Civil Engineering May 2021 | Vol 29 No 4

are very complex and predicting future beach erosion and accretion is not straight- forward. Based on historical surveys and erosion at the existing gabion wall, the replacement structure needed to be robust enough to withstand the coastal processes and excessive scour. Moreover, the public walkway and several private erven are located im- mediately landwards of the gabion wall, which limited the available footprint for a new structure. SELECTING THE MOST SUITABLE COASTAL PROTECTION OPTION Your typical coastal protection options are broadly divided into foreshore and onshore structures. Foreshore structures, such as groynes and breakwaters, run perpen- dicular to the coastline and are designed to prevent erosion. Onshore structures, such as seawalls and revetments, generally run parallel to the shoreline and are intended to protect the “landside” infrastructure and services from flooding and wave overtop- ping. These onshore structures are subdi- vided into “soft” solutions (natural dune systems and geocontainer revetments) and “hard” structures (rock revetments or vertical walls). The potential options were assessed against the following criteria: Q Q Capital cost versus maintenance cost Q Q Impact on the adjacent coastline and infrastructure Q Q Available space/footprint Q Q Physical characteristics of the site (i.e. rocky or sandy) Q Q Coastal processes (i.e. waves, currents and sediment movement). For Big Bay, five options were considered, including a (soft) frontal dune, rock revet- ment, gabion revetment, geocontainer revetment and vertical wall. The location of a frontal dune would have been limited by the highwater mark, which means that the largest portion of the dune would have had to extend onto the already developed private property. Even if the City managed to purchase these properties (which would incur significant time and cost implications), the sewer line would have had to be rerouted. Moreover, due to the limited space, the dune would still have been in very close proximity to the highwater mark and vulnerable to ero- sion during future storm events. The required footprint for a rock revetment, in turn, would have fallen outside the available coastal protection footprint (see Figure 2). Since that would have necessitated a time-consuming environmental impact assessment, and considering the urgency of the matter, this option was eliminated. Any type of vertical wall would have had a potential impact on the coastline, again necessitating lengthy and expensive assessments. Moreover, the structure itself would have been very expensive and likely exceeded the available footprint, as the bedrock is not shallow and scour protec- tion in front of the wall would be required. Ultimately, it came down to a choice between replacing the existing gabion structure or constructing a new geocon- tainer revetment. Since the emergency structure had to last at least five years to allow the City time to plan and implement a long-term solution, the geocontainer revetment was selected as the most feasible option. GEOCONTAINERS “Geocontainers” is short for “geosyn- thetic sand-filled containers”, which in layman’s terms are massive sandbags. Geocontainers are manufactured from special multi-layered geotextiles and reinforced with a high-strength composite fabric. The material is resistant to punc- ture, abrasion (such as rocks slamming into the bags) as well as UV rays. The containers come in various sizes. Those used for the Big Bay revetment were 2.5 tonne bags, with in-situ size of 1.8 m × 1.6 m × 0.48 m (a standard size product), relatively easy to handle/place, and manufactured locally. The bags specified Nearly dwarfed by the enormous geocontainers used to build the revetment are (left) Allan Klaassen, Southern Oceaneering contractor, and (right) Pierre Roux, City of Cape Town coastal engineer and project manager Civil Engineering May 2021 13 Figure 2 Typical sections of alternative options 8 7 6 5 4 3 2 1 0 –1 –2 Elevation (m) against mean sea level Distance length (m) 20 18 16 14 6 12 10 8 4 2 Disturbed footprint Undisturbed footprint (beach) Beach level 2017 Beach level 2019 Maximum sea level Wall and foundation Geocontainers Rock revetment Big Bay Coastal Protection options and beach levels Geocontainer length approximately 8 m Scour protection of vertical wall