WAYS TO SOLVE LADYSMITH’S CHRONIC FLOODING PROBLEMS

Located at the toes of Drakensberg mountains, Ladysmith has experienced flooding for over 170 years. In modern times, the situation has been exacerbated by climate change, the most recent example being in January 2022 when the town experienced flooding on three different occasions, resulting in extensive loss of life and property damage. By Dr Nezar A Eldidy, Pr Eng*

The town’s location is part of the problem, as it is situated on the floodplain of the Klip River in the Thukela catchment area, with its central business district (CBD) and a large part of its residential areas situated within the river's flood basin.

Relocating the town is not an option, though, chiefly due to its places of historic and cultural interest, with at least 21 sites situated adjacent to the N11 route passing through the town. These include Later Iron Age sites, Anglo-Boer War period sites, homesteads, farmsteads, public buildings, one memorial, and two contemporary places of worship.

One of the interventions in subsequent years was the construction of the Qedusizi Dam in 1996 on the Klip River. The dam has a 32 m high wall, which was designed to manage flood peaks and delay floodwater from the upper region of the Klip River's catchment area. It has two openings without gates that allow a discharge of 450 m3/second, limiting large-scale damage and providing an adequate evacuation warning period.

However, it was meant to work in tandem with the nearby Windsor Dam, which was built to retain floodwater exceeding a flow of 700 m3/second. However, the dam was decommissioned in 1998 due to siltation and a lack of maintenance of the meanders of the Klip River.

As a result, during severe floods, Qedusizi Dam comes under major pressure. To illustrate the point, historical data suggests a 70% probability of a flow of about 1 000 m3/second every f years upstream of the Qedusizi Dam, while a flood of about 1 500 m3/second may occur every 100 years.

On top of this, a study of the cross-sectional capacity of the Klip River’s meander reveals that the average carrying capacity has decreased to less than 50% of the maximum discharge from the Qedusizi Dam, and some areas have only 23% of their original carrying capacity.

Since sections of the river winding through the town are situated at higher elevation above the CBD, when the banks are breached the stormwater heads towards the CBD’s lowest point.

Geological formations and soil types

The situation is compounded by the underlying geology, which forms part of the Vryheid

Formation of the Ecca group. It consists of shales, mudstones and fine-grained sandstones, intruded by doleritic sills and dykes of Jurassic age.

On top of the sandstone/shales subsoil lies a layer of loamy clay. These clay-rich soils have the largest pore space, and hence the highest water retention capacity, which is not conducive for stormwater dispersion.

The typical coefficient of seepage for such soil is 0.8 m3/hour, which is an average value for this type of soil. In contrast, the estimated seepage into the soil from the meander and the surface of the CBD is 1 700 m3/hour and 1 800 m3/hour, respectively.

Another factor to consider is groundwater. Investigations in the vicinity of Ladysmith, where the soil has the same structure, show groundwater level fluctuations and a rainfall variation of about 3.2 m and 15%, respectively. In general, the response of groundwater levels to changes in rainfall across the province has a lag time of two to four months, so extensive surface water build-ups add to the drainage challenge.

Ladysmith’s CBD

The existing drainage pipes convey stormwater from the CBD into the Klip River through a system of duckbill check valves. However, in a post-flood inspection, 11 out of the 24 check valves were either damaged or completely pulled out.

Although these are manufactured as nonreturn valves, in practice, they come under passive pressure during floods. Turbulence occurs at the outlet, which leads to valve damage.

What transpires during a flood event?

These and other findings formed part of the detailed step-by-step analysis needed to design a sustainable mitigation solution. That included gaining an understanding of how past flood events panned out.

Typically, the records show that during storm events, water gradually rises behind Qedusizi Dam. As momentum builds, the discharge from the dam increasingly raises the level of the Klip River. At peak discharge, the water level can reach up to 9 m in height, with the following ripple effects:

Water backflows through the drainage system: the current drainage pipe outfalls located above the riverbed have diameters of 900 mm to 1 200 mm. During a rain event, the duckbill valves discharge water from the CBD drainage gravity line (under active pressure). At the same time, the water level in the river will continue to rise until it exceeds the top of the pipe or the upper tip of the valve.

When the water level in the river exceeds the level of Forbes Street (one of the CBD’s lowest points), water flows back into the drainage system and into the CBD, resulting in a backflow that can reach 72 000 m3/hour.

This can put the lowest area in the CBD under approximately 2 m of water within

3.5 hours of reaching the high water level in the river due to pressure difference.

Water overflowing the banks: when water flowing from Qedusizi Dam reaches its maximum discharge of 450 m3 /second and is joined by a discharge of 50 m3/second from the Flagstonespruit, the water will breach the banks of the meander and pour at a rate of about 350 m3/second towards the CBD due to the reduction in carrying capacity of the meander/river cross section. The run-off of the CBD: it contributes about 60 000 m3/hour during the storm, but the controlling factor is the level of the Klip, and its backflowing into the CBD.

Slow drainage through the soil: as discussed above, the loamy clay retains water and takes considerable time to drain, which increases the surface run-off through the town.

Flood solutions for Ladysmith’s CBD

The reality is that even with all the infrastructure solutions installed to date, Ladysmith – and its 250 000 plus inhabitants – will always be exposed to flood risks because of the town’s location.

Three solutions were investigated, namely:

• partial dredging and lining of the channel

• construction of an aqueduct for water transfer from upstream to downstream. The first solution was the most practical and attainable with the support of national government funding.

The recommended route is as follows:

• the dredging of about 1 million m3 of silt and deposits from the river to enlarge and deepen the cross-sections

• the installation of a pump station on Queen Street – the station will host three pumps of about 1 m3/second discharge and a head of not less than 10 m

• the building of a discharge and calming lagoon with gabions and lined with geotextiles to discharge water downstream of the CBD

• replacement of the duckbell valves with automated sluice gates or with flap valves

• the installation of an additional 900 mm collector pipeline with concrete chambers – the pipe will be at a higher level than the current drains

• the installation of two 600 mm steel pipelines to evacuate water from the pump station to the discharge lagoon.

Conclusion

Selecting the appropriate equipment for the drainage system can help prevent flooding, but the need for yearly maintenance of the river channel is crucial. In this respect, combatting silt build-up behind the Qedusizi Dam wall is an essential part of this process.