South Africas Van Ryneveldspas dam on the Sundays River at Graaf Reinett has a catchment area of 13,382 km2. By 2009 we estimate there will be around 49.1 million m3 of sediment in the reservoir with a remaining storage capacity of only about 29 million m3.
South Africa is heavily dependent on these storage reservoirs to maintain reliable water supplies at times of water stress but recent estimates suggest that the country already allocates over 98% of its available water resources.
We are currently working on the idea that small farm reservoirs constructed over the last ~100 years in these large catchments have dramatically reduced the amount of sediment reaching large storage reservoirs. A conservative estimate from mapping in the nearby Sneeuberg Mountains suggests that in a catchment of the size of the Sundays River there could be as many as 10,000 reservoirs with potential to store around 500 million m3 of sediment. Many of these dams are falling into disrepair and stored sediment is being evacuated through breaches in dam walls often caused by major floods. About 40% of these reservoirs are already full of sediment and about 28% are breached. Our collaborative research with the Universities of Aberdeen, Oxford and Sheffield in the UK and Cape Town and Rhodes in South Africa will evaluate the extent to which release of stored sediment is a major threat to future water resource provision.