Modeling sedimentation rates of Malilangwe reservoir in the south-eastern lowveld of Zimbabwe

Tatenda Dalu,Tamuka Nhiwatiwa,Lenin Dzibakwe Chari,Bruce Clegg,Edwin Munyaradzi Tambara

doi:10.1007/s13201-012-0067-9

Abstract

Modelling the sedimentation rates using the Wallingford (2004) equations with the aid of NDVI (remote sensing) to assess land degradation was carried out for Malilangwe reservoir catchment in the south eastern lowveld of Zimbabwe. Siltation life of the reservoir was determined from rate of incoming sediment, trap efficiency and reservoir capacity using the Wallingford method. The average rainfall of the study area was about 560 mm while runoff from the catchment ranged from 0.3 mm (minimum) to 199 mm (maximum) with an overall average runoff of 50.03 mm. Results showed that the overall mean annual sediment concentration was approximately 2,400 ppm. The reservoir capacity to inflow ratio was estimated at 0.8 with a sedimentation rate of 120.1 tkm -2 year -1 . Calculated probability of the dam filling is 26.8 %. Results also showed that the siltation life of the reservoir was (100 years according to the Wallingford method. The Normalised Difference Vegetation Index (NDVI) showed progressive decline (p 0.05) of the vegetation health from 2000 to 2009. While acknowledging the limitations of techniques used, this study demonstrates in part the effectiveness of sedimentation modelling and remote sensing as a tool for the production of baseline data for assessment and monitoring levels of land degradation in the Malilangwe reservoir catchment.

Highlights

In Zimbabwe, sediment load has exceeded normal design limits in many reservoirs, reducing storage capacity and shortening their useful life for human benefit
Geographical Information Systems (GIS-Normalised Difference Vegetation Index (NDVI)) images were used for monitoring vegetation changes in the Malilangwe catchment (MC)
The catchment showed progressive decline in NDVI values as shown by the decrease in cover especially along the Malilangwe Wildlife Reserve boundary meaning that vegetation was being cleared or greenness of plants was decreasing

Summary

Introduction

In Zimbabwe, sediment load has exceeded normal design limits in many reservoirs, reducing storage capacity and shortening their useful life for human benefit. According to van der Wall (1986) and Mambo and Archer (2007), Africa stands for rapid land degradation, declining fertility, soil erosion and drought. Sedimentation of reservoirs, in the light of man accelerated erosion, is according to the Zimbabwean Government a major time bomb (van der Wall 1986; Mambo and Archer 2007). Land degradation in Zimbabwe has been caused mainly by the decline of forest areas through cutting down of trees for agriculture and fuel (van der Wall 1986; Mambo and Archer 2007). The widespread impacts of deforestation are reflected at a national and regional level through vastly elevated soil erosion rates, sedimentation of major waterways and an increased frequency and severity of floods (Adger 1992; Ewers 2006).

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