PORTS & HARBOURS

Building stable beaches – a new tool to help

18 years ago

HR Wallingford in the UK has developed a new software package, in ArcGIS, to help coastal engineers model artificial beaches. The Beach Design and Management Tool (BDMaT) allows engineers to apply the necessary parabolic equations computationally, saving much laborious number crunching and giving a visual display of the results. BDMaT helps in the design of stable beaches, offering an intuitive environment with simple inputs and instant visualisation of the results.

When designing an artificial beach, it is desirable to avoid (or minimise) future maintenance commitments. This can be achieved by designing the initial beach plan-shape in such a way that it remains stable (or in dynamic equilibrium) under the incident wave climate.

Crenulate bays, where a sandy beach is held between two erosion-resistant headlands, can evolve to such a stable beach plan-shape. There is little movement of the beach contours over the long term. Such bays tend to form naturally and attain dynamic stability most effectively when swell or storm waves are experienced from a narrow directional sector.

Several empirical bay shape equations have been derived to fit curves to the shoreline planform of headland bay beaches. One of the most widely used is the parabolic bay shape equation. This is helpful in evaluating beach stability as well as predicting shoreline changes that might arise when structures are built on a curved beach. Applying the equation, however, can involve labour intensive manual methods, including hand calculation. Results then have to be traced onto a map or aerial photograph. This can be a repetitive and laborious process.

The new software, BDMaT, calculates the idealised shoreline planform of a headland-bay beach in static equilibrium, based on the parabolic equation. It then presents the results graphically within GIS, overlaying an image or map of the existing or proposed beach. By comparing the existing shoreline with the static equilibrium planform the software allows planners to make visual assessments of beach stability and in terms of designing an artificial beach, anticipate the volume of sand required to achieve the stable planform.