Environmental impact assessment for coastal protection and erosion control on west of runway 18, Malé International Airport, Hulhulé, Kaafu Atoll, Maldives

Abstract

This report addresses the environmental concerns of the proposed quaywall strengthening and erosion control of western coast of Runway 18, Malé International Airport, Hulhulé, Kaafu Atoll. The primary objective of the project is to enhance the safety of the airstrip at the turning pad area, which is prone to flooding during rough weather due to wave overtopping from existing sheetpile structure. As a result there is scour behind the sheetpile wall increasing the threat of subsidence or structural weakening of the western edge of the turning pad area. Therefore, there is an immediate need to protect this area by strengthening existing sheetpile quaywall with additional protection measures. Hence, different options for the coastal protection have been evaluated and the most practicable options not entailing excessive costs have been recommended. Since this is an immediate and temporary measure, minimal protection has been proposed, which is the protection of the immediate area behind the turning pad where sheetpile structure has been placed. The length of protection is about 100m. The other areas with concrete gravity seawalls have not been considered for protection as these areas are not severely affected and the entire area, including the 100m coastline which is proposed for immediate protection, would be reclaimed in the near future, as per the Airport Master Plan. The different options for the protection of the proposed 100m coastline immediately behind the turning pad area include revetments along the coastline or offshore breakwaters covering a length of about 250m on the dead reef flat at about 100m from the sheetpile coastline. Two types of materials have been considered for the revetments: single layer Core-Loc armour units and 2.5m3 geotextile containers filled with sand. The Core-Loc units have very high permeability (50-60% voids) to absorb wave energy while the geotextile containers have no voids when placed together. Therefore, the design using geotextile containers have been revised to incorporate about 15% voids, thereby improving the wave energy absorption capacity. Based on the costs and wave energy absorption potential of each type of material, it is recommended to use the Core-Loc units. However, these units are not readily available in the Maldives, therefore, geotextile containers have been recommended due to the urgency of the project. In both cases, the revetments have been designed to provide adequate wave runup to minimize the force of the wave. Additional protection would not be required. For the breakwater also, the above two types of materials have been considered and submerged breakwaters as well as emerged breakwaters have been considered. The breakwater option is expensive given the length of the breakwater. Also, the distance between the potential breakwater location and the shore area to be protected is about 100m, which makes it less effective as wind-generated waves will reoccur inside the lee of the breakwater. Other options such as groynes have not been considered because the lagoon on this side has been dredged. Imported rock boulders could also be used for both the revetment option and the breakwater option. However, the cost of boulders and the time to deliver to site makes it impracticable and has not been considered. Moreover, a structure with rock boulders would be less permeable (about 30-40%) than the proposed Core-Loc armour units. Hence, cost was the deciding factor in choosing the Core-Loc units and geotextile containers over rock boulders. The reef flat at the area in which the proposed offshore breakwater would be constructed is almost entirely dead with high levels of sediment resuspended in the water column. This is due to the increased sediment level from dredging that had taken place in the area. There is also the cumulative effect of sediment resuspension resulting from the Hulhumalé reclamation, but this is expected to be small. The reef slope and edge in this area would not be severely affected as no machinery would be used in or closer to these areas during the implementation of the proposed project activities. All machinery would be used on the shallow reef flat area, which consists of bedrock and a few dead coral porites. Given the level of dredging, reclamation and coastal protection works that have been carried out in the area, the cumulative impacts of the proposed project would be negligible. Therefore, this project is not considered to have adverse environmental or social impacts but there would be economic impacts that would be positive in that the dangers of potential subsidence of the airstrip turning pad is minimized with greater flood control and protection afforded by the proposed temporary coastal protection structures. However, mitigation measures to minimize any damage to reef will be in place and the project components will be well planned considering minimal aesthetic impact although this area is not aesthetically sensitive. Environmental monitoring is not recommended for the project under consideration except monitoring of the effectiveness of the structures for at least one year after construction. However, it is recommended that the Maldives Airports Company in association with other operators in Hulhulé initiate and conduct an island-wide environmental monitoring programme which would cover the environmental monitoring needs of all projects undertaken on the island.