Assessing the Impact of Drainage Trenches on Flood Extent in Zanzibar: A Geospatial Case Study of the Urban West Region

This study assesses the construction of trenches and their variations under flood scenarios in Zanzibar by using GIS technology. This cross-sectional study was conducted in five Shehias in Zanzibar, namely Chumbuni, Mtumwajeni, Mwanakwerekwe, Migombani, and Jan’gombe. In order to determine the consequences of floods and to visualize floods effect in each Shehia, geospatial data was collected. The first data set comprised geographic coordinates of sampled flood areas, acquired through field surveysusing the Global Positioning System (GPS) and participatory GIS (P-GIS). The second data set involved the map obtained from the Google Earth application. These datasets were utilised to find the area, buffer or perimeter of the flood area of each Shehia, and to determine changes in the flood before and after building the trenches. The findings of the study indicated that there is a decrease in the effect of floods and the area or perimeter after building the trenches. This observation is substantiated by the overall findings, which reveal that prior to the construction of the trenches, the flood-affected area measured 423,316.52 m². Following the construction works, this area was reduced to 108,759.85 m², demonstrating a decline of more than 70% in flood impact attributable to the trench intervention. Additionally, the study found that there are variations in floods prior to the trenches. Prior to the trenches' construction, flood effects were seen to be more severe than they were afterwards. The study suggests that the results should be used to emphasise the importance of existing policies for disaster events.