WATER SUPPLY & DEMAND
NEVER WASTE A GOOD DISASTER Over the past few weeks, many celebratory videos and photos have been shared of overflowing supply dams for the Nelson Mandela Bay Municipality (NMBM). Kirsten Kelly talks to Matthew Hills, an engineer at NMBM, about avoiding dry taps, the mitigation measures put in place from an engineering perspective, and – most importantly – the lessons learnt.
T
he local dams supplying water to the NMBM were last full in November 2015. While droughts are not new to the metro, NMBM has never experienced such a long, protracted drought like this one. There was a slight reprieve with good rains in September 2018 but from there, the dam levels continued to drop to levels below 10%. This put the metro on a knife’s edge with regard to balancing water demand and supply,” says Hills. While many parallels have been drawn between NMBM’s recent water crisis and that of Cape Town in 2017, the major differences are threefold. First, the duration of the hydrological drought was significantly longer (it’s been eight years since the dams lasts spilled). Second, NMBM has access to significant volumes of raw water from
Two barges were commissioned and deployed at the Impofu Dam, with a total combined maximum abstraction capacity of 60 Mℓ/day
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NOV E MB E R /DEC EM BER 2023
Matthew Hills, engineer at NMBM
a surface water catchment not affected by the local drought – the Gariep Dam on the Orange River. And lastly, both NMBM and consumers at large failed to collectively reduce their consumption as dramatically as Cape Town managed to do.
How close were dry taps?
“The lowest combined dam capacity of 9.98% was reached on 21 July 2021, with only 3.43% of that water available when accounting for dead storage. I do not think anyone in NMBM’s Water and Sanitation Department slept much during that period. It was frightening. It was unprecedented. We had roughly 36 days left until complete failure of two of our major supply dams. We reached the lowest recorded dam levels in our history. But municipal officials, engineers, scientists, community members, and businesses pulled together, and several projects that were implemented to push back ‘Day Zero’ began to deliver results. Fortunately, after the recent rainfall within the local
dam catchment areas, four of our five dams are now overflowing,” states Hills.
Water restrictions and consumption
Despite the good rainfall, water restrictions remain in place, as the Impofu Dam – crucial in the NMBM water reticulation system – is below 45% full. “We are being cautious because weather predictions provided by the South African Weather Service (SAWS) moving forward indicate that El Niño (ENSO) conditions are predicted to arrive in early summer. This will mean below-average rainfall and above-average temperatures during this period, resulting in high levels of evaporation. We need to build a reliable and dependable water reserve. It is also important to note that none of the municipality’s local catchments fall within NMBM boundaries, and rainfall is needed in those catchment areas,” says Hills. NMBM has been heavily criticised for its failure to dramatically reduce consumption during the recent drought period. Before the drought, NMBM was consuming on average 340 Mℓ/day. After the implementation of a holistic water conservation and water demand management strategy, which included 15 separate work streams, this consumption was brought down to as low as 260 Mℓ/day. However, likely due to drought fatigue kicking in and political instability, consumption increased, and the metro has battled to sustain consumption to below the 280 Mℓ/day mark.
