RESIDENTS of and frequent visitors to Small Bay in Bloubergstrand will be only too aware of the major construction project underway and the closure of the Small Bay Park and the adjacent access road.
CBN reached out to the City to determine the nature of the project and Gregg Oelofse , Manager: Coastal Management in the City’s Environmental Management Department, obliged with the details of this R55-million coastal protection project.
Project timeline:
The upgrade of the sea wall at Small Bay, and the work to relocate the sewer main along this stretch of the coastline, started in February 2023. If all goes as planned, the work should be complete by September 2024.
Background information:
The City’s Coastal Management Branch commenced with the upgrade of coastal infrastructure and reconstruction of the sea wall at Small Bay in Bloubergstrand to protect coastal properties and important services infrastructure from wave overtopping and flooding. The existing sea wall will be reconstructed from the Small Bay Park, along Pelegrini and Popham Streets to Ferguson Street – all-in-all, a section of about 190m. In addition, the existing sewer line will be rerouted landward, away from the sea wall, from the park to the De Mist ablution building and a portion of Pelegrini and Popham Streets along the seawall will be reconstructed.
Motivation for the project:
The existing concrete seawall and walkways which extend from the play park in Small Bay to the Popham and Ferguson Streets intersection, have been undermined for a number of years, leading to collapse, resulting in damage to the adjacent road infrastructure. This was due to the age of the infrastructure, as well the historical development footprint which resulted in hard infrastructure and services located in the tidal- and wave breaking zone. Of specific concern was the exposure of the sewer line which was located within the existing seawall along Popham and Pellegrini Streets and the resultant risk of ongoing sewer spills.
Due to the close proximity of the seawall and other infrastructure (roads and services) in relation to the high-water mark, as well as the physical and coastal characteristics of the site, the only feasible long-term solution to protect the existing infrastructure and mitigate wave overtopping was to replace the derelict seawall with a robust coastal protection structure.
Project constraints and mitigation measures
The project has significant constraints which had to be considered to ensure the most feasible solution in terms of coastal protection, recreational use and constructability in such a dynamic environment. Some of the key constraints and mitigation thereof included the following:
Constructing a robust seawall defence structure on a very small existing footprint:
Due to the emergency nature of the project and to mitigate any potential negative impact on the natural environment and coastline, it was essential to ensure the re-constructed seawall footprint shall not encroach seawards. In order to work within the allowable footprint, the existing beach access and configuration were maintained (like for like). A great and important addition is to ensure that the new ramp access has a flatter gradient – thus, once reconstructed, it will allow for better wheelchair access to the beach. The pedestrian walkway makes allowance for universal access along its length.
Allowance for pedestrian movement and seating:
The old seawall configuration comprised of a “stepped landscape area” along a portion of Popham Street. Unfortunately, this stepped landscape will be lost when the new seawall is constructed. The key issue that the City faced is that by allowing historic development to encroach so far seawards, we now have reached a pinch point where there is so little space left that our choices for coastal protection have become severely limited. The lesson from Small Bay is development and services must be set back.
After completion of the project, the formal beach access will remain at the beach adjacent to the park, where the ramp will remain in place to provide access to the beach. Given the urgency of the work, the severe consequences of a sewer line failure that will have wider impacts beyond Small Bay, and the presence of other beaches accessible to the public in close proximity, the City is of the view that access to the beach adjacent to the Park via the ramp is sufficient and appropriate.
Height of new seawall:
The existing seawall is only +2.5 m Mean Sea Level and therefore high wave overtopping volumes shall occur during storm events, which pose a significant risk to the landside essential services, infrastructure and safety of pedestrians and vehicles. Complete prevention of wave overtopping would require a seawall height of greater than +5 m Mean Sea Level, which would result in a new wall of 2 to 3 meters higher than the existing seawall. However this would have significant impacts on views, public sense of place and private properties. As such, a measured approach was accepted that protects key infrastructure from inundation, allows for some overtopping while minimising visual impacts in order to mitigate wave overtopping and protect the sewer line. In order to address the above, a 900 mm high cope (concrete unit on top of the seawall which is curved to deflect waves) is required along the wall, meaning that the top level of the hard structure will be 900 mm higher than present. However, it will not affect public access to the beach via the access ramp provided. Thus, formal access to the beach is in no way inhibited.
Flooding during storm events:
The new seawall will not provide full protection to the infrastructure or properties behind the seawall. There will still be significant wave overtopping and flooding behind the seawall during storm and high-water conditions even with the cope units. Periodic flooding will take place behind the seawall due to the relatively low road and property levels behind the seawall and the flooding will increase in time due to the effects of sea level rise. However, if the increase of 900mm to the top of the coastal defence structure is not installed along the entire new seawall, as well as the portion of the play park section, then the level of coastal protection will be compromised in this area, resulting in:
- More frequent and higher levels of flooding events over the play park;
- Greater potential for damage to the park, park facilities and sewer pump station;
- More frequent clean-up operations and maintenance would be required after significant storm events;
- Poor design and implementation not aligned to the City’s approach to climate change; and
- Wash out or erosion of park area.
Design requirements:
A very important consideration in this project is the requirement to implement a capital project that meets the City’s commitment to a resilient future. The City has acknowledged climate change as one of the key risks. Due to the very high cost of building coastal infrastructure as well as the extensive lead up time required for these kinds of interventions, this project (like all coastal infrastructure projects of this nature) is required to be designed and meet the minimum following requirements in terms of structural design:
- A minimum 50 year horizon. In other words this seawall must remain effective and functional all the way until 2080. This 50 year horizon makes the project cost effective. A shorter design life duration would not meet cost effective budget expenditure.
- Expected/predicted sea level rise and climate change must be a key informant to the final design in terms of design loads and flooding events.
- Key infrastructure must be protected. This includes accounting for future risks. In this case this key infrastructure includes the sewer line, pump station, public road and public park
- The new seawall foundation must be designed with enough structural stability to allow for raising the wall height in future if required.
Due to this very complex project a highly professional team was appointed and comprised:
- Scoping and Feasibility studies: Coastal Management Coastal Engineers (in-house)
- Technical assessment: Feasibility phase: KZR Consulting and WSP Coastal
- Conceptual & Detail design of preferred solution: HHO Consulting engineers (road and sewer) & PRDW Consulting
- Main contractor: CSV Construction
- Specialist marine sub-contractor: Southern Oceaneering
Challenges belie modest project value
Coastal infrastructure construction always has the underlying challenge of the elements, which has a direct bearing on sea conditions. For projects such as this – demolition and reconstruction of a sea wall, realignment of the sewer line and other services, and rehabilitation of the adjacent roadway means that work can generally take place for six hours during low tide. To conform to strict environmental standards, extensive use is made of geo containers to not only protect the works by dispersing the energy of the waves, which buys time, while preventing the washout of construction debris into the ocean.
Vital sequencing
So sequencing the work within a very constricted work space has been a vital consideration, with the project divided into five manageable sections. The decision was taken to start work on section five first – perhaps the most complicated – in front of the Play Park and pump station, followed by section four, both of which will be completed before the December shutdown.
An added complication on this project is the location of the sewer pump station located 5m below mean sea level which is the collection point for properties located both to the north and to the south and has been subject to flooding from ground water ingress.
Furthermore while remedial work was carried out on the south sewer line during a previous sea wall protection contract, the northern line which was encased in the buttress of the old sea wall had to be temporarily rerouted due to the collapse of a portion of Pelegrini Street (see below), which could have caused catastrophic spills.
Safety paramount
The Small Bay beach area is popular with residents and visitors and the control of inquisitive and curious onlookers have been a constant issue to ensure their and our own personnel’s safety, especially during inclement weather where high tides overtopping is a frequent occurrence.
The new sea wall comprises 73 concrete panels mounted on a concrete platform and fixed to the bedrock by 10m long pre-stressed steel anchors. One hundred and three precast coping units will be grouted to the panels. Aesthetic considerations dictate that the new wall and coping have a high quality concrete finish.
The aggressive environment, confined working area, unpredictable weather conditions, proximity of residents and curious onlookers means a high priority of site safely has to be applied and thus far, we have completed 25 000 injury free hours.
Pelegrini Road collapse
On Friday 23 June 2023 a section of the concrete road pavement in Pelegrini Street collapsed under the wheels of one of the contractor’s delivery vehicles, fortunately without injury to anyone. This portion of road is the only vehicle and pedestrian access to the park area and private properties – with significant impact on local residents. However, the incident was resolved quickly – the contractor evacuated residents immediately and repaired the broken road.
On investigation it was found that the underlying fill had been severely eroded leaving a chasm approximately 8m long and up to 4m wide under the concrete slab where the remaining slabs in that area had no support at all. Further to that, a 100mm asbestos-cement water main, immediately below the slab was equally suspended and vulnerable.
The decision taken was to tackle the situation immediately with materials available on site to mitigate further disruption to residents on Pelegrini Street.
Staff from the CoCT water and Sanitation Department end-capped the vulnerable water main to allow repair works to proceed. The undermined section of roadway was demolished, and the rubble cleared away. The excavation was backfilled using soil and rockfill and compacted in layers. The entire fill was enclosed with geofabric to prevent loss of fine material under wave action. A 150mm thick gravel layer was laid and compacted as a temporary riding surface. The roadway was reopened at 22:00 on 23 June 2023.
