Purpose
The significance of small farm dams in regulating water and sediment flows to downstream water storage reservoirs is identified as an important issue in South Africa where water shortages are a major current and likely future problem. The role of farm dam breaching, subsequent release of stored sediment and re-connection of the upstream sediment production areas to the downstream channels is a neglected topic in geomorphology.
Materials and methods
We have mapped the location and estimated the volume of sediment stored in small farm dams in a sample area of ~100 km2 in the Sneeuberg uplands. Detailed studies at four cored dams have used 137Cs, 210Pb, mineral magnetism and sediment stratigraphy to date sediments, correlate time-synchronous levels in the deposits, estimate sediment yields to the dams and identify significant changes in sediment sources. Dam breaching episodes have been recorded and related to local rainfall. We use published data on the water and sediment capacity of a nearby large water storage reservoir to illustrate the potential threat of small dam breaching to the long-term sustainability of water resource provision.
Results and discussion
The high density of small dams in the study area (~1 dam km−2), and the fact that almost 50% are full of sediment, suggests a high potential for breaching. Breaching has already occurred at ~30% of small dams. Extrapolation of the dam density and stored sediment volumes to a nearby catchment of a water storage reservoir, the Nqweba dam, suggests that up to 72 million m3 of sediment in small farm dams could be released through breaching. The current rate of sediment input into the reservoir of ~1 million m3 year−1 will fill the remaining capacity by 2025. This does not take into account the effects on sediment yield of dam breaching, climate and land use change, or the re-connection of upper with lower catchments as a result of breaching.
Conclusions
Serious issues of water supply in South Africa need to address the potential for enhanced sedimentation rates in major reservoirs due to the breaching of small farm dams. The risk of breaching will be exacerbated by changing farm economies, neglect of dam maintenance and climate change, particularly the increase in rainfall intensity which has already been observed in the region.