Assessment of the influence of sand gradation on site suitability for construction of sand dam; a case study of Kitui south Sub-county, Kenya

Abstract

This study was undertaken in Kitui South in Kitui County in the south eastern parts of Kenya. The main objective of the study was to develop a spatial model that can be used for selection of suitable sites for sand dam in Kitui South. Data analysis was done using three approaches. The first approach involved the use of the Remote Sensing (RS) technique where data was preprocessed and analyzed using Erdas Imagine software and Geographical Information Systems (GIS) for spatial modelling. Sand samples were collected from seasonal rivers which included Mwila, Kakya, Wiitu, Nguni, Ngunyumu, Muvuko, Kanzilu, Masaa, Katiliku, Ngulungu, Nzeeu, Koma and Katitika. The results from the spatial model which integrated all factors shows that 16% of the studied sites were fairly suitable for construction of sand dams while 79% were classified as suitable and 5% are categorized as very suitable. Several sites in the central and the entire eastern parts of Mutha Ward were found not suitable for sand dam construction due to poor conditions such as the lack of suitable sand particles, lack of bedrock exposure on the riverbeds, flat terrain, very shallow stream banks, and weak soils on the riverbanks. Suitable sites for sand dams were found on the western and central areas of Kitui South where production and high accumulation of coarse sands while few excellent sites were observed in Ikanga ward and near Mutomo hills. Optimum accumulation of sands along the seasonal streams was identified at a slope ranging between 1.5 and 6%. Accumulation of different grades of sand especially the medium sands (r= 0.76) and coarse sands (r=0.75) were found to be strongly influenced by the changes in elevation along the stream. Out of 80 investigated sites, 59% of them were dominated with uniform sands (Cu<3), 30% were intermediate sands and only 11% were well-graded sands (Cu>5). The study also found that86% of the sites along the seasonal streams were dominated with medium sands while 10% were dominated with fine sands and the rest (4%) dominated by coarse and gravelly sands. From the results this study concludes that 59% of the 80 sites that were investigated in this study have a high potential for providing suitable sites for sand dams in Kitui South Sub-County.