A remote sensing and GIS based study in assessment of the degradation risk of the Kolonnawa marsh

A majority of the Sri Lankan wetlands is under threat of degradation mainly due to urbanization, population pressure and industrialization. This study was conducted to detect the changes and to assess and map the degradation risk of the Kolonnawa marsh located in the Western Province of Sri Lanka by analyzing the data collected through field surveys, remote sensing and GIS techniques. A questionnaire survey was conducted within 11 Grama Niladhari (GN) Divisions around the marsh to obtain information on household characteristics, interactions with the marsh, awareness on marsh degradation and marsh conservation. According to the temporal change assessment performed with Landsat TM (1992), Landsat ETM+ (2002) satellite images, 32 % of the marsh area has been reduced in 1992 - 2002 period. According to Worldview 2 image, the total marsh area including the associated waterways in 2010 was 144.56 ha. A risk index was formulated considering the risk factors prevailing in the study area. Risk mapping, which was carried out based on the risk index resulted in three risk classes; moderate, high and very high. Mapping based on the risk index was helpful to identify the severity of the degradation risk of the marsh at GN Division level. The study identified that all surrounding GN Divisions pose risk on degradation of the marsh while the highest impact was reported from the Obesekarapura GN Division. The risk matrix was identified as an important tool in prioritizing the areas for conservation.