Civil Engineering May 2021 | Vol 29 No 4

20 May 2021 Civil Engineering A vertical seawall is usually placed on a rubble-mound berm used as toe protec- tion for the seawall (Figure 1). In front of the seawall, the rubble-mound berm consists of a core, filter layer, and armour layer of rock. Underneath the concrete seawall, a screed layer is placed on top of the core rock to level part of the berm (Figure 1). The function of the screed layer is to ensure good contact and fric- tion between the seawall and the berm. Scouring of the screed layer is one of the main causes of seawall failure. Forty-two tests were conducted at the Stellenbosch University Water Laboratory (Figure 2) to determine design guidelines to minimise the scouring of the screed layer (Streuderst, 2021). The area of the screed layer washing out from underneath the seawall (the scour area) for each experiment was computed using a new method developed by Streuderst (2021) using the Image Processing Toolbox in MATLAB. Wave celerity increases as the wave period increases, leading to a rise in the rate of wave energy transmission through the toe berm. This results in more scour developing as more wave energy reaches the screed layer. The scour area for wave periods ranging between 6 s and 12 s are narrowly grouped, whereas the scour areas for the 16 s and 18 s wave period are signifi- cantly higher. In one of the most extreme cases tested, an 18 s wave period caused 83% of the screed layer to be washed out. The rubble-mound berms (toes) with the highest crest provided the best protection. For an 18 s peak wave period, the largest structure experienced a 20% scour area, whereas the lowest berm experienced 80% scour. Increasing the filter layer before the armour layer proved to be effective and economical. Adding two layers of rock to the filter layer (19% increase in the total crest height) led to a 50% decrease in the scoured area. A thicker layer generates an irregular surface, resulting in better interlocking and increased porosity which improves wave energy dissipation and armour layer stability. Additionally, a larger median rock mass in the underlayers enhanced the energy dissipation and structural sta- bility. The filter criterion stating that the underlayer rock mass should be a tenth of the upper (armour) layer proved to be the most effective in most experiments. As a first approximation, the dynamic pressure head was measured at different elevations in the rubble-mound berm and converted into velocity to determine the energy distribution. Even though Laboratory tests to aid the design of a rock toe as scour protection for a vertical seawall Anchen Buys (nee Streuderst) Master’s student: port and coastal engineering Stellenbosch University astreuderst@prdw.com Koos Schoonees Pr Eng Lecturer: port and coastal engineering Stellenbosch University kooss@sun.ac.za Figure 1 Vertical seawall with its rock toe Sea Land Constant thickness Screed layer subject to scour Fill *Varies *Varies Armour layer Median rock mass constant Filter layer Median rock mass constant Core Median rock mass constant