Detailed Island Risk Assessment in Maldives, L
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Detailed Island Risk Assessment in Maldives Volume III: Detailed Island Reports L. Gan – 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 2 1. Geographic background 1.1 Location Gan is located on the eastern rim of Laamu Atoll, at approximately 73° 31' 50"E and 1° 52' 56" N, about 250 km from the nations capital Male’ and 3.5 km from the nearest airport, Kadhdhoo (Figure 1.1). Gan is the largest island in terms of land area and population amongst 13 inhabited islands of Laamu atoll. It’s nearest inhabited islands are Kalhaidhoo (7 km), Mundoo (10 km) and Atoll Capital Fonadhoo (10 km). Gan forms part of a stretch of 4 islands connected through causeways and bridges and is the second largest group of islands connected in this manner with a combined land area of 9.4km 2. The island is exposed to NE monsoon generated winds and waves, and occasional storm activities originating from the cyclone belt of Indian Ocean. Gan is also believed to be located in an area where offshore ocean bathymetry could create a ‘funnelling’ effect due to wave refraction during tsunami events originating from Sumatran Ridge (Shifaz, 2004). E E ' ' 5 0 1 3 ° ° 3 3 7 Isdhoo 7 Dhanbidhoo Maaen'boodhoo 2° 00' N Mundoo Indian Ocean Hadhdhunmathi Atoll (Laamu Atoll) Gan Maavah Kadhdhoo N Fonadhoo Location Map of Gan Gaadhoo 0 5 10 Hithadhoo kilometers Kunahandhoo Figure 1.1 Location map of Gan. 3 1.2 Physical environment Gan is the largest island in the Maldives with a surface area of 600 Ha (6 km 2). It has a length of 7.2km and a width of 1.5km at its widest point. The island is wider in the north (1500m) and narrower in the south (400m). There are three settlements on the island, Thundi (northeast), Mathimaradhoo (east) and Mukurimagu (south). In additional there is a zone designated as Industrial Development Zone, which has a number of structures, located within it. All the settlements are located along the coastline but only the Thundi settlement is located away from the oceanward coastline. Entire settlement of both the Mathimaradhoo and Mukurimagu are within 300m of oceanward coastline while Thundi is located approximately 900m away from it. Gan has been connected to the adjacent island, Maandhoo through land reclamation. Together the two islands form a land area of 670 Ha (6.7 km 2) and covers 21km of coastline. In addition, the islands of Kadhoo (airport) and Funadhoo (Atoll Capital) are connected to Maandhoo Island through causeways and bridges. The total length of the island group is approximately 16km. The reef of Gan is a large reef system with a surface area of 4500 Ha (45km 2), covering 70% of the eastern rim of Hadhunmathi Atoll and stretching to approximately 29km. The reef also hosts 5 inhabited islands, an Airport island (Kadhoo), 2 industrial islands and 8 uninhabited islands, totalling a 1220ha (12.2 km 2) of land. It is the largest concentration of land in a single reef and Gan comprises half of its land area. Gan is oriented slightly in a northeast-southwest direction and is located in the middle of the reef system. The island is located approximately 250m from the oceanward reefline and 350m from the lagoonward reefline. The reef system is exposed to wind generated waves during NE monsoons and long distance swell waves from the southeast Indian Ocean. In spite, of its size, Gan is a low lying island with an average height of +0.9m MSL. The oceanward coastline is long and low, exposing the island abnormal rises in sea level. Vegetation cover on the island is very high but large tracts of land have been cleared for agriculture and forestry. There are substantial variations in the topography of the island including a large wetland area, which plays a major role in the drainage system, especially during rainfall and ocean induced flooding events. 4 The proportion of Gan developed for human settlement is small. However, the impact of human settlement can be found throughout natural environment of the island. Parts of the natural environment have been modified to meet the development requirements of the settlement and the atoll population. Terrestrial modifications have been undertaken around the entire island for agricultural development, while coastal modifications have mainly been undertaken in three main points along the western shoreline which nonetheless have contributed to change coastal processes around Gan. Low areas within the island have been settled without proper levelling, leading to flooding in some of these areas. 5 2. Natural hazards This section provides the assessment of natural hazard exposure in Feydhoo Island. A severe event history is reconstructed and the main natural hazards are discussed in detail. The final two sections provide the hazard scenarios and hazard zone maps which are used by the other components of this study as a major input. 2.1 Historic events The island of Gan has been exposed to multiple hazards in the past although its exposure has been limited. A natural hazard event history was reconstructed for the island based on known historical events. As highlighted in methodology section, this was achieved using field interviews and historical records review. Table 2.1 below lists the known events and a summary of their impacts on the island. The historic hazardous events for Gan showed that the island faced the following multiple hazards: 1) flooding caused by heavy rainfall and 2) swell surges, 3) windstorms and 4) tsunami. Impacts and frequency of these events vary significantly. Flooding caused by rainfall is the most commonly occurring hazard events. Windstorms have also been reported as frequent especially during the southwest monsoon. Swell surges have been reported as infrequent and as having little impact. Table 2.1. Known historic hazard events of Gan Metrological Dates of the Impacts hazard recorded events Flooding caused Events commonly There are areas in the 3 settlements by Heavy rainfall occurring during SW (Thundi, Mathimaradhoo and Mukurimagu) monsoon. which are prone to rainfall flooding. All these settlements have wetland areas in close proximity to the settlement. As settlements expand to the low areas exposure to flooding becomes imminent. Impacts from these events are usually minor with damage to household goods and disruption to daily activities such as businesses and schools. Flooding caused • 1950’s (exact There was one major flooding event by swell surges date unknown) reported for Gan, which is dated back to • 5 July 1966 1950’s. Exact date is not known, but residents say there were reports of fish near the northern wetland area, which is located 400m inland. No substantial 6 damage to the settlements was reported. Windstorms • 11 July 1966 1 No major recent events have been • 5 May 1977 reported. • 12 May 1978 Written records show damage to • 28 Sept 1984 vegetation and crops. Little damage to property was reported. Droughts No major event have been reported Earthquake No major event have been reported Tsunami 26 th Dec 2004 At least 70% of the island was flooded during the tsunami of 2004. Flood heights were recorded at 2.0m (maximum). Flood heights and their distances in Mathimaradhoo are as following 2.0m – at a distance of 30m from shoreline 1.5m at a distance of 100m from shoreline 1.0m at a distance of 150m from shoreline less than 0.5m – at a distance between 300m and 600m from shoreline The primary reason for tsunami inundation may be found in the low ridge of the island and the presence of very low areas towards the centre of the island. 2.2 Major hazards Based on the historical records, meteorological records, field assessment and Risk Assessment Report of Maldives (UNDP, 2006) the following meteorological, oceanic and geological hazards have been identified for Viligilli. • Heavy rainfall (flooding) • Swell waves and wind waves • Windstorms • Tsunami • Earthquakes • Climate Change 1 All dates in italics are adopted from MANIKU, H. A. (1990) Changes in the Topography of Maldives, Male', Forum of Writers on Environment of Maldives. And news paper reports. 7 2.2.1 Swell Waves and Wind Waves Being located on the eastern rim of Laamu atoll, Gan is relatively protected from the year round swell waves approaching from a west to southerly direction. There are no specific wave studies undertaken for Gan, but studies undertaken around the country reports a predominantly southwest to a southerly direction for swell waves (Kench et. al (2006), Young (1999), DHI(1999) and Binnie Black & Veatch (2000)). A similar pattern could be expected for swell waves reaching Laamu Atoll. Laamu atoll is also one of the most closed atolls in Maldives with only 6 major reef passes, 5 of which are narrower than 600m. The widest channel (4km wide), is located in the southern rim facing a south easterly direction. Hence the probability of swell waves, approaching from the southwest, propagating through the atoll is very limited.