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RESIDENTS of and frequent guests to Small Bay in Bloubergstrand will probably be solely too conscious of the most important building venture underway and the closure of the Small Bay Park and the adjoining entry street.
CBN reached out to the Metropolis to find out the character of the venture and Gregg Oelofse , Supervisor: Coastal Administration within the Metropolis’s Environmental Administration Division, obliged with the small print of this R55-million coastal safety venture.
Venture timeline:
The improve of the ocean wall at Small Bay, and the work to relocate the sewer essential alongside this stretch of the shoreline, began in February 2023. If all goes as deliberate, the work needs to be full by September 2024.
Background data:
The Metropolis’s Coastal Administration Department commenced with the improve of coastal infrastructure and reconstruction of the ocean wall at Small Bay in Bloubergstrand to guard coastal properties and necessary companies infrastructure from wave overtopping and flooding. The present sea wall will probably be reconstructed from the Small Bay Park, alongside Pelegrini and Popham Streets to Ferguson Road – all-in-all, a piece of about 190m. As well as, the present sewer line will probably be rerouted landward, away from the ocean wall, from the park to the De Mist ablution constructing and a portion of Pelegrini and Popham Streets alongside the seawall will probably be reconstructed.
Motivation for the venture:
The present concrete seawall and walkways which lengthen from the play park in Small Bay to the Popham and Ferguson Streets intersection, have been undermined for a lot of years, resulting in collapse, leading to injury to the adjoining street infrastructure. This was because of the age of the infrastructure, as effectively the historic improvement footprint which resulted in laborious infrastructure and companies situated within the tidal- and wave breaking zone. Of particular concern was the publicity of the sewer line which was situated inside the present seawall alongside Popham and Pellegrini Streets and the resultant threat of ongoing sewer spills.
As a result of shut proximity of the seawall and different infrastructure (roads and companies) in relation to the high-water mark, in addition to the bodily and coastal traits of the location, the one possible long-term resolution to guard the present infrastructure and mitigate wave overtopping was to interchange the derelict seawall with a sturdy coastal safety construction.
Venture constraints and mitigation measures
The venture has important constraints which needed to be thought-about to make sure essentially the most possible resolution by way of coastal safety, leisure use and constructability in such a dynamic surroundings. Among the key constraints and mitigation thereof included the next:
Establishing a sturdy seawall defence construction on a really small present footprint:
As a result of emergency nature of the venture and to mitigate any potential adverse affect on the pure surroundings and shoreline, it was important to make sure the re-constructed seawall footprint shall not encroach seawards. In an effort to work inside the allowable footprint, the present seaside entry and configuration had been maintained (like for like). A fantastic and necessary addition is to make sure that the brand new ramp entry has a flatter gradient – thus, as soon as reconstructed, it can enable for higher wheelchair entry to the seaside. The pedestrian walkway makes allowance for common entry alongside its size.
Allowance for pedestrian motion and seating:
The outdated seawall configuration comprised of a “stepped panorama space” alongside a portion of Popham Road. Sadly, this stepped panorama will probably be misplaced when the brand new seawall is constructed. The important thing subject that the Metropolis confronted is that by permitting historic improvement to encroach to date seawards, we now have reached a pinch level the place there’s so little area left that our selections for coastal safety have develop into severely restricted. The lesson from Small Bay is improvement and companies have to be set again.
After completion of the venture, the formal seaside entry will stay on the seaside adjoining to the park, the place the ramp will stay in place to offer entry to the seaside. Given the urgency of the work, the extreme penalties of a sewer line failure that may have wider impacts past Small Bay, and the presence of different seashores accessible to the general public in shut proximity, the Metropolis is of the view that entry to the seaside adjoining to the Park by way of the ramp is adequate and acceptable.
Top of recent seawall:
The present seawall is simply +2.5 m Imply Sea Degree and subsequently excessive wave overtopping volumes shall happen throughout storm occasions, which pose a big threat to the landside important companies, infrastructure and security of pedestrians and autos. Full prevention of wave overtopping would require a seawall peak of better than +5 m Imply Sea Degree, which might end in a brand new wall of two to three meters larger than the present seawall. Nonetheless this may have important impacts on views, public sense of place and personal properties. As such, a measured method was accepted that protects key infrastructure from inundation, permits for some overtopping whereas minimising visible impacts as a way to mitigate wave overtopping and defend the sewer line. In an effort to tackle the above, a 900 mm excessive cope (concrete unit on prime of the seawall which is curved to deflect waves) is required alongside the wall, that means that the highest stage of the laborious construction will probably be 900 mm larger than current. Nonetheless, it won’t have an effect on public entry to the seaside by way of the entry ramp offered. Thus, formal entry to the seaside is on no account inhibited.
Flooding throughout storm occasions:
The brand new seawall won’t present full safety to the infrastructure or properties behind the seawall. There’ll nonetheless be important wave overtopping and flooding behind the seawall throughout storm and high-water situations even with the cope items. Periodic flooding will happen behind the seawall because of the comparatively low street and property ranges behind the seawall and the flooding will enhance in time because of the results of sea stage rise. Nonetheless, if the rise of 900mm to the highest of the coastal defence construction isn’t put in alongside the complete new seawall, in addition to the portion of the play park part, then the extent of coastal safety will probably be compromised on this space, leading to:
Extra frequent and better ranges of flooding occasions over the play park;
Higher potential for injury to the park, park amenities and sewer pump station;
Extra frequent clean-up operations and upkeep can be required after important storm occasions;
Poor design and implementation not aligned to the Metropolis’s method to local weather change; and
Wash out or erosion of park space.
Design necessities:
A vital consideration on this venture is the requirement to implement a capital venture that meets the Metropolis’s dedication to a resilient future. The Metropolis has acknowledged local weather change as one of many key dangers. As a result of very excessive price of constructing coastal infrastructure in addition to the in depth lead up time required for these sorts of interventions, this venture (like all coastal infrastructure tasks of this nature) is required to be designed and meet the minimal following necessities by way of structural design:
A minimal 50 12 months horizon. In different phrases this seawall should stay efficient and purposeful all the best way till 2080. This 50 12 months horizon makes the venture price efficient. A shorter design life length wouldn’t meet price efficient price range expenditure.
Anticipated/predicted sea stage rise and local weather change have to be a key informant to the ultimate design by way of design masses and flooding occasions.
Key infrastructure have to be protected. This consists of accounting for future dangers. On this case this key infrastructure consists of the sewer line, pump station, public street and public park
The brand new seawall basis have to be designed with sufficient structural stability to permit for elevating the wall peak in future if required.
As a consequence of this very complicated venture a extremely skilled group was appointed and comprised:
Scoping and Feasibility research: Coastal Administration Coastal Engineers (in-house)
Technical evaluation: Feasibility section: KZR Consulting and WSP Coastal
Conceptual & Element design of most popular resolution: HHO Consulting engineers (street and sewer) & PRDW Consulting
Principal contractor: CSV Building
Specialist marine sub-contractor: Southern Oceaneering
Challenges belie modest venture worth
Coastal infrastructure building at all times has the underlying problem of the weather, which has a direct bearing on sea situations. For tasks similar to this – demolition and reconstruction of a sea wall, realignment of the sewer line and different companies, and rehabilitation of the adjoining roadway signifies that work can typically happen for six hours throughout low tide. To evolve to strict environmental requirements, in depth use is made from geo containers to not solely defend the works by dispersing the vitality of the waves, which buys time, whereas stopping the washout of building particles into the ocean.
Important sequencing
So sequencing the work inside a really constricted work area has been a significant consideration, with the venture divided into 5 manageable sections. The choice was taken to begin work on part 5 first – maybe essentially the most sophisticated – in entrance of the Play Park and pump station, adopted by part 4, each of which will probably be accomplished earlier than the December shutdown.
An added complication on this venture is the placement of the sewer pump station situated 5m under imply sea stage which is the gathering level for properties situated each to the north and to the south and has been topic to flooding from floor water ingress.
Moreover whereas remedial work was carried out on the south sewer line throughout a earlier sea wall safety contract, the northern line which was encased within the buttress of the outdated sea wall needed to be briefly rerouted because of the collapse of a portion of Pelegrini Road (see under), which might have prompted catastrophic spills.
Security paramount
The Small Bay seaside space is well-liked with residents and guests and the management of inquisitive and curious onlookers have been a relentless subject to make sure their and our personal personnel’s security, particularly throughout inclement climate the place excessive tides overtopping is a frequent prevalence.
The brand new sea wall includes 73 concrete panels mounted on a concrete platform and stuck to the bedrock by 10m lengthy pre-stressed metal anchors. 100 and three precast coping items will probably be grouted to the panels. Aesthetic concerns dictate that the brand new wall and coping have a top quality concrete end.
The aggressive surroundings, confined working space, unpredictable climate situations, proximity of residents and curious onlookers means a excessive precedence of website safely needs to be utilized and so far, we have now accomplished 25 000 harm free hours.
Pelegrini Street collapse
On Friday 23 June 2023 a piece of the concrete street pavement in Pelegrini Road collapsed underneath the wheels of one of many contractor’s supply autos, happily with out harm to anybody. This portion of street is the one automobile and pedestrian entry to the park space and personal properties – with important affect on native residents. Nonetheless, the incident was resolved shortly – the contractor evacuated residents instantly and repaired the damaged street.
On investigation it was discovered that the underlying fill had been severely eroded leaving a chasm roughly 8m lengthy and as much as 4m huge underneath the concrete slab the place the remaining slabs in that space had no help in any respect. Additional to that, a 100mm asbestos-cement water essential, instantly under the slab was equally suspended and susceptible.
The choice taken was to deal with the state of affairs instantly with supplies accessible on website to mitigate additional disruption to residents on Pelegrini Road.
Employees from the CoCT water and Sanitation Division end-capped the susceptible water essential to permit restore works to proceed. The undermined part of roadway was demolished, and the rubble cleared away. The excavation was backfilled utilizing soil and rockfill and compacted in layers. All the fill was enclosed with geofabric to stop lack of fantastic materials underneath wave motion. A 150mm thick gravel layer was laid and compacted as a short lived driving floor. The roadway was reopened at 22:00 on 23 June 2023.
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