Detailed Island Risk Assessment in Maldives
Volume III: Detailed Island Reports
S. Hithadhoo – 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
Hithadhoo Island is located on the western rim of Addu atoll, at approximately 73° 05' 37"E and 0° 37' 06" S, about 533km from the nations capital Male’ and 11km from the nearest airport, Gan (Figure 1.1). Hithadhoo is one of the few inhabited islands facing the western Indian Ocean and exposed to the southwest monsoon swells. Hithadhoo is the atoll capital amongst six inhabited islands in the atoll. It’s nearest inhabited islands are Maradhoo, Maradhoo-Feydhoo and Feydhoo. Hithadhoo forms part of a stretch of 5 islands connected through causeways and bridges and is the second largest group of islands connected in this manner. Addu atoll is the southern most atoll of Maldives and is located south of the equator. It sits along the southern half of the laccadive-chargos ridge, exposing the entire atoll to direct wave action from both east and west directions in Indian Ocean. However, it locations in the heart of the doldrums is makes the island relatively safe from major climatic hazard events. 73°E 15'
Meedhoo Hithadhoo Hulhudhoo
Addu Atoll (Seenu Atoll)
Maradhoo
Maradhoo-Feydhoo Viligilli 0° 40' S N Feydhoo
Gan (Airport) Location Map of Hithadhoo
0 2.5 5 kilometers
Figure 1.1 Location map of Hithadhoo. 1.2 Physical environment
Hithadhoo is the second largest island in the Maldives with a surface area of 525.7 Ha (5.3 km 2). It has a length of 8.6km and a width of 1.8km at its widest point. It is also the second largest inhabited island in Maldives. The reef of Hithadhoo is large with a surface area of 4152 Ha (41.5 km 2) and cover the entire western rim of Addu Atoll, stretching to approximately 18km. The reef also hosts 3 large inhabited islands and the Airport island (Gan), totalling a 1011ha (10.1 km 2) of land, half of which forms Hithadhoo Island. It is one of the largest concentrations of land in a single reef. The reef and the islands on them are oriented in a northwest-southeast direction. Hithadhoo is located on the northern end of the reef system. The settlement is located approximately 100m from the western reefline and 1000m from the eastern reefline.
Unlike all the other islands in Addu Atoll, Hithadhoo Island is exposed to wave action from both west and eastern side. There are a group of small uninhabited islands on the eastern reef flat of the island which performs the functions similar to that of barrier islands absorbing whatever wave energy reaching over the eastern reef line, especially during northeast monsoon.
Hithadhoo has some of the most unique coastal and terrestrial features found in the Maldivian islands in relation to natural hazards mitigation. Notable features include one of the best natural defence systems against ocean induced flooding found anywhere in Maldives and a well established drainage system dominated by two major wetland areas. The northern most wetland area is rich in biodiversity and has been declared a Protected Area by Ministry of Environment, Energy and Water. The natural defence system includes a 3.5m high ridge and strong, well established layer of coastal vegetation. In addition, a group of barrier islands protect the main island from wave activity on the eastern side and the location on the doldrums keeps the island free of major storm activity. However, the composition of coastal sediments and geomorphology of coastal ridges suggest that Hithadhoo is located in a very high energy zone, especially its western shoreline.
Hithadhoo is a highly urbanised settlement with a registered population over 13,600 inhabitants, and is the second largest settlement in Maldives. The high level of urbanization also meant that the natural environment of the island is highly modified to meet the development requirements of the settlement and the atoll. Majority of the terrestrial modifications are undertaken in the northern half of the island and coastal modifications are undertaken on the east. Almost 80% of the eastern coastline has been modified through land reclamation, harbour development, coastal protection, dredging and quay wall development activities. A large area of wetland have so far been reclaimed for housing and are earmarked for future reclamation. In contrast, the western coastline is very much in pristine condition except for a kilometre stretch where the settlement is located close to the shoreline. Much of the coastal vegetation has been kept intact and no developments have been undertaken along the shoreline. The general vegetation cover on the island is high compared to islands with similar population densities, owing to the large size of plots. 2. Natural hazards
This section provides the assessment of natural hazard exposure in Hithadhoo Island. A severe event history is reconstructed and the main natural hazards are discussed in detail. The final two sections provide the hazard scenarios and hazard zone maps which are used by the other components of this study as a major input.
2.1 Historic events
The island of Hithadhoo has been exposed to multiple hazards in the past. A natural hazard event history was reconstructed for Hithadhoo based on known historical events. As highlighted in methodology section, this was achieved using field interviews and historical records review. Table 2.1 below lists the known events and a summary of their impacts on the island.
The historic hazard events for Hithadhoo showed that the island experienced relatively few hazard events in the past. The island records and interviews with islanders only revealed 4 major events. Other records were obtained from historical sources such as Maniku (1990) and newspaper reports. These events have been marked in italic in the table.
The following multiple hazards were identified: 1) windstorms, 2) flooding caused by heavy rainfall, 3) swell surges, 3) and tsunami. Impacts caused by these events and frequency of occurrence vary significantly. Windstorms and flooding caused by rainfall were the most commonly occurring hazard events. Events could only be traced back 35 years, beyond which no reports of serious events were recorded.
Table 2.1 Known historic hazard events in Hithadhoo. Metrological Impacts Dates of the hazard recorded events
Flooding caused Rainfall related flooding on this island is 6-7 May 1978 by Heavy rainfall mainly limited to the eastern side of the 12 October 1981 island and around reclaimed wetland 14 October 1985 areas. It was reported that flooding on the 3rd –8th October 2005 eastern side has been exacerbated since the construction of the link road between Gan and Hithadhoo. Flooding incidents have caused damage to houses, personal belongings and blockage of the sewerage networks.
Flooding caused There are no official reports of wave surge 14 October 1984 by swell surges flooding on the island, except in 2007. 3 June 1987 However historical documents showed 9-10 September 1987 three major events. Impacts have been 15 May 2007 moderate to severe on property, crops and personal belongings
Windstorms The island reports frequent windstorms. 15 January 1970 One major wind storm that was reported to 29-30 April 1971 have 90miles/hr affected the island during 6-7 May 1978 1989 severely impacted the mango plants 12 October 1981 on the island that are a major source of 14 October 1985 income for some of the families on the 29 March 1989 island. It was reported that over 1000 20 July 2003 mango trees were affected by this event. Over 400 breadfruit trees and 4200 banana plants were destroyed. 50 houses had their roofs blown off. This incident caused road blockages for a couple of days on many of the roads. The schools were closed for a week and the cleanup operation took a week.
Droughts No major event have been reported
Earthquake No major event have been reported
Tsunami There has been only one known event. 26 th Dec 2004 This event flooded the eastern shoreline of the island to a height of approximately 0.3m. The tsunami however did not flood the island with any significant force and therefore the impacts of the tsunami flooding have been very minimal.
3.2 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 Hithadhoo. Given the location of Hithadhoo and the hazard exposure of nearby islands like S.Feydhoo and Gn.Fuvahmulah, Hithadhoo at first is expected to experience numerous hazard events. However, the lack of historical events tends to suggest that the island has some protective features. The issues are further explained explored in the physical environment section.
• Swell waves and wind waves • Heavy rainfall (flooding) • Windstorms • Tsunami • Earthquakes • Climate Change
3.2.1 Swell Waves and Wind Waves
Origins and Occurrence of waves in Hithadhoo
The wave regime around Maldives, especially around the western line of atolls is partially influenced by swell waves originating from the Southern Indian Ocean ( Kench et. al (2006), Young (1999), DHI(1999) and Binnie Black & Veatch (2000)). The Southern Indian Ocean is notorious for developing the most intense storms found anywhere on earth which are capable of generating swell waves throughout the year. Abnormal storm events in this regional could generate waves capable of causing flooding in the low lying islands of Maldives.
Hithadhoo Island is the southernmost inhabited island of Maldives. Its proximity to the southern Indian Ocean combined with the location on the southwest corner of Addu Atoll exposes the island to southern swell waves. The presence of swell waves around the region was confirmed by DHI(1999) during a wave study in the neighbouring Fuvahmulah Island (see Table 2.2).
Table 2.2 Wave regimes in neighbouring Fuvahmulah Atoll. Season Total Long Period Short Period
Predominantly from E-S. Mainly E-NE. High NE - Monsoon From S-SW High Waves from W waves from W
Transition Period 1 Mainly from SE-E From S-SW Mainly from NE-SE
From SE-SW. Mainly Mainly from SE-S. High SW - Monsoon from S. High Waves also From S-SW waves from West from W
From SE-W. Higher Transition Period 2 As SW monsoon From S-SW waves from West
The occurrence of abnormal swell waves on Hithadhoo 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. 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.
Topographic data of Hithadhoo reveals one of the highest ridge systems found in Maldives reaching 3.6 m above MSL. The presence of this ridge might explain lack of swell related flooding events compared to Feydhoo Island which lies just a few kilometres south and within the same reef system. There have however been 4 known occasions where the ridge system was over-topped. None of these occasions caused major damage in the oceanward side. Comparison of historic flood events with South Indian Ocean storm data revealed no relationships with cyclonic events (see Table 2.3 and 2.4). It does however link with known extra tropical depressions especially the 1987 and 2007 events, the two most severe swell events experienced in Maldives. It was also noted that these 4 events also caused had a major impact on Feydhoo Island but not Hithadhoo Island.
Table 2.4 shows major flooding events in Hithadhoo and related major storm events in South Indian Ocean.
Table 2.3 Historical flood events and possible links with storm events. Flooding Cyclone Date Maximum Distance Direction Tide Level event Name of Category Storm Event 14 th October unknown Data not 1985 available
2nd & 3 rd June unknown Median tide 1987 9-10 unknown NA September 1987 15 - 17 May unknown 13 -19 Extra 5630 SW Peak tide of 2007 May tropical the month 2007 Depression
Table 2.4 Cyclones within 1500km of Hithadhoo and of category 3 strength (source: Unisys and JTWC (2004) and University of Hawaii Tide Data). Wind Tide Level Flooding Cyclone Speed (monthly) reported Name Date (knots) Longitude 1963-01-09 12/01/1963 70 70.4 NA No 1971-07-09 09/07/1971 NA 72.0 NA No 1979-11-25 29/11/1979 100 73.7 NA No 1979-12-10 18/12/1979 110 79.9 NA No 1982-01-06 12/01/1982 115 76.5 NA No 1982-04-23 29/04/1982 100 77.9 NA No 1984-04-03 5/04/1984 75 69.5 NA No 1986-01-07 9/01/1986 80 81.6 NA No 1987-03-02 9/03/1987 75 73.7 NA No 1988-10-30 2/11/1988 75 77.3 low No 1988-11-05 14/11/1988 100 80.5 High No 1989-03-26 1/04/1989 100 70.0 Highest No 1990-01-30 3/02/1990 65 69.7 NA No 1991-03-20 26/03/1991 90 81.2 NA No 1993-01-16 24/01/1993 110 70.0 Low No 1993-04-29 4/05/1993 90 68.8 High No 1994-03-26 4/04/1994 70 79.2 Highest No 1994-11-21 26/11/1994 115 72.7 Medium No Low- No 1995-01-31 6/02/1995 65 71.0 medium Medium - No 1995-03-28 1/04/1995 95 70.5 High Medium- No 1996-04-06 13/04/1996 135 64.8 High 1996-10-15 18/10/1996 65 79.7 Low No Medium - No 1996-10-28 6/11/1996 125 81.0 High 1996-11-20 26/11/1996 65 80.5 Medium No Medium - No 2001-01-06 12/01/2001 100 69.1 High DINA 18/01/2002 70 71.2 High No IKALA 26/03/2002 65 73.2 Medium No BOURA 17/11/2002 75 69.2 High No KALUNDE 8/03/2003 140 71.7 Low No BENI 12/11/2003 105 74.5 Low No AROLA 9/11/2004 75 77.1 NA No BENTO 23/11/2004 140 76.5 NA No
Flooding is also known to be caused in Hithadhoo’s lagoon ward side by a gravity wave phenomenon known as Udha . These events are common throughout Maldives and especially the southern atolls of Maldives. 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.
The origins of the udha waves as yet remain scientifically untested. 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 Gan 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.
The specific relationship with udha on the lagoon ward side of the island remains to be further researched as such gravity waves usually occur on the oceanward reef flat. Perhaps the refraction or even high tides could cause water level to rise on the lagoonward side. Nonetheless, the lagoon ward side experiences low levels of flooding, usually up to 20m inland during the udha period and poses no major hazard to the settlement.
Wave Surge related historical flood impacts
The common flooding area as a result of surges on Hithadhoo is identified to be within 50m of the western coastline. The eastern shoreline has also recorded a few incidents especially after the land reclamation. Flooding on the western shoreline has been directly linked with swell waves from the South Indian Ocean. Flooding on the eastern side has been linked generally with both udha and swell waves. Intensity of waves is extremely high on the western shoreline which is exposed to southern swells, whereas the waves on the eastern shoreline are very subdued. None of the flood events have been reported to be severe events with little structural damage and no fatalities. The lack of major flooding events and their impact in Hithadhoo is attributed to the high coastal ridge. This ridge could mitigate waves approaching from the oceanward side up to 4 m on the reef flat. The existence of these ridges may also be related to persistence of swell waves in the area. In contrast the eastern shoreline is very low with less than 1.0 m above MSL. However, the eastern shoreline is generally protected from the direct impact of the swell waves. Waves refracted around the Hithadhoo ‘headland’ combined with high tides are believed to have been responsible for low levels of flooding on the eastern side.
It was interesting to note that during May 2007 wave heights reached over 4 m on the Hithadhoo reef flat, while the wave data for Gan shows that heights were 3.0 m. Waves overtopped the 3.6 m high ridge, which would mean that waves on reef would have to be at least 4.0 m. Judging from the beach ridge heights of Feydhoo and Hithadhoo, both on the same reef system, it may seem that swell waves reaching Hitadhoo are generally raised around the northern part of Hithadhoo. Perhaps this effect is caused by wave refraction and setup due to the orientation of the reef system against swell waves. HISTORICAL FLOOD EVENTS AND ESTIMATED GENERAL WAVE PROPAGATION IN HITHADHOO REGION
NE Monsoon Waves, & potential tsunami
Historical Flood Events
SW monsoon waves Southern Swell Waves
Figure 2.1 Historical flood events and probable wave propagation patterns near Hithadhoo and its reef flat.
Future event prediction
Hithadhoo is likely to be effected if wave heights on reef flats exceed 4.0 m. Due to the low probability of wave heights reaching a damaging 5.0 m on the reef flat, it is unlikely that any substantial damage could occur on the western shoreline of Hithadhoo, especially in the north western region. However it is still probable that waves could diffract around the northern end of Addu Atoll and cause flooding on the eastern shoreline of Hithadhoo, especially if it coincided with high tide. Intensity of such events is considered to be low. Possible range of direction of swell waves in Feydhoo: South to West South West
Figure 2.2 Historical storm tracks (1945-2007) and possible direction of swell waves for Hithadhoo Island
At present, it is very difficult to forecast the exact probability of swell hazard event and their intensities due to the unpredictability of swell events and lack of research into their impacts on Maldives. However, since the hazard exposure scenario is critical for this study a tentative exposure scenario has been developed based on the historical events. In this regard there is a probability of major swell events occurring every 20 years in Hithadhoo with probable water heights less than 0.5 m on its western side and every 10 years with probable water heights of less than 0.5 m on its eastern coastline. Events with water heights less than 0.3 m are likely to occur more frequently on the eastern side due to high tides and low reclamation heights. A flooding probability of 20% was also observed for the eastern side, when the monthly peak tide reaches 2.3 m or more. These tides usually occur in March, April, October or November. Tides alone may not have caused the flooding but its occurrence with swell waves would have triggered the events. Land reclamation at lower heights may have exposed the eastern side more tidal flooding. This trend may continue with more planned reclamation activities, if the present reclamation practices are followed.
2.2.2 Heavy Rainfall
The rainfall pattern in the Maldives is largely controlled by the Indian Ocean monsoons. Generally the NE monsoon is dryer than the SW monsoon. Rainfall data from the three main meteorological stations, HDh Hanimaadhoo, K. Hulhule and S Gan shows an increasing average rainfall from the northern regions to the southern regions of the country (Figure 2.3). 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
Fig 2.3 Map showing the mean annual rainfall across the Maldives archipelago.
The mean annual rainfall of Gan is 2299.3mm with a Standard Deviation of 364.8mm 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 2.4 ).
Fig 2.4 Mean Monthly Rainfall (1978-2004).
Historic records of rainfall related flooding on the island of Hithadhoo indicates that this island is often flooded although the intensity of the floods is low. Records for all incidents have not been kept but interviews with locals and research into newspaper reports show that localised levels of flooding within areas of Hithadhoo has been experienced dating back to late 1970’s. Assessment of flood events against abnormal departure in annual rainfall does not show a major relationship except for the 1978 event (see Figure 2.5). Flooding caused by rainfall on the island has been reported to reach up to 0.4 m above the ground level.
Figure 2.5 Standard departure of rainfall from normal levels.
It would be possible to identify threshold levels for heavy rainfall for a single day that could cause flooding in Hithadhoo, through observation of daily rainfall data. Unfortunately, we were unable to acquire daily historical data from the Department of Meteorology due to the newly introduced user-pays-policy and lack of resources to acquire them.
Hithadhoo’s exposure to rainfall flooding is partially due to the drainage patterns and partially due to human activities. As will be discussed in the physical environment section of this report, there are two main wetland areas in the island which acts as drainage for most of the island. Houses constructed around the paths of these drainage patterns are directly impacted during heavy rainfall. Furthermore, a number of reclamation activities have been carried out on the wetland areas and on the reef, often without considering the consequences on drainage system. Similarly, since the 1960s taro pits were dug across a number of housing plots in the islands. These activities have left low elevations across the island, specifically inside the backyards, leading to heavy rainfall related flooding, Introduction of vehicles and extensive use of roads led to the top soil to be hardened, creating puddles and occasionally wide scale retention of water in the lower roads. As a remedy, roads were maintained by levelling, re-levelling and infilling using extra sand. Over the years, most roads have been raised and often stand higher than the surrounding houses. Heights of about 0.3m were observed in some roads. To add to the problem, the old taro pits further serves as a drainage area from the roads. Majority of the taro pits have since been refilled, although most the refilled areas are still lower than the surrounding roads.
The probable maximum precipitations predicted for Gan by UNDP (2006) are as shown 2.5:
Table 2.5: Probable Maximum Precipitation for various Return periods in Gan. Return Period 50 year 100 year 200 year 500 year 218.1 238.1 258.1 284.4
The maximum precipitation for 24 hour period in Maldives has been recorded as 219.8mm in Kaadedhoo airport 133km north of Gan. Based on the field observations and correlations with severe weather reports from Department of Meteorology ((DoM, 2005) the following threshold levels were identified for flooding. These figures must be revised once historical daily rainfall data becomes available (Table 2.6).
Table 2.6 Threshold levels for rainfall related flooding in Hithadhoo. Thresho ld level Impact (daily rainfall) 50mm Puddles on road, flooding in low houses. 100mm Flooding in low houses; a number of roads flooded; minor damage to household items especially in the backyard areas 150mm Widespread flooding on roads and low lying houses. Minor to moderate damage to household goods, possible school closure. 200mm Widespread flooding on roads and houses. Moderate to major damages to household goods, possible school closure, damage to crops, gullies created along shoreline, possible damage to road infrastructure. 250+mm Widespread flooding around the island. Major damages to household goods and housing structure, schools closed, businesses closed, damage to crops, damage to road infrastructure,
Quite often heavy rainfall is associated with multiple hazards especially strong winds and possible swell waves. It is therefore likely that a major rainfall event could inflict far more damages those identified in the table.
2.2.3 Wind storms and cyclones
Maldives being located within the equatorial region of the Indian Ocean is generally free from cyclonic activity. There have only been a few cyclonic strength depressions that have tracked through the Maldives, all which occurred in the northern regions. According to the hazard risk assessment report (UNDP, 2006), Hithadhoo falls within the least hazardous zone for cyclone related hazards. There are no such records for the southern region, although a number of gale force winds have been recorded due to low depressions in the region.
Based on historical records, windstorms are the most frequent hazard event in Hithadhoo. The most intense storm recorded occurred in 1989 when winds over 80knots were reported. Unfortunately wind data were unavailable for this study. Historic records for Hithadhoo have also indicated that even strong breeze – near gale force winds (Table 2.7) have caused significant damage to property and trees on the island. One such event that is observed in the available meteorological records (records for the years 2002 and 2003) was the strong breeze that occurred on the 20th of July 2003. This event was recorded to have attained an average wind speed of 23knots.
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. Estimates have therefore been made using the only available data: 2002 and 2003.
Analysis of all the wind speed data for the years 2002 and 2003 indicates that the probability of occurrence of wind speeds greater than 23 knots is 1.3days (0.36%) in a year (Table 2.8). The analysis also indicated that highest winds blow from SSW – W (Fig 2.6).
Table 2.7 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.
Table 2.8 Probability of occurrence of wind at different speeds in Addu Atoll (based on hourly records for the years 2002 and 2003).
Probability of occurance Direction Speed range <=10 kts >10 - 20kts >20 - 30kts >30kts 0 - 22.5 0.0881 0.0002 22.5 - 45 0.0529 0.0007 45 - 67.5 0.0278 0.0002 67.5 - 90 0.0304 0.0003 90 - 112.5 0.0216 0.0011 112.5 - 135 0.0253 0.0024 135 - 157.5 0.0246 0.0011 157.5 - 180 0.0419 0.0015 180 - 202.5 0.0615 0.0027 202.5 - 225 0.0655 0.0149 0.0002 0.0001 225 - 247.5 0.0645 0.0343 0.0002 247.5 - 270 0.1407 0.0838 0.0031 270 - 292.5 0.0769 0.0088 292.5 - 315 0.0619 0.0034 315 - 337.5 0.0545 0.0027 337.5 - 360 Total 0.8381 0.1583 0.0035 0.0001
Figure 2.6 Windrose chart for Gan, Addu Atoll, using the hourly data for years 2002 and 2003.
The threshold levels for damage are predicted based on interviews with locals and housing structural assessments provided by risk assessment report (UNDP, 2006) as shown in Table 2.9.
Table 2.9 Threshold levels for wind damage based on interviews with locals and available meteorological data. Wind speeds Imp act 1-10 knots No Damage 11 – 16 knots No Damage 17 – 21 knots Light damage to trees and crops 22 – 28 knots Breaking branches and minor damage to open crops, some weak roofs damaged 28 – 33 knots Minor damage to open crops and houses 34 - 40 knots Minor to Moderate to major damage to houses, crops and trees 40+ Knots Moderate to Major damage to houses, trees falling, crops damaged
It should also be noted that historic records for Hithadhoo indicates there are very few strong wind events that have caused any significant impact on the island. This contrasting difference between Feydhoo and Hithadhoo, could be due to the presence of dense and wide coastal vegetation belt and the high ridge on the windward side of Hithadhoo. The dense vegetation growing on the ridge would absorb a lot of wind energy below the roof level of most of the houses in the area. The normal roof height for a single story house in the Maldives is between 3 to 4m. The windward side ridge system on the island has a height more than 2.5m from the average height of the island. The height of vegetation at the ridge is approximately 2m from the ground level at the ridge. This makes the height of vegetation over the ridge to be higher than the roof level of most houses on the island.
2.2.4 Tsunami
UNDP (2006) reported the region where Hithadhoo is geographically located to be a moderate tsunami hazard zone. The tsunami of December 2004 had very little impact on the eastern side of Hithadhoo. There was no reported flooding of the island from this event. The tide gauge at Gan in Addu Atoll recorded the tsunami of December 2004 as a wave of height 1.4 m within the atoll lagoon (Figure 2.7). Plotting the maximum water level recorded at Gan tide gauge (0.8 m +MSL) over the cross-sectional profile of Hithadhoo clearly shows that the tsunami wave of December 2004 was just a few centimetres above than the ground level at the northern side of Hithadhoo (Figure 2.8). Comparatively lower wave height recorded at Gan is partly due to the refraction of the wave caused by the Indian Ocean bathymetry as it travelled west toward Maldives and due the relative orientation and distance from the earthquake epicentre which triggered the tsunami. 200 150 100 50 0 -50 -100
Water depth (cm)MSL rel depth Water -150 -200 0 100 200 300 400 500 600 700 800 900 1000 1100 Elapsed time (min) since 00:00hrs (UTC) of 26-12-2004
Figure 2.7 Water level recordings from the tide gauge at Gan, Addu Atoll indicating the wave height of tsunami 2004.
5
4 Tsunami induced tide level recorded at Gan, Addu Atoll (December 2004) 3
2
1
0 0 200 400 600 800 1000 1200
Height rel MSL rel (m) Height -1
-2 Island profile
-3
-4 Distance from oceanward shoreline (m)
Figure 2.8 Maximum water level caused by tsunami of December 2004 plotted across the island profile of Hithadhoo evidently showing the reason why there was so little flooding caused by this event.
The absence of impact during the 2004 tsunami doesn’t mean that the island is not exposed to tsunamis. The predicted probable maximum tsunami wave height for the area where Hithadhoo is located is 0.8 – 2.5 m (based on UNDP (2006)). Examination of the flooding that will be caused by a wave run-up of 2.5 m for the island of Hithadhoo indicates that such a magnitude wave will flood the island up to an extent of approximately 1000m from the lagoonward shoreline and that the first 100 m from the shoreline will be a moderately destructive zone (see Figure 2.9). The main advantage for Feydhoo against tsunamis is that it is located on western coastline of Addu Atoll and that no major atoll passes exist directly east of the atoll. The main source of tsunamis for Maldives is Sumatran trench on the eastern side.
However, due to the atoll shape and island orientation, parts of the eastern coastline of Hithadhoo is located parallel to potential tsunami waves approaching from the east. Furthermore, it is also well understood that the tsunami wave will also diffract into the atoll lagoon through atoll passes which will cause the water level in the atoll lagoon to rise. Hence if the atoll lagoon water level rises 2.5 m above MSL then island will flood from its lagoonward, as noted above. The ration between maximum tide level to maximum wave height for the tsunami of 2004 is 0.57. When this ratio is applied to the maximum tsunami wave height predicted within the lagoon for this region of the country results in a 1.8 m water level rise within the atoll lagoon.
5.0 Tsunami induced water level within the atoll lagoon for a wave of height 3.2m 4.0
3.0
2.0
1.0
0.0 0 200 400 600 800 1000 1200
Height rel MSL rel Height (m) -1.0
-2.0 Island profile
-3.0
-4.0 Distance from oceanward shoreline (m)
Figure 2.9 Tsunami related flooding predicted for Hithadhoo based upon theoretical water level rise within lagoon for the maximum probable tsunami wave height at Hithadhoo 2.2.5 Earthquakes
There hasn’t been any major earthquake related incident recorded in the history of Hithadhoo or even Madives. However, during 16th July 2003 an earthquake of unknown (but possibly of very small) magnitude are known to have caused tremors in Hithadhoo.
The Disaster Risk Assessment Report (UNDP 2006) highlighted that Addu Atoll is geographically located in the highest seismic hazard zone of the Maldives. According to the report the rate of decay of peak ground acceleration (PGA) for the zone 5 in which Hithadhoo is located has a value less than 0.32 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 3.9 table below). The MMI is a measure of the local damage potential of the earthquake. See table 3.10 for the range of damages for specific MMI values. Limited studies have been performed to determine the correlation between structural damage and ground motion in the region. The conversion used here is based on United States Geological Survey findings. No attempt has been made to individually model the exposure of Hithadhoo Island as time was limited for such a detailed assessment. Instead, the findings of UNDP (2006) were used.
Table 2.10 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 2.11 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
1 Based on KATZFEY, J. J. & MCINNES, K. L. (1996) GCM simulation of eastern Australian cutoff lows. Journal of Climate , 2337-2355. 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 the threshold for damage is very limited even in a 475 year return earthquake. It should however be noted that the actual damage may be different in Maldives since the masonry and structural stability factors have not been considered at local level for the MMI values presented here. Usually such adjustments can only be accurately made using historical events, which is almost nonexistent in Maldives.
2.2.6 Climate Change
The debate on climate change, especially Sea Level Rise (SLR) is far from complete. Questions have been raised about SLR itself (Morner et al., 2004, Morner, 2004) and the potential for coral island environments to naturally adapt (Kench et al., 2005, Woodroffe, 1993). However the majority view of the scientific community is that climate is changing and that these changes are more likely to have far reaching consequences for Maldives. For a country like Maldives, who are most at risk from any climate change impacts, it is important to consider a cautious approach in planning by considering worst case scenarios. The findings presented in this section are based on existing literature. No attempt has been made to undertake detailed modelling of climate change impacts specifically on the island due to time limitations. Hence, the projection could change with new findings and should be constantly reviewed.
The most critical driver for future hazard exposure in Maldives is the predicted sea level rise and Sea Surface Temperature (SST) rise. Khan et al. (2002, Woodroffe, 1993) analysis of tidal data for Gan, Addu Atoll shows the overall trend of Mean Tidal Level (MTL) is increasing in the southern atolls of Maldives. Their analysis shows an increasing annual MTL at Gan of 3.9 mm/year. These findings have also been backed by a slightly higher increase reported for Diego Garcia south of Addu Atoll (Sheppard, 2002). These calculations are higher than the average annual rate of 5.0 mm forecasted by IPCC (2001), but IPCC does predict a likely acceleration as time passes. Hence, this indicates that the MTL at Hithadhoo by 2100 will be nearly 0.4m above the present day MTL .
Similarly, Khan et al. (2002) reported air temperature at Addu Atoll is expected to rise at a rate of 0.4C per year, while the rate of rise in SST is 0.3C.
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 Addu Atoll where Hithadhoo is located.
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 Hithadhoo 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 Ocean 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 Hithadhoo 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 Hithadhoo and surrounding region is summarised below. It should be cautioned that the values are estimates based on most recent available literature on Gan 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 2.12 Summary of climate change related parameters for various hazards. Element Predicted Predicted change (overall rise) Possible impacts on rate of Hazards in Hithadhoo Best Case Worst Case change
SLR 3.9-5.0mm 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 0.4°C / Yr 2050: decade +1.72° Yr 2100: +3.72° SST 0.3°C / Yr 2050: Increase in storm decade +1.29° surges and swell wave related flooding, Coral Yr 2100: bleaching & reduction +2.79° in coral defences Rainfall +0.14% / Yr 2050: Increased flooding, yr (or +1384mm could affect coral reef +32mm/yr) growth Yr 2100: +2993mm Wind gusts 5% and Yr 2050: +3.8 Yr 2050: Increased windstorms, 10% / Knots +7.7Knots Increase in swell wave degree of Yr 2100: +8.3 Yr 2100: +16.7 related flooding. 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 provided in section 3.2 above, the following event scenarios have been estimated for Hithadhoo Island (Table 2.13, 2.14, and 2.15).
Table 2.13 Rapid onset flooding hazards
Hazard Max Impac t thresholds Probability of Occurrence Prediction
Low Moderat Sever Low Moderate Severe e e Impact Impact Impact Swell Waves – NA < 4.0m > 4.0m 2 > 5.0m Modera Low Very western side te Low
(wave heights on reef flat – Average Island ridge height +3.6m above reef flat) Tsunami 3.0m < 2.0m > 2.0m 3 > 3.0m Modera Low Very te low (wave heights on reef flat) SW monsoon 2.0m < 3.0m > 3.0m > 4.0m Very Very low Unlikely high seas High
Heavy Rainfall 284mm <75m >75mm >175m High Moderate Low
2 Impact on southern half of island will be severe if floods higher than 3.0m. The northern half has higher ridge. 3 If tsunami approaches from within the atoll lagoon impact can be severe beyond 2.5m. (For a 24 hour m m period)
Table 2.14 Slow onset flooding hazards (medium term scenario – year 2050)
Hazard Impact thresholds Probability of Occurrence
Low Moderate Severe Low Moderate Severe
SLR: Tidal < > 2.5m > 3.0m Moderate Very Low Very Flooding 2.5.0m Low
SLR: Swell < 4.0m > 4.0m > 5.0m Very high Moderate Low Waves – western side SLR: Heavy <75mm >75mm >175mm Very Moderate Low Rainfall High
Table 2.15 Other rapid onset events
Hazard Max Impact thresholds Probability of Occurrence Prediction
Low Moderate Severe Lo w Moderate Severe
Wind storm NA <28 > 28 knts > Very Moderate Low knts 40Knts High Earthquake III < IV > IV > VI Low Unlikely none (MMI value 4)
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 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
4 Refer to earthquake section above components of this study only events above the severe threshold were considered. Hence, the hazard zones should be interpreted with reference to the hazard scenarios identified above.
2.4.1 Swell waves and SW monsoon high Waves
The intensity of SW monsoon udha is predicted to be highest 50m from the eastern coastline (see Figure 2.11). It is unlikely that the western beach ridge will be overtopped by udha events unless accompanied by swell waves. The eastern side however remain exposed during high tide and udha period due to low elevation.
Intensity of swell waves is expected to be highest 50m from the western coastline and 150m from the eastern side. Swell waves higher than 4.0m on reef flat are predicted to overtop the oceanward ridge and penetrate 50-100m from coastline. There will also be a tendency for flood waters to flow rapidly eastward due to low topography especially if the duration of swell incident is longer or waves higher than 5.0m on reef flat.
There is a small probability of swell waves propagating through the south western reef pass of the atoll if waves are oriented parallel to the pass. Such waves could affect the southern half of Hithadhoo up to 100m. Hazard Zoning Map Swell Waves, High Seas
Intensity Index
Low 1 2 3 4 5 High
Contour lines represent intensity index based on a severe event scenario (+4.0m on reef flat & +0.5m on land)
High Coastal Ridge Newly Reclaimed Low areas
Revetment to protect Addu Link Road
Predominant Swell Wave Direction
Figure 2.11 Hazard zoning map for swell waves and southwest monsoon high seas.
3.4.2 Tsunamis
When a severe threshold of tsunami hazard (>3.0m on reef flat) is considered, the north eastern side of the island is predicted to receive the highest intensity (Figure 2.12). This is due to the low elevation of coastline and its possible direct exposure to tsunami wave trains. The small islands on the northeast are expected to absorb much of the wave energy but the southern half is expected to receive full energy. 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. Unlike Feydhoo Island which is protected from the direct path of tsunami waves, Hithadhoo is expected to receive higher intensity waves. Along with the eastern rim islands of Hulhudhoo, Meedhoo, Herethere and Viligilli are expected to experience the brunt of any large tsunami event.
Hazard Zoning Map Tsunami
Intensity Index
Low 1 2 3 4 5 High
Contour lines represent intensity index based on a severe event scenario (+3.5m on reef & +1.5m on coastline)
Figure 2.12 Hazard zoning map for tsunami flooding.
2.4.3 Heavy Rainfall
Heavy rainfall above the severe threshold is expected to flood low lying areas of the island especially near wetland areas, reclaimed wetland areas and reclaimed reef areas (Figure 2.13). The reclaimed wetland in the northern and southern have experienced the worst floods, while the reclaimed areas on the east are also very likely to be flooded in the future due to the runoff patterns and low elevation in the region. The area around the Addu Link Road is also reported to be particularly susceptible due the blockage of surface runoff towards the sea. At present the drainage system is reported to function poorly due to high levels of sedimentation and lack of arrangement from the community and authorities to regularly clean them. The inner areas of the islands are likely to experience low levels of flooding due to remnants of taro pits and improper road maintenance activities. The rainfall hazard zones are approximate and based on the extrapolation of topographic data collected during field visits. A comprehensive topographic survey is required before these hazard zones could be accurately established.
Hazard Zoning Map Heavy Rainfall
Intensity Index
Low 1 2 3 4 5 High
Contour lines represent intensity index based on a severe event scenario (+200mm in a 24 hour period)
Figure 2.13 Hazard zoning map for heavy rainfall related flooding.
2.4.4 Strong Wind
The coastal areas of the western shoreline are predicted to receive the strongest winds both as a first contact point and due to the significantly high coastal ridge (Figure 2.14). As explained earlier predicted strong wind direction is W to SSW. In the general the entire island is exposed to strong wind. However a narrow strip adjacent to the high ridge is expected to experience a slightly reduced intensity due to the protection provided by the ridge 2.0m higher than this zone. Much of the impact on the eastern half of the island could be from secondary impacts such as falling trees.
Hazard Zoning Map Strong Wind
Intensity Index
Low 1 2 3 4 5 High
Contour lines represent intensity index based on a severe event scenario (+40 Knots)
Figure 2.14 Hazard zoning map for strong wind.
2.4.5 Earthquakes
The entire island is a hazard zone with an intensity rating of 2.
2.4.6 Climate Change
Establishing hazard zones specifically for climate change is impractical at this stage due to the lack of topographic and bathymetric data. However, the predicted impact patterns and hazard zones described above are expected to be prevalent with climate change as well, although the intensity is likely to slightly increase.
2.4.6 Composite Hazard Zones
A composite hazard zone map was produced using a GIS based on the above hazard zoning and intensity index (Figure 2.15). The coastal zone approximately 100m on the western coastline and 250m from eastern coastline is predicted to have the highest intensity of hazard events. The inner part of the island is also exposed to multiple hazards although at a small scale. Hazard Zoning Map Multi Hazards
Intensity Index
Low 1 2 3 4 5 High Contour lines represent intensity index based on multiple hazards (Swell waves, high seas, heavy rain strong wind, tsunami and Sea lvel rise)
Figure 2.15 Composite hazard zone map.
2.5 Limitations and recommendation for future study
The main limitation for this study is the incompleteness of the historic data for different hazardous events. The island authorities do not collect and record the impacts and dates of these events in a systematic manner. There is no systematic and consistent format for keeping the records. In addition to the lack of complete historic records there is no monitoring of coastal and environmental changes caused by anthropogenic activities such as road maintenance, beach replenishment, 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 Hithadhoo 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 Hithadhoo.
References
BINNIE BLACK & VEATCH (2000) Enviromental / Technical study for dredging / reclamation works under Hulhumale' Project - Final Report. Male', Ministry of Construction and Public Works. DEPARTMENT OF METEOROLOGY (DOM) (2005) Severe weather events in 2002 2003 and 2004. Accessed 1 November 2005,
3. Environment Setting and Vulnerabilities
3.1 General environment Conditions
3.1.1 Terrestrial Environment
Topography
The topography of Hithadhoo was assessed using four island profiles (see Figure 3.1). Given below are the general findings from this assessment.
0 500 1,000 meters
TOPGRAPHIC PROFILE LOCATIONS P3
P1
P2
P4
.
Figure 3.1 Topographic survey locations.
Hithadhoo has one of the highest elevations in Maldives along its western shoreline (see Figures 3.2 and 3.3). The ridge system which is believed to be a response to high wave energy in the area reaches to +3.6 m MSL along the surveyed lines. The only similar ridge system recorded in Maldives is in the neighbouring atoll-island, Fuvahmulah (CDE, 2006). The average height of the ridge system is estimated at +3.4 m MSL and the average width is estimated at 100 m. The height of the ridge decreases southwards and is estimated to be around 1.9-2.4 m in the southern half of the island. The ridge system stands out prominently in the island section profile and is 2.0-3.0 m higher than the lower areas in the island, which is a substantial variation.
Apart from the coastal ridge, the island is generally low lying with an average elevation of +1.0 m MSL along the surveyed island profiles (see figures 3.2-3.4). This finding was reconfirmed from the shallow depths of ground water table around the island and considerably deeper depths along the ridge. The houses located on the ridge have their bathrooms and toilets located 1.5-2.0 m below ground level, in order to have access to water. The lowest on dry land is found along the newly reclaimed areas which have not considered levelling during their implementation. The reclaimed areas along the wetlands were found to be lower than those reclaimed on the reef.
Low beach Oceanward (0.7m) G Ridge (+3.2m) Unlevelled Possible reclaimed land reclaimed wetland G’ Dredged Shoreline Original Main Road area Link Road 1980’s Shore line
125m wide 1m ridge system
0 Lagoonward Side Approximate Mean Sea Level Oceanward Side
Reclaimed Land
0 100 200 300 400 500 600 700 Figure 3.2 Topographic profile P1 Oceanward G Depth of Ridge well: -2.6m. Multiple Area floods Very few houses Ridges (+3.6m) during spring allocated on ridge. high tides G’ Verylow Coastline (+0.65m) Drainage line Link Road Main Road
1m 120m wide ridge system Reclaimed area 0 Lagoonward Side Approximate Mean Sea Level Oceanward Side
0 200 400 600 800 1000 1200
Figure 3.3 Topographic profile P2 Northen Wetland areas
0 500 1,000 meters
ESTIMATED DRAINAGE PATTERNS
Reclaimed w etland Wetland Lakes
Southern w etland Areas
Figure 3.4 Wetland areas and estimated drainage patterns
There are two major low areas on the island which form wetland areas: the northern most wetland area called “eedhigali Kilhi” and south central wetland area called “Maa kilhi”. These areas appear to have a major influence on the drainage system of the island along with the western ridge system which causes a general west to east runoff pattern. The fact that the original wetland areas were reclaimed at a lower elevation than the surrounding land causes runoff into these zones during heavy rainfall and eventually leads to flooding (see Figure 3.5). Similarly, reclamations on the reef flat are lower than the original island causing frequent but low impact flooding from accelerated surface runoff. Lowest elevation G in a house End of Main Road original (unreclaimed part) +0.2m wetland area Lagoonward G’ shoreline Link Road
Flooding due to surface runoff from main island and rising water table 1m
0 Approximate Mean Sea Level Lagoonward Side 140m from Oceanward Side Reclaimed wetland
0 100 200 300 400 500 Figure 3.5 Profile (P3) of a reclaimed wetland area
As characteristic of large islands, considerable variations in topography were observed in Kulhudhuffushi. Unfortunately, the roads around Kulhudhuffushi have been considerably modified as part of the road maintenance programme. As a result most of the roads have been levelled, and may not represent the true topography of the island. The road maintenance programme does not level the surrounding houses and as years of road levelling has caused a number of houses to be located lower than the road.
Vegetation
The vegetation cover in Hithadhoo Island is high compared to islands with similar population densities. Figure 3.6 shows the dense vegetation distribution in Hithadhoo Island. The remaining areas of the island have sparsely distributed and shorter species. The majority of the area around the southern wetland areas is covered with shorter species, while that in settlement area is covered with backyard fruit and shade trees.
The coastal vegetation around the island, apart from the modified eastern shoreline is very dense and mostly in their natural state. On average the coastal vegetation belt is 30m wide with some areas reaching over 80-100m. All in all, the coastal vegetation system is healthy and functioning well compared to most other inhabited islands with similar population densities.
The height of the coastal ridge also seems to limit the type of vegetation located along it. Only salt tolerant species with longer root systems were located along the higher areas of the ridge and none of the introduced species such as bread fruit trees seemed to survive the conditions. The islanders reported difficulty in growing larger trees in the area, although natural growth of larger trees had occurred to some extent. 73.1194°E 73.0744°E 73.0969°E
0.584562°S 0 500 1,000 meters
VEGETATION COVER
0.607045°S
0.629528°S
Figure 3.6 Vegetation cover in Hithadhoo
Ground Water and Soil
Hithadhoo Island has a substantial layer of fresh water (MoFT, 1999). Water lens depth varies across the island based on topography. Generally the water table could be reached with less than 1m at median tide in all areas other than the ridge system. This could decrease to 0.5m during spring high tides or more during heavy rainfall, especially in reclaimed wetland areas. The water lens along the wetland areas are above ground level while that of the ridge system can only be reached at -2.0 to -2.5m.
Hithadhoo’s ground water was reported to be in generally in good quality although traces of contamination were reported in random locations around the island (MoFT, 1999). There were no shortages of potable water in the past due to the good quality of ground water and heavy rainfall in the region.
The soil conditions were not assessed across the island due to time limitation. Hithadhoo is expected to have comparatively good soil due to vast size of the island and the general low elevation of the island. The reclaimed wetland area in the north seemed particularly fertile with more than half of the islands mango trees being located in this zone.
3.1.2 Coastal Environment
General Characteristics
The coastal environment of Hithadhoo has contrasting characteristics on its western and eastern shoreline. Figure 3.7 summarises the coastal characteristics of Hithadhoo. The eastern shoreline, which is less exposed to natural hazards, has been largely modified by development activities. The coastal processes on the eastern shoreline have been altered significantly due to obstructions such as solid jetties, coastal protection and dredging activities. The western shoreline on the other hand is very much in its natural state with a properly functioning coastal system.
The western coastline has one of the most well established defence systems against sea induced natural hazards. The +3.6m high ridge has been developed over time possibly due to strong wind and very high wave energy, which could be either due to strong wind generated waves during southwest monsoons or long distance swells arriving from storm activity in southwest Indian Ocean. The fact that the islands located on the southern half of the same reef system does not have similar ridge systems, suggests that the ridge formation is associated with a combination of factors including exposure to southwest monsoons, storm events and possibly biological characteristics of the reef system near Hithadhoo.
The beach composition on the island varies dramatically from north to south along the western shoreline. The northern half, especially the northwest corner is characterised by washed-up large coral pieces (predominantly table coral pieces). This pattern usually indicates two natural processes: 1) the area is exposed to strong wave energy from severe storm activity or wind generated waves, and 2) the coral decomposing organisms in the region are low. Judging from the geomorphic features such as multiple ridges and height of those ridges, it is very likely that the area is mostly exposed to strong wave energy.
The high ridges along with the strong coastal vegetation belt form the natural defensive system of the island against future sea induced flooding events.
The group of islands on the north east corner of the island performs the functions of barrier islands by absorbing wave energy coming from the east during NE monsoon.
possible historical NE monsoon strom High Multiple %%%%%% wind generaed 0 500 1,000 activity ridges swells meters COASTAL ENVIRONMENT %%%%%% Natural barrier against NE monsoon Very high wave energy %%%%%% +3.4m ridge waves High Energy 30m coastal veg Moderate Energy Moderate to low energy
+3.3m ridge %%%%%% 20m Coastal veg %%%%%% Very low area %%%%%% SW monsoon floods upto 10m wind generated at sprint tides swells +3.6m ridge %%%%%% +80m coastal veg heavily modified %%%%%% coastal envrionment
low ridge %%%%%% uninhabited area
Figure 3.7 General features of the coastal environment
Coastal erosion
It is difficult to undertake a detailed assessment of the Hithadhoo coastal erosion patterns due the unavailability of historical data, large size of the island and substantial coastal modifications on the eastern shoreline. In general, it appears that the western shoreline undergoes seasonal and periodic erosion cycles. Large stretches of coastline showed evidence of past erosion from beach berms and exposed roots of vegetation. Erosion in one area is associated with proportional accretion along another portion of the coastline. During field observations, the northern areas along the western shoreline were seen to be undergoing moderate erosion while the southern areas were undergoing accretion. The relatively large size of material along the northern end of the island makes them immobile during regular wave activity. Hence, there is no movement of sediment from the western side of Hithadhoo to the eastern side or vice versa.
The barrier islands in the north east of Hithadhoo appear to have undergone significant erosion. The sediments removed from the islands appear to be deposited in the lagoon between Hithadhoo and the islands.
Erosion was not reported by islanders as a major environmental issue, probably due to the large size of the island. There were about 5 structures located within 5m of the western coastline which could be under threat if the present erosion and accretion trends continue.
3.1.3 Marine environment
General Reef Conditions
General historical changes to reef conditions were assessed anecdotally, through interviews with a number of fishermen. The general agreement amongst the interviewees was that the quality of reef areas on the lagoonward declined considerably over the past 50 years following the construction of causeways between Gan, Feydhoo and Maradhoo-Feydhoo. During this period lowering of coral cover and reduction in fish numbers, were reported. Since the causeways were replaced by bridges, fish abundance was reported to be increasing dramatically. Reef conditions on the oceanward reef line were reported to be in relatively good condition. In fact the north western corner of the reef was reported to be one of the best coral reefs in Maldives, in terms of biodiversity and live coral cover. Patches of seagrass can be found on the eastern lagoon and has been prevalent at least since the 1960’s.
3.1.2 Modifications to Natural Environment
Coastal Modifications 73.1194°E 73.0744°E 73.0969°E
0.584562°S 0 500 1,000 meters COASTAL & TERRESTRIAL Reclaimed Wetland MODIFICATIONS Area Reclaimed land Lakes Access Channel Other wetlands Quaywalls Settlement areas Dredged Areas Harbour closes to Reef Line western coastlline artificial sand pier Coastal Protection
0.607045°S Artificial drainage and floodway Reclaimed road Coastal protection seperating for Link Road two wetland areas
Reclaimed Wetland Area New land reclmation (2007)
vegetation Commercial Harbour cover reduced (Solid Jetty) 0.629528°S
Figure 3.8 Coastal modifications around Hithadhoo
As noted earlier, much of the coastal modifications have been undertaken on the eastern shoreline of the island. Below is a summary of major modifications. • The major coastal modification activity undertaken on Hithadhoo is land reclamation. Despite its large natural size, almost 73 ha of land have been added through reclamation which is more than the size of the island of Feydhoo, Maradhoo or Maradhoo-Feydhoo in Addu Atoll. Perhaps the need for reclamation comes from the fact that almost 20% of Hithadhoo is comprised of wetland areas. The current practices in land reclamation projects usually make the resulting coastal environment unfavourable for prevailing coastal processes. Location of dredge areas close to shoreline, improper coastline shaping and improper shoreline profiling are some of the practices that are likely to effect the natural adjustment of coastal processes. In addition, the newly reclaimed land in Hithadhoo are reclaimed without considering the drainage patterns in the island and without an artificial drainage system, causing rainfall related flooding in the these areas. The coastline still appears to be part of an ongoing reclamation project with remnants of dredge materials and dredge areas. The low elevations in these areas now cause flooding up to 10m inland during high tides. A new land is being currently reclaimed in the southern part of the island for the establishment of a fish canning factory. Such new additions are likely to continue in the future but the adhoc manner of reclamation, without proper long term planning is likely to have major negative implications on the island coastal environment’s natural adjustment.
• A stretch of the eastern shoreline has coastal protection developed on them as a measure to protect the Addu Link Road. These breakwaters constructed using boulders are highly likely to become redundant if the adhoc reclamation practices continue. The breakwaters are only designed to stabilise shoreline rather than protect the road from flooding.
• A solid jetty (commercial harbour) has been constructed perpendicular to the shoreline on the southeast corner of the island. This structure essentially has halted all movement of sediments along the shoreline. Prior to the new reclamation activities, seasonal erosion was observed on either side of the jetty location.
• A number of dredged areas can be found close to the island shoreline which acts as sediment dumps causing small but consistent loss of sediments. • A harbour has been developed along with 500m of quay wall and a 1.5km long harbour basin. This area was dredged as part a sediment source for the past reclamation projects.
Terrestrial Modifications
• The terrestrial environment of the island has been considerably modified to meet the settlement expansion across the entire island.
• The coastal vegetation of the island has been drastically reduced in the settlement area but the vegetation cover is considered to be higher than similar high density settlements.
• Much of the coastal vegetation on the western coastline is intact, but there area where the coastal vegetation has been encroached by development activities, especially construction of new houses.
• Land reclamation of wetland areas without considering the elevations and impacts on drainage systems has caused such areas to flood regularly during heavy rainfall.
• The increase in rainfall related flooding in the low areas of the island prompted the authorities to undertake road maintenance activities, which primarily involved levelling and raising roads. This has led to some houses in the island to be lower than the road, especially in the low lying areas, causing flooding during heavy rainfall.
• The newly reclaimed areas from the reef have poor vegetation cover. This pattern is typical in land reclaimed from reefs across Maldives. This may be partly due to the high alkalinity of the soil following reclamation and partly due to lack of re-vegetation activities following land reclamation projects.
3.2 Environmental mitigation against historical hazard events.
3.2.1 Natural Adaptation
Hithadhoo is perhaps one of the best examples of natural adaptation of coral islands to prevailing natural hazards in Maldives. The adjustment of ridges, coastal processes and drainage patterns are evident from initial assessments and require further empirical assessments to understand the adaptation processes. The defensive mechanisms established for the storms are critical for mitigating a number of other sea induced hazards as well. Preservation of this these natural defensive mechanisms and minimal alteration of physical processes that help develop these systems are critical in natural hazard mitigation planning.
3.2.1 Human Adaptation
Hithadhoo Island has number of mitigations undertaken to prevent impacts of natural hazards. Most of these measures are to protect impacts on infrastructures rather than the settlement or physical environment. The main coastal mitigation measures include foreshore breakwater to protect the Addu Link Road and nearshore breakwaters to protect harbour. The foreshore breakwaters were constructed specifically to mitigate potential coastal erosion hazards. A number of measures have also been undertaken to prevent rainfall relation flooding. These include raising the roads in newly reclaimed low areas to prevent flooding, construction of a floodway to mitigate flooding in southern wetland areas and construction of an artificial drainage system around the Addu Link Road to mitigate impacts of potential rainfall related flooding on the road.
3.3 Environmental vulnerabilities to natural hazards
3.3.1 Natural Vulnerabilities
Natural Vulnerabilities
• The low elevation along the eastern shoreline exposes the area to tsunamis, possible surges and predicted sea level rise.
• The Northwest-southeast orientation along with low elevation and reef shape exposes the majority of the island’s eastern shoreline to tsunami related flooding hazards.
• There are substantial topographic variations within Hithadhoo. During times of heavy rainfall the drainage patterns causes flooding in structures and roads located close to low areas. The agricultural land located in the low wetland areas (north) is particularly vulnerable to such flooding.
• Hithadhoo is exposed to swell waves and monsoon generated waves from South West Indian Ocean due to its location on the western rim (based on Naseer (2003)). Although the existing settlement area is protected by a high ridge, the uninhabited southern part may not enjoy the same protection due to the low elevation of ridges. Hence, new developments in the southern area may be comparatively more exposed to sea induce flooding.
• Hithadhoo is located in a high rainfall zone. Combined with substantial variations in topography, the island is exposed to rainfall related flooding during periods of heavy rainfall.
• Hithadhoo is located in an earthquake prone zone due to its proximity to Carlsberg Ridge (UNDP 2006).
• The wetland areas in the north and south of the island could expand with the projected sea level rise and associate rise in water table, leading to more frequent rainfall induced flooding events. Similarly, there is a possibility that salt water could seep into the water table, especially the freshwater reservoirs in the south of the island.
• The wetland areas in the north are separated from the ocean only by a narrow stretch of coral deposits. This rim could be breached by a major storm event in the future although it is highly unlikely.
• Reef width appears to play an important role increasing or decreasing the impacts of ocean induced wave activity. The proximity of Hithadhoo Island coastline to reef edge may increase the exposure of the island to certain sea induced Hazards. Implications of the existing distance needs to be studied further to establish a concrete relationship.
3.3.2 Human induced vulnerabilities
• The main impacts from human induced activities have come from improper land reclamation on the eastern side of the island. These include alteration of drainage system, impacts of dredging on the reef system, alteration of coastal processes and general failure to mitigate the negative impacts of reef reclamation. Increased exposure to the following hazards was identified as a direct result of these activities.
o Lack of consideration for island topography has resulted in the newly reclaimed land to be lower than the existing island. This has exposed the newly reclaimed areas to frequent rainfall related flooding and possible sea induced flooding during a tsunami or surge. The artificial drainage system established for the Addu Link Road has been unable to function properly, perhaps owing to the large volume of water flow mixed with sediments.
o The reclamation activities on the eastern side seem to be adhoc and without a scheduled plan of implementation. This causes major disruptions in coastal processes which require considerable time to adjust to changes.
o The quality of reef on the eastern side of the island has been reported to have declined considerably following development activities. Interviews with fishermen revealed a decline in live coral cover over the past 50 years probably owing to the development activities and over exploitation of reef resources.
• Similar to the reclamation of reef areas, improper reclamation of wetlands have exposed the island to rainfall related flooding. The reclamation process appears to have failed to address the implications on topographic variations and the resulting drainage patterns.
• Almost 80% of Hithadhoo’s eastern coastline does not have proper coastal vegetation on them. This is primarily due to the development activities in the area.
• The eastern coastline is now an artificial environment due to dredging activities, quay walls and reclamation activities. The island building processes no longer functions properly in this region. It would require continuous human intervention to mitigate natural hazards such as erosion.
• Past continuous road maintenance activities on the island to mitigate rainfall flooding has caused the road to be raised higher than the surrounding housing plots. As a result, houses in some parts of the island experiences severe flooding during heavy rainfall.
• Encroachment of settlement areas close to wetland areas has caused some structures to be located along the natural drainage zone. This has in the past exposed such plots to rainfall induced flooding. This pattern was most noticeable in the northern parts of the island and close to the southern wetland areas. 3.4 Environmental assets to hazard mitigation
• Hithadhoo is the second largest island in Maldives. The size of the land area along is one crucial characteristic of the island which could help reduce the impacts of flooding events.
• The location of Hithadhoo on western rim of Addoo Atoll and close to the equator protects the island from direct exposure to the most damaging sea induced events such as tsunamis and storm surges. It should however be noted that the maximum predicted tsunamis of 6m height may still inflict devastation in Hithadhoo mainly along its eastern coastline due to the low elevation of the area. The shape of reef formation on the eastern side of Hithadhoo also partly exposes the eastern coastline to ocean induced hazards arriving from the east.
• Hithadhoo has one of the highest natural ridges found in any island of Maldives. These ridges are believed to be a response to major storm events or continued exposure to strong wave action in the region. These ridges are also capable of mitigating sea induced flooding events up to 3.5m high for much of the island. Based on the historical records, prevailing climatic patterns and tectonic settings in West Indian Ocean, it is highly unlikely that flooding event exceeding 3.5m may reach the western shoreline of Hithadhoo. Nonetheless, these ridges could be considered the biggest defensive asset of Hithadhoo against any possible sea induced hazards from the west.
• Hithadhoo has the strongest coastal vegetation belt found in the nine islands studied in this project. Along with the high ridge, the strong coastal vegetation belt forms a formidable defensive system against ocean induced flooding and strong winds.
• There is a well established drainage system, dominated by wetland areas in the north and south reducing the impact of rainfall related flooding.
• Due to the low density with in the settlement area, Hithadhoo retains a large portion of its vegetation cover. It is very likely that these patches of vegetation help reduce the exposure of structures to strong winds.
• The coastal processes along the western coastline of the island appear to be functioning well without much human intervention. Although much of the eastern side of the island has been modified considerably, the fact that the coastal processes of the western and eastern side are largely unrelated in Hithadhoo, helps to maintain the natural processes responsible for natural hazard mitigation intact.
• The presence of barrier islands in the north eastern part of Hithadhoo protects the settlement from sea induced events such as storm surges and tsunamis, arriving from the east.
3.5 Predicted environmental impacts from natural hazards
The natural environment of Hithadhoo and islands in Maldives archipelago in general appear to be resilient to most natural hazards. The impacts on island environments from major hazard events are usually short-term and insignificant in terms of the natural or geological timeframe. Natural timeframes are measured in 100’s of years which provides ample time for an island to recover from major events such as tsunamis. The recovery of island environments, especially vegetation, ground water and geomorphologic features in tsunami effected islands like Laamu Gan provides evidence of such rapid recovery. Different aspects of the natural environment may differ in their recovery. Impacts on marine environment and coastal processes may take longer to recover as their natural development processes are slow. In comparison, impacts on terrestrial environment, such as vegetation and groundwater may be more rapid. However, the speed of recovery of all these aspects will be dependent on the prevailing climatic conditions.
The resilience of coral islands to impacts from long-term events, especially predicted sea level rise is more difficult to predict. On the one hand it is generally argued that the outlook for low lying coral island is ‘catastrophic’ under the predicted worst case scenarios of sea level rise (IPCC 1990; IPCC 2001), with the entire Maldives predicted to disappear in 150-200 years. On the other hand new research in Maldives suggests that ‘contrary to most established commentaries on the precarious nature of atoll islands Maldivian islands have existed for 5000 yr, are morphologically resilient rather than fragile systems, and are expected to persist under current scenarios of future climate change and sea-level rise’ (Kench, McLean et al. 2005). A number of prominent scientists have similar views to the latter (for example, Woodroffe (1993), Morner (1994)). In this respect, it is plausible that Hithadhoo may continue to naturally adapt to rising sea level. There are two scenarios for geological impacts on Hithadhoo. 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 Hithadhoo (based on (Yamano 2000)). The question whether the coral islands could adjust to the latter scenario may not be answered convincingly based on current research. However, it is clear that the highly, modified environments of Hithadhoo, especially the eastern coastline, stands to undergo substantial change or damage (even during the potential long term geological adjustments), due to potential loss of land through erosion, increased inundations, and salt water intrusion into water lens (based on Pernetta and Sestini (1989), Woodroffe (1989), Kench and Cowell (2002)).
Hithadhoo has particular vulnerability to sea level rise due to the presence of wetland areas. Since wetland areas in coral islands are linked to the tide and sea level, an increase in sea level may result in increase in size of such areas and a subsequent reduction in land (Woodroffe 1989).
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 Hithadhoo.
Hazard Scenario Probability Potential Major Environmental Impacts at Location Tsunami (maximum scenario) 2.5m Low • Moderate damage to coastal vegetation (Short-term) • Long term or permanent damage to selected inland vegetation in northern low areas especially common backyard species such as mango and breadfruit trees • Minor salt water intrusion into wetland areas and island water lens causing minor loss of some flora and fauna. • Contamination of ground water if the sewerage system is damaged or if liquid contaminants such as diesel and chemicals Hazard Scenario Probability Potential Major Environmental Impacts at Location in the boat yard are leaked. • Minor damage to backyard crops (short- term) • Moderate damage to crops around northern wetland • Moderate to major damage to coastal protection and island access infrastructure such as breakwaters and quay walls. • Short-medium term loss of soil productivity (north eastern part) • Minor damage to coral reefs (based on UNEP (2005)) Storm Surge (based on UNDP, (2005)) 0.60m (1.53m Very Low • Minor damage to coastal vegetation (north storm tide) eastern side) • Minor to moderate damage to coastal protection infrastructure • Minor geomorphologic changes in the north eastern shoreline and lagoon Strong Wind 28-33 Knots Very High • Minor damage to very old and young fruit trees • Debris dispersion near waste sites. • Minor damage to open field crops 34-65 Knots Low • Moderate damage to vegetation with falling branches and occasionally whole trees • Debris dispersion near waste sites. • 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. • Minor changes to coastal ridges Heavy rainfall 187mm Moderate • Minor to moderate flooding in low areas, including roads and houses. 284mm Low • Widespread flooding but restricted to low areas of the island. Drought Low • Minor damage to backyard fruit trees Earthquake Low • Minor-moderate geomorphologic changes to land and reef system. Sea Level Rise by year 2100 (effects of single flood event) Medium Moderate • Widespread flooding during high tides and (0.41m) surges. • Loss of land due to erosion. • Loss of coastal vegetation • Major changes to coastal geomorphology. • Saltwater intrusion into wetland areas and Hazard Scenario Probability Potential Major Environmental Impacts at Location 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
At the time of this study, no detailed plans have been developed for establishing Hithadhoo as a safe island. Presented below are some of the considerations that need to be made in developing Hithadhoo as a safe island in the future.
• Hithadhoo Island has a well established defensive system against sea induced natural hazards on its western coastline. It is vital that this system be maintained and enhanced in the new safe island development plan. Specific attention should be given in the land use plan to avoid developments within this zone including coastal protection.
• Due to its large size, Hithadhoo Island relies on a functioning drainage system to reduce rainfall induced flooding around the island. The proposed reclamation of wetland areas in the south of the island may have major implications for the drainage system and subsequent exposure to rainfall related flooding. Reclamation of this area should only be considered after careful assessment of the implications for drainage systems, topography, soil, vegetation systems and biodiversity. Experiences from GA. Villigili and G.DH. Thinadhoo shows that improper reclamation of such areas today will lead to rainfall hazard exposure in the future. The current practices in land reclamation planning and implementation has major flaws from an environmental point of view and need to be revised using proper assessments. Until the impacts of reclaiming wetland areas can be discerned with a high degree of certainty and an appropriate reclamation processes to mitigate the impacts could be established, it is not recommended to modify the wetland areas of Hithadhoo.
• The proposed new land reclamation under the present land use plan is expected to have implications on island environment and island exposure to natural hazards. The following points were noted on the proposed reclamation project.
o The reclamation is highly likely to cause further damage to the outer reef and the protected areas due to its proximity and current land reclamation practices. This may reduce the defensive capacity of the reef system and expose Hithadhoo to long term climate hazards. Proper reclamation practices need to be put in place prior to considering reclamation activities.
o The soil composition of a reclaimed area may need to be properly established. Soil in coral islands of Maldives has specific profiles which dictate the suitability to vegetation and perhaps drainage.
o The elevation of the newly reclaimed area should be inline with the existing island topography or should consider establishing a functioning drainage system to mitigate flooding hazards resulting from modified topography, especially where the new reclamation joins the existing island.
• A re-vegetation plan needs to be incorporated into the safe island development plan to ensure minimal exposure to strong winds and discomforts from future climate change. These include re-vegetating previously reclaimed land.
• Although the eastern side of the island is considered the lagoonward side of the island, the reef shape exposes the eastern side to direct wave actions arriving from the east. It is therefore important to treat the eastern side partly as an oceanward side. Hence, an Environment Protection Zones may be required wherever possible on the eastern coastline.
3.7 Limitations and recommendations for further study
• The main limitation of this study is the lack of time to undertake more empirical and detailed assessments of the island. The consequence of the short time limit is the semi-empirical mode of assessment and the generalised nature of findings.
• The lack of existing survey data on critical characteristics of the island and reef, such as topography and bathymetry data, and the lack of long term survey data such as that of wave on current data, limits the amount of empirical assessments that could be done within the short timeframe.
• The topographic data used in this study shows the variations along three main roads of the island. Such a limited survey will not capture all the low and high areas of the island. Hence, the hazard zones identified may be incomplete due to this limitation.
• This study however is a major contribution to the risk assessment of safe islands. It has highlighted several leads in risk assessment and areas to concentrate on future more detailed assessment of safe islands. This study has also highlighted some of the limitations in existing safe island concept and possible ways to go about finding solutions to enhance the concept. In this sense, this study is the foundation for further detailed risk assessment of safe islands.
• There is a time scale mismatch between environmental changes and socio- economic developments. While we project environmental changes for the next 100 years, the longest period that a detailed socio-economic scenario is credible is about 10 years.
• Uncertainties in climatic predictions, especially those related Sea Level Rise and Sea Surface Temperature increases. It is predicted that intensity and frequency of storms will increase in the India Ocean with the predicted climate change, but the extent is unclear. The predictions that can be used in this study are based on specific assumptions which may or may not be realized.
• The following data and assessments need to be included in future detailed environmental risk assessment of safe islands.
o A topographic and bathymetric survey for all assessment islands prior to the risk assessment. The survey should be at least at 0.5m resolution for land and 1.0m in water.
o Coral reef conditions data of the ‘house reef’ including live coral cover, fish abundance and coral growth rates.
o At least a years data on island coastal processes in selected locations of Maldives including sediment movement patterns, shoreline changes, current data and wave data.
o Detailed GIS basemaps for the assessment islands.
o Coastal change, flood risk and climate change risk modeling using GIS.
o Quantitative hydrological impact assessment. o Coral reef surveys
o Wave run-up modelling on reef flats and on land for gravity waves and surges.
References
Commerce Development and Envrionment (CDE) (2006). EIA for the Proposed Tourist Hotel Development on Gnaviyani Atoll, Fuvahmulah Island, Maldives. Male', One and Half Degree Maldives Private Limited.
IPCC (1990). Strategies for Adaptation to Sea-Level Rise: Report of the Coastal Management Subgroup. Strategies for Adaptation to Sea-Level Rise: Report of the Coastal Management Subgroup . IPCC Response Strategies Working Group. Cambridge, University of Cambridge.
IPCC (2001). Climate Change 2001: Impacts, Adaptation, and Vulnerability . Cambridge, United Kingdom and New York, NY, USA, Cambridge University Press.
Kench, P. S. and P. J. Cowell (2002). "Erosion of low- lying reef islands." Tiempo 46 : 6- 12.
Kench, P. S., R. F. McLean, et al. (2005). "New model of reef-island evolution: Maldives, Indian Ocean." Geology 33 (2): 145-148.
Ministry of Finance and Treasury (MoFT) (1999). Final Report for the Atoll Development Project. Atoll Development Project, Volume 2: Working Papers . Opus International Consultants Limited. Male', Ministry of Finance and Treasury, Government of Maldives.
Naseer, A. (2003). The integrated growth response of coral reefs to environmental forcing: morphometric analysis of coral reefs of the Maldives. Halifax, Nova Scotia, Dalhousie University : 275.
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
Hithadhoo Island is predominantly exposed to rainfall floods with high frequency. Swell wave/surge floods may occur on the eastern coast of the island. No significant tsunami inundation is expected due to the location of the island on the western rim of Addoo Atoll.
4.1 House vulnerability
Around 250 houses are identified as vulnerable on Hithadhoo Island, which accounts for about 13.4% of the total existing houses on the island. Houses with extremely poorly physical conditions account for 8%.
4.1.1 House vulnerability
The house vulnerability of Hithadhoo Island is dominantly attributed to two vulnerability factors: weak physical structure and low plinth elevation with respect to the adjacent road surface. As shown in Fig. 4.1 , 60% of the vulnerable houses identified are weak in their physical structure and around 40% low in their plinth level. In contrast, only less that 10% of the vulnerable houses are found poor in protection from ocean-originated floods. The distribution of vulnerability factors implies that most of the vulnerable houses on Hithadhoo Island may be frequently subjected to household-wide floods and subjected to slight damage.
70.0 60.0 50.0 40.0 30.0 Houses 20.0 10.0
%of Total Vulnerable 0.0 WB PP LE Indicator group
Fig. 4.1 Type of house vulnerability.
4.1.2 Vulnerable houses
The vulnerable houses can be divided into 4 major groups: weak houses (45%), weak houses with low plinth (15%), houses with low plinth (26%) and houses with poor protection (11%), as shown in Fig. 4.2. Just a few houses (3%) are weak in both physical structure and poor in protection.
Hithadhoo WB
WBPP 0% WBLE 26% WBPPLE 45% PP
11% LE 0% 15% 3% PPLE
Fig. 4.2 Distribution of vulnerable houses.
4.2 Houses at risk
Houses on Hithadhoo Island are highly exposed to rainfall floods. As shown in Fig. 4.3, up to 60% of the existing houses may be affected by rainfall flood. According to the land use plan, more plots are allocated in the rainfall flood-prone in the central part of the island. In contrast, less than 10% of the houses are exposed to wave/surge inundations.
Despite of the high exposure, physical damage to houses is relatively minor. As shown in Table 5.1, less than 10% of the exposed houses may be subjected to slight damage. No population displacement is expected due to flooding events.
Although Hithadhoo Island is located in Seismic Hazard Zone 5 and exposed to a GPA of 0.18-0.32 (UNDP, 2006), no significant damage to houses is expected.
Table 4.1 Houses at risk on S. Hithadhoo. Exposed Vulnerable Potential Damage Hazard houses houses Serious Moderate Slight Content type # % # % # % # % # % # % TS ------W/S 134 7.3 6 4.5 0 0 0 0 6 4.5 128 95.5 RF 1045 56.6 123 11.8 0 0 0 0 97 9.3 948 90.7 Flood Earthquake 1846 100 156 8.5 Wind 1846 100 156 8.5 ------Erosion
Fig. 4.3 Houses at risk associated with rainfall floods (left) and swell wave/surge floods (right). 4.3 Critical facilities at risk
Although exposed to both rainfall and wave/surge floods (Fig. 4.4, Table 4.2), most critical facilities, such as schools, mosques, administration offices, and communication sites, are not vulnerable and subjected to any physical damage. Given a flooding event of 0.5 m water depth, however, some schools may be affected in their contents.
Table 4.2 Critical facilities at risk on S. Hithadhoo Island. Critical facilities Potential damage/loss Hazard type Monetary Exposed Vulnerable Physical damage value Tsunami - - - - 4 mosques, 3 None Content-affected schools, 4 admin Wave/Surge offices, 1 communication site, 1 TV cable Flood 12 mosques, 7 None Content-affected schools, 5 admin Rainfall offices, 2 communication sites Earthquake All facilities None No Wind - - - - Erosion - - - -
4.4 Functioning impacts
The functioning of most critical facilities is hardly disrupted during flooding, except for that of the sewerage systems of the island, which may fail to function for days. In addition, road flooding may cause inconvenient to transportation, to some degree.
Table 4.3 Potential functioning impact matrix Flo od Function Earthquake Wind Tsunami Wave/surge Rainfall Administration 1)
Health care
Education 1-2 days
Religion
Sanitation 3) 3-5 days
Water supply
Power supply
Transportation A few days
Communication 2)
Note: 1) Administration including routine community management, police, court, fire fighting; 2) Communication refers to telecommunication and TV; 3) Sanitation issues caused by failure of sewerage system and waste disposal.
4.5 Recommendations for risk reduction
According to the physical vulnerability and impacts in the previous sections, the following options are recommended for risk reduction of S. Hithadhoo:
• Enhance building codes in the rainfall flood-prone areas, in particular the flooding zone in the south of the island. • Avoid maintaining the roads by raising the road surface. • Mitigate swell wave/surge flooding on the western coast by setting up an EPZ with a proper width. Many houses and proposed critical facilities are too close to the shoreline and may be subjected to wave overtopping flooding. • Mitigate rainfall flooding on the eastern coast by improving the existing drainage system that was probably degraded due to the construction of the road along the eastern coast. • Retrofit vulnerable houses by raising their plinth level to a proper height.
Fig. 4.4 Critical facilities at risk associated with rainfall floods (left) and swell wave/surge floods (right).