Colombo Flood Detention Areas

Location: 70 0’ 0 N and 790 50’ 15 E to 70 0’ 0 N and 800 0’ 0 E; 773909 N and 371132 E to 773866 N and 389543 E; in the Colombo District, of the Western Province.

Area: Approximately 400 ha (Kolonnawa marsh = 214.3 ha; Kotte marsh = 97.4 ha; Heen marsh = 87.7 ha)

Altitude: Below mean sea level to 1.0 m above sea level

Overview: The Colombo marshes constitute three interconnected marshes that form a unique environment in the Greater Colombo area. The marshes are extremely important for flood detention within the Colombo City. They are a part of a large marsh system connected to the Attidiya and Bellanwila Sanctuary and also provide refuge for wildlife within the urban areas of Colombo. However, due to their urban location, the marshes are under threat from pollution and land filling.

Physical features: The area belongs to the Southwestern Precambrian group and probably has a geological history similar to that of the Negombo estuary and the Bellanwila-Attidiya marsh which are also located within the greater Colombo region. The major soil groups found in the area are Red-Yellow Podzolic Plinthite Soil, Half Bog Soil and Alluvial Soils. The marshes lie within the wet zone and receive a mean annual rainfall of 2,000mm. The average annual temperature ranges between 250 C and 270 C.

Ecological features: The marshes are surrounded by urban landscapes, degraded natural habitats, and managed habitats such as paddy fields and home gardens. Natural areas contain extensive reed beds and Annona glabra dominated scrubland habitats.

Noteworthy fauna: The marshes harbour several species of endemic fish, including Esomus thermoicos, Clarias brachysoma, Aplocheilus dayi, Channa orientalis and Puntius singhala. Among the amphibians, Polypedates cruciger and Rana gracilis are endemic to Sri Lanka. Aquatic reptiles include freshwater turtles (Melanochelis trijuga, Lissemys punctata) endemic serpents (i.e. Xenocrophis asperrimus) and the crocodile (Crocodylus palustris). Common water birds include Pelecanus philippensis, Dendrocygna javanica, Alcedo atthis, Amaurornis phoenicurus, Phalacrocorax niger, Mesophoyx intermedia, Egretta garzetta and Ardeola grayii. Rare mammals such as Prionailurus viverrinus and Lutra lutra also inhabit these marshes, while troops of the endemic Trachypithecus vetulus can be found in the bordering trees.

Noteworthy flora: Floating Nymphaea spp. occur in a patchy manner. Patches of Syzygium spp. and Pandanus spp. are also scattered around the wetlands. Several species of grasses (i.e. Cynodon dactylon) and sedges (i.e. Fimbristylis spp., Eleocharis spp.) are also present.

Land use: The wetlands are not used extensively for human activities although some small-scale fisheries, agriculture and animal husbandry are carried out. The wetlands are also used for recreational purposes. Additionally, large areas of the wetlands are used as land fills for development and dumping of waste.

Possible changes in land use: Increased land reclamation for urbanisation will reduce the importance of the wetlands for other uses.

Hydrological and biophysical values: The marshes are extremely important for flood detention during heavy rains, and are a vital part of the drainage system of the greater Colombo area.

Social and cultural values: The proximity of the wetlands to Colombo City provides easy access for bird watching, conservation education and recreational activities. The present parliament of Sri Lanka is also located within the wetland system. The Diyawanna Oya and surrounding marshes were important natural defensive barriers for the ancient capital city of Kotte and are therefore of historical importance, and the area has been described in ancient poetry.

Scientific research and monitoring: The wetlands have been studied by a number of universities and have been the focus of numerous student research projects.

Conservation education: Educational programmes, especially for school children have been conducted by a number of state organisations, NGO’s and volunteer organizations.

Recreation and tourism: Some areas of the wetlands are popular with families as recreational areas. A large country club and a golf course are also located nearby.

Conservation measures taken: In 2006 the CEA in collaboration with the MoE commenced a programme to conserve and manage the wetland system including canals and surrounding areas. The marshes have been included in the Greater Colombo Flood Control and Environment Improvement Project being executed by the SLLRDC. The CEA has also published a wetland site report and conservation management plan for the Colombo Flood Detention Areas in 1995.

Conservation measures proposed: Detailed conservation measures to conserve critical wetland habitats and manage water quality have been outlined in the CEA conservation management plan.

Disturbance and threats: The major threat to the wetland system is land reclamation activities and landfills for large-scale housing schemes due to the increasing urban population and resulting demand for land. Clearing of vegetation and undergrowth in peripheral areas is also a problem, while the spread of invasive alien plant species such as Lantana camara, Annona glabra, Eichhornia crassipes, Salvinia molesta and Typha angustifolia is a major threat to native plant species. The spread of carnivorous invasive alien fish such as Chitala ornata, and Poecilia reticulata has affected the populations of endemic fish species. Haphazard dumping of solid waste and sedimentation have also led to pollution of the waterways, reduced water exchange and the flood detention capacity. Need urgent actions to mitigate existing threats.

Land tenure: The four flood detention areas, Kolonnawa North Marsh, Kolonnawa East Marsh, Kotte Marsh and Heen Marsh are all state owned.

Management authority and jurisdication: Sri Jayawardenapura Kotte DS, Kolonnawa DS, and the SLLRDC.

References: CEA/ARCADIS Euroconsult (1995a); Perera and Perera (2006)