Investigations and design of an appropriate drainage system for the University of Nairobi's veterinary farm-Kanyariri.

Abstract

The study was carried out in the University of Nairobi's Veterinary Farm at Kanyariri. The area lies in the upper catchment of Mathare river four Kilometres north of upper Kabete campus. The farm is approximately 50 ha of which 6 ha are seasonally waterlogged. The area under study is a bottom land lying below a springline where the water table is within 50 ern for at least two months every wet season. The present study was carried out to establish the causes of the poor drainage in the bottom land, determine relative contribution of each causative factor and to give remedial measures to improve the drainage situation in the area. Drainage investigations which involved monitoring of ground water behaviour in the bottom land, water balance analysis to ascertain the sources of the excess water, soil and topographic studies to ascertain the causes of the drainage problem were carried out. It was found that upslope seepage into the bottom land and direct rainfall were the main sources of the excess water. Low hydraulic conductivity, infiltration rates and presence of depressions within the bottom land were the major contributors to the poor surface drainage whereas the virtually impermeable clay (glei) layer, over most of the profile, resulting to low intemal drainage was the cause of the poor subsurface drainage. Soil loosening accompanied by mole drainage was recommended as a solution owing to the small drain spacings (7.5 m) required due to the low hydraulic conductivity value of 0.06 m/day. Based on long term rainfall data as well as storm rainfall data from a neighbouring weather station, surface drainage system showing drain dimensions, grades and spacings were proposed. An interceptor drain was proposed to serve both as a cutoff and interceptor drain for subsurface flow. The designed system was thought to be adequate to control ground water rise and prevent waterlogging in the 50 em root zone. The drainage coefficient as determined from the rainfall analysis was 51.7 mmlday giving a drainage modulus of 6 l/s/ha for the surface drainage system.