Department of Civil Engineering

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Browsing Department of Civil Engineering by Author "Abel, Nicolaas Cornelius"

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    Determining the effects of nearshore conditions on rip current behaviour using hydrodynamic numerical modelling
    (Stellenbosch : Stellenbosch University, 2023-02-28) Abel, Nicolaas Cornelius; Theron, Andre Karl; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: At many beaches around the world, narrow seaward flowing currents of water, known as rip currents, are commonly found. Unfortunately, rip currents, which are vital for the transport and cross-shore mixing of nutrients, sediment, biological species, heat as well as pollutants, can easily entrap unsuspecting bathers and rapidly carry them to deeper water. In these circumstances, a combination of exhaustion, panic and fear can result in drowning. The focus of this research is not on the positive aspects of rip currents as vital transport mechanism, but rather on their dangers to bathers. Rip currents are the leading cause of sea rescues among beachgoers, resulting in more than 65% of all nearshore rescues worldwide. Thus, there is a need to develop an operational rip current forecasting model. The purpose of this research is to use numerical modelling to gain a better understanding of the behaviour and severity of rip currents for a range of nearshore conditions, specifically nearshore wave heights, tidal elevation and intermediate beach states. Using hydrodynamic numerical modelling software packages such as MIKE21 Coupled FM, the behaviour of rip currents concerning varying nearshore wave conditions, tidal elevation, and intermediate beach states can be studied. Such studies add to the existing body of knowledge on rip current behaviour and can ultimately have a significant positive outcome on bather safety. Rip currents are primarily driven by wave breaking and are influenced by water levels, originating within the surf zone and extending beyond the breaker line. Several rip current types exist, defined by their various forcing mechanisms. Bathymetry-controlled channel rip currents are the focus of this research. Thirty five hydrodynamic numerical simulations were conducted comprising of various combinations of nearshore wave-, bathymetry-, and water level characteristics. The results showed that wave-breaking and wave-bathymetry interaction are equally important in rip current behaviour. A rip current cannot form if the wave-bathymetry interaction does not allow for wave breaking. Ultimately the bathymetry controls the location and existence of the rip current. Rip current behaviour is controlled by the water level (tidal elevation) and the wave energy acting on the bar-rip morphological template. Higher rip flow velocities are present at lower water levels (≤ Mean Sea Level), with larger wave periods (≥ 12 s) and greater incident wave heights (≥ 2 m). Adult bathers experiencing a moderate 0.2 m/s flow velocity at a still water depth of 1.6 m are exposed to a significant danger of being overpowered by the rip current. Subsequently, a safety threshold is identified and defined as the cross-shore distance from the shoreline to the location where a bather is exposed to significant danger. The smallest safety threshold (30 m) was achieved on a transverse bar and rip bathymetry, imposed with a 2 m incident wave height with a 12 s wave period at a water level of -1 m MSL. The rip current flow velocity alone is not dangerous. Combining higher flow velocities (≥ 0.2 m/s) with significantly deep water (~≥ 1.6 m) is what makes rip currents treacherous. Further studies are required on the behavioural characteristics of rip currents. More tests are recommended on possible wave characteristics combinations and to validate the results by conducting physical modelling. Future studies should also include nearshore morpho dynamics.