12 key findings from the Seven Mile Beach erosion study
Author: isi网站管理员-刘成 Source: Updated: 2024-03-20
 Compass Contributor

An aerial view of the southern end of Seven Mile Beach shot in September for the Compass.

The following is extracted from the executive summary of DHI’s beach erosion report.

A comprehensive study was carried out to analyse the cause of the observed coastal erosion along Seven Mile Beach and to define adequate mitigation measures to halt the erosion and restore the beach. The study included advanced mathematical modelling of waves, coastal hydrodynamics and sediment transport and was supported by high-resolution bathymetric data.

The modelling study included a long term (43 years) analysis of nearshore wave conditions, and the resulting sediment transport, under normal conditions and during nor’westers.

Special attention was also given to the generation and propagation of waves during hurricanes. A total of 23 tropical storms and hurricanes within the period 2000-2022 were simulated and their contribution to the coastal sediment balance along SMB was calculated.

An analysis was made of the beach erosion due to cross shore sediment transport during storms and hurricanes. During these events sediment is eroded from the upper part of the beach and deposited in deeper water further seaward. This type of erosion, often referred to as “acute” erosion can cause considerable damage to buildings and other coastal infrastructure located close to the shore.

The following main conclusions were drawn from the study:

  1. The coastal sediment balance at SMB is highly event-based and is determined by two key factors: 1) – Extratropical cyclones (nor’westers) that cause wave-driven currents and sediment transport from north to south along SMB and, 2) Tropical storms and hurricanes that mainly drive sediment from south to north.
  2. The analysis of historic wave data (1979-2022) suggests that the intensity of the nor’westers has decreased significantly since the late 1990s. This has led to a decrease in sediment transport from north to south along SMB. At the same time, no major hurricanes with direct impact on SMB were recorded in the period between 2008 (Paloma) and Laura (2020). As a result, both northward and southward directed transport rates were reduced. Consequently, no major changes in the sediment balance that could lead to chronic beach erosion occurred in that period.
  3. Since 2020 a series of storms and hurricanes have affected Grand Cayman and caused excessive sediment transport from south to north along SMB. The net northward directed transport during the past years has caused a sediment deficit in the southern part of SMB. This has resulted in beach erosion. The sediment deficit, and the associated shoreline retreat, is migrating along SMB towards north.
  4. During tropical storms and hurricanes, waves and strong winds cause complex sediment transport patterns across the entire shoal, a part of the sediment is transported across the shoal towards the edge of the reef, causing sediment loss to deep water. The loss of sediment to deep water occurs along the entire SMB but is concentrated in the northern part.
  5. During storms and hurricanes, cross shore sediment transport can cause significant acute erosion where volumes of up to 30 m3/m can be removed from the shoreline within a few days. This results in a retreat of the mean water line of around 20m and vertical displacements of the beach of up to 1m. Often this type of erosion is reversible as the displaced sediment can be transported back to the shore during a calm period after the storm. However, much damage can be done if properties are located too close to the shore.
Storms are one key factor in causing beach erosion on Seven Mile.
  1. If the present trend in meteomarine conditions continues, then the observed erosion along the southern part of SMB will get worse and gradually spread towards north. If no measures are taken, then chronic erosion along the entire SMB is likely to occur the coming years/decades.
  2. The best way to mitigate the erosion along SMB is through artificial beach nourishment. It is recommended to feed the sand to the beach in two designated spots in the southern part of SMB. These so-called sand engines will gradually feed the neighboring beaches. A major advantage of the use of sand engines instead of integral beach nourishment is that the establishment of the nourishment does not require the use of heavy machinery on the beach. The sand engines can be created using floating pipelines where sand is pumped directly from the dredger to the beach.
  3. An initial nourishment volume of 200,000 m3 sand is recommended, equally distributed between the two sand engines. The sand must be distributed along a 500m long stretch. The initial increase in beach width at the sand engine is around 40m. After the initial charging, the sand will gradually be dispersed by waves and currents to neighboring areas.
  4. The nourishment requires maintenance through periodic re-charging of the sand engines. The recommended frequency of re-charging the sand engines is once every 5 years but can be adjusted if desired. In any case, maintenance depends on the frequency and intensity of storms.
  5. It is strongly recommended to initiate a shoreline monitoring program where the beach width is measured twice a year, at the beginning and the end of the hurricane season. Good quality information of the sediment distribution along the shore makes the maintenance activities easier to plan and more efficient.
  6. It is recommended to consider the use of sediment traps to re-use sand that is transported during hurricanes and reduce sediment loss to deep water. A sediment trap is a strategically located, dredged pit. Currents generated during storm events will carry the sediment to the traps where it is deposited.
  7. Sand areas near the reef, in water depths between 10m and 20m, can be sources for nourishment. The use of sand from these areas will not have a negative impact on shoreline stability.
  8. Sand areas near the reef, in water depths between 10m and 20m, can be sources for nourishment. The use of sand from these areas will not have a negative impact on shoreline stability.

(Source: https://www.caymancompass.com/)

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