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Detailed Island Risk Assessment in Maldives
Volume III: Detailed Island Reports
G. Dh. Thinadhoo – Part 1
DIRAM team
Disaster Risk Management Programme UNDP Maldives
July 2008
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Table of contents
1. Introduction
1.1 Geographic location
1.2 Physical environment
1.3 Built environment
2. Natural hazards
2.1 Historic hazard events
2.2 Major natural hazards
2.3 Hazard 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
References
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1. Introduction This report is the first part of the entire detailed island report covering the physical
aspects of the assessment, i.e. natural hazard profile, environmental vulnerability,
and the structural vulnerability of buildings. It aims to provide background information
for decision and policy making in the areas of safe island planning, population
consolidation, economic development, infrastructure development, as well as island
disaster risk management.
This report is designed to be stand -alone and includes all information specific to the
island. In addition, some regional background on natural hazards is also included so
that eventual users, such as island planners and administrators reading a single
island would find it most convenient and avoid referring to the other reports of the
project. Moreover, such an arrangement would also make it easy for this report to be
extended as an island disaster risk management master plan in the future. However,
the similarities in hazard profiles and study limitations amongst islands necessitate
repetition of information across the reports. Readers of multiple island reports are
alerted to this fact, specifically in Subsections 2.2, 2.5, 3.5 and 3.7.
The field survey, conducted in February 1-3, 2007, by Jianping Yan, Ahmed Shaig,
Mohamed Aslam, and Bhupendra Gauchan, covers historic event inventory, hazard
zoning, simple topographical survey, environmental investigation, and inventory of
vulnerable building stocks and critical facilities.
1.1 Geographic location Thinadhoo Island is located on the western rim of Gaafu Dhaalu atoll, at
approximately 72° 59' 50" E and 0° 31' 49" N, about 410 km from the nation’s capital
Male’ and 4.5 km from the nearest airport, Kadedhdhoo (Figure 1.1). It is one of the
few inhabited islands facing the western Indian Ocean and exposed to the southwest
monsoon related wave action. The island fo rms part of the natural atoll called
Huvadhoo Atoll, which is considered the second largest atoll in the world. Thinadhoo
is the atoll capital amongst 10 other inhabited islands. It’s nearest inhabited islands
are Madaveli (7.5 km) and Hoadedhdhoo (9 km). Huvadhoo atoll is the nearest atoll
in Maldives to the equator and sits along the southern half of the laccadive -chargos
ridge, exposing the entire atoll to direct wave action from Indian Ocean. However, it
location in the heart of the doldrums makes the island relatively safe from major
climatic hazard events.
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10
N
Location Mapof Thinadhoo
0 5
kilometers
Hoadedhdhoo
GanGadhdhoo
Vaadhoo
Fares Maathodaa
Fiyoaree
Rathafandhoo
Nadella
Madaveli
Kaadedhdhoo (Airport)
Thinadhoo
Kanduhulhudhoo
South Huvadhu Atoll(Gaafu Dhaalu Atoll)
North Huvadhu Atoll(Gaafu Alifu Atoll)
Figure 1.1 Location map of Thinadhoo.
1.2 Physical environment
Thinadhoo Island has undergone substantial human modifications including land
reclamation, dredging activities and coastal infrastructure development projects. At
present the island is almost rectangle shaped with width ranging from 745 m to 900
m and length ranging from 1590 m to 1351 m. The total surface area of the island at
present is 115.5 ha (1.16 km2). The island is oriented in a north-south direction.
The original island had a land area of approximately 39 ha (0.39 km2) and had a
wetland area covering 16 ha (0.16 km2). The land reclamation process which started
in the 1980’s reclaimed the entire wetland area and parts of the reef flat.
Approximately 71 ha or 61% of the present island is reclaimed and the island of
Thinadhoomaahutta or Maahutta has also been joined to form the present
Thinadhoo.
The reef of Thinadhoo is fairly large with a surface area of 1150 ha (11 km2)
extending to about 7.5km in a northeast-southwest orientation. Thinadhoo is now the
only island within the reef system following the joining of the only inhabited island
(Thinadhoomaahutta). Thinadhoo is located on the southern tip of reef system, next
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to a major reef entrance (Thinadhoo Kandu). The depth of the reef flat is quite
shallow averaging less than -1 m MSL. The distance between the island shoreline
and oceanward reef rim varies from 83 m in the southwest corner to 200 m in the
west. The average distance to reef edge is approximately 170 m.
Thinadhoo is a highly urbanised settlement with over 8000 inhabitants. It is
considered the main urban centre in Huvadhoo Atoll and amongst the largest
population centres in the Maldives. The high level of urbanization also meant that the
natural environment of the island is highly modified to meet the development
requirements. Land reclamation activities were undertaken to relieve land shortages.
The land reclamation activities have resulted in the modification of the entire
coastline, while the vegetation is sparse and almost absent in the newly reclaimed
areas. There are major variations in topography caused by the reclamation activities,
which has resulted in drainage issues and flooding during heavy rainfall. The newly
reclaimed areas do not have a coastal vegetation belt increasing the risk of erosion
and impacts from ocean induced flooding events. Environmental issues associated
with urbanisation are being experienced by its inhabitants including, ground water
contamination, improper waste disposal, degradation of coastal areas, depletion of
vegetation and coastal erosion.
While Thinadhoo has a low incidence of historical natural disasters, the present
environmental characteristics in the island have a number of weaknesses which may
expose the island to future hazards.
1.3 Built environment
Thinadhoo has 1357 allocated plots , 73 living plots, and 177 plots available.
Thundi Avah is located in the northwestern corner; Mathimaradhoo Avah and
Sinaat Sarahadhu in the central eastern coastline; and Mukurimagu Avah in
the south of the island. According to the new landuse plan, a new residential
area is being built in the northwesatern part of the island for population
consolidation.
Most house stocks in the residential areas are masonry built using traditional
construction techniques. However, there are two-storey houses recently built
as well.
6
The key facilities of L. Gan Island have 4 schools, 1 regional hospital in the
center of the island. Each residential area has its own power station with
backup generators .
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2. Natural hazards
This section provides the assessment of natural hazard exposure in G.Dh Thinadhoo
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 rest of this study as a major input.
2.1 Historic hazard events Thinadhoo Island has experienced frequent multiple hazards. 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 lists the known events and a summary of their
impacts on the island.
The historic records showed that the Thinadhoo faced the following multiple hazards:
1) flooding caused by heavy rainfall and 2) swell and storm surges, 3) windstorms
and 4) tsunami. Impacts and frequency of these events vary significantly. Flooding
caused by ra infall and swell surges are the most commonly occurring hazard events,
which however, can only traced back 20-30 years. Windstorms have also been
reported as frequent especially during the southwest monsoon. Since the elderly in
the island cannot recall events beyond 1978, it is highly plausible that severe events
came to the attention of inhabitants only with the rapid expansion of settlement
especially towards the hazard prone western coastline of the island and following the
land reclamation activities.
Table 2.1 Known historic hazard events of Thinadhoo. Metrological hazard
Dates of the recorded events
Impacts
Flooding caused by Heavy rainfall
• 13 July 19831 • 8th Sep 2001 • 9-10th July 20022 • 12 December
2002 • 14 th Nov 2003 • 6th Dec 2003 • 26 th-27th Nov
2006
This is an island that frequently experiences heavy rainfall. According to the island chief, heavy rainfall is most common during mid to late Southwest (SW) monsoon. These events are reported to cause heavy flooding of the reclaimed marsh area on the southern end of the island and the northern half of the natural island. Even a few hours of continuous heavy rainfall causes the houses and the roads in these areas to flood up to a height of 0.3 m – 0.6 m. It has been
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. 2 Heaviest rainfall for a 24 hour period recorded in the country
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reported that the flood waters sometimes have lasted more than a day. The major impacts are:
- Blocked sewerage networks within the flooded zones for up to 3 -5 days.
- Severe damages to the backyard crops such as bananas, chillies etc.
- Damaged personal property such as children’s text books and household goods.
- Disrupted daily life including economic activities, school functioning and transportation.
Flooding caused by surges
• 7 May 19783 • 7 April 1984 • 3 June 1987 • 10 Sept 1987 • June – July 1991 • June – July 2003 • 5th May 2004 • June – July 2005 • 15-19 May 2007
The island is reported to experience frequent (once every few years) flooding caused by wave surges and sometimes large swell waves generated far offshore from the costs of the Maldives. Events are also reported to occur during mid SW monsoon. Surge waters often reaches up to 300 m inland along much of the length of western shoreline. These surge waters have flooded the impact zone up to a height of 0.5 m. The major impact from these events is damages to the backyard crops within the impact zone.
Windstorms 13 July 1983 A number of windstorm events were reported by elders; however, no dates could be determined.
Droughts No major event have been reported
Earthquake Fire
1848
No major event have been reported There was one major event wh ich is known to have caused widespread damage to Thinadhoo. It is not known whether the event was caused by a natural event or human activity.
Tsunami 26th Dec 2004 There has been only one known event. This event flooded the harbour area, the coastal roads and some houses near the harbour to a height of 0.3 m. The tsunami however did not flood the island with any significant force and therefore the impacts of the flooding have been very minimal.
2.2 Major natural hazards
3 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.
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The following major hazards have been identified for Thinadhoo. This finding is
based on the historical records, meteorological records, field assessment and
Disaster Risk Profile of Maldives (UNDP, 2006).
• Swell waves, storm surges and udha
• Heavy rainfall (flooding)
• Windstorms
• Tsunami
• Earthquakes
• Climate Change
The next part of this section will expand on the characteristics, past events impacts
and future event prediction for these hazards.
2.2.1 Swell waves, storm surges and udha
2.2.1.1 Swell waves and storm surges
The geographic location of Thinadhoo Island, with its proximity to the southern Indian
Ocean and location on the western rim of Huvadhoo Atoll, exposes it to year round
swell waves. The presence of swell waves in the region was confirmed by DHI(1999)
during a wave study in the neighbouring Fuvahmulah Island (see Table 2.2). This
study confirms the consistent pattern of wave approach from the South–Southwest
sector, as identified by other swell wave studies of the Indian Ocean (Young, 1999).
The main concern for Thinadhoo Island is the occasional occurrence of abnormal
swell wave events which has the potential to overtop coastal ridges and flood the
island. The events of 1987 and 2005 were caused by known swell waves (table 2.1).
Table 2.2 Wave regimes in neighbouring Fuvahmulah Atoll Season Total Long Period Short Period
NE - Monsoon Predominantly from E-S. High Waves from W From S-SW Mainly E-NE. High
waves from W
Transition Period 1 Mainly from SE-E From S-SW Mainly from NE-SE
SW - Monsoon From SE-SW. Mainly
from S. High Waves also from W
From S-SW Mainly from SE-S. High
waves from West
Transition Period 2 As SW monsoon From S-SW From SE-W. Higher waves from West
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Thinadhoo is identified as a relatively safe island from cyclone related storm surges,
due to its proximity to the equator (UNDP, 2006). However, the development of
localised storm events, with high wind speeds over long periods, has the potential to
generate storm surges and storm tides. Such events have frequently affected
Thinadhoo and can be differentiated from long distant swell waves, due to the
extreme weather conditions prevailing at the time of flooding. The impacts from such
an event would be similar to swell waves, although its duration may be shorter and
intensity higher. Limited records available for this study suggest that the events
during 2003 and 2005 (see table 2.1) were the direct results of storm surges.
The site specific occurrence of abnormal swell waves on Thinadhoo reef flat is
dependent on a number of factors such as the wave height, location of the original
storm event, tide levels and reef geometry. These issues are explained in more detail
in Volume 2, chapter 3.
Past event impacts
The common inundation zone due to swell waves and storm surges is identified as
the oceanward coastline (Figure 2.1). The inland extent of flooding is greatest along
the newly reclaimed areas and could be attributed to the topographically lower
elevations and absence of natural ridge system4. The small area on the southwest
corner of the island with natural ridges reaching +1.8 m MSL were found to be
protected during flood events.
The maximum inundation depth reported on the island during flood events is 1.0 m.
However, it is likely that the depth at the shoreline would have exceeded 1.0 m. This
height is consistent with flood heights reported from swell or surge related waves in
other parts of the country. The event of May 2007 was one of the severest, especially
since the reclamation of the oceanward reef flat.
4 Historical flood events prior to 1989 had to be removed from the assessment due to substantial land reclamation and coastline alteration.
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Reclaimed Land
Original Island
Pro
bab
le w
ave
pro
pag
atio
n p
atte
rns
RE
EF
FL
AT
A generally floodresilient zonedue to high ridges
Historical Flood Events& Estimated Wave
Propagation patternsaround Thinadhoo
HISTORIC EVENTS
NE monsoonWind waves
0 150 300
meters
Figure 2.1 Historical flood events and probable wave propagation patterns in
Thinadhoo and its reef flat.
Swell wave event relationship to storm events
An attempt was made to link the major swell events with extratropical storm events in
the South Indian Ocean. Table 2.3 shows major flooding events in Thinadhoo and
concurrent major storm events in South Indian Ocean.
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Table 2.3 Historical flood events and possible links with storm events. Flooding
event Cyclone Name
Date of Storm Event
Maximum Category
Distance Direction Tide Level
7 May 1978 unknown Data not available
7 th April 1984 7/03/1984 4 Apr – 14 April 1984
3 1450km WSW-S Data not available
3 rd June 1987 unknown Median tide
10 September 1987
unknown NA
June 1991 8th May 1993 Konita 29 Apr
- 07 May 1993
3 1350km SSW High – 2 days after Peak tide of May
June 2003 unknown Peak tide of June
5 May 2004 Juba 4 May 2004
2 1000km WSW-S Medium Tide
June 2005 unknown Peak tide 15 - 17 May
2007 Unknown 13 -19
May 2007
Extra tropical Depression
5630 SW Peak tide of the month
The assessment revealed no concrete results but three events appear to have
occurred concurrently. They were all category 2 or larger extra-tropical cyclones
within 1500 km of Thinadhoo. The event of May 2007 was not classified as a
cyclone. The flood events identified in the table but not associated with the cyclonic
events are also likely to have originated from such depressions, for which data is
limited. Not all the events listed Table 2.3 is expected to be the result of swell waves.
It could also have resulted from storm surges from localised storm events.
Unfortunately we do not have access to localised storm data. However, a common
factor in all these flood events is that they occurred during or close to peak tide of the
month.
To further evaluate these patterns against potential swell wave events, all storms
within 1500 km of Thinadhoo above category 3 were analysed against tide and
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reported flood events (see Table 2.4). There are no clear patterns evident from these
data. Detailed assessment using synoptic charts of the South Indian Ocean
corresponding to major flooding events are required to delineate any specific trends
and exposure thresholds for Thinadhoo.
Table 2.4 Cyclones within 1500 km of Thinadhoo and of category 3 strength (source:
Unisys and JTWC (2004) and University of Hawaii Tide Data)
Cyclone Name Date
Wind Speed (knots) Longitude
Tide Level (monthly)
Flooding reported
1963-01-09 12/01/1963 70 70.4 NA No 1971-07-09 09/07/1971 NA 72.0 NA No 1979-11-25 29/11/1979 100 73.7 NA No 1979-12-10 18/12/1979 110 79.9 NA No 1982-01-06 12/01/1982 115 76.5 NA No 1982-04-23 29/04/1982 100 77.9 NA No 1984-04-03 5/04/1984 75 69.5 NA Yes 1986-01-07 9/01/1986 80 81.6 NA No 1987-03-02 9/03/1987 75 73.7 NA No 1988-10-30 2/11/1988 75 77.3 low No 1988-11-05 14/11/1988 100 80.5 High No 1989-03-26 1/04/1989 100 70.0 Highest No 1990-01-30 3/02/1990 65 69.7 NA No 1991-03-20 26/03/1991 90 81.2 NA No 1993-01-16 24/01/1993 110 70.0 Low No 1993-04-29 4/05/1993 90 68.8 High Yes 1994-03-26 4/04/1994 70 79.2 Highest No 1994-11-21 26/11/1994 115 72.7 Medium No
1995-01-31 6/02/1995 65 71.0 Low-
medium No
1995-03-28 1/04/1995 95 70.5 Medium -
High No
1996-04-06 13/04/1996 135 64.8 Medium-
High No
1996-10-15 18/10/1996 65 79.7 Low No
1996-10-28 6/11/1996 125 81.0 Medium -
High No
1996-11-20 26/11/1996 65 80.5 Medium No
2001-01-06 12/01/2001 100 69.1 Medium -
High No
DINA 18/01/2002 70 71.2 High No IKALA 26/03/2002 65 73.2 Medium No BOURA 17/11/2002 75 69.2 High No KALUNDE 8/03/2003 140 71.7 Low No BENI 12/11/2003 105 74.5 Low No JUBA 4/05/2004 75 71.0 Medium Yes AROLA 9/11/2004 75 77.1 NA No BENTO 23/11/2004 140 76.5 NA No
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Future event prediction
Swell wave direction is mainly from south to west-south-west as shown in Figure 2.2 .
Events beyond this arch may not influence Thinadhoo or could have reduced impact
due to the protection offered by the southern and eastern rim of the atoll.
Possible range ofswell wave directionin Thinadhoo:South to West South West
Figure 2.2 Historical storm tracks (1945-2007) and possible direction of swell waves
for Thinadhoo Island
Strom surge events are most likely to approach from the west, although it is also
likely that low levels of surges could be generated within the atoll due to fetch within
atoll.
Based on current knowledge, it is difficult to forecast the exact probability of a swell
or storm surge events and their intensities. However, since a hazard exposure
scenario is critical for this study, a tentative event scenario has been developed
based on the historical events. In this regard, there is a probability of major swell
events occurring every 5 years in Thinadhoo , with probable water heights of less
than 1.0 m and every 3 years with probable water heights of 0.5 -0.75 m. Events with
water heights less than 0.5 m and greater than 0.2 m are likely to occur annually.
These figures are based on the lowest ridge height on the western coastline and
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hence, the extent of flooding will depend on the actual ridge height at any given
location. The timing of swell events is expected to be predominantly between
November and June, based on historic events and storm event patterns.
The exposure of Thinadhoo to frequent flooding can be partially attributed to land
reclamation. Land reclamation within the past decade has extended the islands
shoreline up to a distance of just 200 m from the reef edge. Moreover, the probability
of flooding is further increased due to the absence of coastal vegetation and the low
crested nature of the modified dune system. The crest level, width of beach dune,
and the coastal vegetation are primary factors that may control surge related flooding
on the islands.
2.2.1.2 Udha
As explained in chapter 3 of Volume 2, Udha events are classified in this study as
low impact annual flooding events, which is characterised by low levels of flooding
within a few meters of the coastline. These events usually occur during SW
monsoon. Based on the flooding patterns from storm surges and swell waves, it is
difficult to discern the difference between these three types of events and more
specific studies need to be undertaken to determine the exact difference. However,
udha events as defined by this study are unlikely to cause substantial impacts on the
island, except disruption to daily life in households close to the southern coastline.
2.2.2 Heavy rainfall
Thinadhoo is located in the highest rainfall region of the Maldives. The nearest
meteorological station is Kaadedhoo airport which became operational in 1993.
Unfortunately this study does not have access to Kaadedhoo data. Moreover,
Kaadedhoo data may be limited for long term trend observation due smaller number
of observation years. Hence, to resolve the issue, data from Gan and severe weather
event reports from Kaadedhoo has been used. It is recommended that further
assessment be made once Kaadedhoo data becomes available.
The mean annual rainfall of Gan is 2299.3 mm with a Standard Deviation of 364.8
mm and the mean monthly rainfall is 191.6 mm. Rainfall varies throughout the year
with mean highest rainfall during October, December and May and lowest between
February and April (See Figure 2.3 below).
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Figure 2.3 Mean Monthly Rainfalls (1978-2004).
Past event impacts
Historic records indicate that the island is often flooded during heavy rainfall.
Records for all incidents have not been kept but interviews with locals and research
into newspaper reports show that localised levels of flooding within areas of
Thinadhoo has only become prominent since the 1990’s. This coincides with the
extensive land reclamation undertaken on the island. Heavy rainfall flooding has
been reported to reach up to 0.6 m above the ground level and based on available
records, Thindhoo is amongst the most intensely flooded island in Maldives.
Thinadhoo’s exposure to flooding has been increased manifold due to human
activities. Since 1988 , land reclamation has been undertaken in the southern
wetlands and around the island within the lagoon (sees section on Physical
Environment Vulnerability). These topographic modifications failed to take into
account the drainage patterns and created highly exposed topographic lows. As a
result, during heavy rainfall, the lowly areas are regularly flooded. Furthermore, to
mitigate flooding on roads, they were raised with extra sand without considering the
flooding implications for surrounding houses. At present some houses are about 0.4
m lower than the adjacent roads. With no artificial drainage system for the roads, the
surrounding houses in the low areas are at constant risk from flooding.
The impacts of flooding so far reported has not been disastrous, but has had
continued impacts on infrastructure, socio-economic functions and vulnerable groups
(poor) living within the low areas. Damages are not structural but losses to personal
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belongings and disruptions to daily life during heavy rainfall are a constant risk for the
vulnerable groups.
By observing daily rainfall data, it would be possible to identify threshold levels for
heavy rainfall for a single day that could cause flooding in Thinadhoo. Unfortunately,
we were unable to acquire daily historical data. However, available records shows
that Kaadedhoo received a maximum precipitation of 219.8 mm for a 24 hour period,
the highest recorded anywhere in Maldives since recording began. This event
caused widespread damage to personal property, road infrastructure, sewerage
infrastructure, backyard crops and harbour quay wall, and led to school closure,
business closure and evacuation from some houses. The worst affected area was
the reclaimed southern half of the island where flood heights reached 0.6-0.7 m.
Later in December that year rainfall of up to 100 mm caused further flooding. During
the flooding events of November and December 2003, the recoded rainfall in
Kaadedhoo for the 24 hour period was 64.4 mm and 60.3 mm (DoM, , 2005). These
two events caused disruption to businesses, school and minor damage to household
goods.
Future event and impact threshold prediction
The probable maximum precipitations predicted by UNDP (2006) for the nearest
major weather station (S. Gan) are as follows (Table 2.5):
Table 2.5 Probable Maximum Precipitation for various Return periods in Gan Return Period 50 year 100 year 200 year 500 year 218.1 238.1 258.1 284.4 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. These figures must be revised once historical daily rainfall data
becomes available (Table 2.6).
Table 2.6 Threshold levels for heavy rainfall flooding in Thinadhoo Threshold level (daily rainfall)
Impact
60 mm Puddles on road, flooding in low houses, minor damage to household goods in most vulnerable locations, disruption to businesses and primary school in low areas.
100 mm Moderate flooding in low houses; all low lying roads flooded; moderate damage to household items especially in the backyard areas
150 mm Widespread flooding on roads and low lying
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houses. Moderate to major damage to household goods, School closure.
200 mm Widespread flooding on roads and houses. Major damages to household goods, sewerage network, backyard crops, School closure, gullies created along shoreline, possible damage to road infrastructure.
250+mm Widespread flooding around the island. Major damages to household goods and housing structure, schools closed, 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 damage than those identified in the table.
2.2.3 Wind storms and cyclones
According to the Disaster Risk Assessment Report (UNDP, 2006) Thinadhoo falls
within the least hazardous zone for cyclone related hazards. There are no records of
cyclones in the southern region, although a number of gale force winds have been
recorded due to low depressions in the region. Winds exceeding 34 knots (gale to
strong gale winds) were reported in Kaadedhoo annually between 2002 and 2005 -
all caused by known low pressure systems near Maldives rather than the monsoon.
Thinadhoo is however exposed to strong winds from monsoonal variations and
localised storm events. Figure 2.4 shows the description of wind speeds and
predominant monthly directions for the period between 1978 and 2001.The
monsoonal winds are generally weaker in the south and more uniform in yearly
distribution in wind speed (Naseer, 2003). However, the occasional strong
monsoonal activity or localised low depressions generate wind speeds capable for
causing substantial damage to vegetation and weak housing structures.
19
Figure 2.4 Description of wind speed data from Gan Weather Station for the period
1978-2001. (Source: Naseer (2003)).
The data from Gan (1978 to 2001) reports a maximum of 63 km/h. The data also
shows that there were four similar events - albeit of smaller intensity - over this
period. The reports for the period 2001 to 2007 provide a different picture, however.
During this period, individual events reaching 70 km/h or more have been report for
each of the 7 years (DoM, 2005). The differences between the reports and data from
the two periods are most likely to be due to the resolution at which data was
analysed. It is highly unlikely that such a substantial jump in high wind speed events
could have occurred since 2001. On the other hand, reports of severe damage
resulting from strong winds increased after 2001. More high resolution data is
required to confirm the occurrence in this unprecedented increase.
Based on the DoM reports, the maximum wind speed experienced in Kaadedhoo is
96 km/h. These are extremely high wind speeds and are equivalent to a category 2
cyclone according to the beaufort scale (see table 2.7)
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Table 2.7 Beaufort scale and the categorisation of wind speeds.
Beau- fort No DescriptionCyclone category
Average wind speed (Knots)
Average wind speed
(kilometres per hour)
Specifications for estimating speed over land
0 Calm Less than 1 less than 1 Calm, smoke rises vertically.
1 Light Air 1 -3 1 - 5 Direction of wind shown by smoke drift, but not by wind vanes.
2 Light breeze 4 - 6 6 - 11Wind felt on face; leaves rustle; ordinary wind vane moved by wind.
3 Gentle breeze 7 - 10 12 - 19Leaves and small twigs in constant motion; wind extends light flag.
4Moderate breeze 11 - 16 20 - 28 Raises dust and loose paper; small branches moved.
5 Fresh breeze 17 -21 29 - 38Small trees in leaf begin to sway; crested wavelets form on inland waters.
6 Strong breeze 22 - 27 39 - 49Large branches in motion; whistling heard in telegraph wires; umbrellas used with difficulty.
7 Near gale 28 - 33 50 - 61Whole trees in motion; inconvenience felt when walking against the wind.
8 Gale Category 1 34 - 40 62 - 74 Breaks twigs off trees; generally impedes progress.
9 Strong gale Category 1 41 - 47 75 - 88Slight structural damage occurs (chimney pots and slates removed).
10 Storm Category 2 48 - 55 89 - 102Seldom experienced inland; trees uprooted; considerable structural damage occurs.
11 Violent storm Category 2 56 - 63 103 - 117Very rarely experienced; accompanied by widespread damage.
12 Hurricane Category 3,4,5 64 and over 118 and over Severe and extensive damage.
Past event impacts
Historic records for Thinadhoo have indicated that near gale force winds (see Table
2.7) have caused significant damage to property and trees on the island. Hence
during the high winds between 2002 and 2005, a number of moderate to major
damages were reported to property, vegetation and backyard crops.
Future event and impact threshold prediction
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.
In order to fill this gap, temporary impact thresholds could be defined using past
impacts on the island and experiences from other islands (see table 2.8). These
descriptions need to be modified using high resolution wind data, when they become
available.
21
Table 2.8 Threshold levels for wind damage based on interviews with locals and
available meteorological data
Wind speeds Impact 1-10 knots No Damage 11 – 16 knots No Damage 17 – 21 knots Light damage to trees and crops 22 – 28 knots Breaking branches and minor damage to
open crops, some weak roofs damaged 28 – 33 knots Minor damage to open crops and houses 34 - 40 knots Minor to Moderate to major damage to
houses, crops and trees 40+ Knots Moderate to Major damage to houses, trees
falling, crops damaged
2.2.4 Tsunami
UNDP (2006) reported that the region where Thinadhoo is located is a moderate
tsunami hazard zone. The tsunami of December 2004 had very little impact on the
island . There was flooding of the island from its lagoonward side. The nearest tide
gauge, at Gan in Addu Atoll, recorded the tsunami of December 2004 as a wave of
height 1.4 m within the atoll lagoon (Figure 2.5). Plotting the maximum water level
recorded at Gan tide gauge (0.8 m +MSL) over the cross-sectional profile of
Thinadhoo clearly shows that the tsunami wave of December 2004 was just higher
than the average ground level of Thinadhoo (Figure 2.6). Comparatively lower wave
height recorded at Gan is partly due to the refraction of the wave caused by the
Indian Ocean bathymetry as it travelled westwards the Maldives, and due the relative
distance from the earthquake epicentre which triggered the tsunami.
-200
-150
-100
-50
0
50
100
150
200
0 100 200 300 400 500 600 700 800 900 1000 1100
Elapsed time (min) since 00:00hrs (UTC) of 26-12-2004
Wat
er d
epth
(cm
) rel
MS
L
Figure 2.5 Water level recordings from the tide gauge at the nearest tide station (Gan, Addu Atoll) indicating the wave height of tsunami 2004.
22
-4
-3
-2
-1
0
1
2
3
4
5
0 100 200 300 400 500 600 700 800
Distance from oceanward shoreline (m)
Hei
ght r
el M
SL (m
)
Island profile
Tsunami induced tide level recorded at the nearest tide station (December 2004)
Extent of inundation
Figure 2.6 Maximum water level caused by tsunami of December 2004 plotted across the island profile of Thinadhoo evidently showing the reason why there was flooding from the island’s lagoonward side. Future event and impact threshold prediction
The predicted probable maximum tsunami wave height for the area where Thinadhoo
is located is 0.8–2.5 m (UNDP, 2006). Examination of the flooding tha t will be caused
by a wave run-up of 2.5 m indicates an inundation at least up to 500 m inland and
with the maximum run-up covering the entire island. The first 20–50 m from the
shoreline will be a severely destructive zone. However, it should be noted tha t
Thinadhoo is fairly protected by the eastern rim of Huvadhoo Atoll from the direct
impacts of tsunami waves. Hence, impacts resulting from water level rise in the atoll
lagoon are more likely to cause significant in Thinadhoo.
It is well understood that the tsunami wave will travel into the atoll lagoon and cause
the water level in the atoll lagoon to rise. The rising of water level would cause
inundation from the lagoonward side of the island, if the water level rises above the
height of the island. The tsunami of December 2004 which raised the water level
within the atoll lagoon by approximately 0.8 m above MSL was just above the
average island elevation. The ration between maximum tide level to the maximum
wave height for the tsunami of 2004 is 0.57. When this ratio is applied the maximum
tsunami wave height predicted for this region, it results in a 1.4 m water level rise
23
within the atoll lagoon. This would flood the island entire island (see figure 2.7
below).
Thinadhoo
-4.0
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
4.0
5.0
0 100 200 300 400 500 600 700 800
Distance from oceanward shoreline (m)
Hei
ght r
el M
SL (m
)
Theoretical flood decay curve
Threshold level of flooding for severe structural damage
Island profile
Figure 2.7 Tsunami related flooding p redicted for Thinadhoo based upon theoretical water level rise within lagoon for the maximum probable tsunami wave height at Thinadhoo
2.2.5 Earthquakes
There hasn’t been any major earthquake related incident recorded in the history of
Thinadhoo or even the Maldives. However, there have been a number of anecdotally
reported tremors around the country.
The Disaster Risk Assessment Report (UNDP 2006) highlighted that Huvadhoo Atoll
is geographically located in the second highest seismic hazard zone of the Maldives.
According to the report, the rate of decay of peak ground acceleration (PGA) for the
zone 4 in which Thinadhoo is located has a value less than 0.18 for a 475 years
return period (see Table 2.9). PGA values provided in the report have been
converted to Modified Mercalli Intensity (MMI) scale (see column ‘MMI’ in Table 2.9).
The MMI is a measure of the local damage potential of the earthquake. See Table
2.10 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
24
Thinadhoo Island as time was limited for such a detailed assessment. Instead, the
findings of UNDP (2006) were used.
Seismic hazard zone
PGA values for 475yrs return period
MMI5
1 < 0.04 I 2 0.04 – 0.05 I 3 0.05 – 0.07 I 4 0.07 – 0.18 I-II 5 0.18 – 0.32 II-III
Table 2.9 Probable maximum PGA values in each seismic hazard zone of Maldives (modified from UNDP, 2006).
MMI Scale
Intensity Description of Damage
I Instrumental Not felt. Marginal and long period effects of large earthquakes.
II Feeble Felt by persons at rest, on upper floors, or favourably placed.
III Slight Felt indoors. Hanging objects swing. Vibration like passing of light trucks. Duration estimated. May not be recognized as an earthquake.
IV Moderate 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 upper range of IV, wooden walls and frame creak.
V Slightly strong
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.
VI-XII Strong - Catastrophic
Light to total destruction
Table 2.10 Modified Mercalli Intensity description (Richter, 1958). According to these findings the threshold for damage is very limited even in a 475
year return earthquake. 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.
5 Based on KATZFEY, J. J. & MCINNES, K. L. (1996) GCM simulation of eastern Australian cutoff lows. Journal of Climate, 2337-2355.
25
2.2.6 Climate change
The parameters relating to climate change in Maldives is described in detail in
chapter 3 of volume 2. Table 2.1 1 below, summarizes these impacts.
Table 2.11 Summary of climate change related parameters for various hazards. Predicted change (overall rise) Element Predicted
rate of
change Best Case Worst Case
Possible impacts on
Hazards in
Thinadhoo
SLR 3.9-5.0mm /yr
Yr 2050: +0.2m
Yr 2100: +0.4m
Yr 2050: +0.4m
Yr 2100: +0.88m
Tidal flooding, increase in swell wave flooding, reef drowning
Air Temp 0.4°C / decade
Yr 2050: +1.72°
Yr 2100: +3.72°
SST 0.3°C / decade
Yr 2050: +1.29°
Yr 2100: +2.79°
Increase in storm surges and swell wave related flooding, Coral bleaching & reduction in coral defences
Rainfall +0.14% / yr (or +32mm/yr)
Yr 2050: +1384mm
Yr 2100: +2993mm
Increased flooding, Could effect coral reef growth
Wind gusts 5% and 10% / degree of warming
Yr 2050: +3.8 Knots
Yr 2100: +8.3 Knots
Yr 2050: +7.7Knots
Yr 2100: +16.7 Knots
Increased windstorms, Increase in swell wave related flooding.
Swell Waves
Frequency and intensity changes. (exact values not known)
Increase in swell wave related flooding.
26
2.3 Event scenarios
Based on the discussion provided in section 2.2 above, the following event scenarios
have been estimated for Thinadhoo Island (Tables 2.12-14).
Table 2.12 Rapid onset flooding hazards
Hazard Max
Predict
ion
Impact thresholds Probability of Occurrence
Low Moderate Severe Low
Impact
Moderate
Impact
Severe
Impact
Swell Waves & storm surges
(wave heights on reef flat – Average Island ridge height +1.7m above reef flat)
NA < 2.0m
> 2.0m6 > 3.0m High Low Very Low
Tsunami
(wave heights on reef flat)
3.0m < 2.0m
> 2.0m > 3.0m Moderate
Low Very low
SW monsoon high seas
(wave heights on reef flat)
2.0m < 2.0m
> 2.0m > 3.0m Very High
Very low Unlikely
Heavy Rainfall
(For a 24 hour period)
284mm <60mm
> 60mm >175mm High Moderate Low
Table 2.13 Slow onset flooding hazards (medium term scenario – year 2050)
Hazard Impact thresholds Probability of Occurrence
Low Moderate Severe Low Moderate Severe
SLR: Tidal Flooding
< 2.0m
> 2.0m > 3.0m Moderate Very Low Very Low
SLR: Swell < 2.0m > 2.0m > 3.0m Very high Moderate Low
6 Impact on southern western corner of the island will only be moderate if waves reach 2.5 m, due to the high natural ridge. The rest of the reclaimed coastline is on average 1.5 m higher than the reef flat.
27
Hazard Impact thresholds Probability of Occurrence
Waves 1.0.1.
SLR: Heavy Rainfall
<60mm >60mm >175mm Very High
Moderate Low
Table 2.14 Other rapid onset events
Hazard Max
Prediction
Impact thresholds Probability of Occurrence
Low Moderate Severe Low Moderate Severe
Wind storm NA <30 knts
> 30 knts > 44Knts
Very High
High Moderate
Earthquake
(MMI value7)
II < IV
> IV > VI Very Low
Unlikely none
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 and hazard
impact threshold levels. The index ranges from 0 to 5 where 0 is ‘no impact’ and 5 is
‘very severe impact’. 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 event scenarios
identified section 2.3 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 100m
from the oceanward coastline (see Figure 2.8). Swell waves higher than 3.0 m on
reef flat are predicted to penetrate the island 300 -500 m inland. The runoff on to the
island is facilitated by the low topography and bare land on the newly reclaimed
parts. The uneven contours of the hazard zones are a result of variations in
topography and presence of obstructions. Areas where the original islands meet the
reclaimed areas could have a faster runoff due to the considerably low topography.
7 Refer to earthquake section above
28
The south western corner of the island has high natural ridges protecting the area
form severe flooding. However the presence of the area may cause refraction and
flood the low reclaimed areas behind it.
Flooding from the southern end is less predictable and is largely dependent on the
approach of the waves. There have been occasions where the sou thern half has not
been flooded at all.
There is a low probability of wind waves generated within the atoll during NE
monsoon to cause low levels of flooding. However the winds during this period are
comparatively low compared to southwest monsoon.
SW monsoon high waves (udha ) are not expected to have an impact beyond 100 m
of the coastline.
0 150
meters
300
Low
1 2 3 4 5 High
Contour lines represent intensityindex based on a severe eventscenario (+3.0m on reef flat &
+1.3m to +0.3m on land)
HAZRAD ZONING MAPLong distance swell waves
and wind waves
Figure 2.8 Hazard zoning map for swell waves and southwest monsoon high seas.
29
2.4.2 Tsunamis
When a severe threshold of tsunami hazard (>3.0 m on reef flat) is considered the,
the eastern and southern half of the island is predicted to receive the highest
intensity (Figure 2.9). The effected zone dependent on the distance from coastline
and minor variations in topography as it advances inland . Inundation dept 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 depths compared to the
coastline. Even in the worst case scenario , the tsunami wave intensity is expected to
be low in Thinadhoo as it is protected by the eastern rim of the atoll and away from
the direct impact from predicted tsunamis.
0 150
meters
300
Low 1 2 3 4 5 High
HAZRAD ZONING MAPTsunami
Contour lines represent intensityindex based on a severe eventscenario (+3.0m on reef flat &
+1.0m to +0.3m on land)
Figure 2.9 Hazard zoning map for tsunami flooding.
30
2.4.3 Heavy rainfall
Heavy rainfall above the severe threshold is expected to flood most parts of the
island except close to the oceanward shoreline (Figure 2.10). The areas predicted for
severe intensity are the reclaimed former wetland areas in the south and the low
areas along the intersection between the original island and newly reclaimed land.
These areas act as drainage basins for the surrounding higher areas. The reclaimed
areas in general are lower than the existing islands, except the northern half where
reclamation was carried out at the existing island level. The higher areas of the island
are expected to have a lower intensity although the impact of flooding may be felt
due to the increased height of roads compared to surrounding houses and lack of an
artificial drainage system on the roads.
meters
0 150 300
Low 1 2 3 4 5 High
HAZRAD ZONING MAPHeavy Rainfall
Contour lines represent intensityindex based on a severe eventscenario (+175mm in a 24 hour
period)
Figure 2.10 Hazard zoning map for heavy rainfall flooding .
31
The rainfall hazard zones are approximate and based on the extrapolation of
topographic data collected during field visits. A comprehensive topographic survey is
required before these hazard zones could be accurately established.
2.4.4 Strong wind
Due to the comparative lack of vegetation cover on the island and the uniform east-
west orientation of the roads, the intensity of the strong wind across the island is
expected to remain fairly constant. Smaller variations may exist between the west
and the east side, where by the west side receives higher intensity due to the
predominant westerly direction of strong winds. The entire island has been assigned
an intensity index of 4 for strong winds.
3.4.5 Earthquakes
The entire island is a hazard zone with equal intensity. An intensity index of 1 has been assigned. 3.4.6 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.
3.4.7 Composite hazard zones
A composite hazard zone map was produced using a GIS based on the above
hazard zoning and intensity index (Figure 2.11). The coastal zone approximately 100
m on the oceanward coastline and 50 m from lagoonward coastline is predicted to
have the highest intensity of hazard events. The inner part of the island is also
exposed to multiple hazards. This pattern of exposure is expected due to the small
size of the island and due to the use of severest threshold for exposure.
32
1500
meters
300
Low 1 2 3 4 5 High
Contour lines represent intensityindex based on a severe event
scenarios
HAZRAD ZONING MAPMultiple Hazards
Figure 2.11 Composite hazard zone map
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,
33
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 at least 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 Thinadhoo 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 Thinadhoo.
34
3. Environment vulnerabilities and impacts
3.1 Environment settings
3.1.1 Terrestrial environment
Topography
The topography of Thinadhoo was assessed using three island profiles (see Figure
3.1). Given below are the general findings from this assessment.
P1
P2
P3
0
Topographic Survey Locations
200
metres
400
72.9924°E
72.9969°E
73.0014°E
73.0059°E
0.535099°N
0.530602°N
0.526106°N
0.521609°N
Figure 3.1 Topographic survey locations
The island has an average elevation of +1.1 m MSL along the surveyed profiles. The
maximum height was observed on the south west corner of the island with a +1.9 4 m
MSL ridge (see Figure 3.2). The lowest point was observed along the southern end
of the island with +0.5 m MSL (see Figure 3.3). These trends were further
reconfirmed from the ground water depths.
35
GG’
0 100 200 300 400 500 600 700 800
1m
0Approximate Mean Sea LevelOceanward Side Lagoonward Side
G
G ’Profile P1
Reclaimed Land
Reclaimed Land
Original Island
Quaywall
Oceanward Ridge
(+1.65m)
Original island Ridge
(+1.5m)
Reclaimed Area(+1m)
Reclaimed Area
(+0.8m)
Figure 3 .2 Topographic profile P1
1m
0
0 100 200 300 400 500 600 700 800
Approximate Mean Sea Level
Oceanward Side Lagoonward Side
G G’
Reclaimed wetlandOriginal Island Original Island
Lowest recordedPoint
(+0.5m) Primary SchoolReclamationarea begins
DamagedBreakwater
Oceanward Ridge
(+1.94m)Second ridge(+1.65m)
G
G’
Profile P2
Floodzones
Figure 3.3 Topography profile P2
36
0 200 400 600 800 1000 1200 1400
Approximate Mean Sea Level
South North
1m
0
Original Island
G G’
Uninhabited Island(Maahutta)Merged duringreclamation
G G’
Profile P2
Reclaimed wetland Reclaimed Reef
Low areaswhere originalisland and newreclamations
meet
Reclaimedland
(+1.53m)
Manualdrainage
duct
Low areaswhere original
island and newreclamations
meet
OriginalIsland
(+1.53m)
Lowly ReclaimedWetland(+0.7m)
Low coastline(+0.8m)
Drainage
Floodzones
Figure 3.4 Topography profile P3
As evident from Figures 3.2 to 3.4, there are substantial topographic variations in the
island. Much of the variations can be attributed to land reclamation activities. Since
the 1980’s, approximately 80ha (0.8 km2) of new land has been reclaimed comprising
approximately 70% of the present island (see Figure 3.7 below). These include 16 ha
(0.16 km2) of wetland area, 57ha (0.57 km2) of reef area and 7ha of the uninhabited
island Maahutta, which was merged to Thinadhoo during reclamation.
Reclamation of the wetlands did not include topographic levelling. As a result, these
areas are substantially lower - at some points 0.8 m lower - than th e original island
(see Figure 3.4). Similarly, low areas were inadvertently established along the
shoreline intersections of original island and reclaimed areas (see Figure 3.4), and
along the new coastline (see F igure 3 .2).
The oceanward coastline topography shown in Figure 3.1 appears to be a natural
development, since the original reclamation had a flat elevation. This finding can be
reconfirmed from the soil composition and an approximately 10 m shift inland in the
coastline, since land reclamation. The soil composition of the reclaimed area
contains coarse sand and coral remnants, while the newly developed ridge system
has considerably fine sediments. The natural development of the ridge is most likely
the result of a combination of wave and wind assisted deposition over the last 4-5
years. It is also highly likely that the ridge system may continue to grow if left
unmodified and if similar climatic conditions prevail.
Implications on natural hazard exposure of Thinadhoo Island, due to the existing
topographic variations are numerous. The presence of low lying areas and the newly
developed drainage patterns are causing regular flooding during heavy rainfall (see
37
Figure 3.3 and 3.4). Combined with the high rainfall in the region and the lack of an
artificial drainage system, heavy rainfall flooding has become the most frequent
natural hazard in Thinadhoo. Similarly, the low elevation of newly reclaimed land
especially on the oceanward side could have implications for future sea induced
flooding events.
Vegetation
Thinadhoo Island vegetation is sparse. As explained earlier, majority of the island
has recently been reclaimed and there were no major re-vegetation programmes
conducted following the reclamation , while natural growth has been slow.
Observation of the vegetation distribution patterns reveals that the vegetation cover
is only present in the original Thinadhoo and Mahuttaa Island. Vegetation on original
Thinadhoo Island itself is sparse due to high population density and is limited to
backyards and open spaces. Vegetation on Mahutta Island is dense , although
substantial parts of the vegetation are now being removed for industrial development
in the area.
The coastal vegetation is almost non-existent. The only area of substantial coastal
vegetation cover is the south west corner of the island. The newly reclaimed coastal
areas are completely bare apart from a small growth of pioneer vegetation. These
pioneer vegetations have a long way to go before they naturally develop in to a full
fledged coastal vegetation system.
It was interesting to note the lack of vegetation in the reclaimed wetland areas of
Thinadhoo. In other reclaimed wetland areas across the country, vegetation re -
growth has been rapid. The case Gaafu Alifu Viligilli and Seenu Hithadhoo is a prime
example and one which has been studied under this project. There are number of
possible reasons for the lack of vegetation growth in Thinadhoo. The most obvious
reason perhaps is the recentness of reclamation in Thinadhoo. Another explanation
may be the ‘saltiness’ of Thinadhoo wetland as it was more openly connected to the
lagoon before reclamation.
38
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2000 400
meters
Open Areas
≅ Α Large Trees
72.9
924°
E
72.9
969°
E0.530602°N
0.5351°N
73.0
014°
E
Figure 3.5 Vegetation distributions in Thinadhoo
Ground water and soil
No attempt was made in this study to undertake a quantitative analysis of the soil and
ground water conditions but a visual assessment was made based on similarities
with other islands in Maldives.
The original islands of Thinadhoo and specifically Maahutta had a substantial layer of
humus followed by fine and whiter material above the water table. The reclaimed
areas on the other hand had no humus layer and did not have any grading in their
soil profile. The entire profile up to the hard reef flat was represented by a single
layer of coarse sand and large coral pieces.
Thinadhoo ground water was reported to be generally in moderate to poor condition.
A number of houses reported effects of saltiness and contamination. They reported
bad taste and smell as main concerns. The ground water conditions in the newly
reclaimed areas of the reef were reported to be poor, perhaps due to the lack of time
39
for an established groundwater aquifer. In general, the Thinadhoo Island does not
rely on ground water for drinking but is used for all other purposes. There were
reports of occasional water shortage and a desalination plant is planned for the
future.
3.1.2 Coastal environment
Beach and beach erosion
It is difficult to assess the coastal erosion in Thinadhoo due the recentness of the
land reclamation activities. Land reclamation activities are generally associated with
rapid onset of erosion at selected locations. This process occurs in the short-term
and stabilise once equilibrium in natural forces are achieved. It may however
continue to be a chronic long term problem if the natural processes are unable to
adjust in the short-term. The areas currently experiencing coastal erosion is
described below.
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400
Areas undergoingerosion at present
2000
meters
72.9
924°
E
72.9
969°
E
0.530602°N
0.5351°N
73.0
014°
E
Figure 3.6 Present erosion patterns in Thinadhoo.
40
• Almost the entire coastline of the newly reclaimed areas is undergoing
erosion. This is most likely a part of the natural adjustment process brought
about by the abrupt alterations to the lagoon and coastline. Much of the
eroded material on the oceanward side was observed to be naturally re -
utilised for the shoreline adjustment including development of coastal ridge.
• The settlement areas have been mainly exposed to severe erosion around
the southeast corner of the island. Breakwaters and revetments have been
developed to prevent erosion. The coastline is barely 5 m from the housing
structures in this region.
• Substantial erosion was observed on the northern coastline of former
Maahutta Island. This is also believed to be part of the natural adjustment
process and may take a number of years to fully adjust.
3.1.3 Marine environment
General reef conditions
General historical changes to reef conditions were assessed anecdotally, though
interviews with a number of fishermen. The general agreement amongst the
interviewees was that the quality of reef areas in general had declined considerably
over the past 50 years with a lowering of coral cover and reduction in fish numbers.
Reef conditions on both the oceanward and lagoonward side were reported to be in
poor condition with dead corals and over sedimentation. It is highly likely that the
reported sedimentation may have occurred during land reclamation activities.
3.1.4 Modifications to natural environment
Coastal modifications
• The main and most obvious modification to the natural environment of
Thinadhoo is the extensive land reclamation. As shown in figure 3.7, almost
the entire coastline of the present day Thinadhoo - with the exception of the
southwest corner and parts of old Maahutta Island - have been modified
through land reclamation.
• Coastal protection against erosion has been undertaken only in the south
east corner of the island. Much of the island has been left unprotected after
the reclamation activities. A considerable amount of time may be required for
the new coastline to achieve equilibrium in coastal processes and hence,
severe coastal erosion is imminent, at least in the short-term.
41
• As in most inhabited islands of Maldives, access infrastructure development
activities have been undertaken in Thinadhoo. These include a dredged
harbour, quay wall and a breakwater - all developed on the eastern coastline.
Their presence limits the movement of sediment across the eastern shoreline.
400
Post 2000 reclamation
Original Island
Reclaimed wetland
Pre-2000 reclamation
Breakwater
Dredged Areasfor landreclmation
2000
meters
Quaywall
Maahutta
Original Island
Remnants ofreef access roaddeveloped duringdredging activities
Breakwaterto mitigateerosion
72.9
969°
E
72.9
924°
E
0.530602°N
0.5351°N
73.0
014°
E
Figure 3.7 Natural environment modifications in Thinadhoo.
• The methods used during the reclamation activities have left a number of
undesired coastal modifications that at present does not perform any useful
function for the island. These include a large dredged area just 20 m off the
existing coastline which was used as a dredged material source and the
remnants of a reclaimed access road used to transport dredge material. The
deep dredged areas are highly likely to have a major impact on the sediment
transport and therefore, may facilitate coastal erosion in the short-term. The
remnants of a so lid and perpendicular ‘reef access’ road in the south east
corner of the island may have implications for coastal erosion. The structure
42
is currently being used to mine sand from the reef for construction and road
maintenance purposes.
• In summary, much of the coastline around Thinadhoo is no longer in a natural
state.
Terrestrial modifications
• The terrestrial environment of the island has been considerably changed due
to the land reclamation activities.
• The topography across the island has been changed due to the lack of
consideration given to proper levelling during reclamation projects. This has
led to inappropriate changes in topography, hindering the natural drainage
patterns that existed on the island. Low elevation of reclaimed areas has
resulted in regular flooding in large parts of the island, especially in the old
wetland areas and areas where the old coastline meets the newly reclaimed
reef areas.
• The coastal vegetation around the island has been removed entirely except
for a small patch in the south western corner of the island and the island of
Maahutta. The newly reclaimed areas do not have a coastal vegetation
system which could potentially expose the coastline to erosion and increase
the impacts of ocean induced flooding in these areas.
• The vegetation on the island has been considerably cleared for human
settlement. The only area with substantial vegetation on the island is the
newly added Maahutta Island. Vegetation cover on Maahutta Island itself is
on the decline, following the designation of the area as an industrial zone.
The newly reclaimed areas of the island have less than 10% vegetation
cover. This is perhaps due to the lack of natural growth in reclaimed soil and
the absence of a re-vegetation programme following reclamation activities. It
is highly likely that natural re-growth would take a number of years.
• The increase in heavy rainfall flooding due to the reclamation activities
prompted the authorities to undertake road maintenance activities, which
primarily involved levelling and raising roads. This has led to some houses in
the island to be lower than the road, especially in the low lying areas, causing
flooding in these houses during heavy rainfall.
43
3.2 Environmental mitigation against historical hazard events
3.2.1 Natural adaptation
It is difficult to ascertain past adaptation due to the intense modifications on the
island. The only remaining natural part of the island, the southwest corner, does have
some evidence of past adaptation activities. It appears that the area has experienced
strong wave action and possibly major storm events. There are multiple ridges in the
region. The composition and location indicates that the outer ridge was a response to
a single or a series of subsequent wave events. The inner ridge on the other hand
appears to be a response to general strong wave activity in the region. Hence, it is
apparent that the western side of Thinadhoo has in the past experienced strong wave
action and that the island had adapted to the strong conditions by developing ridges.
3.2.2 Human adaptation
Thinadhoo has a number of mitigation measures undertaken to prevent natural
hazards. The following are the key measures.
• Coastal protection has been developed to prevent erosion in the southeast
corner of the island. This is also the settlement areas closest to the coastline
with distances less than 10 m. These measures are likely to remain in the
future and may require further enhancement, as the natural processes has
been altered almost to an artificial level. Other areas around the island
experiencing severe erosion may require such measures in the future as the
settlement expands closer to the coastline.
• Artificial drainage channels have been dug around the major flood prone
locations in the island. These measures create obstruction to vehicle
movement during rainfall season but partly mitigate moderate to high rainfall
impacts.
• Emergency measures have been developed to mitigate impacts from very
heavy rainfall, which has an annual occurrence. These measures include
acquiring wa ter pumps and designation of Fire Service to lead the flood
mitigation activities. If heavy rainfall is predicted, water pumps are pre-
emptively deployed to reduce flooding impacts.
• Roads around the low areas have been raised to make them usable during
the rainy season. Subsequently, low lying houses around the island gets
regularly flooded. Such houses have responded by either raising the entire
44
plot or constructing low flood barriers at the door. The backyard of most of
these houses is unusable during heavy rainfall.
3.3 Environmental vulnerabilities to natural hazards
3.3.1 Natural vulnerabilities
• The low elevation generally makes the island susceptible to swell waves and
predicted sea level rise. The reclaimed wetland areas on the southern side
will get frequently flooded during high seas in southwest monsoon and high
tides, if the predicted medium or high projections for sea level rise become a
reality.
• North -south orientation exposes the majority of the island’s western coastline
to flooding Hazards.
• Thinadhoo Island is exposed to swell waves and monsoon generated waves
from South West Indian Ocean (Naseer 2003), due to its location on the
western rim of Huvadhoo Atoll.
• Thinadhoo is located in a high rainfall zone. Combined with substantial
variations in topography, the island is frequently exposed to heavy rainfall
flooding .
• Thinadhoo is also located in an earthquake prone zone due to its proximity to
Carlsberg Ridge (UNDP 2006).
• Reef width appears to play an important role increasing or decreasing the
impacts of ocean induced wave activity. The proximity of Thinadhoo Island
coastline to reef edge may increase the exposure of the island to certain sea
induced Hazards. Implications of the existing distance needs to be studied
further to establish a concrete relationship.
3.3.2 Human induced vulnerabilities
• The major impact from human induced activities has come from improper
land reclamation. These include lack of consideration for maintaining a proper
drainage system, impacts of dredging on the reef system and coastal
processes, and failure to assess the impacts of oceanward reef reclamation.
Increased exposure to the following hazards was identified as a direct result
of improper reclamation.
45
o Lack of consideration for island topography and drainage systems has
caused large areas of the island to be frequently flooded during heavy
rainfall. Thinadhoo is perhaps one of the worst cases of heavy rainfall
flooding events in Maldives. The location of island in a high rainfall
zone has not helped in easing this vulnerability. With the predicted
climate change, an increase in rainfall could have major implications
for Thinadhoo islands rainfall hazard exposure.
o The extension of ocean ward coastline close to the reef edge,
especially without consideration to the natural island topography has
increased the exposure of the newly reclaimed land to ocean induced
flooding. The natural island ridges observed on the south west corner
of the island with the same distance to reef edge is approximately 1.0
m higher that the artificial coastline in the newly reclaimed area.
Hence, during past flooding incidents only the reclaimed land gets
flooded while the areas with natural ridges remain resilient.
o Reclamation activities have caused the existing natural coastal
processes to change dramatically. The coastal processes may still be
in a process of change in search of equilibrium in prevailing
conditions. Hence, coastal erosion has been a major part of the
coastal change since the reclamation process. At present the
islanders does not consider erosion to be significant as it does not
affect most of the inhabited areas. Significant areas have been lost
from the newly reclaimed land, however. The fact that the coastline is
developed in an artificial shape may not help in speedy adjustment of
processes. It is highly likely that it’ll take a number of years
o The quality of reef around the island has been reported to have
declined considerably following the land reclamation activities and this
may have implications for reef adaptation against sea level rise.
• Similar to the reclamation of reef flat areas, improper reclamation of wetland
areas has exposed the island to severe rainfall flooding . The reclamation
process appears to have failed to address the implications on topographic
variations and the resulting drainage patterns. As a result, the reclaimed
wetland areas on the south side of the island experiences rainfall floods of up
to 1 m and regular pumping is required during heavy rainfall to mitigate
severity of flooding impact.
46
• Almost 90% of Thinadhoo’s coastline does not have proper coastal vegetation
on them. This is primarily due to the amount of land reclamation done on the
island. Much of the reclaimed western coastline of the island, which is
considered the major hazard zone for sea-induced flooding, remains totally
devoid of vegetation. Since reclamation works were completed manual re -
vegetation has not been undertaken. The natural re-growth of coastal
vegetation would take a number of years and perhaps could take longer than
a natural island due to the artificial nature of the soil profile. At present, only
pioneer vegetation species were observed in the area which only indicates
the beginning of vegetation development. Combined with the low elevation of
the ridge and the absence of any coastal vegetation on the western side,
Thinadhoo in considerably exposed to ocean induced flooding and continues
to experience major flooding events during SW monsoon.
• The general lack of vegetation on the island exposes structures and weaker
vegetation 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. Perhaps the lack of vegetation may owe
to the fact that almost 61% of the island is recently reclaimed land. The
evidence of other reclaimed areas around Maldives shows that it is highly
unlikely that natural vegetation growth can occur in a short timeframe without
human intervention.
• The eastern coastline is now an artificial environment due to dredging
activities, quay walls and reclamation activities. The island building processes
no longer functions properly in this region. It would require continuous human
intervention to mitigate natural hazards such as erosion.
• Past continuous road maintenance activities on the island to mitigate heavy
rainfall flooding has caused the road to be raised higher than the surrounding
housing plots. As a result the houses have become the drainage areas for the
road causing considerable flooding during heavy rainfall.
3.4 Environmental assets to hazard mitigation
• The large land area of Thinadhoo Island is a major asset against regular
flooding and predicted medium impact scenarios of sea induced flooding.
However, much of the land area is reclaimed land which has predominantly
47
involved inappropriate reclamation design and implementation, leading to an
increase in exposure to certain hazards.
• The location of Thinadhoo on western rim of Huvadhoo Atoll and close to the
equator protects the island from direct exposure to the most damaging sea
induced flooding events such as tsunamis and storm surges
• High natural ridge on the south western corner of the island helps prevent
ocean induced flooding up to +2.0 m in the area.
3.5 Predicted environmental impacts from natural hazards
The natural environment of Thinadhoo 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 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 Thinadhoo may continue to naturally adapt to rising
sea level. There are two scenarios for geological impacts on Thinadhoo. First, if the
48
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 Thinadhoo (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, modified environments of Thinadhoo 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)).
Thinadhoo may be particularly vulnerable to sea level rise due to the artificial nature
of the island and substantial alterations brought to the natural processes around the
island. It remains to be seen whether the natural adaptation processes can function
properly provide natural mitigation measures for Thinadhoo Island against sea level
rise.
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
Thinadhoo.
Hazard Scenario Probability at Location
Potential Major Environmental Impacts
Tsunami (maximum scenario) 2.5 m Low • Moderate damage to coastal vegetation
(Short-term) • Long term or permanent damage to selected
inland vegetation in southern low areas especially common backyard species such as mango and breadfruit trees
• Salt water intrusion into wetland areas and island water lens causing minor loss of flora and fauna.
• Contamination of ground water if the sewerage system is damaged or if liquid contaminants such as diesel and chemicals are leaked especially in the industrial area of Maahutta
• Moderate to major damage to coastal
49
Hazard Scenario Probability at Location
Potential Major Environmental Impacts
protection and island access infrastructure such as breakwaters and quay walls.
• Short-medium term loss of soil productivity • Minor damage to coral reefs (based on
UNEP (2005)) Storm Surge (based on UNDP, (2005))
0.60 m (1.53 m storm tide)
Low • Minor damage to coastal vegetation (north eastern side)
• Minor to moderate damage to coastal protection infrastructure
• Minor geomorphologic changes in the western 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
• Debris dispersion near waste sites. • Minor changes to coastal ridges
65+ Knots Very Low • Widespread damage to inland vegetation • Debris dispersion near waste sites. • Minor changes to coastal ridges
Heavy rainfall 187 mm Moderate • Minor to moderate flooding in low areas,
including roads and houses. 284 mm Very Low • Widespread flooding around the island
• Moderate to major damage to vegetation, roads and structures in low areas.
• Geomorphic changes in selected points of the coastline due to artificial runoff channels.
• Possible damage to sewerage system and subsequent contamination of ground water. Similar incidents have occurred in the past.
• Health implications for inhabitants in low areas, if the heavy rainfall persists for a long period (3-7 days).
Drought • Minor damage to backyard fruit trees Earthquake • Minor-moderate geomorphologic changes Sea Level Rise by year 2100 (effects of single flood event)
Medium (0.41 m)
Moderate • Widespread flooding during high tides and storm surges.
• Loss of land due to erosion. • Loss of coastal vegetation • Major changes to coastal geomorphology. • Saltwater intrusion into low areas and
salinisation of ground water leading to water shortage and loss of flora and fauna.
50
3.6 Findings and recommendations for safe island development
• Thinadhoo is an island which has undergone considerable human
modifications in the past. Much of the activities have been undertaken without
the proper technical studies and considerations for the natural environment.
As a result the island is already exposed to a number of natural hazards. Any
attempts to make Thinadhoo a safe island should consider reducing these
hazards as a priority.
• The safe island development project in Thinadhoo proposes to add new land
to the north of the island along with coastal protection and an Environment
Protection Zone for the western end. The implications of these activities are
numerous. Depending on the timing of the new developments, these changes
could further destabilise the adapting coastal processes and may lead to
onset of erosion in other parts of the island.
• The proposed new land reclamation is expected to have the biggest impact
on the already heavily effected island environment and island exposure to
natural hazards. The following points were noted on the proposed reclamation
project.
o Reclamation is being conducted close to the oceanward side and
reaches to within 130 m of the reefline. The implications for moving
the coastline close to the reef line needs to be clearly understood for
both the existing and new reclamation. There is a possibility that the
reduced distance may increase the chances of wave overtopping and
flooding during severe weather events.
o Reclaiming the oceanward side and protecting only the newly
reclaimed area with breakwaters may cause considerable changes in
the unprotected coastlines around the island. This could include rapid
onset erosion at specific points around the island, especially at the
end points of coastal protection and a possible prolonged continuation
of the erosion and accretion until equilibrium in coastal processes are
achieved.
o The reclamation is highly likely to cause further damage to the outer
reef due to its proximity and current land reclamation practices. This
would reduce the defensive capacity of the reef system and expose
Thinadhoo to long term climate hazards.
51
o The soil composition of a reclaimed area may need to be properly
established. Soil in coral islands of Maldives has specific profiles
which dictate the suitability vegetation and perhaps drainage.
o The elevation of the newly reclaimed area should be inline with the
existing island topography or should consider establishing a
functioning drainage system to mitigate flooding hazards resulting
from modified topography, especially where the new reclamation joins
the existing island.
o The proposed shape of the reclamation zone is artificial and does not
represent the coastline shapes of large natural islands. There may be
implications for wave action and foreshore currents. These aspects
need to be properly studied before the proposed shape is approved.
o The flat elevation of a +1.4 m above MSL for the reclaimed land may
not be the most efficient topography for a functioning drainage system.
The costs involved in establishing and maintaining an artificial
drainage system without the assistance of natural slopes may be
considerably higher.
• The function of the low drainage areas in the proposed Environment
Protection Zone (EPZ) needs to be reviewed. Given the limited topographic
variations within the newly proposed reclaimed land, the proposed 0.1 m
variation and the 25 m width in the drainage area may not have the desired
effects on flood control. The function of a low area near the high ridges has
best been performed in other islands if the width of the area is large and if an
appropriate variation in height between the low area and the high areas
exists. Hence it is recommended that a review of the function and
characteristics of the floodway, reconsideration of the flat elevation of +1.4 m
for the island and reconsideration of the 0.1 m variation for the floodway be
undertaken.
• 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
52
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 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. Furthermore, the EPZ zone planned for Thinadhoo has
only been proposed for the new reclamation project. A functioning coastal
vegetation belt is an urgent priority for the previously reclaimed land as well.
• The constant height of the ridge proposed in the present safe island
development concept needs to be reviewed to identify a suitable height to the
wave conditions prevailing around Thinadhoo Island and predicted hazard
scenarios for the region. Adjusting the heights of previously reclaimed land
needs to be undertaken as well.
• A re-vegetation plan needs to be incorporated into the safe island
development plan to ensure minimal exposure to strong winds and future
climate change related temperature increases. These include re -vegetating
previously reclaimed land.
• The EPZ zones needs to be extended around the island.
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 three
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
53
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 socio-
economic 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.
o A topographic and bathymetric survey for all assessment islands prior
to the risk assessment. The survey should be at least at 0.5 m
resolution for land and 1.0 m in water.
o Coral reef conditions data of the ‘house reef’ including live coral cover,
fish abundance and coral growth rates.
o At least a year’s data on island coastal processes in selected locations
of Maldives including sediment movement patterns, shoreline
changes, current data and wave data.
o Detailed GIS basemaps for the assessment islands.
o Coastal change, flood risk and climate change risk modelling using
GIS.
o Quantitative hydrological impact assessment.
o Coral reef surveys
o Wave run-up modelling on reef flats and on land for gravity waves and
surges.
54
4. Structural vulnerability and impacts
Thinadhoo Island is exposed to all three major floods, although located on the
western rim of the atoll. Rainfall floods prevail in the south and north of the
island and occur every year; swell wave/surge floods hit the island from the
western coast, occurring once every few years; and tsunami inundates the
eastern coast of the island with a period of more than 200 years and a
maximum magnitude of 2.5 m on the shoreline.
4.1 House vulnerability
80 houses are identified as vulnerable on Thinadhoo Island, accounting for
11% of the total existing houses of the island. Houses with poorly physical
structure make up to 7% of the total houses.
4.1.1 Vulnerability typ
The house vulnerability of G.dh. Thinadhoo is dominantly attributed to the
weak physical structure and the low elevation with respect to their adjacent
road surface. As shown in Figure 4.1, 64% of the vulnerable houses are weak
in their physical structure and around 30% are low in their plinth with respect
to their adjacent road surface. In addition, about 20% of the vulnerable
houses are poor in protection.
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
% o
f T
ota
l Vu
lner
able
H
ou
ses
WB PP LE
Indicator group
Figure 4.1 The type of house vulnerability.
55
4.1.2 Vulnerable houses
The vulnerable houses identified can be divided into four major groups:
houses with weak structure, houses with low elevation, houses with weak
structure and low elevation, and houses with poor protection. As shown in Fig.
4.2, most vulnerable houses are weak in their physical structure, accounting
for 50% of the total vulnerable houses and followed by those with poor
protection with a percentage of 20%. Vulnerable houses with low elevation
and those that is weak in physical structure and low in their plinth level
account for 15%, respectively. The distribution of vulnerable houses
implicates that around half of the vulnerable houses are mis-located, either
too close to shoreline in the ocean-originated flood-prone area or lower than
their adjacent road surface.
Thinadhoo
50%
0%14%0%
21%
15% 0%
WB
WBPP
WBLE
WBPPLE
PP
LE
PPLE
Figure 4.2 Distribution of vulnerable houses.
4.2 Houses at risk
4.2.1 Rainfall flood
At present, 213 houses in total, accounting for around 30% of the total
existing houses, are exposed to rainfall floods (Figure 4.3, left). The exposure
increases slightly in the future. According to the land use plan, around 37 new
plots will be allocated in the rainfall flood-prone area.
56
Among the exposed houses, 11 are found vulnerable, which accounts for only
5% of the exposed houses. Given a water depth of 0.5 m and the duration of
3 – 5 days, 2 houses may be subjected to a slight damage due to their
extremely poor physical condition, whereas most exposed houses (99%) are
content-affected, such as back yard crops, house furniture, and other
household goods.
A single heavy rainfall flooding event may result in minor damage to property.
However, the accumulative damage/impacts can be significant because
heavy rainfall flooding is a very frequent event occurring once every year. In
the context of accelerated sea-level rise, it can be expected that rainfall floods
in the southern part of the island will be dramatically enhanced in both
intensity and frequency. Combined with the increasing exposure of houses,
heavy rainfall flooding impact has been becoming one of the unignorable
issues of Thinadhoo Island.
4.2.2 Swell wave/surge flood
Currently, 60 houses (8% of the total existing houses) are located in the swell
wave/surge flood-prone area (Figure 4.3, right), but only 2 are vulnerable. The
potential damage may be very minor, given an inundation of 0.5 m. The
exposure of houses will dramatically increase in the future. According to the
land use plan, around 213 new plots will be allocated in the hazard-prone
area.
4.2.3 Tsunami floods
A significant number of houses are exposed to tsunami inundation with a
water depth varying from 2.5 m at shoreline to 0.5 m in most of the inundation
area (Figure 4.4). As shown in Table 4.1, around 16% of the existing houses
on the island are located in the tsunami inundation area. However, the
exposure will be dropped off to 12% according to new land use plan for the
island.
57
Around 14% of the exposed houses may be subjected to moderate damage
and 3% to slight damage. The overall damage will lead to 0.4 of population
displacement only.
4.2.4 Earthquake
Located in Seismic Hazard Zone 3 and exposed to a GPA of 0.07 (UNDP,
2006), around 52 houses of Thinadhoo Island, accounting for 7% of the total
existing houses, may be subject to a slight to moderate damage due to their
weak structure. In worse case, some houses may be completely destroyed
during an earthquake.
Table 4.1 Houses at risk on G.dh. Thinadhoo.
Potential Damage Exposed
houses
Vulnerable
houses Serious Moderate Slight Content Hazard
type # % # % # % # % # % # %
TS(p) 116 15.6 19 16.4 0 0 16 13.8 3 2.6 97 83.6
TS(f) 91 12.3 18 19.8 0 0 6 6.6 12 13.2 73 80.2
W/S 60 8.1 2 3.3 0 0 0 0 1 1.7 59 98.3
Flo
od
RF 213 28.7 11 5.1 0 0 0 0 2 0.9 211 99.1
Earthquake 742 100 51 6.9
Wind 742 100 51 6.9 - - - - - - - -
Erosion
4.3 Critical facilities at risk
Critical facilities that are exposed to 3 major flooding hazards include island
court, hospital, schools, mosques, and power house (Figure 4.5 and 4.6).
However, none of them are structurally vulnerable to inundations of 0.5-1.0 m
water depth (Table 4.2). Some of facilities such as mosques are not even
content-affected due to their high plinth, 0.5 m above the ground.
Table 4.2 Critical facilities at risk on Thinadhoo Island.
58
Critical facilities Potential damage/loss Hazard type
Exposed Vulnerable Physical damage Monetary
value
Tsunami
1 island court, 1
hospital, 1 mosque, 1
warehouse
None Content-affected
Wave/Surge
2 proposed
mosques, 2
proposed nursery
schools
None ?
Flo
od
Rainfall
2 schools, 4
mosques, 1 power
house
None Content-affected
Earthquake All facilities None No
Wind - - - -
Erosion - - - -
Note: “-“ means “not applicable”.
4.4 Functioning impacts
The functioning impacts of most exposed facilities are minor, just a few hours
to a day at maximum. However, the drainage system may stop functioning for
days and road flooding may affect routine activities of the island. Table 5 is a
summary of some of the potential functioning impacts caused by flooding.
Table 4.3 Potential functioning disruption matrix
Flood Function
Tsunami Wave/surge Rainfall Earthquake Wind
Administration1) A day
Health care A day
Education A day A day A few days
Religion A day A day A few days
Housing 0.4%
Sanitation3) A few days
59
Water supply
Power supply
Transportation days
Communication2)
Note: 1) Administration including routine community management, police, court, fire fighting; 2) Communication
refers to telecommunication and TV; 3) Sanitation issu es 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 Thinadhoo:
• Mitigate rainfall floods in the southern part of the island by
setting up effective drainage systems or proper leveling of the
area. For swell wave/surge floods on the western coast, a ridge
with 0.5 m higher can mitigate the flooding significantly. For the
southeastern corner of the island, a proper EPZ is required,
although tsunami inundation is a rare hazard.
• Enhance building codes in the rainfall flood-prone area by
raising the plinths of houses by at least 0.5 m, and in the ocean-
originated flood-prone area by strong boundary wall, together
with a buffer zone with reasonable width, say, 20 m.
• Avoid protecting roads from flooding by raising the road surface.
60
61
Figure 4.3 Houses at risk associated with rainfall floods (left) and wave/surge floods (right).
62
Fig. 4.4 Houses at risk associated with tsunami floods: present-left and future-right.
63
Figure 4.5 Critical facilities at risk associated with rainfall floods (left) and swell wave/surge floods (right).
64
Figure 4.6 Critical facilities at risk associated with tsunami floods: Present (left) and future (right).
65
References DEPARTMENT OF METEOROLOGY (DOM) (2005) Maldives Climate:
Annual and Monthly Reports. Accessed 1 February 2008, <http://www.meteorology.gov.mv/>, Department of Meteorology, Male', Maldives.
DHI (1999) Physical modelling on wave disturbance and breakwater stability,
Fuvahmulah Port Project. Denmark, Port Consult. IPCC (1990) Strategies for Adaptation to Sea-Level Rise: Report of the
Coastal Management Subgroup. IN IPCC RESPONSE STRATEGIES WORKING GROUP (Ed.) Strategies for Adaptation to Sea-Level Rise: Report of the Coastal Management Subgroup. Cambridge, University of Cambridge.
IPCC (2001) Climate Change 2001: Impacts, Adaptation, and Vulnerability,
Cambridge, United Kingdom and New York, NY, USA, Cambridge University Press.
KATZFEY, J. J. & MCINNES, K. L. (1996) GCM simulation of eastern Australian cutoff lows. Journal of Climate, 2337-2355.
KENCH, P. S. & COWELL, P. J. (2002) Erosion of low- lying reef islands.
Tiempo, 46, 6-12. KENCH, P. S., MCLEAN, R. F. & NICHOL, S. L. (2005) New model of reef-
island evolution: Maldives, Indian Ocean. Geology, 33, 145-148. MANIKU, H. A. (1990) Changes in the Topography of Maldives, Male', Forum
of Writers on Environment of Maldives. 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.
PERNETTA, J. & SESTINI, G. (1989) The Maldives and the impact of
expected climatic changes. UNEP Regional Seas Reports and Studies No. 104. Nairobi, UNEP.
RICHTER, C. F. (1958) Elementary Seismology, San Francisco, W.H.
Freeman and Company. UNEP (2005) Maldives: Post-Tsunami Environmental Assessment. United
Nations Environment Programme. 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.
UNITED NATIONS DEVELOPMENT PROGRAMME (UNDP) (2005) Disaster
Risk Profile for Maldives., Male', UNDP and Government of Maldives.
66
WOODROFFE, C. D. (1989) Maldives and Sea Level Rise: An Environmental Perspective. Male', Ministry of Planning and Environment.
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.
YAMANO, H. (2000) Sensitivity of reef flats and reef islands to sea level
change, Bali, Indonesia.