BREAKWATERS
Breakwaters serve a fundamentally different purpose from sea dikes. Where a dike is a land-based structure that excludes water, a breakwater is a partially or fully submerged structure that dissipates wave energy in the water column before it reaches the shore. In Mekong Delta the term encompasses three distinct structure types used for different coastal problems.
Detached breakwaters for mangrove establishment, this is the application most specific and important to Mekong Delta's situation. The paradox of mangrove restoration on an eroding shoreline is that the plants that would best protect the shore cannot survive on the shore long enough to establish — wave energy uproots seedlings before they develop root systems. A detached breakwater placed 50–150 m offshore creates a sheltered, lower-energy zone immediately landward of it. In this calmer water, fine sediment settles out and accumulates, forming a mudflat. Mangrove propagules can colonise the mudflat and root successfully. Within 3–5 years, the mangrove belt is dense enough to attenuate waves on its own, and the breakwater structure has served its purpose as a nursery infrastructure.
Groynes, shore-perpendicular groynes trap longshore sediment transport, building up the beach on the updrift side. In Mekong Delta this can be effective where longshore drift is the primary erosion mechanism — principally along the eastern Tiền Giang and Bến Tre coast, where the dominant south-to-north longshore current moves sediment away from exposed headlands. The critical limitation is downdrift starvation: by trapping sediment that would otherwise continue moving along the shore, a groyne creates or accelerates erosion immediately downdrift of its tip. A single groyne converts a diffuse erosion problem into a concentrated one. Groynes are therefore only viable as a system — requiring a planned field of structures with carefully calculated spacing — and even then, require continuous monitoring of the downdrift shoreline response.