BARELY two days after the Roads Authority (RA) announced the temporary closure of the Rosh Pinah-Aussenkehr road due to the flooding caused by heavy rainfall in South Africa, Namibians in the southern part of the country have been warned to prepare for floods.
The Water Affairs Department of South Africa has issued flood warnings for the downstream users on the lower Orange River.
According to Namibia’s Ministry of Agriculture spokesperson, Jona Musheko, the report from South Africa gave flood warnings as the dams in South Africa are already filled to capacity.
Musheko stated that there will not be much room to absorb more flow; hence Namibians should expect more spills and releases from upstream.
“The situation currently requires close monitoring and alertness from both authorities of Namibia and South Africa for eminent flooding impact to the downstream users on the lower Orange River usually affected by these type floods,” he said.
According to the South African Weather Service, rain probability is at 60% in the Upper Vaal and Middle Vaal River catchment, while rain probability at Lower Vaal and Middle Vaal River catchment is at 30%.
In addition, two gates are still opened at the Grootdraai dam due to continuing substantial inflow of about 375.3 m³/s into the dam.
“The 281.8 m³/s release from the Grootdraai Dam primarily contributed most of the inflow into Vaal dam,” it is said in the report.
The Vaal Dam level is at about 108.6% but decreasing.
The weather report indicated that both the Gariep and Vanderkloof Dams must be allowed to overflow freely over the spillway during floods.
Hence, the gates must be opened only under exceptional circumstances such as when the dams are facing imminent failure due to structural vulnerabilities or when forecasts show that dam levels will definitely rise to or beyond the non-overspill crest, the top of the wall.
Levels along the Riet River upstream of the confluence with the Modder River and downstream of the Kalkfontein dam are low. Hence the contribution of the Riet River to the Lower Vaal River is small, as well as that of the Harts River.
Flow levels along the main stem of the Lower Vaal are medium (871 m³/s at Schmidsdrift) and are increasing in response to increased outflow from the Bloemhof Dam.
Flows along the Caledon and Orange Rivers which constitute the inflow into the Gariep Dam have decreased to become significantly lower than outflow. Hence, the dam level is continuing to decrease.
The water levels along the entire Orange River downstream of the Vanderkloof Dam remain medium to high but have peaked and are currently decreasing at Katlani, Prieska, Upington, Neusberg and Blouputs. Levels are still rising further downstream at Sendelingsdrift.
Warnings targeted at downstream communities exposed or likely to be affected by river flow changes due to dam water releases for flood control and their likely impacts are issued by dam operators timely when such releases are scheduled.