The City of Cape Town’s rehabilitation of the Strand sea wall recently won a Technical Excellence Project Award in the Marine and Harbour category at the 2019 South African Institution of Civil Engineering (SAICE) Western Cape awards ceremony. The new sea wall was designed to withstand storms by deflecting water and sand along Beach Road, and to endure the impact of climate change for the next 30 years.
“The refurbishment of the Strand sea wall is certainly one of the most innovative and impressive projects the city has delivered. Together with our partners, we were able to display the engineering creativity the construction design required. Apart from the critical protection of the beachfront, we have also upgraded the promenade and trading area to the benefit of all who live and work here,” said the city’s mayoral committee member for transport Felicity Purchase.
Some of the criteria for the award included whether the project displayed unique features; how the impacts on the environment were managed during the implementation thereof; and whether the project displayed engineering ingenuity.
The city has invested R103m 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 will be rehabilitated in future phases.
Protecting coastal infrastructure
A multifaceted solution was needed 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 which 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 about 600 L-shaped precast 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 tonnes each.
Furthermore, on the seaward side of the wall, the city placed gabion mattresses on the foundation to protect the wall units against erosion that may result from tidal action. These gabion mattresses and concrete wall units are now covered with sand and only about 0.8m 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 deemed necessary due to climate change.
A five metre-wide promenade was also created along the seaward side of Beach Road, stretching from the 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 market place, which initially consisted of ad hoc informal traders located on the Beach Road sidewalk.
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 sand bags in order to create the berms required to protect the excavation against the high tides and waves during construction. Thereafter the sand in the berms was finally used to re-establish the beach level and backfill behind the sea-wall in order to preserve the beach sand as required.