Sediment Yield and Bank Erosion Assessment of Pra River Basin

Abstract

The Pra River Basin has been engulfed by certain anthropogenic activities particularly illegal small scale mining (popularly called galamsey) and serious concerns have been raised by stakeholders within the basin of the level of pollution due to the release of chemicals and sediments into the water bodies. Fluvial sediment yield data is an essential requirement for informed decision making on water resources development and management. However, information on the sediment load of most rivers is very rare due to the lack of financial resources to regularly undertake sediment yield studies. This study was undertaken to assess the sediment yield levels, sediment sources and bank erosion within the Pra Basin through field data collection and spatial modelling to ascertain stakeholder’s perceptions and suggest remedial measures to the problem. Suspended sediment concentration measurements were undertaken for 9 months in selected stream discharge measuring stations within the basin. Daily mean suspended sediment concentration was determined from which monthly and annual suspended sediment yields were derived. Sediment source tracking was done using a single tracer 210 Pb and the relative contribution of surface and bank sediments to the fluvial sediment transport was determined using the simple mixing model. Lead-210 was analysed using the Atomic Absorption Spectrophotometer (AAS). Bank erosion was assessed using erosion pins. The spatial patterns in soil erosion and sediment yield were modelled using the revised universal soil loss (RUSLE) equation integrating it into Geographic Information System (GIS). Suspended sediment concentration and sediment yield of the Pra Basin were found to be very high resulting in a high annual specific suspended sediment yield. Bank erosion measurement revealed very active bank erosion and deposition within the river channel and bank erosion was observed to increase downstream. Sediment source analyses showed that bank material was the dominant sediments which accounted for over 60% of suspended sediment loads. However, predicted sediment yields using the RUSLE were very low as compared to observed data. To promote coordinated development and sustainable management of the resources of the basin, there is the need to resource agencies in charge of regulating natural resource utilization in the basin to control land use activities particularly galamsey to ensure the sustainability of vital ecosystems. The Government also needs to resource financially and improve upon staff strength of the Hydrological Services Departments and the Sediment Unit of the Water Research Institute of CSIR to enable them maintain and monitor critical stations for flow and sediment discharge measurements. Also future research works in sediment yield modelling should consider deploying a model that is capable of modelling both surface and concentrated sediment discharges as this will give a better perspective to a comparative assessment between observed and simulated sediment yield within the Pra Basin.