Using SAR Images in Modelling River Hydrodynamic Processes in the Mara Wetlands
Mtamba, Joseph¹; van der Velde, Rogier²; Vekerdy, Zoltán²; Mtalo, Felix W.^3
1University of Dar es Salaam and University of Twente, TANZANIA, UNITED REPUBLIC OF; ²Faculty of Geo-Information and Earth Observation, University of Twente, NETHERLANDS; ³University of Dar es Salaam, TANZANIA, UNITED REPUBLIC OF

One of the important problems associated with flood monitoring is a flood extent extraction from satellite imagery, since it is impractical to acquire the flood area through field observations. Instantaneous synthetic aperture radar (SAR) derived flood extent map helps retrieving the distributed conveyance parameters in one-dimensional flood routing models. These models are generally calibrated on the sole use of ground data. In our research, we use earth observation (EO) data in order to determine relevant flooding pattern in the Mara wetlands, providing an alternative model calibration technique.

The Mara wetlands in Mara Basin are the floodplains of the River Mara, a few kilometres upstream of Lake Victoria. They accommodate a multitude of ecosystems with a large diversity in fauna and flora. They provide a variety of environmental services to the traditional livelihood of the local communities but are also the basis of a tourism industry that generates substantial revenue. For the global community, the wetlands sustain a tremendous pool of biodiversity which include endangered fish species. In the upstream river catchment, there are a lot of economic activities that have resulted in significant land use changes leading to changes in the hydrologic regime of the Mara River and its water quality. It is anticipated that ecological status of Lake Victoria and the Mara wetlands will also change. To predict the impacts on the wetland, the hydraulic processes of the Mara River including its floodplains have to be assessed at different temporal scales and the governing factors have to be identified.

Spatial structures of the wetland vegetation is studied with optical EO images and textural analysis of both optical and SAR images using the Grey Level Co-occurance Matrix (GLCM) technique. This information is used in the parametrization of the flood resistance in a spatially distributed manner.

Instantaneous flood boundaries derived from ERS, RADARSAT and Envisat radar scenes are used to delineate flood areas. By combining EO data with ground based data in the calibration procedure, the EO data provides more reliable flood extension predictions. Based on time series of SAR images, the temporal inundation pattern of the Mara Wetland is quantified statistically for each pixel of the inundated areas. The model is calibrated against the spatial distribution of inundation frequencies, as well as some selected flood extent maps.