Flushing Fine Sediment from the Beach Face

Summary

Since its construction in 2019, this project is located in a sheltered coastal area in Asia and faces significant sedimentation challenges. Fine sediment accumulation degraded the beach's natural beauty and user experience, causing swamp-like conditions and dissatisfaction among visitors. This issue not only disrupted aesthetics but also posed environmental risks and affected local tourism and property values. Coastal Revive Solutions (CRS) implemented an innovative, cost-effective solution using custom wave generation to resuspend and remove fine sediments, preventing the high costs of replacing the beach’s top layer with new sand. The method restored beach quality, maintained stability, and minimized disruption, ensuring long-term sustainability with minimum regular maintenance.

Introduction

The beach was initially developed by the local government to meet the growing demand for recreational spaces, serving both residents and tourists. It was part of a larger coastal development initiative to create a safe, accessible, and eco-friendly public beach for swimming, sunbathing, and water sports. The beach is equipped with various amenities, including changing rooms, showers, lifeguards, and food kiosks, making it a popular destination for day visitors.

The Issue

The beach is in a sheltered with high suspended sediment concentration and quickly showed issues related to the buildup of fine sediment since its construction. Over time, the accumulation of silt and mud degraded the natural appeal of the beach, creating swamp-like conditions that negatively impacted the visitor experience. This ongoing sedimentation not only affected the beach’s aesthetics but also caused a decline in user satisfaction, as the previously pristine shoreline became increasingly muddy and unattractive.

This issue extended beyond aesthetics. Fine sediment also trapped more nutrients and organic material, creating favourable conditions for macroalgae blooms, which were particularly observed during the summer months, noticeable in 2022. Economically, the sedimentation problem reduced the beach's appeal to both residents and tourists, which in turn affected nearby property values and local businesses dependent on beach tourism.

Site conditions

During development, 65,000 cubic meters of sand were imported to create a 200-meter artificial beachfront. The sand, with a D50 of around 0.25 mm (containing under 3% fines), was selected for its grain size, colour, and texture to ensure compatibility with the local coastal environment and promote long-term beach stability. The design incorporated two groynes and a 1:15 slope to help maintain stability and reduce erosion.

The significant wave height at the site is typically below 0.4 meters, with occasional larger waves during typhoon events. Suspended sediment concentrations range from 32 mg/L to 165 mg/L, with corresponding Secchi depths of 0.65 m to 3.8 m, depending on hydrodynamic conditions. Due the combination of relatively high suspended sediment concentration and low wave energy, the beach has experienced rapid accumulation of fine sediment, leading to dissatisfaction among beachgoers.

3. The Approach and Solution

The local authorities turned to Coastal Revive Solutions (CRS) for an alternative, as replacing the sediment layer was deemed costly and unsustainable as the issue will recur. Upon inspecting the area, CRS identified a mud layer on the beach face below depths of around 0.5m, ranging from 5 cm to 13 cm thick, with an average of 16% fine material.

CRS developed a tailored solution using two 26-foot boats operating between 10 to 25 knots and 100 to 300m from the beach, generating wave heights between 0.80 to 1.20 meters. The operation was intensified during spring tides, when tidal variation reached about 2.5 meters, to ensure the sediment across the beach face was resuspended and transported beyond the depth of closure.

Each boat produced 4 to 6 waves per run and operated between 8 p.m. and 4 a.m. to avoid disturbing beachgoers. The method effectively resuspended and cleared the top layer of mud, gradually restoring the beach’s sandy surface. Within five months of operations, most of the mud layer was removed from the shallow part of the beach, with 50% of the material settling in deeper waters between -2.1 mCD and -3.3 mCD, where the mud layer thickened. Meanwhile, the sand remained on the beach, maintaining a stable profile without being transported offshore. After the process, sediment samples showed a significant reduction in fine material, dropping to just 2% in the shallow areas.

Example of sediment grains size distribution before and after the campaign.

4. Conclusion

CRS’s tailored approach replicated the beneficial effects of natural storm-driven waves, successfully clearing the beach face of accumulated mud, thus saving significant costs associated with sand replacement. The solution minimized disruption to daily beach activities while ensuring long-term beach stability.

The strategy involved optimizing boat selection, speed, design, ballast, route, and wave frequency to maximize the sediment-cleansing effect without causing erosion or unnecessary operational expenses. Real-time monitoring allowed for continuous adjustments, further enhancing the process’s effectiveness.

CRS now conducts regular maintenance, with monthly operations during the rainy season and bi-monthly during the dry season. This ongoing management keeps the beach clean, optimizing user experience, reducing costs, and maintaining local revenue streams and property values.