Detailed Island Risk Assessment in Maldives
Volume III: Detailed Island Reports
K. Thulusdhoo – Part 1
DIRAM team Disaster Risk Management Programme UNDP Maldives
December 2007 Table of contents
1. Geographic background 1.1 Location 1.2 Physical Environment 2. Natural hazards 2.1 Historic events 2.2 Major hazards 2.3 Event Scenarios 2.4 Hazard zones 2.5 Recommendation for future study 3. Environment Vulnerabilities and Impacts 3.1 General environmental conditions 3.2 Environmental mitigation against historical hazard events 3.3 Environmental vulnerabilities to natural hazards 3.4 Environmental assets to hazard mitigation 3.5 Predicted environmental impacts from natural hazards 3.6 Findings and recommendations for safe island development 3.7 Recommendations for further study 4. Structural vulnerability and impacts 4.1 House vulnerability 4.2 Houses at risk 4.3 Critical facilities at risk 4.4 Functioning impacts 4.5 Recommendations for risk reduction 1. Geographic background
1.1 Location
Thulusdhoo is located on the eastern rim of North Male’ atoll at approximately 73° 39' 3"E and 4° 22' 28" N, just 27 km from the nations capital Male’ and 24 km from the Male’ International Airport (Fig. 1.1). Thulusdhoo is the Atoll Capital of North Male’ Atoll, amongst a group of 9 inhabited islands. It’s nearest inhabited islands are Huraa (7 km), Dhiffushi (10 km) and Himmafushi (11 km). Thulusdhoo is in a strategic location due to its proximity to Male’Urban Region and a large number of resort islands. The island falls within the South East quadrant of the atoll where more than 60% of the islands are located.
Location Map of Thulusdhoo Gaafaru 0 5 10 kilometres
NORTH MALE' ATOLL
Dhiffushi
Thulusdhoo Huraa Himmafushi
Hulhumale' Male' International Airport Male' Fig. 1.1 Location map of Thulusdhoo.
1.2 Physical environment
Thulusdhoo is medium-sized island with a length of 1000 m and a width of 550 m at its widest points. The total surface area of the island is 38 Ha (0.38 km 2). The reef of Thulusdhoo is fairly large with a surface area of 6.3 km 2. The reef also hosts two other islands which are presently developed as tourist resorts. Parts of the island are located fairly close to the oceanward reef edge with the closest distance being 80 m from wave break zone. The widest distance from the beach to wave break zone has about 500 m. The average depth of the reef flat is -1 m MSL or less. The existing island surface is approximately +0.8 – 1.6 m MSL. The island is generally low towards its western side.
Thulushoo is oriented in an east-west direction and has a small islet (Thulusdhoo Irumathee huraagandu) on the eastern end of the island. This islet has been stable for at least over 70 years and it has been naturally vegetated during the last 50 years. The islet also acts as natural protection for the eastern areas of the island and plays a major role in shaping the island. The island is growing consistently towards west while the eastern areas remain stable.
Thulusdhoo has a natural harbour due the extensive lagoon on the western side of the island. The lagoon extends to about 2300 m within the reef. The growth of the island towards west has meant that sand is constantly deposited within the deep lagoon hence creating a steep slope on its western end and allowing vessels to approach close to the shoreline.
The existing natural environment of the island has been considerably modified although the extent of coastal modifications is small compared to most of other inhabited islands. 2. Natural hazards
This section provides the assessment of natural hazard exposure in K.Thulusdhoo 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 remaining components of this study as a major input.
2.1 Historic events
The island of Thulusdhoo has been exposed to multiple hazards in the past although its exposure has been limited. A natural hazard event history was reconstructed for the island based on known historical events. As highlighted in methodology section, this was achieved using field interviews and historical records review. Table 2.1 lists the known events and a summary of their impacts on the island.
The historic hazardous events for Thulusdhoo showed that the island faced the following multiple hazards: 1) swell surges, 2) udha , 3) windstorms, 4) flooding caused by heavy rainfall and 5) tsunami. Impacts and frequency of these events vary significantly. Flooding caused by rainfall and udha events is the most commonly occurring hazard events. Windstorms have also been reported as frequent especially during the southwest monsoon. Swell surges have been reported but very infrequent and as having little impact.
Table 2.1. Known historic hazard events of Thulusdhoo. Metrological Dates of the recorded Impacts hazard events
Flooding caused • Frequent events The island is also reported to experience by Heavy rainfall commonly occurring rain fall related flooding. These flooding during SW monsoon. are limited the topographic low area in the centre of the island.
Flooding caused • Late 1950’s There have been 2 major flooding events by swell surges • March 1987 but only one of them is directly linked to • Once in every few wave surges. Waves surge usually occur years flooding on the from the east (oceanward) side of the eastern shoreline. island. Flood waters have been reported to have reached up to 200 m inland from the eastern shoreline. Damages to backyard crops and temporary salinisation of groundwater in the flooded areas have been recorded as man impacts. Regular flooding caused by udha is only limited to the few 10’s of meters inland from the eastern shoreline.
Windstorms No specific records but The effect of strong winds is not limited to reported as frequent a particular area of the island. The effect of strong winds is however is greatest on the western side of the island as these events are mainly caused by the monsoon winds during SW monsoon.
Droughts No major event have been reported
Earthquake No major event have been reported
Tsunami 26 th Dec 2004 There has been only one known event. This event flooded a large area of the island with great force. The tsunami wave was observed to have a height of approximately 2m above ground level when it reached the eastern shoreline of the island. This event flooded over 80% of the island and its impacts include: - Salinisation of groundwater - Damage to the sewerage network - Destroyed a large percentage of large trees - Destroyed a large percentage of backyard crops - Sever to minor structural damage to many houses
2.2 Major hazards
Based on the historical records, meteorological records, field assessment and Risk Assessment Report of Maldives (UNDP, 2006) the following meteorological, oceanic and geological hazards have been identified for Thulusdhoo. • Windstorms • Heavy rainfall (flooding) • Swell waves and udha • Storm Surges • Tsunami • Earthquakes • Climate Change 2.2.1 Swell Waves and Udha
Studies on wave patterns around the country reports a predominantly southwest to a southerly direction for swell waves (Kench et. al (2006), Young (1999), DHI(1999) and Binnie Black & Veatch (2000)). Being located on the eastern rim of Male’ atoll, and on the eastern line of atolls with the archipelago, Thulusdhoo is relatively protected from predominant swell waves in the region. However, the island is still exposed to abnormal swell waves originating from intense storms in the southern hemisphere between 73°E and 130°E longitude. Waves generated from such abnormal events could travel against the predominant swell propagation patterns in the Indian Ocean (Goda, 1998), causing flooding on the eastern rim island of Maldives (Figure 2.1). Much of the historical flood events on the eastern coastline are likely to be the result of such waves.
Male' Atoll
s
n
SW monsoon e o
Thulusdhoo v a o
s
Wind waves w
n
d o
n
i M
W E N
eeesss vvveee aaavvv ll l ww eeellll l wweee sssww EEE sss SSSEEE lll S SS aaalll SSS rrrmm SSSWW ooorrrmm WW nnnooo SSS ooonnn ww bbb eee AA lllll lll W W aaavvv vvveee eeesss Estimated wave propagation patterns around Kudahuvadhoo
Vaavu Atoll
020 40 kilometres
Figure 2.1 Estimated (predominant) wave propagation patterns around Thulusdhoo.
The occurrence of abnormal swell waves on Thulusdhoo reef flat is dependent on a number of factors such as the wave height, location of the original storm event with in the South Indian Ocean, tide levels and reef geometry. Figure 2.2 illustrates the estimated wave propagation and behaviour patterns around Thulusdhoo. The orientation of the island in a NNE to SSW direction, parallel to the predominant wave direction could facilitate wave run-up on the island from oceanward side. Similarly the presence of a channel north of the island may cause waves to refract around the island and flood along the northeastern shoreline.
Historical flood extents
Lagoon Possible Possible refraction refraction
Wave dissipation on reef flat
s ay e r av w Estimated swell wave ell sw propagation patterns al orm near Thulusdhoo & bn A historic flood extents SE
0 300 600 metres
Figure 2.2 Estimated behaviour of swell waves around Thulusdhoo.
It is often difficult to predict occurrence of such abnormal events as there is only a small probability, even within storm events of similar magnitude, to produce waves capable of flooding islands. Moreover, based on the current data available it is impossible to link the swell incidents to the known cyclonic events in the Indian Ocean. 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 Thulusdhoo from southern swells. Unfortunately this study does not have the resources and time to undertake such an assessment but is strongly recommended for any future detailed assessments.
Unlike the swell waves, both the oceanward and lagoonward coastlines of Thulusdhoo are exposed to monsoonal wind waves. During the NE monsoon between November and March, the eastern (oceanward) coastline may receive strong waves. Wave studies done in similar settings in nearby K.Hulhule’ (Binnie Black & Veatch, 2000) reported wave heights less than 2.0 m and with wave periods of 2-4 seconds. The west coast is exposed to wind generated waves during SW monsoon, originating within the atoll due to the 30 km fetch and usually with wave heights less than or about 0.5 m. The orientation of the island is expected to offer some protection from the SW windwaves (See Figure 2.3).
Udha
Flooding is also known to be caused in Thulusdhoo by a gravity wave phenomenon known as Udha . These events are common throughout Maldives and especially the southern atolls of Maldives during the SW monsoon.
The intensity and impacts of udha waves are usually very low with flooding occurring within 10m of coastline at less than 0.3 m height above the ground. It is not expected to be a major hazard in the short-term.
The origins of the udha waves as yet remain scientifically untested. No specific research has been published on the phenomenon and has locally been accepted as resulting from local wind waves generated during the onset of southwest monsoon season. The relationship has probably been derived due to the annual occurrence of the events during the months of May or June. It is highly probable that waves originate as swell waves from the Southern Indian Ocean and is further fuelled by the onset of southwest monsoon during May. The timing of these events coincides as May marks the beginning of southern winter and the onset of southwest monsoon. The concurrent existence of these two forms of gravity waves during the southwest monsoon is confirmed by Kench et. al (2006) and DHI(1999) . It is also questionable whether the southwest monsoon winds waves alone could cause flooding in islands since the peak tide levels on average are low during May, June and July. Furthermore the strongest mean wind speeds in Male’ has been observed for November and is more consistent during October to November than during May and June period (Naseer, 2003). This issue needs to be further explored based on long term wave and climatological data of the Indian Ocean before any specific conclusions can be made. However if the relationship does exists, this phenomena could prove to be a major hazard in the face of climate change since the intensity of southern Indian Ocean winter storms is expected to increase.
Storm Surges
The Disaster Risk Assessment report of 2006 (UNDP, 2006), reported that Thulusdhoo was located in a moderate storm surge hazard zone with probable maximum event reaching 0.6m above MSL or 1.53m with a storm tide. The combined historical records of nearby islands in Male’ and Vaavu Atoll does not show any flooding caused by a storm surge. Similar to the swell waves, the occurrence of any storm surge on Thulusdhoo reef flat is dependent on a number of factors such as the wave height, location of the original storm event within the Indian Ocean, tide levels and reef geometry.
Future swell event prediction
Due to its location, abnormal swell related flooding events should be considered a serious hazard for Thulusdhoo. The island is expected to be exposed to storm waves mainly from south and south east as shown in the map below. Events beyond this arch may not influence the island due to the protection offered by surrounding atolls. Possible range of swell wave direction in K.Thulusdhoo: SE to S
Historic storm events 1945 - 2007
Figure 2.3 Historical storm tracks (1945-2007) in Indian Ocean and possible direction of swell waves for Thulusdhoo Island.
Due to the unpredictability of these swell events and lack of research into their impacts on Maldives, right now it is impossible to forecast the probability of swell hazard event and their intensities. Assessment in Thulusdhoo is further limited by the lack of historical events. However, since the hazard exposure scenario is critical for this study a tentative exposure scenario has been estimated for the island. There is a probability of major swell events occurring every 15 years with probable water heights above 0.7 m and every 5 years with probable water heights of 0.4 m. Events with water heights less than 0.2 m are likely to occur annually especially as Udha.
The timing of swell events is expected to be predominantly between November and June, based on historic events and storm event patterns (see Table 2.2).
Table 2.2 Variation of Severe storm events in South Indian Ocean between 1999 & 2003 (source: (Buckley and Leslie (2004)).
Severe wind event variation Longitude band Winter Summer 30 °E to 39 °E 12.5 17 40 °E to 49 °E 7.5 10 50 °E to 59 °E 7.5 26 60 °E to 69 °E 6 14 70 °E to 79 °E 6 6 80 °E to 89 °E 12 6 90 °E to 99 °E 12 8 100 °E to 109 °E 8 3 110 °E to 119 °E 15 7 120 °E to 130 °E 13.5 2
The reclamation plans for Thulusdhoo were incomplete at the time of this study. The existing drafts show land reclamation on the south and southwestern half of the island. After this development the reef flat width will be reduced to approximately 230 m. This reduction will increase the percentage of occurrence of gravity wave energy on the reef flat to approximately 30% and therefore increasing the probability of flooding caused by surges by 20%. Similarly the impact of flooding will increase relative to encroachment of settlement to coastal areas, even if the probability of flood events remains constant. Potential increase in frequency and intensity of flood events are also probable with climate change and is addressed in a latter section.
3.2.2 Heavy Rainfall
The rainfall pattern in the Maldives is largely controlled by the Indian Ocean monsoons. Generally the NE monsoon is dryer than the SW monsoon. Rainfall data from the three main meteorological stations, HDh Hanimaadhoo, K. Hulhule and S Gan shows an increasing average rainfall from the northern regions to the southern regions of the country (Figure 2.4). The average rainfall at S Gan is approximately 481mm more than that at HDh Hanimadhoo.
3500
3000
2500
2000
1500
1000
Meanannual rainfall (mm) 500
0 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 Year
Gan Hulhule Hanimadhoo Figure 2.4 Map showing the mean annual rainfall across the Maldives archipelago.
The closest meteorological station to Thulushdhoo is the National Meteorological Centre in Hulhule Island. Unfortunately this study does not have access to daily data for Hulhule’.
The mean annual rainfall in Hulhule’ is 1991.5 mm with a Standard Deviation of 316.4 mm and the mean monthly rainfall is 191.6mm. Rainfall varies throughout the year with mean highest rainfall during October, December and May and lowest between February and April (See Figure 2.5).
Fig 2.5 Mean Monthly Rainfall in Hulhule’(1975-2004). Historic records of rainfall related flooding on the island of Thulusdhoo indicates that this island is often flooded and its intensity is comparatively high in certain areas. The overall intensity of is usually moderate. Records for all incidents have not been kept but interviews with locals and research into newspaper reports show that localised levels of flooding within sections of the island. These areas usually correspond to topographic lows within the island. Moreover, substantial topographic variations exist within the island, as is common on larger islands of Maldives. Heavy rainfall related flooding has been reported to reach up to 0.3m above the ground level in central areas of the island.
The impacts of flooding so far reported has not been disastrous, but has had continued impacts on the community such as disruptions to socio-economic functions such as temporary school and business closures, occasional damage to personal property and crops. Of particular importance is the location of Atoll Education Centre in a flood prone location.
It would be possible to identify threshold levels for heavy rainfall for a single day that could cause flooding in Thulusdhoo, through observation of historic daily rainfall data. Unfortunately, we were unable to acquire daily historical data from Hulhule’. Available limited severe weather reports shows that Hulhule and Male’ received a maximum precipitation of 110.8mm for a 24 hour period on 21th November 2004 (DoM, 2005). Based on interviews with locals both events caused moderate levels of flooding for 2 days. The interviewees were unable to recall a single event with significant impacts, reinforcing the view that flood events were of low intensity.
The probable maximum precipitations predicted for Hulhule’ by UNDP (2006) are shown in Table 2.3.
Table 2.3 Probable Maximum Precipitation for various Return periods in Hulhule’ . Station Return Period 50 year 100 year 200 year 500 year Hulhule’ 187.4 203.6 219.8 241.1
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, as shown in Table 2.4. These figures must be revised once historical daily rainfall data becomes available. Table 2.4 Threshold levels for rainfall related flooding in Thulusdhoo. Threshold level Impact (daily rainfall) 60mm Puddles on road, flooding in low houses, occasional minor damage to household goods in most vulnerable locations, disruption to businesses and primary school in low areas. 110mm Moderate flooding in low houses; all low lying roads flooded; minor damage to household items, temporary (minor to Moderate) disruptions to socio-economic functions for less than 24 hours 160mm Widespread flooding on roads and low lying areas. Moderate damage to household goods, disruptions to socio-economic functions for more than 24 hours. 200mm Widespread flooding on roads, low areas and houses. Moderate damage to household goods, sewerage network, backyard crops, disruption to socio economic functions for more than 24 hours, gullies created along shoreline, possible damage to road infrastructure. 230+mm Widespread flooding around the island. Major damages to household goods and housing structure, socio economic functions disrupted for more than 48 hours, businesses closed, damage to crops, damage to road infrastructure, sewerage network and quay wall.
Quite often heavy rainfall is associated with multiple hazards especially strong winds and possible swell waves. It is therefore likely that a major rainfall event could inflict far more damages those identified in the table.
2.2.3 Wind storms and cyclones
Maldives being located within the equatorial region of the Indian Ocean is generally free from cyclonic activity. There have only been a few cyclonic strength depressions that have tracked through the Maldives (UNDP, 2006). Thulusdhoo falls within the second most hazardous zone for cyclone related hazards and has a maximum predicted cyclonic wind speeds of 84.2 Kts (see Figure 2.6). There are no records of such high wind intensity resulting from a cyclone for the Male’ region, although a number of gale force winds have been recorded due to low depressions in the region. Winds exceeding 35 knots (gale to strong gale winds) were reported as individual events in Hulhule annually between 2002 and 2006, all caused by known low pressure systems near Maldives rather than the monsoon (DoM, 2005). The maximum wind speed in Hulhule during this period was approximately 46 kts.
Kulhudhufushi Fonadhoo
Thulusdhoo
Kudahuvadhoo Vilufushi Gan
probable maximum Hazard Zones cyclone wind speed (kts) Villingili 5 96.8 Thinadhoo 4 84.2 3 69.6 2 55.9 Hithadhoo Feydhoo 1 0.0
Figure 2.6 Cyclone hazard zones of the Maldives as defined by UNDP (2006).
Interviews with the locals have indicated that the island has been affected by numerous wind storms. Unfortunately records have not been kept for these events, especially their dates or its impacts. However two events have been identified that had moderate impact on the island: June 1987 and December 1992 events. The event of June 1987 affected a number of islands across of Maldives causing damage to crops, vegetation and housing structures. The December 1992 event reached wind speeds over 25 knots and caused moderate damage to backyard crops and vegetation.
Hence, wind speeds close to near gale winds (see Table 2.5) have caused moderate damage to property and trees on the island. Thulusdhoo now have sparse vegetation cover, resulting primarily from settlement expansion related clearing. Moreover, the remaining large trees within the settlement contain a large proportion of wind vulnerable species, especially breadfruit trees (Artocarpus altilis).
In order to perform a probability analysis of strong wind and threshold levels for damage, daily wind data is crucial. However, such data was unavailable for this study.
Table 2.5 Beaufort scale and the categorisation of wind speeds. Average wind Cyclone Average wind speed Beau- fort No Description Specifications for estimating speed over land category speed (Knots) (kilometres per hour)
0 Calm Less than 1 less than 1 Calm, smoke rises vertically. Direction of wind shown by smoke drift, but not by wind 1 Light Air 1 -3 1 - 5 vanes. Wind felt on face; leaves rustle; ordinary wind vane moved 2 Light breeze 4 - 6 6 - 11 by wind. Leaves and small twigs in constant motion; wind extends 3 Gentle breeze 7 - 10 12 - 19 light flag. Moderate 4 breeze 11 - 16 20 - 28 Raises dust and loose paper; small branches moved. Small trees in leaf begin to sway; crested wavelets form on 5 Fresh breeze 17 -21 29 - 38 inland waters. Large branches in motion; whistling heard in telegraph 6 Strong breeze 22 - 27 39 - 49 wires; umbrellas used with difficulty. Whole trees in motion; inconvenience felt when walking 7 Near gale 28 - 33 50 - 61 against the wind.
8 Gale Category 1 34 - 40 62 - 74 Breaks twigs off trees; generally impedes progress. Slight structural damage occurs (chimney pots and slates 9 Strong gale Category 1 41 - 47 75 - 88 removed). Seldom experienced inland; trees uprooted; considerable 10 Storm Category 2 48 - 55 89 - 102 structural damage occurs. Very rarely experienced; accompanied by widespread 11 Violent storm Category 2 56 - 63 103 - 117 damage. 12 Hurricane Category 3,4,5 64 and over 118 and over Severe and extensive damage.
The threshold levels for damage, summarized in Table 2.6, are predicted based on interviews with locals and housing structural assessments provided by risk assessment report (UNDP, 2006).
Table 2.6 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, minor to moderate damage to vegetation, probability of damage to property due to falling trees. 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 the region where Thulusdhoo is geographically located to be a very high tsunami hazard zone. According to official reports 50% of the island was flooded during the 2004 tsunami. Field surveys and photographs immediately after the event revealed that approximately 75-80% of the island was flooded. Flooding occurred mainly from the southern side and penetrated more than 350 m inland. Flood waters also approached from the lagoonward side due to refraction and the tsunami related tide surge.
There were a number of structural damages close to the southern coastline. These range from complete devastation in some properties to partial damage. A large number of houses were flooded causing loss of personal property. Other damages include salinisation of groundwater for a period of 1 month, damage to vegetation, backyard crops and sewerage network.
The tsunami run-up height on the southern shoreline was reported at 2.0 m reducing to 0.1 m, 250 m inland. Tsunami induced tide level within the lagoon predicted using the tide data from the nearest tide station at Hulhule’ shows that the island experienced water heights higher than 0.2 above the average northern and southern coastline (Figure 2.7 and 2.8). The small levels of flooding from the northern side, most likely reflects this rise.
Figure 2.7 Water level recordings from the tide gauge at Hulhule’ indicating the wave height of tsunami 2004 (source: University of Hawai’i SeaLevel Centre, http://ilikai.soest.hawaii.edu/uhslc/iot1d/male1.html).
5 Tsunami induced tide level recorded at 4 the nearest tide station (December 2004)
3
2
1
0 0 50 100 150 200 250 300 350 400 450 500
Height rel MSL rel (m) Height -1
-2
-3
-4 Distance from oceanward shoreline (m)
Figure 2.8 Maximum water level within the atoll lagoon induced by tsunami of December 2004 plotted across the island profile of Thulusdhoo.
The predicted probable maximum tsunami wave height for the area where Thulusdhoo is located is 4.5 m (UNDP, 2006). Examination of the flooding that will be caused by a wave run-up of 4.5m for the island of Thulusdhoo indicates that such a magnitude wave will flood at least up to 300m inland from the oceanward shoreline. The first 50 – 100 m from the shoreline will be a severely destructive zone (Figure 2.9). The theoretical tsunami flood decay curve was plotted for a wave that is applied only for the direct wave from the oceanward side of the island. However, it is well understood that the tsunami wave will also travel into the atoll lagoon which will cause the water level in the atoll lagoon to rise. Rising of water level in the atoll lagoon would also cause flooding of the island from the lagoonward side of the island, if the atoll lagoon water level rises above the height of the island. Hence the entire island is predicted to be flooded with a maximum predicted tsunami.
Theoretical 5.0 flood decay 4.0 curve Threshold level of flooding for severe structural damage 3.0
2.0
1.0
0.0 0 50 100 150 200 250 300 350 400 450 500
Height rel MSL rel (m) Height -1.0
-2.0 Extent of most -3.0 destructive zone
-4.0 Distance from oceanward shoreline (m)
Figure 2.9 Tsunami related flooding predicted for Thulusdhoo based upon theoretical flood decay curve and the maximum probable tsunami wave height at Thulusdhoo.
2.2.5 Earthquakes
There hasn’t been any major earthquake related incident recorded in the history of Thulusdhoo or even Maldives. However, there have been a number of anecdotally reported tremors around the country.
The Disaster Risk Assessment Report (UNDP 2006) highlighted that Male’ Atoll is geographically located in the lowest seismic hazard zone of the entire country. According to the report the rate of decay of peak ground acceleration (PGA) for the zone 1 in which Thulusdhoo is located has a value less than 0.04 for a 475 years return period (see table below). PGA values provided in the report have been converted to Modified Mercalli Intensity (MMI) scale (see column ‘MMI’ in Table 2.7). The MMI is a measure of the local damage potential of the earthquake. See Table 2.8 for the range of damages for specific MMI values. Limited studies have been performed to determine the correlation between structural damage and ground motion in the region. The conversion used here is based on United States Geological Survey findings. No attempt has been made to individually model the exposure of Thulusdhoo Island as time was limited for such a detailed assessment. Instead, the findings of UNDP (2006) were used.
Table 2.7 Probable maximum PGA values in each seismic hazard zone of Maldives (modified from UNDP, 2006). Seismic PGA values for MMI 1 hazard zone 475yrs return period 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 2.8 Modified Mercalli Intensity description (Richter, 1958). MMI Shaking Description of Damage Value Severity I Low Not felt. Marginal and long period effects of large earthquakes. II Low Felt by persons at rest, on upper floors, or favourably placed. III Low Felt indoors. Hanging objects swing. Vibration like passing of light trucks. Duration estimated. May not be recognized as an earthquake. IV Low 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 Low 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 Light - Light to total destruction Catastrophe
According to these findings it is unlikely that Thulusdhoo will receive an earthquake capable of causing destruction. It should however be noted that the actual damage may be different in Maldives since the masonry and structural stability factors have not been considered at local level for the MMI values presented here. Usually such adjustments can only be accurately made using historical events, which is almost nonexistent in Maldives.
2.2.6 Climate Change
1 Based on KATZFEY, J. J. & MCINNES, K. L. (1996) GCM simulation of eastern Australian cutoff lows. Journal of Climate , 2337-2355. The debate on climate change, especially Sea Level Rise (SLR) is far from complete. Questions have been raised about SLR itself (Morner et al., 2004; Morner, 2004) and the potential for coral island environments to naturally adapt (Kench et al., 2005; Woodroffe, 1993). However the majority view of the scientific community is that climate is changing and that these changes are more likely to have far reaching consequences for Maldives. For a country like Maldives, who are most at risk from any climate change impacts, it is important to consider a cautious approach in planning by considering worst case scenarios. The findings presented in this section are based on existing literature. No attempt has been made to undertake detailed modelling of climate change impacts specifically on the island due to time limitations. Hence, the projection could change with new findings and should be constantly reviewed.
The most critical driver for future hazard exposure in Maldives is the predicted sea level rise and Sea Surface Temperature (SST) rise. Khan et al. (2002) analysis of tidal data for Hulhule’, Male’ Atoll shows the overall trend of Mean Tidal Level (MTL) is increasing in the southern atolls of Maldives. Their analysis shows an increasing annual MTL at Hulhule’ of 4.1 mm/year. These findings have also been backed by a slightly higher increase reported for Diego Garcia south of Addu Atoll (Sheppard, 2002). Moreover, IPCC (2001) predict a likely acceleration as time passes. Hence, this indicates that the MTL at Hulhule’ by 2100 will be nearly 0.5m above the present day MTL .
Similarly, Khan et al. (2002) reported air temperature at Male’ Atoll is expected to rise at a rate of 0.5°C per year, while the rate of rise in SST is 1.1°C.
Predicted changes in extreme wind gusts related to climate change assumes that maximum wind gusts will increase by 2.5, 5 and 10 per cent per degree of global warming (Hay, 2006). Application of the rate of rise of SST to the best case assumption indicates a 15% increase in the maximum wind gusts by the year 2010 in southern Atolls .
The global circulation models predict an enhanced hydrological cycle and an increase in the mean rainfall over most of the Asia. It is therefore evident that the probability of occurrence and intensity of rainfall related flood hazards for the island of Thulusdhoo will be increased in the future. It has also been reported that a warmer future climate as predicted by the climate change scenarios will cause a greater variability in the Indian monsoon, thus increasing the chances of extreme dry and wet monsoon seasons (Giorgi and Francisco, 2000). Global circulation models have predicted average precipitation in tropical south Asia, where the Maldives archipelago lies, to increase at a rate of 0.14% per year (Figure 2.10).
12
10
8
6 Rate of increase = 0.135% per year 4
2
Increase of precipitation (%) 0 2010 2020 2030 2040 2050 2060 2070 2080 2090 Year
Figure 2.10 Graph showing the rate of increase of averaged annual mean precipitation in tropical south Asia (Adger et al., 2004).
There are no conclusive agreements over the increase in frequency and intensity of Southern Indian Ocean Storms. However, some researchers have reported a possible increase in intensity and even a northward migration of the southern hemisphere storm belt (Kitoh et al., 1997) due rise in Sea Surface Temperatures (SST) and Sea Level Rise. If this is to happen in the Southern Indian Ocean, the frequency of and intensity of storms reaching Thulusdhoo Island coastline will increase and thereby exposing the island more frequent damages from swell waves. The increase in sea level rise will also cause the storms to be more intense with higher flood heights.
The above discussed predicted climate changes for Thulusdhoo and surrounding region is summarised in Table 2.9. It should be cautioned that the values are estimates based on most recent available literature on Maldives which themselves have a number of uncertainties and possible errors. Hence, the values should only be taken as guide as it existed in 2006 and should be constantly reviewed. The first three elements are based climate change drivers while the bottom three is climatological consequences.
Table 2.9 Summary of climate change related parameters for various hazards. Element Predicted Predicted change (overall rise) Possible impacts on rate of Hazards in Best Case Worst Case change Thulusdhoo
SLR 4.1-5.0mm Yr 2050: Yr 2050: +0.4m Tidal flooding, increase /yr +0.2m in swell wave flooding, Yr 2100: +0.88m reef drowning Yr 2100: +0.4m Air Temp 0.5°C / Yr 2050: decade +2.15° Yr 2100: +4.65° SST 1.1°C / Yr 2050: Increase in storm decade +4.73° surges and swell wave related flooding, Coral Yr 2100: bleaching & reduction +10.3° in coral defences Rainfall +0.14% / Yr 2050: Increased flooding, yr (or +1204mm Could affect coral reef +28mm/yr) growth Yr 2100: +2604mm Wind gusts 5% and Yr 2050: +3.8 Yr 2050: Increased windstorms, 10% / Knots +7.7Knots Increase in swell wave degree of related flooding. Yr 2100: +8.3 Yr 2100: +16.7 warming Knots Knots Swell Frequency Increase in swell wave Waves expected related flooding. to change. Wave height in reef expected to be high
2.3 Event Scenarios
Based on the discussion in section 2.2 above, the following event scenarios have been estimated for Thulusdhoo Island (Table 2.10, 2.11, 2.12).
Table 2.10 Rapid onset flooding hazards. Hazard Max Impact thresholds Probabil ity of Occurrence Prediction
Low Moderat Sever Low Moderate Severe e e Impact Impact 1.0.1. I mpact Swell Waves NA < 2.3m > 2.3m > 3.0m High Moderate Low (wave heights on reef flat – Average Island ridge height +2.0m above reef flat) Tsunami 4.5m < 2.3m > 2.3m > 3.0m Modera Low Very te low (wave heights on reef flat) SW monsoon 0.5m < 2.3m > 2.3m > 3.0m High Very low Unlikely high seas 1.0.2. Heavy Rainfall 241mm <60m > 60mm >175m High Moderate Low m m (For a 24 hour period)
Table 2.11 Slow onset flooding hazards (medium term scenario – year 2050). Hazard Impact thresholds Probability of Occurrence
Low Moderate Severe Low Moderate Severe
SLR: Tidal < 2.3m > 2.3m > 3.0m Moderate Very Low Very Flooding Low
SLR: Swell < 2.3m > 2.3m > 3.0m Very high Moderate Low Waves
SLR: Heavy <60mm >60mm >175mm Very Moderate Low Rainfall High
Table 2.12 Other rapid onset events. Hazard Max Impact thresholds Probability of Occurrence Predictio n
Lo Moderat Sever Lo Moderat 1.0.3. Sever w e e w e e
Wind NA <30 > 30 knts > Ver High Moderate storm knts 45Knts y Hig h Earthquak I < IV > IV > VI Ver Unlikely none e y
Low (MMI value 2)
3.4 Hazard zones
Hazard zones have been developed using a hazard intensity index. The index is based on a number of variables, namely historical records, topography, reef geomorphology, vegetation characteristics, existing mitigation measures (such as breakwaters) and hazard impact threshold levels. The index ranges from 0 to 5 where 0 is considered as no impact and 5 is considered as very severe. In order to standardise the hazard zone for use in other components of this study only events above the severe threshold were considered. Hence, the hazard zones should be interpreted with reference to the hazard scenarios identified above.
3.4.1 Swell waves and SW monsoon high Waves
The intensity of swell waves and SW monsoon udha is predicted to be highest 30 m from the coastline on the ocean ward side (see Figure 2.11) and 15 m from the lagoonward side. Swell waves higher than 3.0 m on reef flat are predicted to penetrate inner island up to or beyond 200m from the coastline. The longest run-up would be from the oceanward coastline where it could penetrate 250 m inland. The run-up on the island
2 Refer to earthquake section above is controlled by topography. Waves smaller than 2.0 m MSL will be controlled by the dual ridges on the southern coastline but not totally stopped.
The lagoonward side is relatively safe from swell related flooding due to the protection provided by the atoll rim and island orientation. However, waves could refract around the reef system through the reef entrance north of the island. This could cause flooding from northern half of the island within the lagoon. Such impacts are predicted to be limited to 10-30 m from the lagoonward coastline and their intensity is expected to remain low. Moreover, the presence of the breakwater will control flood waters from the northern side.
SW monsoon high waves ( udha ) are not expected to have an impact beyond 20m of the coastline and are likely to influence the coastline right around the island.
0 100 200 Lagoon metres
Intensity Index Hazard Zoning Map Contour lines represent Swell waves, Udha & intensity index based on Surges Low 1 2 3 4 5 High severe event scenarios
Figure 2.11 Hazard zoning map for swell wave, storm surges and southwest monsoon high seas.
3.4.2 Tsunamis When a severe threshold of tsunami hazard (>3.0 m on reef flat) is considered, 80% of the island is expected to be flooded (Figure 2.12). If the waves reach beyond 4.0 m MSL the entire island is highly likely to be flooded due the prevalent tide levels. High intensity waves will flush through the island from the eastern and southern side while tide related surges will occur within the atoll lagoon, flooding the northern coastline. The intensity of flood waters will be highest 100-150 m from the shoreline.
Wave height around the island will vary based on the original tsunami wave height, but the areas marked as low intensity is predicted to have proportionally lower heights compared to the coastline.
0 100 200 Lagoon metres
Intensity Index Hazard Zoning Map Contour lines represent Tsunami intensity index based on Low 1 2 3 4 5 High severe event scenarios
Figure 2.12 Hazard zoning map for tsunami flooding.
3.4.3 Heavy Rainfall
Heavy rainfall above the severe threshold is expected to flood parts of the settlement (Figure 2.13). The areas predicted for severe intensity are the topographic lows in the central parts of the island. These areas act as drainage basins for the surrounding higher areas. The intensity is generally expected to be low in most locations. The hazard zone presented in the map below is based on limited topographic surveys done on the island. Due to the large size of the island it was impossible to assess the topographic variation across the entire island during this project. Hence the hazard zones shown below should be considered as the most prominent zones only. More detailed assessment is required once high resolution topographic data becomes available.
0 100 200 Lagoon metres
Intensity Index Hazard Zoning Map Contour lines represent Heavy Rainfall intensity index based on Low 1 2 3 4 5 High severe event scenarios
Figure 2.13 Hazard zoning map for heavy rainfall related flooding.
3.4.4 Strong Wind
The intensity of the strong wind across the island is expected to remain fairly constant. Smaller variations may exist between the west and east side where by the west side receives higher intensity due to the predominant westerly direction of abnormally strong winds. The entire island has been assigned an intensity index of 4 for strong winds during a severe event.
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.
3.4.6 Composite Hazard Zones
A composite hazard zone map was produced using a GIS based on the above hazard zoning and intensity index (Figure 2.14). The coastal zone approximately 150m from the oceanward coastline and the topographically low areas within the island are predicted to be the most intense regions for multiple hazards. The eastern side is particularly identified as a hazard zone due to the exposure to swell waves, storm surges, udha and tsunamis.
0 100 200 Lagoon metres
Hazard Zoning Map Intensity Index Contour lines represent Multiple hazards intensity index based on Low 1 2 3 4 5 High severe event scenarios
Fig 2.14. Composite hazard zone map.
3.5 Limitations and recommendation for future study
The main limitation for this study is the incompleteness of the historic data for different hazardous events. The island authorities do not collect and record the impacts and dates of these events in a systematic manner. There is no systematic and consistent format for keeping the records. In addition to the lack of complete historic records there is no monitoring of coastal and environmental changes caused by anthropogenic activities such as road maintenance, beach replenishment, causeway building and reclamation works. It was noted that the island offices do not have the technical capacity to carry out 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 Thulusdhoo 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 Thulusdhoo.
References
BINNIE BLACK & VEATCH (2000) Enviromental / Technical study for dredging / reclamation works under Hulhumale' Project - Final Report. Male', Ministry of Construction and Public Works. BUCKLEY, B. W. & LESLIE, L. M. (2004) Preliminary climatology and improved modelling of South Indian Ocean and southern ocean mid-latitude cyclones. International Journal of Climatology, 24 , 1211-1230. DEPARTMENT OF METEOROLOGY (DOM) (2005) Severe weather events in 2002 2003 and 2004. Accessed 1 November 2005,
3. Environment Vulnerabilities and Impacts
3.1 Environment Settings
3.1.1 Terrestrial Environment
Topography
The topography of Thulusdhoo was assessed through three island profiles and a beach profile (see figure 1). Given below are the general findings from this assessment.
Atoll Lagoon Atoll Pass Present Accretion Present Erosion Topographic Profiles Ground water assessment
Lagoon 0 100 200 metres
P3
P2 P1
P4 Islet (Huraa) Growing Zone
Reef Flat < -1.0m MSL Indian Ocean
Figure 3.1 Field assessment location and general geographic setting.
The island is generally low lying with an average elevation of +1.4 m MSL. There is a higher ridge on the oceanward sides of the island reaching +1.8m MSL (see Figures 3.1 and 3.2) and averaging +1.6 m MSL. This ridge extends to the north, east and southern parts of the island (Figure 3.3). The width of the ridge ranges from 50 m in single ridge areas to 150 m in double ridged areas. The centre of the island is generally lower with some areas 0.68 m lower than the edges. To some extent parts of the island has the typical saucer shape topography found in most islands of Maldives.
One of the main topographic characteristic of the island is a general declining trend of island elevation from east to west. The ridge on the eastern side reaches +1.8 m while the elevation in the eastern end was observed to be barely +0.8 m MSL. Profile 2 shows the declining trend even towards the centre of the island. A profile across the entire length of the island was not possible as a clear view was not available for the survey. These patterns in elevation could be explained in terms of the island’s geomorphologic evolution. The eastern ridges on the oceanward side are exposed to stronger wave action due to the proximity of eastern areas to reef edge. The surfs off Thulusdhoo reef are predominantly high and are even known to be amongst the best surfing spots in North Male’ Atoll. They may also have been exposed storm activities in the past. The western end, on the otherhand, represents the growing end of the island. This area is highly dynamic. Over the past 60 years an areas the size of 1.3 ha has been displaced along with vegetation. The vegetation, soil and coastal characteristics (such as beach rocks) bear further evidence of an unstable and a growing region. The low elevation is also a sign of a relatively calm zone in terms of wave action. Further assessment of this region is provided in the coastal erosion section.
It is impossible to identify the topography of the entire island based on island profiles. However, it is possible to estimate trends in topography based on the profiles and visual observations. Figure 3.4 shows the low and high areas estimated for Thulusdhoo.
N G’ Profile P1 Water Level - Well Multiple Oceanward Ridges Ground Water level Most likely due to recent accretion G Sea Level Intersection point +1.7m of two profiles G G’ +1.28m Lagoonward Side Oceanward Side
1m
Water Level 0 12:45pm Approximate Mean Sea Level Low Tideline
0 100 200 300 400 500 600
Figure 3.2 North-south topographic section of Thulusdhoo along its width.
N G Profile P2 Oceanward Ridge G’ Intersection point + 1.82m of two profiles Oceanward Side +1.27m G G’ Lagoonward Side High Area 1 m
0 Approximate Mean Sea Level Water Level 5:30 pm
Figure 3.3 East-West topographic section of Thulusdhoo along its Length.
Thulusdhoo Island Estimated Topography
0 100 200
metres
Low Areas Highest areas
High Areas
Figure 3.4 Estimate of major topographic variations in Thulusdhoo Island.
Vegetation The vegetation of Thulusdhoo is fairly sparse occupying just 35% of the total land area. Much of the vegetation has been cleared for human settlement and industrial establishments. There are two significant zones of vegetation; a coconut plantation next to the industrial zone and a coastal vegetation zone on the eastern end of the island (see Figure 3.5).
Coastal vegetation is very sparse. Approximately 46% of the 2.6km long coastline has just a thin strip of vegetation often barely exceeding 10m. Much of the undergrowth and smaller coastal vegetation has been removed leaving a thin stretch of coconut palms. This is most evident in the northern half and parts of the southern areas of the island (see Figure 3.5).
Thulusdhoo Island Coastal Vegetation
0 100 200 metres
Coastal Vegetation Coconut Plantation
Figure 3.5 Thulusdhoo island vegetation.
Ground Water and Soil
Thulusdhoo is expected to have a substantial layer of fresh water. Water lens depth varies across the island based on topography. Towards the low-lying centre of the island water lens was reached at 0.5-0.6 m at high tide. This could decrease to 0.4 m during spring high tides or more during heavy rainfall. There are no areas above water table or wetland areas within Thulusdhoo.
Thuludhoo’s ground water is of moderate quality. There were complaints of smell and saltiness in the inhabited parts of the island. They face major environmental issues related to ground water pollution and over extraction due to human activities in the industrial zone.
The soil conditions vary from east to west. The geomorphologically established eastern part of the island has larger layer of humus and is more compacted probably due to human habitation. The western end on the other hand is characterised by loose and young soil with a smaller layer of humus. These two areas seem to cope differently to heavy rainfall. The western zone, although low, has a higher percolation rate than the eastern areas.
3.1.2 Coastal Environment
Beach and Beach Erosion
Thulusdhoo island beach environment is highly dynamic. The islands western and eastern halves are very active while the southern and northern halves are relatively stable. Thulusdhoo beaches consist of fine sand except on the south east section where coarser sand was observed. The small islet consists entirely of coral outcrops on its eastern side. Analysis of Thulusdhoo islands coastal changes for the past 40 years reveals the dynamism of its beach environment. Figure 3.6 shows the coastline changes between 1969 and 2004. 0 100 200 meters
Beach Line 1969 Beach Line 2004 Land gains and Losses: 1969-2004 Losses Vegetation Line 2004 Gains
Figure 3.6 Coastal erosion in Thulusdhoo.
During this period approximately 1.89 hectares of vegetated land has been eroded from the southern and eastern ends of the island. However, almost all the eroded sediments have been reworked around the island and approximately 1.97 hectares of new vegetated land has been added within the same period. Hence, although erosion appears to be significant, in terms of net gain and losses, the island has slightly gained. These findings have been confirmed from the island topographic profiles and beach rock areas around the island.
As identified on the map, major erosion hazard zones are western and south-eastern and eastern sections of the island. During the past five years substantial erosion began occurring on the eastern end of the island. The islanders consider the eastern section to be the most critical erosion hazard zone in the island. However, this concern is more related to perception of erosion rather than the extent of physical changes. Analysis of settlement growth over the last 5-10 years shows substantial encroachment towards the eastern side of the island. Plots were allocated near the newly accreted land as the beach appeared permanent. This has inadvertently set in motion a series of modifications that would alter the coastal dynamics of eastern zone and the island as a whole. When the new erosion cycle arrived the newly developed areas fell within the erosion zone, hence a number of mitigation measures had to be put in place protect the new developments. These modifications discussed in a latter section.
3.1.3 Marine environment
General Reef Conditions
General historical changes to reef conditions were assessed anecdotally, though interviews with a number of fishermen and young snorkelers. The general agreement is that the number of fish species and number of fishes on the eastern and northern sides have declined while live coral cover has declined to half over the last 50 or so years. Conversely, the eastern side was reported to be in good and same condition for the last 50 years. These findings are coarse but it does give a general pattern on the changing conditions of the reef, which is critical for future natural adaptation assessment of the island to sea level rise hazard.
A number of sea grass patches are growing in the eastern side of the island. These patches now occupy close to 40 percent of the eastern coastal area and substantial increase from 1998 when it occupied just 5 percent. These patches are expected to expand on the eastern side.
3.1.4 Modifications to Natural Environment
Coastal Modifications
• As in most inhabited islands of Maldives, access infrastructure has been developed in Thulusdhoo island. These include a piled jetty for general access, a piled landing area for commercial uses and dredged channel across the reef for lagoon access. Thulusdhoo Island is blessed with a natural harbour adjacent to its coastline and hence has not undertaken any major coastal access infrastructure works that could have major impact on coastal processes. Perhaps this is one reason why the dynamic nature of the beaches around the island is preserved.
• A number of erosion mitigations measures have been developed in the island over the last 10 years. The most prominent structure is breakwater off the north eastern side of island, built in response to severe erosion in the late 1990’s. Since then erosion in the area has stabilised although there is no significant accretion as well. Erosion mitigation measures have also been developed in the eastern section of the island where substantial erosion has occurred in the past. Groynes have been built along a 350m stretch on the eastern side.
• Dredging activities have been undertaken close to shoreline for sand mining and boat access.
Dredged channel
Coastal Protection Dredged Reef Entrance Lagoon Lagoon Dredged Area Main Jetty Groynes
Commercial Landing Area
0 150 300 meters
Figure 3.7 Coastal modifications in Thulusdhoo Island.
Terrestrial environment modifications 0 150 300 Vegetation Cov er 1969 Major Vegetation 2004 meters Settlement 1969
Figure 3.8 Terrestrial Environment modifications.
• Human settlement expansion in Thulusdhoo over the years has meant that substantial modification to the terrestrial environment was necessary. Vegetation cover has been reduced to 30% of the island land area within just 30-35 years. Thulusdhoo Island lacks large trees within its settlements area, which would otherwise act as barriers to wind hazard. Coastal vegetation has been reduced to a critical level with some areas having barely 10m of vegetation or no vegetation at all.
• The modification to topography is minimal. Some roads have been raised in the past due rainfall related flooding in the low areas of the island.
• Saltwater intrusion in the water lens is minimal a present, but there are some location within the island especially near the industrial zones which report poor water quality. The ground water around the industrial zone is also reported to be contaminated from number of sources such as chemical leaks, oil leaks and poor hazardous waste disposal methods.
3.2 Environmental mitigation against historical hazard events
3.2.1 Natural Adaptation Thulusdhoo Island in the past has adapted its coastal environment to changing climatic and wave conditions by adjusting sediments around the island. This process in still very active and as result has made the Thulusdhoo coastal environment highly dynamic. Similarly there is evidence that the island has in the past adjusted its ridges according to the prevailing conditions and that it continues to do so today.
3.2.1 Human Adaptation
A number of mitigation measures have been developed on Thulusdhoo against coastal erosion. They include a nearshore breakwater in the northeast and groynes along the eastern shoreline. These structures help to stablise erosion in areas where it has reached critical levels by altering the coastal processes. However, these measures only help to mitigate short term cycles in erosion and the permanent nature of thee structures form a major hindrance to the long-term coastal changes. No mitigation measures have been developed against other natural hazards.
3.3 Environmental vulnerabilities to natural hazards
3.3.1 Natural Vulnerabilities
• Island generally low lying and therefore exposed to flooding from the southern side of the island. The high ridge on the eastern side is not prominent on the southern side.
• A topographic variation in the central part of the island exposes the area to heavy rainfall associated flooding and creates condition for flood run-up during ocean induced flooding events. Currently the low areas experience rainfall related flooding almost regularly effecting island functions such as schools and economic activities (see Figure 3.9). Analysis of flood extents during tsunami and two other major flood events show the effects of topographic lows were prominent in the wave run up (see Figure 3.10).
N G’ Effect of Heavy Rainfal (Kinbigasmagu) l
G
G G’ Telecom NearestHouse School Power House Atholhuge NearestHouse
1
0
Low Tideline
0 100 200 300 400 500 600
Figure 3.9 Effect of topography on rainfall related flooding (along Kinbigasmagu).
• It’s location on the eastern rim generally exposes the island to flooding events and tsunamis.
• Island is located in a strong wind hazard zone and is vulnerable to major wind storms.
3.3.2 Human induced vulnerabilities
• As discussed earlier, the lack of coastal vegetation in Thulusdhoo is a major concern in terms of exposure to natural hazards. Coastal vegetation including the undergrowth acts as natural barrier against tsunami’s, other ocean induced flooding events and wind storms. A wider coastal vegetation belt would absorb wave energy from a tsunami or a flooding event reducing the impact on infrastructure and human settlement. This has been proven from findings across the nine islands studied under this project. In Thulusdhoo Island itself, past major flooding events such as 1987 floods failed to cause any damage due to the strong vegetation belt that acted as buffer between the settlement and shoreline. Stronger coastal vegetation also reduces wind energy during wind storms and protects settlement areas near the coastline. Coastal vegetation also plays a critical role in stabilising the beach areas and assists in controlling erosion. Hence lack of coastal vegetation along the north, south and north east shoreline exposes these areas to above mentioned hazards.
• Similar to the lack of coastal vegetation, the removal of vegetation from the settlement area exposes the structures to the direct effects of strong wind. The effects of climate change and global warming could be felt more strongly due to the apparent increase in temperature within the settlement area.
• Present land use has contributed significantly to the exposure of settlement and population to tsunamis, ocean induced hazards and strong wind damage. Settlement areas have expanded to within 10m of the coastline at some locations while the average distance is just 15m from the coastline. Damages from past major flooding incidents may have been low due to the considerable barrier between settlement and ocean ward coastline (see figure below). For example during the 1987 flooding incident the damage to the island structures were limited to just 3 houses due the approximately 100m wide vegetation belt. If the same event was to occur today, it is logical to consider the extent of damage would be much higher as the distance between the coastline and the nearest structure in barely 10m. Similarly land use does not seem to take into account the volatility of the island beaches. Settlement has now expanded to within 10m of the north eastern coastline which is essentially a dynamic zone following a geological cycle of erosion and accretion. The new plots in this erosion and accretion zone are naturally exposed to coastal erosion hazards. With the predicted climate change and sea level rise, these zones may require considerable space to naturally adjust to the changing conditions. Unfortunately, present land use patterns on the island are continually exposing settlement areas to coastal erosion and sea induced flooding events. N G’ Water Level - Well Ground Water level Historical Flood Events (Kinbigasmagu) Sea Level G 1967 Extent of Settlement footprint Telecom NearestHouse
G School Power House Atholhuge NearestHouse G’ 2004 1950’s
1987 1 Regular 0 (Udha) 2006 1987 1967 Low Tideline
0 100 200 300 400 500 600
Figure 3.10 Historical flood events and relationship with topography and extent of human settlement.
• The response to coastal erosion in erosion and accretion zones of the island is to construct breakwaters or groynes to stabilise beaches. These structures do help to stabilise the beaches to the prevailing climatic conditions of the time but the do have implication for the entire coastal processes of an island. For example the erosion of the north eastern beaches were mitigated using breakwaters in the area, but the structure has also starved the northern and north eastern areas of a continuous supply of sediments. It is also very likely that with a change in the climatic and oceanographic conditions, the breakwater would become a hindrance to sediment flow in other areas as well as erosion of previously unaffected areas. Breakwaters until it is extended right round the island, may act to increase the hazards from sea level rise due the restrictions they impose on parts of the island coastal processes.
• Reefs form the first line of defence in coral islands against waves and predicted sea level rise. A functioning and healthy reef is essential for a number of geomorphologic functions such as sediment production and reef adaptation to rising sea levels. The natural history of Maldives bears evidence of the role reefs played in natural adaptation to varying sea levels. The fact that the eastern side of the reef of Thulusdhoo is in good condition is an asset to the Thulusdhoo. However, the past inappropriate human activities in the reef such as coral mining and the gradual decline of reef condition on the western and northern ends probably would increase the sea level rise hazard in Thulusdhoo.
• Over extraction of ground water was identified as a major issue in the island. At present it has not reached critical levels as is the case in some of the smaller inhabited islands. There is however potential for reduction in ground water quality and salt water intrusion in the future if the ground water continues to be over extracted. This may expose the island to drought (in terms of non-potable water) and sea level rise hazards due to the facilitation of slat water intrusion.
• Improper modification of topography
• Although part of the natural drainage system of the island, the expansion of settle Frequent use of vehicles and road development activities are expected to further reduce the porosity of the surface layer of soil exposing such areas to rainfall related flooding hazards.
3.4 Environmental assets to hazard mitigation
• The east-west orientation of the island can generally be regarded as an asset in terms of exposure to ocean induced hazards and strong wind related hazards. It has been predicted that the most likely source of a tsunami is the Sumatran ridge located towards the east of Thulusdhoo and that islands on an east west orientation experienced far less impacts during the tsunami of 2004.
• The relatively large size is a major natural asset of Thulusdhoo. During flooding events the extent of impact on the settlement may be considerably reduced due the limited reach of wave length during run-up, as has been demonstrated by the tsunami of 2004 and other flooding events (see Natural Hazards Section).
• The small islet (huraagandu) on the eastern and of the island act as a Barrier island for the island in term of ocean induced hazard exposure. During the tsunami of 2004 and other flooding events such as the 1987 flooding, this huraagandu was know to have borne the brunt of flood impacts. The island itself if geomorphologically strong and has stabilised over the past 50 years to establish as a vegetated islet. Apart from reducing the impact of natural hazards, this islet also performs a major function in stabilising the beach environment. The point area of the main island towards the islet is most probably a result of the presence of this islet.
• Strong vegetation on eastern end of the island is a crucial element of the island coastal system especially in the face of reduced coastal vegetation in the areas immediately surrounding it. Such vegetation can hold a dynamic area such as the eastern end stable and possibly reduce the impact of flood events in the eastern region.
• Thulusdhoo Island has a major part of its natural coastal processes intact, especially the eastern side of the island. The island has a better chance of natural adaptation to changing climatic conditions if the natural processes are allowed to function with minimal change.
• Thulusdhoo’s natural harbour is unique feature only present in a few inhabited islands of Maldives. The presence of deep lagoon reduces the need for dredging activities and construction of coastal structures obstructing the flow of sediments around the island. Hence, the coastal erosion and accretion process is allowed to function with minimal obstruction, as has been from coastal changes over the past 35 years.
• The islands eastern side has higher ridge reaching up to +1.8m MSL and averaging +1.6m MSL. This ridge appears mostly a response to the strong wave conditions in the region although effects of storm events and strong wind would probably have played a role in its development. The presence of the ridge is a major defensive asset to the island in terms ocean induced flood hazards. Storms or storm tides up to +1.5m at mean sea level and +0.8m at high tide may not make it far inland in the eastern zone due to the presence of the high ridge. Given that the maximum storm tides predicted for the region is +1.82m (UNDP, 2006), the impacts of storm tides would be considerably reduced in the eastern part of the island. It should however be noted that there were two major historical flooding events in Thulusdhoo in 1955 and 1987. The 1987 flood is now believed to be a result of a long distance swell wave (Goda 1998) rather than a storm surge.
• Reef width appears to play an important role increasing or decreasing the impacts of ocean induced wave activity. The impact of gravity waves such as tsunami’s for example has its impacts reduced based on the length of reef. As has been discussed in the natural hazards chapter, these findings are preliminary and needs further inquiry using detailed empirical research.
3.6 Predicted environmental impacts from natural hazards
The natural environment of Thulusdhoo and islands Maldives archipelago in general are appear to be resilient to most natural hazards. The impacts on island environments 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 impacts from major events such as tsunamis. The recovery of the Gan Island environment, especially vegetation, ground water and geomorphologic features following the tsunami is a good example of such rapid recovery. Different aspects of the natural environment may differ in their recovery. Impacts on marine environment and coastal processes may take longer to recover as their natural development processes are slow. In comparison, impacts on terrestrial environment, such as vegetation and groundwater may be more rapid. However, the speed of recovery of all these aspects will be dependent on the prevailing climatic conditions.
The resilience of coral islands to impacts from long-term events, especially predicted sea level rise is more difficult to predict. On the one hand it is generally argued that the outlook for low lying coral island is ‘catastrophic’ under the predicted worst case scenarios of sea level rise (IPCC 1990; IPCC 2001), with the entire Maldives predicted to disappear in 150-200 years. On the other hand new research in Maldives suggests that ‘contrary to most established commentaries on the precarious nature of atoll islands Maldivian islands have existed for 5000 yr, are morphologically resilient rather than fragile systems, and are expected to persist under current scenarios of future climate change and sea-level rise’ (Kench, McLean et al. 2005). A number of prominent scientists have similar views to the latter (for example, Woodroffe (1993), Morner (1994)).
In this respect, it is plausible that Thulusdhoo may continue to naturally adapt to rising sea level, especially with a relatively unmodified eastern coastline. There are two scenarios for geological impacts on Thulusdhoo. First, if the sea level continues to rise as projected and the coral reef system keep up with the rising sea level and survive the rise in Sea Surface Temperatures, then the negative geological impacts are expected to be negligible, based on the natural history of Maldives (based on findings by Kench et. al (2005), Woodroffe (1993)). Second, if the sea level continues to rise as projected and the coral reefs fail to keep-up, then their could be substantial changes to the land and beaches of Gan (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 Thulusdhoo stands to undergo substantial change (even during the potential long term geological adjustments), due to potential loss of land through erosion, increased inundations, and salt water intrusion into water lens (based on Pernetta and Sestini (1989), Woodroffe (1989), Kench and Cowell (2002)).
As noted earlier, environmental impacts from natural hazards will be apparent in the short-term and will appear as a major problem in inhabited islands due to a mismatch in assessment timeframes for natural and socio-economic impacts. The following table presents the short-term impacts from hazard event scenarios predicted for Thulusdhoo.
Hazard Scenario Probability Potential Major Environmental Impacts at Location Tsunami (maximum scenario) 4.5m Low • Widespread damage to coastal vegetation (Short-term) • Long term or permanent damage to selected inland vegetation especially common backyard species such as breadfruit trees. • Contamination of ground water if the sewerage system is damaged or if liquid contaminants in the industrial zone such as diesel and chemicals are leaked. • Salinisation of ground water lens to a considerable period of time causing ground water shortage. If the rainwater collection facilities are destroyed, potable water shortage would be critical. Loss of some flora and fauna is also very likely. • Widespread damage to crops (short-term) • Widespread damage to coastal protection infrastructure • Short-medium term loss of soil productivity • Moderate changes to geomorphology of island, especially in the eastern corner. • Moderate damage to coral reefs (based on UNEP (2005)) Storm Surge (based on UNDP, (2005)) 0.60m (1.53m Low • Minor damage to coastal vegetation storm tide) • Minor loss of crops Hazard Scenario Probability Potential Major Environmental Impacts at Location • Moderate –high damage to coastal protection infrastructure • Minor geomorphologic changes in the northern eastern shoreline and lagoon 1.32m (2.30m Very Low • Moderate damage to coastal vegetation storm tide) • Long term or permanent damage to selected inland vegetation especially common backyard species such as mango and breadfruit trees. • Contamination of ground water if the sewerage system is damaged or if liquid contaminants in the industrial zone such as diesel and chemicals are leaked. • Salinisation of ground water lens to a considerable period of time causing ground water shortage. If the rainwater collection facilities are destroyed, potable water shortage would be critical. • Widespread loss of crops • Moderate –high damage to coastal protection infrastructure • Minor-moderate geomorphologic changes in the oceanward shoreline and lagoon • Minor-moderate damage to coral reefs 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. • Moderate-high damage to open field crops • Minor changes to coastal ridges 65+ Knots Very Low • Widespread damage to inland vegetation • Debris dispersion near waste sites. • Widespread damage to open field crops • Minor changes to coastal ridges Heavy rainfall 187mm Moderate • Minor to moderate flooding in low areas, including roads and houses. 242mm Very Low • Widespread flooding but restricted to low areas of the island. Drought • Minor damage to backyard fruit trees Earthquake • Minor geomorphologic changes Sea Level Rise by year 2100 (effects of single flood event) Medium Moderate • Widespread flooding during high tides and Hazard Scenario Probability Potential Major Environmental Impacts at Location (0.41m) storm surges. • Loss of land due to erosion. • Loss of coastal vegetation • Major changes to coastal geomorphology. • Saltwater intrusion into water lens and salinisation of ground water leading to water shortage and loss of flora and fauna. • Minor to moderate Expansion of wetland areas
3.6 Findings and Recommendations for safe island development
• A coral islands main defensive ability against frequent natural hazards is perhaps its robust natural adaptive capacity. In order to retain this ability against ocean induced hazards, a proper and functioning coastal environment is essential. It takes a number of years in term of geological time for an island to stabilise and achieve an equilibrium. Once established the island evolves and adapts to the prevailing conditions. The natural history of Maldives bears evidence to such natural adaptation, including the survival through a 2.5m rise in sea level (Reference Kench et.al). It is perhaps the foremost reason why the coral islands of Maldives has survived thus far.
• The proposed safe island development in Thuludhoo proposes to change a functioning coastal environment into a more artificial environment. The implications of this change are numerous especially in the short term. The proposed modifications may require considerable time for the island to achieve an equilibrium in different forces controlling coastal processes. During this period considerable changes to the existing coastal environment may be imminent. In the absence of coastal protection, these changes would be more noticeable. There is a high probability that the proposed coastal modifications would expose Thulusdhoo to the following ocean induced hazards.
There could be a rapid onset of erosion in specific areas of the island in the short-term until the coastal environment achieves an equilibrium. The present shape of the coastline is a result of the prevailing condition within the reef. Considerable changes to unaltered zones of the island are highly probable, especially around the eastern islet. Hence, coastal erosion hazards may in general be increased. • Island topography and resulting drainage systems are critical features of an island in relation to exposure to natural hazards. Safe island development plan of Thulusdhoo should consider the existing topography and implications of modifying the topography on the rainfall related flooding. Thulusdhoo has considerable topographic variations. The most notable being the high ridges on the east and low elevation on the west side. The proposed reclamation on the west side at +1.4m height would mean a substantial difference in elevation between the proposed reclamation and natural island, creating conditions for rainfall induced flooding or ocean induced flood run-up. Similarly, the function of the low drainage areas in the Environment Protection Zone (EPZ) needs to be reviewed. Given the topographic variations within Thulusdhoo, the proposed 0.1m variation in the drainage area may not have the desired effects on flood control. In the eastern areas where the ridges are high the proposed drainage area simply has no function while in the eastern areas it may lead to rainfall related flooding unless siltation-proof drainage systems are installed.
• Based on the 9 islands studies in this project, it has been observed that strong coastal vegetation is amongst most reliable natural defences of an island at times of ocean induced flooding, strong winds and against coastal erosion. The design of EPZ zone needs to be reviewed to consider the important characteristics of coastal vegetation system that is required to be replicated in the safe island design. The width of the vegetation belt, the composition and layering of plant species and vegetation density needs to be specifically looked into, if the desired outcome from the EPZ is to replicate the coastal vegetation function of a natural system. Based on our observations, the proposed width of coastal vegetation 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.
• EPZ needs to be extended around the island where ever possible considering the characteristics of different zones and the functions the coastal vegetation is expected to perform. At present the Thulusdhoo north eastern coastline is exposed due the absence of coastal vegetation. An appropriate EPZ needs to be developed in the area to reduce wave induced flooding, strong wind hazard and coastal erosion exposure. • The topography of island may dictate flood zones as has been shown in figure 9. It is important that location of evacuation centres consider the topographic variations. For example the school in Thulusdhoo was identified as the immediate evacuation centre by the islanders. However, the school is located in a low elevation which caused flood waters to accumulate around it during the tsunami of 2004.
4.7 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.
• This study however is a major contribution to the risk assessment of safe islands. It has highlighted several leads in risk assessment and areas to concentrate on future more detailed assessment of safe islands. This study has also highlighted some of the limitations in existing safe island concept and possible ways to go about finding solutions to enhance the concept. In this sense, this study is the foundation for further detailed risk assessment of safe islands.
• There is a time scale mismatch between environmental changes and 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. A topographic and bathymetric survey for all assessment islands prior to the risk assessment. The survey should be at least at 0.5m resolution for land and 1.0m in water.
Coral reef conditions data of the ‘house reef’ including live coral cover, fish abundance and coral growth rates.
At least a years data on island coastal processes in selected locations of Maldives including sediment movement patterns, shoreline changes, current data and wave data.
Detailed GIS basemaps for the assessment islands.
Coastal change, flood risk and climate change risk modeling using GIS.
Quantitative hydrological impact assessment.
Coral reef surveys
Wave run-up modelling on reef flats and on land for gravity waves and surges.
References
Goda, Y. (1998). Causes of high waves at Maldives in April 1987. Male', Asia Development Bank.
IPCC (1990). Strategies for Adaptation to Sea-Level Rise: Report of the Coastal Management Subgroup. Strategies for Adaptation to Sea-Level Rise: Report of the Coastal Management Subgroup . IPCC Response Strategies Working Group. Cambridge, University of Cambridge.
IPCC (2001). Climate Change 2001: Impacts, Adaptation, and Vulnerability . Cambridge, United Kingdom and New York, NY, USA, Cambridge University Press.
Kench, P. S. and P. J. Cowell (2002). "Erosion of low- lying reef islands." Tiempo 46 : 6-12.
Kench, P. S., R. F. McLean, et al. (2005). "New model of reef-island evolution: Maldives, Indian Ocean." Geology 33 (2): 145-148.
Pernetta, J. and G. Sestini (1989). The Maldives and the impact of expected climatic changes. UNEP Regional Seas Reports and Studies No. 104 . Nairobi, UNEP.
UNEP (2005). Maldives: Post-Tsunami Environmental Assessment, United Nations Environment Programme.
United Nations Development Programme (UNDP) (2005). Disaster Risk Profile for Maldives. Male', UNDP and Government of Maldives.
Woodroffe, C. D. (1989). Maldives and Sea Level Rise: An Environmental Perspective. Male', Ministry of Planning and Environment : 63.
Woodroffe, C. D. (1993). Morphology and evolution of reef islands in the Maldives. Proceedings of the 7th International Coral Reef Symposium, 1992 . Guam, University of Guam Marine Laboratory. 2: 1217-1226.
Yamano, H. (2000). Sensitivity of reef flats and reef islands to sea level change . Bali, Indonesia.
4. Structural vulnerability and impacts
Thulusdhoo is predominantly exposed to all three major flood types prevailing in the Maldives. The island is generally low lying with an average elevation of +1.4 m MSL. Ocean-originated floods can affect most of the island from sorth. In particular, a 4 m tsunami wave may inundate almost all the island. Historically, the island has been flooded two times by swell / surge. A topographic variation in the central part of the island exposes the area to heavy rainfall associated flooding. Currently the low areas experience rainfall related flooding almost regularly effecting island functions such as schools and economic activities. Analysis of flood extents during tsunami and two other major flood events show the effects of topographic lows were prominent.
4.1 House vulnerability
Not surveyed.
4.2 Houses at risk
Houses on Thulusdhoo Island are highly exposed to all three flood types in the Maldives. As shown in Fig. 4.1 and 4.2, more that 100 houses are exposed to tsunami flooding, accounting for 80% of the total houses and 71 houses are located the swell wave/surge flood-prone area in the southern side of the island, accounting 60% of the total houses. In addition, around 20% of the houses are subjected to frequent rainfall floods prevailing in the center of the island.
The exposure of houses will be significantly modified after the safer island plan is implemented. According to the most recently updated land use plan, Thulusdhoo Island will expand westward by reclaiming the reef flat on the western side of the island. An EPZ with a 2.4+ m high ridge is proposed on the southern shoreline, which is believed to effectively mitigate wave/surge flooding, to some degree, tsunami flooding as well. According to the reduced tsunami hazard zone, with a 2.4+ high EPZ, the number of exiting houses exposed to tsunami flooding can be reduced by 20%. However, total exposure will dramatically increase due to new houses being located in this area.
For the time being, it is not able to estimate the physical damage to these exposed houses because during the field survey, no house vulnerability was conducted. However, we still can give a very preliminary estimate of potential damage to houses based on the tsunami hazard zoning maps. In general, a tsunami hazard extent can be divided into three hazard zones - destructive, moderate to serious damage, and slight damage to content affected, in terms of water depth of <0.5 m, 0.5-1.5 m, and >1.5 m. By overlaying the tsunami hazard zone map with house map, 17 houses are identified to be located in the destructive inundation zone, 63 in the moderate damage zone, and 31 in the slight damage zone for the time being. In the future, after a 2.4+ m EPZ is set up, most existing houses will subjected to slight damage to content-affected. No serious damage to the existing houses is expected, where as damage to newly- built houses depends on their building code.
4.3 Critical facilities at risk
As shown in Fig. 4.3 and 4.4, several key critical facilities, i.e. communication sites and waste site are located ocean-originated flood-prone areas. Most facilities are exposed to floods of low intensity, such as rainfall floods and swell wave/surge floods; no physical damage can be expected. However, Wataniya site, located in the southeastern corner of the island and potentially exposed to a more than 1.5 m tsunami flooding, may be subjected to a serious damage because its control chamber is just less than 1 meter above the ground.
In addition, many transformers in the moderate to destructive tsunami hazard zones may be affected as well.
4.4 Functioning impacts
Functional impacts caused by rainfall and wave/surge floods can be minor. During heavy rainfall, the school located in the rainfall flood-prone area may be closed for days. In contrast, functional impacts induced by tsunami floods can be significant and 3 functional impacts are expected. First, shutdown of Wataniya site will disrupt its customers for a few days t o weeks, In worse cases, it will take a long time to recover its full capacity. Second, power distribution will be affected due to the short circuit of transformers. Thirds, flooding of the waste site located in the southwestern corner may result in secondary contamination. The extent of impact depends on its protection measures.
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 Thulusdhoo:
• Enhance building codes and protection in the ocean-originated flood-prone areas. • Retrofit Wataniya site to resist against more than 1.5 m flooding. • Avoid locating waste site in the flood-prone area to avoid secondary contamination.
Table 4.1 Houses at risk on K. Thulusdhoo. Exposed Vulnerable Potential Damage Hazard houses houses Serious Moderate Slight Content type # % # % # % # % # % # % TS(p) 111 79.3 ? 0 0 17 15.3 63 56.8 31 27.9
TS(f) 85 60.7 ? 0 0 1 1.2 68 80 16 18.8 W/S 71 50.7 ? RF 27 19.3 ? Flood Earthquake 140 100 ? Wind 140 100 ? Erosion
Table 4.2 Critical facilities at risk on Thulusdhoo Island. Critical facilities Potential damage/loss Hazard type Monetary Exposed Vulnerable Physical damage value 2 communication Yes Moderate to serious N/A sites, 1 mosque, 1 damage Tsunami office, 1 waste site, transformers 2 communication None No damage N/A Flood Wave/Surge sites, 1 mosque, 1 office Rainfall 1 schools, 1 office None No N/A Earthquake - - - - Wind - - - - Erosion - - - - Note: “-“ means “not applicable”.
Table 4.3 Potential functional impact matrix Flood Function Earthquake Wind Tsunami Wave/surge Rainfall Administration 1)
Health care
Education Schooling affected for days Housing
Sanitation 3) Secondary contamination Water supply
Power supply Limited disruption of power distribution Transportation
Communication 2) A week disruption
Note: 1) Administration including routine community management, police, court, fire fighting; 2) Communication refers to telecommunication and TV; 3) Sanitation issues caused by failure of sewerage system and waste disposal.
Fig. 4.1 Houses at risk associated with Tsunami floods: present (left) and future (right).
Fig. 4.2 Houses at risk associated with rainfall floods (left) and wave / surge floods (right).
Fig. 4.3 Critical facilities at risk associated with rainfall floods.
Fig. 4.4 Critical facilities at risk associated with swell wave/surge and tsunami floods.
67