Dune at Lappiesbaai Beach, Stilbaai

Philip HendricksUncategorizedLeave a Comment

Stilbaai

The Hessequa Municipality appointed the CSIR to update the 1993 Management Plan for the sandspit east of the Goukou Estuary Mouth, the Lappiesbaai public beach and the foredune located seaward of the houses east of the parking area (CSIR, 1993). Where-as the study area for the 1993 Management Plan included the foredune area between the Goukou Estuary mouth eastwards to the municipal parking area at Lappiesbaai, the focus is the managed buffer dune located seawards of the public parking area and the eight privately owned houses located directly to the east of the parking area. The study area comprises of a total alongshore distance of approximately 400 m.

The latest information, including available aerial images and topographical surveys, was used to quantify the dune and beach changes over the 20 year period (1993 to 2013) and to determine improvements to the 1993 plan. From the analysis it was concluded that the beach and foredune are in a dynamically stable condition with the upper beach moving seawards within a band of up to 20 metres over the 20 year period since active buffer dune management was implemented.

The prevailing wind-blown sand potential and the effects of climate change and in particular sea level rise were taken into account and the dimensions of an effective buffer dune system are put forward for the study area.

Sea storms and climate change

Changes in the shape of sandy coastlines depend on a number of factors of which the most important is the availability and distribution of sediment (sand). Sand along the coast is moved mostly by waves, while the waves approaching the coast are in turn affected by, inter alia, the bottom topography. As the sea level rises, existing topographic features will be located in deeper water and will have a different effect on waves approaching the coast. Features landward of the breaker zone will be in deeper water and will either have an amplified or dampened effect on the wave climate compared to the present. Deep water features may deepen to the degree that their effect on the wave climate is negligible. The points of wave energy convergence and divergence will change. The new locations of wave energy convergence could be expected to experience an increase in erosion while those locations currently subject to energy convergence could accrete if they are exposed to less energy in future. Changes in wave approach will change longshore currents and longshore sediment transport.

In conclusion, the primary hazards to physical (abiotic) coastal infrastructure related to sea storms and climate change are:

  • Extreme inshore sea water levels resulting in flooding and inundation of low lying areas.
  • Changes in storm system characteristics, winds and local wave regime resulting in direct wave impacts.
  • Coastal erosion, removal of dunes and subsequent under-scouring of, for example, foundations and structures.
  • System complexities, thresholds and non-linearities, for example related to sand transport.
  • A combination of extreme events, such as sea storms during high tides plus sea level rise, will have the greatest impacts and will increasingly overwhelm existing infrastructure as climate change related factors set in in time.
  • Areas located adjacent to river mouths have the additional effect of possible higher water levels in the estuary and fore dune wash-away due to the river mouth changing direction, often alongshore to the sandpits.

The 1993 Management Plan

The 1993 management plan (CSIR, 1993) is still valid and was used as baseline for the development of the updated plan which brings into consideration the influence of climate change and specifically the potential effect of sea level rise. Specific layout and management refinements are, however, recommended based on the results of regular in-situ observations over the 20 years.

The main metocean drivers related to the above are thus waves and sea water levels (and to a lesser extent winds and currents).

Conceptual plan for the updated management plan for the Lappiesbaai Beach area

duine

Figure 1: Position and dimensions of the required buffer dune at Lappiesbaai Beach

As depicted in Figure 1, the management plan for the Lappiesbaai Beach areas has the following key components:

  • A vegetated buffer dune of at least 30m width is needed to manage the high potential for wind-blown sand influx.
  • The design should allow for an actively managed frontal section of at least 10 m vegetated with pioneer dune grass that can outgrow the influx of wind-blown sand. It is foreseen that wave run-up during storm events, especially when these occur at high tides, will extend into this area. During such times it is natural that the width of the useable upper beach area will be reduced.
  • The backdune section should be at least 20 m wide and will in time be covered by an indigenous dune vegetation community as the pioneer vegetation is naturally replaced.
  • The high potential for wind-blown sand creates a challenge of providing effective pedestrian access pathways. Figure 2 indicates the alignment and positions of proposed raised boardwalks.
    duine 2Figure 2: Conceptual details of the proposed raised boardwalks and viewing deckMaintaining the integrity of the buffer dune system to provide the ecosystem service it is designed for is the key and critical aspect for safeguarding the public and private property located landwards of the buffer dune system against the influence of present and future coastal processes.

    The basic principles of foredune integrity can be summarised as the ability of a vegetated foredune to prevent movement of wind-blown sand as a result of vegetation removal, trampling or die-off as a result of poor management, as well as to secure a large enough volume of sand to effectively counter the offshore erosion due the design sea-storm.

    From CSIR 1986 and 1994 it was shown that there is a high potential of a net landwards movement of wind-blown sand under the prevailing wind regime. In fact it is recommended that a net annual rate of 20 m3/m towards the west to north-western sector be used to design the buffer dune. To manage this potential it is essential that, for example, the dune vegetation component of the buffer dune system be maintained at a high level of integrity.

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