingdom, University of Southampton, .
BINNIE BLACK & VEATCH (2000) Enviromental / Technical study for dredging /
reclamation works under Hulhumale' Project - Final Report. Male', Ministry
of Construction and Public Works.
BUCKLEY, B. W. & LESLIE, L. M. (2004) Preliminary climatology and improved
modelling of South Indian Ocean and southern ocean mid-latitude
cyclones. International Journal of Climatology, 24, 1211-1230.
DEPARTMENT OF METEOROLOGY (DOM) (2005) Severe weather events in
2002
2003
and
2004.
Accessed
1
November
2005,
<http://www.meteorology.gov.mv/default.asp?pd=climate&id=3>,
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DHI (1999) Physical modelling on wave disturbance and breakwater stability,
Fuvahmulah Port Project. Denmark, Port Consult.
GIORGI, F. & FRANCISCO, R. (2000) Uncertainties in regional climate change
prediction: a regional analysis of ensemble simulations with HadCM2
coupled AOGCM. Climate Dynamics, 16, 169-182.
HAY, J. E. (2006) Climate Risk Profile for the Maldives. Male', Ministry of
Envrionment Energy and Water, Maldives.
IPCC (2001) Climate Change 2001: The Scientific Basis, New York, Cambridge,
United Kingdom and New York, NY, USA.
KATZFEY, J. J. & MCINNES, K. L. (1996) GCM simulation of eastern Australian
cutoff lows. Journal of Climate, 2337-2355.
KENCH, P. S. & BRANDER, R. W. (2006) Wave processes on coral reef flats:
Implications for Geomorphology using Australian Case Studies. Journal of
Coastal Research, 22, 209-223.
KENCH, P. S., MCLEAN, R. F. & NICHOL, S. L. (2005) New model of reef-island
evolution: Maldives, Indian Ocean. Geology, 33, 145-148.
KHAN, T. M. A., QUADIR, D. A., MURTY, T. S., KABIR, A., AKTAR, F. &
SARKAR, M. A. (2002) Relative Sea Level Changes in Maldives and
Vulnerability of Land Due to abnormal Coastal Inundation. Marine
Geodesy, 25, 133–143.
KITOH, A., YUKIMOTO, S., NODA, A. & MOTOI, T. (1997) Simulated changes in
the Asian summer monsoon at times of increased atmospheric CO2.
Journal of Meteorological Society of Japan, 75, 1019-1031.
MANIKU, H. A. (1990) Changes in the Topography of Maldives, Male', Forum of
Writers on Environment of Maldives.
MORNER, N.-A. (2004) The Maldives project: a future free from sea-level
flooding. Contemporary South Asia, 13, 149-155.
MORNER, N.-A., TOOLEY, M. & POSSNERT, G. (2004) New perspectives for
the future of the Maldives. Global and Planetary Change, 40, 177-182.
NASEER, A. (2003) The integrated growth response of coral reefs to
environmental forcing: morphometric analysis of coral reefs of the
Maldives. Halifax, Nova Scotia, Dalhousie University.
RICHTER, C. F. (1958) Elementary Seismology, San Francisco, W.H. Freeman
and Company.
SHEPPARD, C. R. C. (2002) Island Elevations, Reef Condition and Sea Level
Rise in Atolls of Chagos, British Indian Ocean Territory. IN LINDEN, O., D.
SOUTER, D. WILHELMSSON, AND D. OBURA (Ed.) Coral degradation in
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3. Environment Vulnerabilities and Impacts
3.1 Environment Settings
3.1.1 Terrestrial Environment
Topography
The topography of Kudahuvadhoo was assessed using three topographic profiles (see
Figure 3.1). Given below are the general findings from this assessment.
Topographic
Profiles
2.67549°N
Location of
w ells
P3 Profile Number
P3
2.671°N
P2
P1
Topographic Survey
Locations
72.8975°E
72.893°E
2.6665°N
0
150
300
metres
Figure 3.1 Topography field survey locations
The island is generally low lying with an average elevation of +0.88 m MSL along the
surveyed island profiles. This finding was reconfirmed from the shallow depths of ground
water table around the island (on average approximately 1 m at median tide). As
characteristic of large islands, considerable variations in topography were observed in
Kudahuvadhoo.
The main topographic features along Profile 1 (P1) are the coastal ridge on the
oceanward side (+1.5 m), a 75m wide low area (+0.45m) and higher ridge like feature
approximately 300 m from coastline (see Figure 3.2). The settlement area is generally
low at about +0.8 m MSL. The high ridge like feature may perhaps represent an old
shoreline during the evolution of the island. The adjacent low area and the present ridge
system are reminiscent of an island undergoing a long period of accretion and
subsequent stabilisation. The area nonetheless appears to be a major natural asset of
the island against flooding events due to the double ridge and low drainage zone. The
low areas were reported to be regular flood zone during heavy rainfall. At present no
developments have been made in the area, except agricultural plots, which appears to
be thriving in the area. Future housing developments have been planned within the low
area and may have implications during major flooding events. These issues will be
discussed in a latter section. Low areas were identified in the existing settlements which
corresponded with reported rainfall related flood zone. Most notable of these were the
areas around the Atoll Education Centre and Pre-school.
G
Profile P1
G’
Harbour
Quay Wall
Extent of
reclaimed land
Extent of
Existing Settlement
High point
(+1.2m)
Possibly and
old ridge
Low Area
(+0.475)
Oceanward
Ridge
(+1.5m)
G’
G
1m
*
Existing settlement
0
Approximate Mean Sea Level
Oceanward Side
Lagoonward Side
0
*
100
200
300
400
500
600
700
800
900
o
No clear sight. Deviation of 5 . T he length does not reflect the island width but accurately represents depth variations
Figure 3.2 Topographic profile P1.
The main topographic features along Profile 2 (P2) are the consistent low elevation (less
than 1.0 m MSL and the presence of a ridge like feature approximately 250 m inland
(see figure 3.3). This high area could be part of the old ridge system described above for
P1since it has a similar heights and distance from coastline. It may be possible that this
ridge extends around the south and east sides of the island. The oceanward ridge along
the survey line has been considerably cleared and modified during the construction of a
waste management centre. Hence, the height of 1.3m may not represent the actual ridge
height. However, observations of coastline further south and north of the area showed
no significant changes in height. If ridge height is a crude indicator of wave power, the
area is considerably less exposed to strong wave action compared to the coastline
facing the southern reef edge.
In general the coastline topography and geomorphology around the island is reminiscent
of islands in low energy settings around Maldives. Hence, it is very likely that
Kudahuvadhoo has low exposure to strong wave action. Some possible reasons for lack
of exposure include island location within the reef, atoll and the archipelago, which on
initial observation appears to protect the island from high energy events and seasonal
wave action.
Profile P2
Relatively
young
Low area
(+0.5m)
G
G’
New Settlement
Low area
(+0.8m)
High Point Low area
(+1.2m) (+0.8m)
Oceanward
Ridge
(+1.3m)
Area modified
for waste
site construction
G’
G
1m
0
Approximate Mean Sea Level
Oceanward Side
Lagoonward Side
0
200
400
600
800
1000
1200
Figure 3.3 Topographic Profile P2.
Based on the topographic profiles, a crude estimate of the island topography was
developed. Figure 3.4 shows the predicted main high and low areas of the island.
High areas
Low areas
Topographic Survey
Locations
0
150
300
metres
Figure 3.4 Predicted high and low areas of Kudahuvadhoo.
Vegetation
Despite being a large island, the vegetation of Kudahuvadhoo is rapidly being depleted.
At present 50% of the island comprises of moderate to dense vegetation. However,
approximately half of the vegetated area is currently being utilised for agriculture and
various human activities. Hence, much of the vegetation cover is low and the amount of
dense vegetation cover is limited to 20% (see Figure 3.5). Much of the undeveloped
areas on the southern half of the island have been cleared for agricultural activities. New
housing developments in the areas have also resulted in vegetation cover to be reduced.
The coastal vegetation on the island is dense but narrow in most locations, especially
along the southern coastline. This has been a result of agricultural plots encroaching
coastal vegetation. Coastal erosion has further narrowed the vegetation strip in the
southern coastline. There is a strong presence of vegetation in both the southeast and
southwest corner of the island, crucial for keeping the erosion prone areas stable. The
planned new developments would remove much of this vegetation, however.
Major vegetation
patches
Agricultural
land and
recreational areas
Settlement in
2004
Reclaimed
land
Current Tsunami
housing project
Planned expansion
of settlement
Vegetation Distribution
0
150
300
metres
Figure 3.5 Distribution of Vegetation.
Ground Water and Soil
Kudahuvadhoo is expected to have a substantial layer of fresh water. Water lens depth
varies across the island based on topography. Generally the water table could be
reached with less than 1m at median tide. This could decrease to 0.5m during spring
high tides or more during heavy rainfall. There are no areas above water table or
wetland areas within Kudahuvadhoo.
Kudahuvadhoo’s ground water was reported to be in generally in good quality (MPND,
2005). The inhabitants reported no shortages of drinking water in the past due to the
good quality of ground water. However, the settlement were reported be experiencing
gradual decline in quality due to contamination and over extraction. The Friday Mosque,
located away for the settlement is still used as a source of ground water during periods
of low rainfall. Rainwater is the main source of drinking water in 2004 (MPND, 2005).
The soil conditions varied from north to south. The main observation was that the
southern half of the island had a large humus layer while the northern parts had
comparatively small humus layer. This finding may generally be explained by the
presence of settlement in the north and agricultural or undeveloped land in the south.
3.1.2 Coastal Environment
Beach and Beach Erosion
Kudahuvadhoo island beach environment has remained relatively stable over the past
45 years. Currently the islands western and southern shorelines are very active while the
northern and eastern shorelines have undergone a reduction in mobility due to coastal
developments. The western shoreline appears to be growing despite the coastal
development in the north. This may be related to the constant sediment supply from the
south, especially during the northeast monsoon, which is considered the most dominant
supply route. It is also highly likely that the major island building processes takes place
during northeast monsoon.
Beach rock was found along an 800m stretch of the southern shoreline (see Figure 3.6).
Part of the beachrock was covered with accretion, indicating seasonal variation in
beachrock exposure. The beachrock areas were located 2-5m away from the present
shoreline further indicating the seasonal nature of erosion.
There are areas of accretion on the west and east sides and areas of erosion on north,
south and east sides. Erosion on north side may not cease in the short-term, as it could
be related to the stabilisation process after land reclamation. The islanders did not
identify erosion as a major problem.
Accretion
Erosion
Exposed Beach Rock
1969 Vegetation Line
1969 Beach Line
2004 Island Area
2000 New ly Reclaimed Land
Figure 3.6 Erosion and accretion in Kudahuvadhoo.
3.1.3 Marine environment
General Reef Conditions
General historical changes to reef conditions were assessed anecdotally, though
interviews with a number of fishermen and young
snorkelers. The general agreement
amongst the interviewees were that the quality of reef areas on the southern reef line
had declined considerably over the past 50 years with a lowering of coral cover and
reduction in fish numbers. Reef conditions on the northern reef line were reported to be
in relatively good condition as is the areas of the reef away from Kudahuvadhoo.
Patches of seagrass can be found on the eastern and southern side of the island and
could soon spread to the western side of the island. Overgrowth of seagrass may
become a major nuisance in the future due to the low currents observed in the region.
3.1.4 Modifications to Natural Environment
Coastal Modifications
Piled Jetty
Land Reclamation
Harbour and
entrance dredging
Breakwaters
Piled Jetty
Dredged area
(boat landing area)
Coastal Developments
0
150
300
metres
Figure 3.7 Coastal modifications in Kudahuvadhoo.
•
As in most inhabited islands of Maldives, access infrastructure has been
developed in Kudahuvadhoo Island. These include a harbour, harbour entrance
channel, breakwaters, dredged areas for boat landing and land reclamation as a
method of dredge material disposal. Two piled jetties were also developed on the
western side of the island prior to the development of harbour. Almost all the
development activities have been located in the northern half of the island.
•
No coastal developments were undertaken on the southern side of the island
•
As a result of these modifications, coastal processes in the northern part of the
island appear to have altered considerably.
Terrestrial Modifications
•
As presented earlier, substantial changes to the vegetation was necessitated due
to the expanding human settlement. Vegetation cover comprising larger trees
have been reduced by almost 50% within the last 45 years and are predicted to
decline further due to the planned settlement expansion and lack of re-vegetation
policies.
•
Coastal vegetation has been considerably reduced over the past 10 years and
may continue to do so with the planned expansion.
•
The modification to topography, especially from road maintenance activities is
minimal. Parts of the eastern coastline have had considerable modifications
along the ridge areas close to shoreline due to past and current waste
management activities.
•
Saltwater intrusion in the water lens was not reported as an issue on the island.
3.2 Environmental mitigation against historical hazard events
3.2.1 Natural Adaptation
There is little evidence that Kudahuvadhoo was in the past exposed to severe storm
events or intense wave activity. The presence of multiple ridges on the southern part of
the island does indicate abrupt changes to coastline in response to changing wave
conditions, however. The height of these ridges is quite low and material quite fine,
indicating a possible lack of intense storm activity or strong wave action.
It is apparent from Kudahuvadhoo’s observation that the island adapts to the limited
long-term hazards it experiences and mitigates them naturally but is still exposed to
infrequent high impact hazards. In this sense the southern coastline is critical zone for
future natural adaptation of the island.
3.2.1 Human Adaptation
Kudahuvadhoo has no major mitigation measures undertaken to prevent exposure to
natural hazards. The main activities include construction of breakwaters to protect the
harbour and the use of coastal protection measures to prevent erosion in the reclaimed
area. Additionally some roads works have been undertaken in the past to mitigate
rainfall related flooding, but there is no continuous road maintenance programme. The
lack of both natural and human adaptation measures could be taken as crude indicator
of the historical exposure of the island to natural hazards, specifically climate related
hazards.
3.3 Environmental vulnerabilities to natural hazards
3.3.1 Natural Vulnerabilities
•
Island is generally low lying and therefore exposed to flooding from the southern
and north eastern side of the island.
•
Topographic variations within the island exposes certain areas to heavy rainfall
associated flooding and creates condition for flood run-up during ocean induced
flooding events. Currently the low areas experience heavy rainfall related
flooding almost regularly, effecting island functions such as schools and
economic activities. There are major low areas in the present uninhabited areas
of the island and could become a major issue due to settlement expansion.
Analysis of flood extents during tsunami shows that the effects of topographic
lows were prominent in the wave run up.
•
The ridges around the island and more importantly on the southern side are not
high enough to prevent the +1.82 or +2.30m storm surges predicted in the
hazard scenarios.
•
Its location facing the Veymandoo Channel and bathymetric features off the
eastern coastline of Maldives exposes the island to effects of tsunamis.
3.3.2 Human induced vulnerabilities
•
The lack of coastal vegetation in certain parts of the coastline is a major concern
in terms of exposure to natural hazards. Coastal vegetation including the
undergrowth acts as natural barrier against tsunami’s, other ocean induced
flooding events and wind storms. A wider coastal vegetation belt would absorb
wave energy from a tsunami or a flooding event reducing the impact on
infrastructure and human settlement. This has been proven from findings across
the nine islands studied under this project. In Kudahuvadhoo Island itself, coastal
vegetation is predicted to have played a major role in preventing the wave run-up
on land due to the vegetation belt and agricultural crops. Stronger coastal
vegetation also reduces wind energy during wind storms and protects settlement
areas near the coastline. Coastal vegetation also plays a critical role in stabilising
the beach areas and assists in controlling erosion. Hence consistent decline in
coastal vegetation along the southern and western shoreline exposes these
areas to above mentioned hazards.
•
Similar to the lack of coastal vegetation, the removal of vegetation from the
settlement area exposes the structures to the direct effects of strong wind. The
effects of climate change and global warming could be felt more strongly due to
the apparent increase in temperature within the settlement area.
•
The northern coastline of the island has been modified to develop a harbour.
Since its development, it was observed that considerable erosion was
experienced by the newly reclaimed land, probably in search of equilibrium in
coastal processes of the region. It was also observed that the southern half of the
island remains more exposed during both monsoons. Developments in northern
coastline would have implications for the stability of southern coastline if enough
sediment is not transported to the southern side during SW monsoon. It may be a
long time before the coastline adapts to the new modifications, by which time it is
highly likely that mitigation measures for coastal erosion will be put in place,
further changing the coastal processes.
•
Reefs form the first line of defence in coral islands against waves and predicted
sea level rise. A functioning and healthy reef is essential for a number of
geomorphologic functions such as sediment production and reef adaptation to
rising sea levels. The natural history of Maldives bears evidence of the role reefs
played in natural adaptation to varying sea levels. The fact that the southern side
of the reef of Kudahuvadhoo is in good condition is an asset. However, the past
inappropriate human activities in the reef such as coral mining and the gradual
decline of reef condition on the eastern and northern ends probably would
increase the sea level rise hazard in Kudahuvadhoo.
3.4 Environmental assets to hazard mitigation
•
The relatively large size of Kudahuvadhoo is its main asset against natural
hazards. During ocean induced flooding events, the extent of impact on the
settlement may be considerably reduced due the limited extent of inundation, as
has been demonstrated by the tsunami of 2004 and other flooding events (see
Natural Hazards Section).
•
The location of the island on the western line of atolls and on the southern end of
Dhaalu Atoll has meant that the island is relatively less exposed to major ocean
induced hazards. It is relatively protected from the long distant swell waves
coming from the east and the monsoon driven waves coming from the west.
Evidence from the ridge heights also shows that the island has not been exposed
to strong wave action or storm activity in the past. Location within the archipelago
has also meant that Kudahuvadhoo is less exposed to storm surges and strong
winds. However, the island is exposed to tsunami due to the ocean topography
off the eastern rim of Maldives towards Kudahuvadhoo (Shifaz, 2004).
•
The east-west orientation of the island can generally be regarded as an asset in
terms of exposure to ocean induced hazards and strong wind related hazards. It
has been found that islands on an east west orientation experienced far less
impacts during the tsunami of 2004 and in previous flooding events.
•
Strong coastal vegetation along the majority of the island’s coastline act as a
strong defence line against ocean induced floods and strong winds.
•
The present agricultural land acts as a buffer between the settlement and
oceanward coastline, protecting the settlement from sea induced floods. The
protection however comes at an expense to the agricultural crops. The tsunami
of 2004 failed to affect the settlement area due the presence of this wide
vegetation belt, but also destroyed much of the agricultural crops.
•
Kudahuvadhoo appears to have two major routes for sediment supply due to its
location on the reef system. This has allowed the island to grow westward
consistently in spite of the presence of a deep lagoon. This also has probably
meant a reduced but a consistent supply of sediments from the southern route
(see existing environment section) even after the construction of the harbour.
More detailed studies are required to confirm this finding.
•
Kudahuvadhoo Island’s coastal environment has remained predominantly stable
over the past 50 years (based on historical information and field evidence).
Perhaps this stability is largely due to a consistent sediment supply and due the
location of the island within the archipelago and reef system. Changes after the
construction of the harbour have not been monitored well, but thus far does not
seem to have led to major net erosion in the original island.
•
The topographic lows on the southern half of the island may act as a drain during
flooding events, preventing wave run-up further inland.
•
Reef width appears to play an important role increasing or decreasing the
impacts of ocean induced wave activity. The impact of gravity waves such as
tsunami’s for example has its impacts reduced based on the length of reef. As
has been discussed in the natural hazards chapter, these findings are preliminary
and needs further inquiry using detailed empirical research.
3.5 Predicted environmental impacts from natural hazards
The natural environment of Kudahuvadhoo and islands in Maldives archipelago in
general appear to be resilient to most natural hazards. The impacts on island
environments from major hazard events are usually short-term and insignificant in terms
of the natural or geological timeframe. Natural timeframes are measured in 100’s of
years which provides ample time for an island to recover from major events such as
tsunamis. The recovery of island environments, especially vegetation, ground water and
geomorphologic features in tsunami effected islands like Laamu Gan provides evidence
of such rapid recovery. Different aspects of the natural environment may differ in their
recovery. Impacts on marine environment and coastal processes may take longer to
recover as their natural development processes are slow. In comparison, impacts on
terrestrial environment, such as vegetation and groundwater may be more rapid.
However, the speed of recovery of all these aspects will be dependent on the prevailing
climatic conditions.
The resilience of coral islands to impacts from long-term events, especially predicted sea
level rise is more difficult to predict. On the one hand it is generally argued that the
outlook for low lying coral island is ‘catastrophic’ under the predicted worst case
scenarios of sea level rise (IPCC 1990; IPCC 2001), with the entire Maldives predicted
to disappear in 150-200 years. On the other hand new research in Maldives suggests
that ‘contrary to most established commentaries on the precarious nature of atoll islands
Maldivian islands have existed for 5000 yr, are morphologically resilient rather than
fragile systems, and are expected to persist under current scenarios of future climate
change and sea-level rise’ (Kench, McLean et al. 2005). A number of prominent
scientists have similar views to the latter (for example, Woodroffe (1993), Morner
(1994)).
In this respect, it is plausible that Kudahuvadhoo may continue to naturally adapt to
rising sea level. There are two scenarios for geological impacts on Kudahuvadhoo. First,
if the sea level continues to rise as projected and the coral reef system keep up with the
rising sea level and survive the rise in Sea Surface Temperatures, then the negative
geological impacts are expected to be negligible, based on the natural history of
Maldives (based on findings by Kench et. al (2005), Woodroffe (1993)). Second, if the
sea level continues to rise as projected and the coral reefs fail to keep-up, then their
could be substantial changes to the land and beaches of Kudahuvadhoo (based on
(Yamano 2000)). The question whether the coral islands could adjust to the latter
scenario may not be answered convincingly based on current research. However, it is
clear that the highly, environments of Kudahuvadhoo, especially the northern coastline,
stands to undergo substantial change or damage (even during the potential long term
geological adjustments), due to potential loss of land through erosion, increased
inundations, and salt water intrusion into water lens (based on Pernetta and Sestini
(1989), Woodroffe (1989), Kench and Cowell (2002)).
As noted earlier, environmental impacts from natural hazards will be apparent in the
short-term and will appear as a major problem in inhabited islands due to a mismatch in
assessment timeframes for natural and socio-economic impacts. The following table
presents the short-term
impacts from hazard event
scenarios predicted for
Kudahuvadhoo.
Hazard Scenario
Probability
at Location
Tsunami (maximum scenario)
3.2m
Low
Potential Major Environmental Impacts
• Widespread damage to coastal vegetation
(Short-term)
• Long term or permanent damage to selected
inland vegetation especially common
backyard species such as mango and
breadfruit trees.
• Contamination of ground water if the
sewerage systems are damaged or if liquid
contaminants such as diesel and chemicals
in the boat yard are leaked.
• Salinisation of ground water lens to a
considerable period of time causing ground
water shortage. If the rainwater collection
Hazard Scenario
Probability
at Location
•
•
•
•
facilities are destroyed, potable water
shortage would be critical.
Widespread damage to crops
Moderate to major damage to coastal
protection and island access infrastructure
such as breakwaters and quay walls.
Short-medium term loss of soil productivity
(southern agricultural zone)
Minor damage to coral reefs (based on
UNEP (2005))
Storm Surge (based on UNDP, (2005))
0.60m (1.53m Very Low
• Minor damage to coastal vegetation
storm tide)
• Minor loss of crops
• Minor damage to coastal protection
infrastructure
• Minor geomorphologic changes in the
southern oceanward shoreline and lagoon
Strong Wind
28-33 Knots
Very High
• Minor damage to very old and young fruit
trees
• Debris dispersion near waste sites.
• Minor damage to open field crops
34-65 Knots
Low
• Moderate damage to vegetation with falling
branches and occasionally whole trees
• Moderate-high damage to open field crops
• Minor changes to coastal ridges
65+ Knots
Very Low
• Widespread damage to inland vegetation
• Minor changes to coastal ridges
Heavy rainfall
187mm
Moderate
• Minor to moderate flooding in low areas,
including roads and houses.
242mm
Low
• Widespread flooding but restricted to low
areas of the island.
Drought
Low
• Minor damage to backyard fruit trees and
crops
Earthquake
Low
• Minor geomorphologic changes to land and
reef system.
Sea Level Rise by year 2100 (effects of single flood event)
Medium
Moderate
• Widespread flooding during high tides and
(0.41m)
surges.
• Loss of land due to erosion.
• Loss of coastal vegetation
• Major changes to coastal geomorphology.
• Saltwater intrusion into water lens and
salinisation of ground water leading to water
Hazard Scenario
Probability
at Location
shortage and loss of flora and fauna.
• Minor to moderate expansion of wetland
areas
3.6 Findings and Recommendations for safe island development plan
•
A coral islands main defensive ability against frequent natural hazards is perhaps
its robust natural adaptive capacity. In order to retain this ability against ocean
induced hazards, a proper and functioning coastal environment is essential. It
takes a number of years in term of geological time for an island to stabilise and
achieve equilibrium in processes around the island. Once established the island
evolves and adapts to the prevailing conditions. The natural history of Maldives
bears evidence to such natural adaptation, including the survival through a 2.5m
rise in sea level (Kench et.al, 2004). It is perhaps the foremost reason why the
coral islands of Maldives have survived thus far.
•
The proposed safe island development in Kudahuvadhoo proposes to change a
functioning coastal environment into a more artificial environment. The
implications of this change are numerous especially in the short term. The
proposed modifications may require considerable time for the island to achieve
equilibrium in different forces controlling coastal processes. During this period
considerable changes to the existing coastal environment may be imminent. In
the absence of coastal protection, these changes would be more noticeable.
There is a high probability that the proposed coastal modifications would expose
Kudahuvadhoo to the following ocean induced hazards.
There could be a rapid onset of erosion in specific areas of the island in the
short-term until the coastal environment achieves an equilibrium. The present
shape of the coastline is a result of the prevailing condition within the reef.
Considerable changes to unaltered zones of the island are highly probable.
Hence, coastal erosion hazards may in general be increased.
•
Island topography and resulting drainage systems are critical features of an
island in relation to exposure to natural hazards. Safe island development plan of
Kudahuvadhoo should consider the existing topography and implications of
modifying the topography on the rainfall related flooding. Such activities include
reclamation activities proposed in the southern end of the island.
•
Kudahuvadhoo has considerable topographic variations. The proposed new
settlement expansion areas fall into significant low areas of the island which in
the future may be exposed to rainfall related flooding. Similarly, the function of
the low drainage areas in the Environment Protection Zone (EPZ) needs to be
reviewed. Given the topographic variations within Kudahuvadhoo , the proposed
0.1m variation in the drainage area may not have the desired effects on flood
control. In the southern areas where there are multiple ridges, the proposed
drainage area simply may have no function while in the western areas it may
lead to rainfall related flooding unless siltation-proof drainage systems are
installed.
•
Based on the 9 islands studies in this project, it has been observed that strong
coastal vegetation is amongst most reliable natural defences of an island at times
of ocean induced flooding, strong winds and against coastal erosion. The design
of EPZ zone needs to be reviewed to consider the important characteristics of
coastal vegetation system that is required to be replicated in the safe island
design. The width of the vegetation belt, the composition and layering of plant
species and vegetation density needs to be specifically looked into, if the desired
outcome from the EPZ is to replicate the coastal vegetation function of a natural
system. Based on our observations, the proposed width of coastal vegetation in
the standard Safe Island Design may not be appropriate for reducing certain
ocean induced hazard exposures. The timing of vegetation establishment also
needs to be clearly identified in the safe island development plan.
3.7 Limitations and recommendations for further study
•
The main limitation of this study is the lack of time to undertake more empirical
and detailed assessments of the island. The consequence of the short time limit
is the semi-empirical mode of assessment and the generalised nature of findings.
•
The lack of existing survey data on critical characteristics of the island and reef,
such as topography and bathymetry data, and the lack of long term survey data
such as that of wave on current data, limits the amount of empirical assessments
that could be done within the short timeframe.
•
The topographic data used in this study shows the variations along two main
roads of the island. Such a limited survey will not capture all the low and high
areas of the island. Hence, the hazard zones identified may be incomplete due to
this limitation.
•
This study however is a major contribution to the risk assessment of safe islands.
It has highlighted several leads in risk assessment and areas to concentrate on
future more detailed assessment of safe islands. This study has also highlighted
some of the limitations in existing safe island concept and possible ways to go
about finding solutions to enhance the concept. In this sense, this study is the
foundation for further detailed risk assessment of safe islands.
•
There is a time scale mismatch between environmental changes and socioeconomic developments. While we project environmental changes for the next
100 years, the longest period that a detailed socio-economic scenario is credible
is about 10 years.
•
Uncertainties in climatic predictions, especially those related Sea Level Rise and
Sea Surface Temperature increases. It is predicted that intensity and frequency
of storms will increase in the India Ocean with the predicted climate change, but
the extent is unclear. The predictions that can be used in this study are
based on specific assumptions which may or may not be realized.
•
The following data and assessments need to be included in future detailed
environmental risk assessment of safe islands.
A topographic and bathymetric survey for all assessment islands prior to
the risk assessment. The survey should be at least at 0.5m resolution for
land and 1.0m in water.
Coral reef conditions data of the ‘house reef’ including live coral cover,
fish abundance and coral growth rates.
At least a years data on island coastal processes in selected locations of
Maldives including sediment movement patterns, shoreline changes,
current data and wave data.
Detailed GIS basemaps for the assessment islands.
Coastal change, flood risk and climate change risk modeling using GIS.
Quantitative hydrological impact assessment.
Coral reef surveys
Wave run-up modelling on reef flats and on land for gravity waves and
surges.
References
UNDP (2006), Disaster risk profile of Maldives
IDPA report
IPCC (1990). Strategies for Adaptation to Sea-Level Rise: Report of the Coastal
Management Subgroup. Strategies for Adaptation to Sea-Level Rise: Report of
the Coastal Management Subgroup. IPCC Response Strategies Working Group.
Cambridge, University of Cambridge.
IPCC (2001). Climate Change 2001: Impacts, Adaptation, and Vulnerability.
Cambridge, United Kingdom and New York, NY, USA, Cambridge University
Press.
Kench, P. S. and P. J. Cowell (2002). "Erosion of low- lying reef islands." Tiempo
46: 6-12.
Kench, P. S., R. F. McLean, et al. (2005). "New model of reef-island evolution:
Maldives, Indian Ocean." Geology 33(2): 145-148.
Ministry of Planning and National Development (MPND) (2005). Infrastructure
Development for Poverty Alleviation, Volume II - L.Gan. Male', Maldives, Ministry
of Planning and National Development.
Pernetta, J. and G. Sestini (1989). The Maldives and the impact of expected
climatic changes. UNEP Regional Seas Reports and Studies No. 104. Nairobi,
UNEP.
UNEP (2005). Maldives: Post-Tsunami Environmental Assessment, United
Nations Environment Programme.
United Nations Development Programme (UNDP) (2005). Disaster Risk Profile
for Maldives. Male', UNDP and Government of Maldives.
Woodroffe, C. D. (1989). Maldives and Sea Level Rise: An Environmental
Perspective. Male', Ministry of Planning and Environment: 63.
Woodroffe, C. D. (1993). Morphology and evolution of reef islands in the
Maldives. Proceedings of the 7th International Coral Reef Symposium, 1992.
Guam, University of Guam Marine Laboratory. 2: 1217-1226.
Yamano, H. (2000). Sensitivity of reef flats and reef islands to sea level change.
Bali, Indonesia.
4. Structural vulnerability and impacts
4.1 House vulnerability
Not surveyed.
4.2 Houses at risk
As shown in Fig. 4.1 and Fig. 4.2, no houses are exposed to flooding for the time
being. However, the situation will be subjected to a small change in the further
after the establishment of new settlements in the eastern side of the island,
according to a new land use plan updated most recently. Even if it is the case,
the exposure of houses to major flood hazards is very limited. Still no houses are
located in the swell wave / surge flood-prone area and only about 19 houses,
accounting for less than 1% of the total houses, are exposed to tsunami floods.
More houses (more than 20%) are affected by rainfall floods, but damage is very
limited due to the low intensity of rainfall floods (Table 4.1). only the contents of
exposed houses may be subjected to some degree affected. In all, houses on
Kudahuvadhoo Island are at very low risk.
Table 4.1 Houses at risk on Dh. Kudahuvadhoo.
Exposed
Vulnerable
houses
houses
Serious
#
%
#
%
#
%
#
%
#
%
#
%
TS
19
8
W/S
0
0
0
0
0
0
0
0
0
0
0
0
Hazard
Flood
type
RF
53
22.5
Earthquake 236
100
236
100
Wind
Erosion
Potential Damage
Moderate
Slight
Content
Fig. 4.1 Houses at risk associated with Tsunami floods.
Fig. 4.2 Houses at risk associated with rainfall floods (left) and wave / surge floods (right).
4.3 Critical facilities at risk
As shown in Fig. 4.3 and 4.4, critical facilities that are exposed to floods are 2
schools, a mosque, a power house, and a proposed waste disposal site and
waster water treatment plant. Located in the low-lying center of the island, 2
schools and a mosque may be subjected to rainfall floods of very low intensity,
but no physical damage is expected, given the physical conditions at present.
Located in the eastern coast, power house is exposed to ocean-originated floods.
Although no physical damage is expected, its contents may be subjected to
flooding. In addition, two waster processing facilities, proposed to be located in
the southern side of the island, will be subjected to ocean-originated flooding as
well. Flooding of these two facilities may cause secondary contamination to
groundwater system and sewerage system.
In all, critical facilities on Kudahuvadhoo Island are at low risk, although located
in hazard-prone areas. However, some mitigation measures are necessary to
retrofit power house and waste sites or reconsider the location of waste sites.
Table 4.2 Critical facilities at risk on Kudahuvadhoo Island.
Critical facilities
Potential damage/loss
Hazard type
Exposed
Monetary
Physical damage
value
none
Content-affected
N/A
none
Content-affected
N/A
2 schools, 1 mosque
none
No
N/A
Earthquake
-
-
-
-
Wind
-
-
-
-
Erosion
-
-
-
-
Flood
Tsunami
Wave/Surge
Rainfall
Power house, waste
Vulnerable
site, boat repair
Power house, waste
site, boat repair
Fig. 4.3 Critical facilities at risk associated with rainfall floods.
Fig. 4.4 Critical facilities at risk associated with swell wave/surge and tsunami floods.
4.4 Functioning impacts
Although causing no physical damage to most critical facility buildings, major
flooding events may impact the functioning of critical facilities (Table 5.3). For
example, power house and associated distribution network may fail to operate for
days wave/surge and tsunami flooding; Inundation of the waste disposal site can
cause a secondary contamination to the sensitive groundwater system of the
island, which further affect the supply of potable water supply.School activities
may be interrupted for days by rainfall floods as well.
Table 4.3 Potential functioning impacts
Flood
Function
Administration
Tsunami
Wave/surge
Rainfall
Earthquake
Wind
1)
Health care
days
Education
Religion
Housing
Sanitation
3)
Secondary contamination
Water supply
A day
Power supply
Transportation
Communication
2)
Note: 1) Administration including routine community management, police, court, fire fighting; 2) Communication refers to
telecommunication and TV; 3) Sanitation issues caused by failure of sewerage system and waste disposal.
4.5 Recommendations for risk reduction
According to the physical vulnerability and impacts in the previous sections, the
following options are recommended for risk reduction of Kudahuvadhoo:
•
Avoid locating proposed waste disposal site and waste water plant
in the flood-prone area;
•
Retrofit the power house on the northeastern coast.

Detailed Island Risk Assessment in Maldives

Transcrição

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