Application of geospatial techniques in sites selection for community small earth dams in Kajiado County, Kenya

Abstract

The study was carried out along Ol Kejuado River basin, which is an ephemeral river that traverses the middle part of Kajiado County in Kenya. There are no rainwater harvesting structures along the reiver’s course. The study applied geospatial techniques to select sites for community small earth dams. The objectives of the study were to identify suitable criteria for the site location and to apply them to map potential sites. Eight variables which included stream order, slope, precipitation, Net Difference Vegetative Index (NDVI), Topographic Wetness Index (TWI), soil texture, road network and market centres were identified for the site selection analysis. From the analysis, 15 potential sites were generated but four of the sixteen communities identified which included Maparasha in the middle part of the basin, and Emotoroki, Lolakel and Lengisim on the lowest part of the basin did not get any suitable site. Four of the potential sites were further evaluated to establish if they met some important dam design characteristics. Their selection was based on their central position to support maximal sharing by communities. The dam characteristics evaluated included size category, site suitability based on area-volume-dam elevation relationship, dam reliability based on precipitation-runoff ratio and challenges of dam siltation. The community water demand which was projected for a 20 years’ design period fell above the initially proposed capacity for small earth dams. The annual water demand volume of 201,185m 3 placed the dams in the category of Class B (Middle Hazard Dams) where a dam failure would result in damage to isolated infrastructure and interruption of important utilities downstream. The dam sites were subjected to reservoir Area-Volume-Elevation (AVE) analysis where the four dam sites had capacities ranging from 1.1 to 5.5 Mm3 (Million cubic metres) at between 7metres and 9metres depths. Results of analysis of precipitation to runoff ratio of the basin between the years 2011 and 2020 indicated that 88.6% of the precipitation was converted to runoff and were sufficient to fill the reservoirs. It was established that soil erosion was high (11.75 to 34.29 tons/Ha) which could compromise the life of reservoir through siltation. The application of geospatial techniques were found to be faster and more economical than the traditionally applied methods. They were also found to be important in aiding decision-making towards site selection for community earth dams