Scientists have warned that some of Durban's main tourist beaches are in danger of being washed away gradually because of rampant river sand-mining operations that supply the construction industry.
Researchers at the Council for Scientific and Industrial Research (CSIR) say the problem is so serious that they recommend all sand-mining operations in Durban's rivers and estuaries should be banned "as soon as practicable" to avoid future beach erosion on the same scale as the March 2007 storm which devastated large sections of the coastline.
They have also called for a detailed cost-benefit study to measure the monetary benefits of sand-mining compared to potential future losses for the Durban tourism industry, property damage, flood control, fishing and erosion control and other issues.
They found that beaches are being starved of nearly 70 percent of the natural yield of river sand because more than one third of this sand is now trapped behind 12 major dams, while another third is being mined commercially for the construction industry at 31 sand mining ventures in 18 Durban rivers.
This meant beaches were receiving only about 30 percent of the sand they used to get before dams were built and rivers were mined - suggesting that Durban's beaches are now being eroded by the sea at a rate too fast to be replenished by coarse river sand washing down from the mountains.
The main author of the report, CSIR coastal engineer Andre Theron, stressed that the impact of coastal erosion would be felt over the next few decades rather than immediately, but there was already evidence of rapid erosion on some parts of the coast linked to sand-mining and dam construction.
For example, data showed that some of the Bluff's beaches had been eroding at a rate of more than a metre a year, suggesting that some beaches had narrowed by more than 40m since the 1970s.
However, Theron acknowledged that his proposal to ban the mining of river sand would be impractical in the short-term, and would also have far-reaching social and political implications unless alternative construction sand could be found.
The CSIR suggested that all future dams should be built according to new designs to reduce the volume of sand and silt trapped behind water storage dams.
In a report prepared at the request of the eThekwini Municipality, the CSIR warned that the present rate of sand mining from Durban rivers and dam-related sedimentation could lead to a mean coastal erosion rate of roughly a metre a year.
Combined with the effects of climate change and sea level rise, this was likely to have "severe consequences in terms of coastal erosion, initially similar to and eventually exceeding the erosion suffered in KwaZulu-Natal during 2007".
"Based on our assessments our strong recommendation would be to ban river sand mining from eThekwini rivers as soon as practicable, while urgently seeking and evaluating other sources of sand."
Theron suggested that one option was to dredge sand from beneath the surface of Durban's biggest dams.
Another possibility was to set up a major sand recovery plant in Durban harbour to make use of the dredger sand presently dumped at sea to reduce harbour sedimentation.
But according to eThekwini Municipality coastal policy project executive Andrew Mather, there had already been a dramatic decrease in the volume of coarse river sand entering Durban harbour since the 1970s.
"We kept getting stories from the harbour authorities that there was not enough sand for them to dredge to replenish the central beaches, and we didn't believe them at first - thinking that perhaps they were trying to cut costs. But when we looked at the soundings we could see that there was much less sand to dredge."
Theron said a third option was to investigate using land-based sources of Berea red sand for building fill material or low-strength concrete.
A fourth option was to ban sand-mining downstream of existing dams and concentrate on mining sand from rivers immediately upstream of the dams.
And when new dams were built, they should be designed or operated in a way which reduced the volume of trapped sand.
Theron's report was commissioned by eThekwini because of growing concerns by municipal officials over the plethora of sand mining permits issued within the city boundaries in recent years.
However, eThekwini does not have legal authority to ban sand-mining or to curtail the number of mining permits issued by the national Department of Minerals and Energy, and sources suggest that concerns from municipal officials are frequently ignored because of the huge demand for sand by the building industry.
City manager Michael Sutcliffe said the CSIR report had been discussed at senior management level, "and we are taking if very seriously".
"We think that a lot of these sand-mining permits have been issued without full consideration of the big picture and this issue needs to be looked at the national level by the mining department.
"Clearly this is a long-term issue and you are not going to end up with no beaches overnight in Durban - but there could be a tipping point from which it becomes very difficult to recover."