Civil Engineering May 2022 | Vol 30 No 4

Civil Engineering May 2022 23 waves in high tide conditions as well as retaining sand material as the ground and founding levels of the houses on the erven are at least 2.5 m higher than the beach levels. The existing seawalls were founded on sand with scour protection mattresses at the front of the wall base. A vertical retaining wall in an active tidal zone causes wave reflection which in turn causes turbulence and suspension of beach sand. If there is no scour protection in front of the wall, the sand will be washed away by longshore currents and lost from the near-shore zone. The loss of sand manifests itself as scour at the base of the wall. The scour protection of the seawall founda- tions failed, resulting in the scouring of the wall founding material by waves. This was evident from the sinkholes behind the walls, as well as the settlement and forward rotation of the existing cantilever concrete wall (see Figures 5, 7 and 8). The loss of the wall founding and backfill mate- rial increased with time as gaps in the wall joints opened up and became larger due to increasing differential settlement and rotation (see Figure 6). Erf 234 Erf 636 Figure 3 Water levels at Erf 636 equal to the top of the old wall in the June 2017 storm Figure 2 Overtopping in September 2017 at Erf 234 and Erf 636 in Small Bay Figure 5 Sinkholes behind Erf 234 wall in September 2017 due to scouring Figure 6 Significant loss of backfill and wall tilting from scouring at Erf 234 wall in March 2018 at commencement of construction Figure 4 Overtopping at Erf 636 in September 2017