Impact of Land Use Change on Shoreline Erosion and Mangrove Dynamics in Watamu Mida Creek, Kenya

Abstract

Watamu Mida creek coastal area is a major attraction site for tourists and also a source of income for the local people. The key identified anthropogenic pressures in the mangrove forest are; encroachment into the forest (from a growing number of hotels, cottages and private holiday houses) and, high dependence from the surrounding villages which has resulted in clearing and selective cutting. The shoreline is also equally affected by human induced change such as physical alteration of the beach through clearing the vegetation, development close to the High Water Mark, and the construction of sea walls. This research assessed the impact of land use change on mangrove dynamics and shoreline erosion as well as the main driving factors that cause these changes in Watamu Mida creek. The study had four objectives; (i) to assess the land use change and, mangrove dynamics (ii) measure the rate of shoreline change and define the drivers, (iii) determine the natural and human induced drivers of land use change, mangrove dynamics and shoreline change (iv) assess the role of relevant government and community level polices on land use and shoreline management. This study used old aerial photographs (1969 and 1989), current high resolution satellite images World view (2010) and ground truthing to analyze the patterns and dynamics of Mida creek mangrove forest changes, shoreline erosion rates and land use change over 41 years between 1969-2010. The data were generated for the mangrove cover by on screen visual digitizing and interpretation using the mosaic aerial photographs and satellite image. In order to assess the biomass of the mangrove, a non-destructive method was used to collect data on 25 sample plots and 934 trees were measured to estimate the above ground biomass and carbon stock of the forest. This study also investigated the trend of shoreline changes, and the factors attributed to these changes. The Digital Shoreline Analysis System (DSAS) in ArcGIS environment was used to create transects and statistical analyses for the shoreline. The 9.8 km long Watamu shoreline was divided into 245 transects with 40 meter spacing in order to calculate the change rates. To identify the primary driving forces of land use change, a multiple regression model was used. For the household questionnaires, a stratified random sampling method was used. The household survey included 60 respondents from different resource users groups and villages.Five Focused Group Discussions (FGDs) were conducted with representatives of the community and eleven Key informants interviews were conducted with the key leading government office representative’s, non-governmental organization, hoteliers, and old residents along the beach. The greatest land use change rate observed between 1969 and 1989 was in, miscellaneous coastal vegetation cover at 2.5%, while coastal bush experienced a significant negative change rate of -6.5%. The main land use changes observed between 1989 and 2010 were; increasing coastal bush, an expansion of town and urban areas, hotels, and private holiday houses. The results of the mangrove analysis showed a decline in mangrove cover (16%) between 1969 and 1989, while between 1989 and 2010 an increment in mangrove cover (9%) was observed. The total above ground biomass and carbon estimated was 296.14 ton·ha-1 and 148.07 ton·ha-1 respectively. Mida creek mangrove forest is largely dominated by the presence of; Rhizophora mucronata and Ceriop tagal and the regeneration of these two species is very high. The result of shoreline erosion from WLR indicated a mean of -0.89 m/year where 69.7% of transects fall under erosion and 30.3% accretion. Shoreline erosion was mainly attributed to anthropogenic factors. These include; construction near the High Water Mark, defensive structures and sea walls, and, destruction of vegetation along the beach front. The main drivers of land use change were human population growth and policy. The research found that the policy instrument review of the existing policy and legal framework indicated a number of gaps and opportunities for the protection of the coastal environment in the study area. The most unique aspect of this research was that it has analyzed a 41 year period of human and naturally induced changes in the study area through a combination of GIS/RS tools and community/key informant interviews, enabling a robust triangulation of the results to be made. As a result the research recommendations provided a firm foundation for improved County/multi stakeholder management in the Watamu Mida creek area. The integrated methodology developed as part of this paper offers other researchers a clear pathway for future comparative studies.