City of Cape Preps for 100 Year Waves

City of Cape Town’s award-winning coastal project

By Gareth Griffiths

First published in To Build, November 2019

An award-winning coastal engineering project has been recognised with a Marine and Harbour Division Award 2019 by the Western Cape Branch of the South African Institution of Civil Engineering.

The project in question is Phase 1 of the planned sea wall in the coastal CBD of Strand, Cape Town, which effectively protects the beachfront and promenade from frequent flooding and windblown sand ingress.

The SAICE annually recognises the most outstanding civil engineering achievements in the industry by awarding projects which portray the art and science of civil engineering to the general public; and those which illustrate innovative solutions to challenging civil engineering problems. These elements are demonstrated by the Strand sea wall to great effect.

Some of the criteria for this award included whether the project displayed unique features; how the impact on the environment was managed during implementation, and whether the project displayed engineering ingenuity.

The problem

Photo above by Gareth Griffiths Imaging: A section of the old sea retention wall in the eastern vicinity of Strand Pavilion, is part of Phase 3 section of the project which is yet to be completed. It graphically illustrates the problem of tidal wave action. The project aims to raise the level of the promenade above the high water mark to +50% of the projected 100 year high level forecast.

For some decades, the Strand CBD marine precinct has been subject to heavy erosion action by the sea and the shift of beach sand, resulting in road closures, the flooding of businesses along the promenade and damage to other infrastructure. Additional consequences have been the blocking of storm water drains by sand, which has created drainage problems within the CBD during winter. Consequently, during the course of Phase 1 of the project, a new 70m drainage pipe was laid out to sea beyond the wave action.

The city has invested R103 million rand in the construction of the sea wall and upgrade of the promenade to date. This formed part of the first phase of the City’s Strand Pavilion Precinct Upgrade Project from the Strand Pavilion precinct to Da Gama Street. The remainder of the wall is being rehabilitated in future phases.

Photo by the City of Cape Town: A special pond used to facilitate the laying of the new storm water drain 70 metres out to sea.

 The project

During the initial phases of the project, as far back as 2011, city officials begun research to determine the best solution possible within the available resources. After due process, a suitable engineering professional partner was appointed, Mott Macdonald Consulting. The engineers further researched the case, involving professional inputs from among others, the University of Stellenbosch, including Geoff Toms, head of Ports and Coastal Engineering at the university, and Piet Badenhorst, a specialist in dune management. Studies were also undertaken on the condition of coastal structures along the 38km of coastline to Zeekoevlei to get a better technical overview.

By 2014/2015, the construction team moved on site to begin what would become an innovative and unique project. The duly appointed contractor by the city was Civils 2000.

The project envisages three phases, beginning with the first 1.1km of beach to the south of the Strand Pavilion. Because this is the most heavily used commercial section of the beachfront, it was prioritised in phase 1.

The city required a complex solution to protect the coastal infrastructure, with due consideration made in the context of climate change and sea level rise; the impact construction would have on the beach aesthetics and pedestrian access; as well as the strict environmental management controls that needed to be implemented. The sea wall had to be high enough to meet the overtopping design requirements, but also allow a reasonable line of sight of the sea for the public.

The refurbished sea wall consists of 1 500 L-shaped precast upper and lower concrete wall units. Each unit was placed onto a foundation designed to support the weight of the precast concrete units, amounting to between 14 and 16 tons each. The upper concrete units “capping” is bull-nosed in shape, facing outward to break the energy of wave action against them.

‘The entire wall fits together like Lego,’ says Tony Cooksey, a senior engineer at Mott MacDonald, who was with the project from the early concept and design stage, through to the execution.

Photo by Gareth Griffiths Imaging: The upper concrete units “capping” is bull-nosed in shape, facing outward to break the energy of wave action against them.

The bottom components of the wall consist of six 6 differently styled L-shaped units.  Above these were placed the coping components, also in various styles of curvature. Cooksey says that the placement of these was done with high precision by the contractors to ensure the entire wall was seamless to the eye and at a constant level – all 1 100 metres of it.

‘It became clear that the main road was the most inconsistent feature, which undulated all the way,’ he adds.

This becomes clear when viewing it from street level looking out to sea where the height of the wall appears to change. This is obviously an optical illusion, but a mistake commonly made by people commenting in public. The pedestrian pavement is, however, a constant, allowing a magnificent recreational feature for strollers on the promenade.

Photo by Gareth Griffiths Imaging: Undulating road level. Straight level promenade walk. Main Road is now protected from flooding.

A local precast supplier, Concrete Units based at the Airport Industria, expertly formed and supplied the custom-designed concrete units to site within tight specifications.

Protecting against climate induced sea level change

On the seaward side of the wall, the city placed gabion mattresses on the foundation to protect the wall units against erosion from tidal action. These gabion mattresses and concrete wall units are now covered with sand and only about 0,8 metres of the concrete wall is visible above the promenade level. The wall units were designed to accommodate a sea level rise of 50% of the projected 100-year forecast and can be removed and raised in a future extension of the wall height if necessary.

An example of a unique and economical solution the engineering team needed to implement during the construction phase, was the use of beach sand in large sandbags to create the berms needed to protect the excavation against the high tides and waves during construction.

According to the engineers, the sandbags were derived from recycled sugar bags that disintegrated at an early stage in the project, placing enormous pressure on the construction team, as well as in clean-up operations.

Subsequently, the sand in the berms was finally used to re-establish the beach level and backfill behind the sea wall to preserve the beach sand as required.

Innovation in the rehabilitation of main storm water pipe

The rehabilitation of the main storm water pipe laid out to sea to a distance of 70 metres, was an interesting challenge in itself. City engineers who oversaw this project went to great lengths to ensure a viable long-term solution.

One of the innovations used was the application of one-way flexible valves at the end of the pipes that empty into the sea. These devices flex and close during the up-cycle of the tide to prevent backflow, and then vent when the wave action withdraws, allowing storm water to drain out. The device in question, a Tideflex seal by American company, Red Valve, was used to great effect on this aspect of the project.

Another feature of the project was the provision of a new five metre-wide promenade along the seaward side of Beach Road, stretching from Da Gama Street in the west to the Strand Pavilion Precinct in the east. The final design and layout of the wall units allowed for renewal and more formal placement of the marketplace, which initially consisted of ad hoc informal traders located on the Beach Road sidewalk.

It is noteworthy that the project was completed under budget, which is exceptional given the technical challenges of the task at hand.

Photo by City of Cape Town: Pump station and storm water pipe site. Drone pic courtesy of the City.