CLEVER engineering to protect the integrity of the Neckertal Dam’s wall and its foundations is behind a wet spot that was observed on the dry side of it during the weekend.
An earthquake that registered 3.8 on the Richter scale occurred ten kilometres south-west of Ai-Ais in the Fish River Canyon on Thursday sparked speculation that the dam might have sprung a leak.
The Geological Survey of Namibia confirmed the tremor and also that three smaller tremors occurred. The smaller tremors did not register on the scale.
The curiosity of many Namibians was piqued when people noticed the wet spot on the dry side of the dam wall.
NamWater, however, belayed any fears of a possible leak by stating that the wet spot on the dry side of the dam wall is caused by a system built into the huge concrete structure to protect the integrity of the dam wall.
To protect the dam wall from becoming unstable on its foundations, several holes have been drilled into the earth below the foundation of the structure.
Water seeping underground is pushed through these holes and pressure forces it up and into two drainage galleries or tunnels that run along the entire length of the dam wall.
The seepage water is collected in sumps and when these fill up the water runs out on the dry side of the wall through special holes.
Engineers had an opportunity to test the foundation draining system when the dam received some inflow while the wall was still under construction. At the time construction machinery was evacuated from the structure and excess water had to be let out through special floodgates to keep the water from spilling over the wall.
The wet spot on the dry side of the wall was caused by water flowing from beneath the earth at a rate of 50 litres per minute.
The Von Bach Dam has a similar system and seeping water flows through its system and out of the dry side of the dam wall at a rate of up to 50 litres per second. The rate of flow is determined by how full the dam is and the pressure exerted by the mass of water at the bottom of the dam.
The more than 750 billion litres of water currently collected behind the dam wall will cause a lot of seepage underground which if allowed to go unchecked could undermine the structural integrity of the wall which will cause it to collapse.
By Sunday evening the water level in the Neckertal Dam surpassed the 90% mark after water that was released from the Hardap Dam since Tuesday reached it on Saturday evening.
The floodgates at the Hardap Dam were finally closed on Sunday morning when the water level in the dam reached 69.9%.
Although the rain in southern Namibia has given residents a breather for the past two days, weather prophets indicated that moderate to heavy rains might return to the catchment areas of the dams later this week.
Rainy weather is, however, predicted to continue over the central, central northern and eastern parts of Namibia tomorrow.
The Namibia Meteorological Service warned that flash floods may occur in the areas of Tsumkwe and Grootfontein.