Coastal Zone Management Unit

THE DESIGN PROCESS

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Arriving at the Final Design Components

A combination of the objective of minimising environmental impacts and the coastal environment (wave exposure, sand transport processes) drove the design process.

The challenge at this site was to provide a stable beach and related coastal structures yet minimise visual impacts. The lower the structure, the more frequent it will be overtopped by waves and exposed to potentially damaging events. This is especially true for the rear slope armour stones on low crested coastal structures. A combination of past physical model results for similar situations, the development of perspective drawings from various positions and numerical modelling of wave events resulted in the selected coastal structures.

Beach stability was assessed using a combination of numerical and past physical models. Baird & Associates elected to undertake two weeks of model testing specific to this project to assess overtopping and interaction with both existing and proposed structures; two-dimensional tests were undertaken. This combination of engineering tools and order of analyses allowed the engineers to design the appropriate coastal structures for the project.

Wave Climate

Deepwater waves were developed for this site considering the three main sources of incident waves:

North Atlantic swells wrapping around the east side of the island;
East and southeast trade wind generated waves; and
The influence of tropical storms and hurricanes, mostly passing to the south of the island.

The first two sources were investigated through the application of the WAVAD model to develop an operational wave climate. The tropical storms and hurricanes were simulated using a more refined grid application of the WAVAD model and eventually combined with the operational waves to create a full time history of waves. As part of this process, far field wave transformation was required to wrap the easterly and northerly waves around the island towards this southwest sector of the Barbados coast.

Storm wave attack is most frequently generated by tropical storms and hurricanes passing to the south. An extreme analysis was used to define deepwater design waves at Welches.

All structures in the project are designed for the 50 year return period event.

Summary of Sediment Processes and Implications for Solutions

Based on the sediment processes analysis, the following summarises key findings:

The net direction of sediment transport at Welches is from east to west.
Longshore sand transport rates at Welches are supply limited and low.
The main sources of sediment for the Welches beach are the reefs directly offshore.
Analysis of beach erodibility using Sunamura and Horikawa’s C_s factor and the 20 year wave climate indicated that the beach is generally accretive. Sediment is transported onshore from depths of up to 8m during swell events.
However, offshore transport events related to hurricanes and tropical storms occur in most years. The regular occurrence (every 1 to 2 years) of offshore transport events suggests that beach nourishment at Welches had to be protected because of the risk that it would be lost offshore within one to two years.