Detailed Island Risk Assessment in Maldives, G. Dh. Thinadhoo
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Detailed Island Risk Assessment in Maldives Volume III: Detailed Island Reports G. Dh. Thinadhoo – Part 1 DIRAM team Disaster Risk Management Programme UNDP Maldives 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 Thinadhoo Island is located on the western rim of Gaafu Dhaalu atoll, at approximately 72° 59' 50"E and 0° 31' 49" N, about 410 km from the nations capital Male’ and 4.5 km from the nearest airport, Gaafu Dhaalu Kadedhdhoo (Figure 1.1). It is one of the few inhabited islands facing the western Indian Ocean and exposed to the southwest monsoon related wave action. The island forms part of the natural atoll called Huvadhoo Atoll, which is considered the second largest atoll in the world. Thinadhoo is the atoll capital amongst 10 other inhabited islands. It’s nearest inhabited islands are Madaveli (7.5 km) and Hoadedhdhoo (9km). Huvadhoo atoll is the nearest atoll in Maldives to the equator and sits along the southern half of the laccadive-chargos ridge, exposing the entire atoll to direct wave action from Indian Ocean. However, it location in the heart of the doldrums makes the island relatively safe from major climatic hazard events. North Huvadhu Atoll (Gaafu Alifu Atoll) Thinadhoo Kaadedhdhoo (Airport) Madaveli Hoadedhdhoo South Huvadhu Atoll (Gaafu Dhaalu Atoll) Kanduhulhudhoo Nadella Gadhdhoo Gan Rathafandhoo Fiyoaree Vaadhoo N Fares Maathodaa Location Map of Thinadhoo 0 5 10 kilometers Figure 1.1 Location map of Thinadhoo. 3 1.2 Physical environment Thinadhoo Island is far from its natural environment due substantial human modifications including land reclamation, dredging activities and coastal infrastructure development projects. At present the island is almost rectangle shaped with a width ranging from 745m to 900m and a ranging from 1590m to 1351m. The total surface area of the island at present is 115.5 Ha (1.16 km 2). The island is oriented in a north-south direction. The original island had a land area of approximately 39 ha (0.39 km 2) and had a wetland area covering 16 ha (0.16 km 2). The land reclamation process which started in the 1980’s reclaimed the entire wetland area and parts of the reef flat. Approximately 71ha or 61% of the present island is reclaimed and the island of Thinadhoomaahutta or Maahutta has also been joined to form the present Thinadhoo. The reef of Thinadhoo is fairly large with a surface area of 1150 Ha (11 km 2) extending to about 7.5km in a northeast-southwest orientation. Thinadhoo is now the only island within the reef system following the joining of the only inhabited island (Thinadhoomaahutta). Thinadhoo is located on the southern tip of reef system, next to a major reef entrance (Thinadhoo Kandu). The depth of the reef flat is quite shallow averaging less than -1m MSL. The distance between the island shoreline and oceanward reef rim varies from 83m in the southwest corner to 200m in the west. The average distance to reef edge is approximately 170 m. Thinadhoo is a highly urbanised settlement with over 8000 inhabitants. It is considered the main urban centre in Huvadhoo Atoll and amongst the largest population centres in the Maldives. The high level of urbanization also meant that the natural environment of the island is highly modified to meet the development requirements of the settlement. Land reclamation activities were undertaken to relieve the shortage in land for housing. The land reclamation activities have resulted in the modification of the entire coastline, while the vegetation is sparse and almost absent in the newly reclaimed areas. There are major variations in topography caused by the reclamation activities, which has resulted in drainage issues and flooding during heavy rainfall. The newly reclaimed areas do not have a coastal vegetation belt increasing the risk of erosion and increased impacts from ocean induced flooding events. Environmental issues associated with urbanisation are being experienced by its inhabitants including, ground water contamination, improper waste disposal, degradation of coastal areas, depletion of vegetation and coastal erosion. 4 While Thinadhoo has a low incidence of historical natural disasters, the present environmental characteristics in the island have a number of weaknesses which may expose the island to future hazards. 5 2. Natural hazards This section provides the assessment of natural hazard exposure in G.Dh Thinadhoo Island. A severe event history is reconstructed and the main natural hazards are discussed in detail. The final two sections provide the hazard scenarios and hazard zone maps which are used by the rest of this study as a major input. 2.1 Historic events The island of Thinadhoo has been exposed to multiple hazards in the past. A natural hazard event history was reconstructed for the island based on known historical events. As highlighted in methodology section, this was achieved using field interviews and historical records review. Table 2.1 lists the known events and a summary of their impacts on the island. The historic records showed that the Thinadhoo faced the following multiple hazards: 1) flooding caused by heavy rainfall and 2) swell surges, 3) windstorms and 4) tsunami. Impacts and frequency of these events vary significantly. Flooding caused by rainfall and swell surges are the most commonly occurring hazard events, which however, can only traced back 20-30 years. Windstorms have also been reported as frequent especially during the southwest monsoon. Since the elderly in the island cannot recall events beyond 1978, it is highly plausible that severe events came to the attention of inhabitants only with the rapid expansion of settlement especially towards the hazard prone western coastline of the island and following the land reclamation activities. Table 2.1 Known historic hazard events of Thinadhoo. Metrological Dates of the Impacts hazard recorded events Flooding • 13 July 1983 1 This is an island that frequently experiences caused by • 8th Sep 2001 heavy rainfall. According to the island Heavy rainfall • 9-10 th July 2002 2 chiefs’ heavy rainfall is most common during • 12 December mid to late South West (SW) monsoon. 2002 These events are reported to cause heavy • 14 th Nov 2003 flooding of the reclaimed marsh area on the • 6th Dec 2003 southern end of the island and the northern • 26 th -27 th Nov half of the natural island. Even a few hours 2006 of continuous heavy rainfall causes the houses and the roads in these areas to flood 1 All dates in italics are adopted from MANIKU, H. A. (1990) Changes in the Topography of Maldives, Male', Forum of Writers on Environment of Maldives. 2 Heaviest rainfall for a 24 hour period recorded in the country 6 up to a height of 0.3m – 0.6m. It has been reported that the flood waters sometimes have lasted more than a day. The major impacts are: - Blocked sewerage networks within the flooded zones for up to 3-5 days. - Severe damages to the backyard crops such as bananas, chillies etc. - Damaged personal property such as children’s text books and household goods. - Disrupted daily life including economic activities, school functioning and transportation. Flooding • 7 May 1978 3 The island is reported to experience caused by • 7 April 1984 frequent (once every few years) flooding swell surges • 3 June 1987 caused by wave surges and sometimes • 10 Sept 1987 large swell waves generated far offshore • June – July 1991 from the costs of the Maldives. Events are • June – July 2003 also reported to occur during mid SW • 5th May 2004 monsoon. Surge waters often reaches up to 300m inland along much of the length of • June – July 2005 western shoreline. These surge waters have • 15-19 May 2007 flooded the impact zone up to a height of 0.5m. The major impact of these events is damages to the backyard crops within the impact zone. Windstorms 13 July 1983 A number of windstorm events were reported by elders; however, no dates could be determined. Droughts No major event have been reported Earthquake No major event have been reported Fire 1848 There was one major event which is known to have caused widespread damage to Thinadhoo. It is not known whether the event was caused by a natural event or human activity. Tsunami 26 th Dec 2004 There has bee n only one known event. This event flooded the harbour area, the coastal roads and some houses near the harbour to a height of 0.3m. The tsunami however did not flood the island with any 3 All dates in italics are adopted from MANIKU, H. A. (1990) Changes in the Topography of Maldives, Male', Forum of Writers on Environment of Maldives. 7 significant force and therefore the impacts of the flooding have been very minimal. 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 Thinadhoo. • Swell waves and wind waves • Heavy rainfall (flooding) • Windstorms • Tsunami • Earthquakes • Climate Change 2.2.1 Swell Waves and Wind Waves Origins and Occurrence of waves in Thinadhoo 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.