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.

The 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 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.

Dams supplying NMBM

(barges) on which pumps and motors were placed to gain access to the dead storage volumes.

Two barges were commissioned and deployed at the Impofu Dam, with a total combined maximum abstraction capacity of 60 M ℓ /day. These barges supplied raw water to the Elandsjagt WTW by means of pumping the raw water into the intake tower.

At times, abstraction had to be halted when the water level underneath the barge pumps was too low. This was done to prevent damage to the barge pumps that could result from sucking up the riverbed. Occasionally, the barges could be moved further downstream from the intake tower to a deeper level where they could resume operations. At one point, a 6 km 800 mm diameter HDPE pipeline was installed to gain access to the deepest part of the dam next to the Impofu Dam wall.

Maximisation of the Nooitgedagt scheme

This work stream included Phase 3 of the Nooitgedagt/Coega Low Level Supply Scheme (NCLLS) to increase the supply of treated water (sourced from the Gariep Dam that was not affected by drought) to NMBM to a total of 210 Mℓ/day.

“Fortunately, on an engineering level, NMBM introduced several successful interventions to augment water supply and push back dry taps. The supply initiatives can be grouped into three major categories: accessing the dead storage in our local dams, maximisation of the Nooitgedagt scheme, and groundwater augmentation,” adds Hills.

Accessing dead storage in local dams

Simply put, dead storage is the volume of water that cannot be abstracted from a dam for treatment; it is typically the water below the intake tower. The intake tower captures water from the dam and conveys it via a pipeline to a water treatment works (WTW). Since most of the dam levels were already near to or below the intake towers, NMBM was struggling to deliver water to the WTWs. To remedy this, the metro built floating platforms

The eastern part of NMBM receives water from the Gariep Dam via a complex system of rivers, canals, and dams over approximately 630 km. These interbasin schemes include the Orange/Fish Transfer Scheme and Lower Sundays River Water Scheme. Located in the Free State province, the Gariep Dam has a capacity of 5 340 000 million litres. All NMBM dams together can fit into the Gariep nearly 20 times over.

Phase 3 comprised a complete standalone 70 Mℓ/day treatment module at Nooitgedagt WTW, a 45 Mℓ balancing reservoir at Olifantskop, installation of cathodic protection systems on both the original Nooitgedagt to Motherwell high-level pipeline, and the low-level pipeline built under Phase 1.

While Phase 3 was under construction, the raw water licensed volume was available but could not be utilised, so NMBM constructed a temporary WTW by modifying the Grassridge reservoir, thereby converting it into the Grassridge WTW. Raw water from the Gariep Dam was pumped via the high-level pipeline, settled, disinfected, and then blended with treated water from the low-level pipeline. Now that Phase 3 is operational, this temporary WTW has been decommissioned and converted back into a reservoir.

“An important aspect to consider with water supply schemes is that they are demand-driven. It’s no good producing more water at your WTW without being able to convey it to the taps of your consumers, as this will just lead to your WTW overflowing/flooding. Key supporting projects to Phase 3 included upgrading the Motherwell and Stanford Road booster pump stations, building the new Kwanobuhle Supply Pump Station, installing new bulk pipelines such as the Bloemendal pipeline, and rezoning sections of the distribution system,” adds Hills.

Groundwater

“NMBM needed to diversify its existing surface-water-dependent water supply with other sources. When assessing and evaluating the possibilities of reuse, desalination, and groundwater utilisation, it was determined that groundwater was the most feasible option. Groundwater projects were found to offer the lowest cost per megalitre and the shortest completion times, presenting an opportunity to mitigate the impacts of the ongoing drought,” explains Hills.

Groundwater is not new to NMBM, with extensive studies dating back to 2004; however, there was a need to accelerate the implementation of these projects. NMBM started by conducting groundwater investigations, drilling over 200 boreholes and exploring the potential of all available aquifers. It was found that seven sites located near existing bulk water infrastructure had high groundwater potential.

One of the most favourable areas for developing a wellfield was within the Groot Winterhoek aquifer due the groundwater being artesian. A decision was therefore taken to develop the Coegakop Wellfield by drilling five production boreholes. While the groundwater from the wellfield is of good quality, it does contain elevated concentrations of dissolved iron and manganese. This necessitated the construction of Coegakop WTW to treat the borehole water to SANS 241 water quality standards. Coegakop WTW is South Africa’s largest biofiltration plant and is designed to treat up to 20 Mℓ/day.

Water resources now available

After the successful implementation of the water augmentation projects mentioned above, the unrestricted licensed abstraction volumes permitted by the Department of Water &

All NMBM dams combined can fit into the Gariep Dam nearly 20 times over

Sanitation (DWS) amount to 432 Mℓ/day.

Supporting this, NMBM has a total water treatment capacity of approximately 600 Mℓ/day, enabling future resilience of the water supply system.

Forward plans

Over the next five to ten years, the metro is focusing on building climate resilience and mitigating the impact of future droughts. This will centre around further groundwater expansion, water reuse projects, as well as desalination.

“Even though some of the dams are full, the NMBM will continue to work to roll out campaigns to encourage the efficient use of water. Furthermore, roughly R1 billion has been allocated over the next three years for climate resilience. El Niño will continue to drive higher than average temperatures and lower than average rainfall and we simply cannot afford to lose momentum,” says Hills.

Fortunately, NMBM has already made significant inroads towards mitigating any future drought impacts. “Before the drought, only 2% of NMBM’s water came from groundwater sources and 30% of our water came from the Nooitgedagt scheme. After our interventions, 65% of our water is from the Nooitgedagt scheme and 15% of our water supply is from groundwater. We have drastically reduced our dependence on local surface water sources. Therefore, as our dams begin to full up, NMBM will minimise how much water is drawn from them and use them as reserves for when there is another drought,” concludes Hills.

By Clive Lipchin (adjunct professor: Environmental Studies, Tel Aviv University) & Richard Friend (senior lecturer: Human Geography, University of York)

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