HOW DEEP IS TOO DEEP TO DREDGE THE ST LUCIA ESTUARY?

The Port Dunford Formation is part of the coastal geological and groundwater system around the Zululand/Maputaland coastal plain and the western shores of Lake St Lucia.

It is not one uniform, solid layer. It contains sandy, clayey, silty and lignite-rich sections. Some sandy sections can store and transmit groundwater, while the finer layers can restrict or slow groundwater movement.

THE KEY QUESTION

Could deep dredging affect the groundwater system as well as the estuary itself?

PRACTICAL RISK-SCREENING GUIDE

These are practical screening values for discussion—not an official legal classification:

1. Up to 1 metre — Shallow dredging, mainly affecting recent bottom sediments.
2. 1–3 metres — Moderate dredging requiring detailed geological and environmental assessment.
3. More than 3 metres — Deep dredging, with a greater possibility of reaching older Pleistocene deposits.
4. 5 metres or deeper — High-concern dredging requiring sediment coring, borehole drilling, groundwater monitoring and hydraulic modelling.

WHY DEPTH MATTERS

The average depth of Lake St Lucia is naturally only about 0.9–1 metre, although deeper channels occur near the mouth. Removing several metres from the bed could significantly alter water circulation, salinity, sediment movement, fish habitat, groundwater exchange and the relationship between the estuary and surrounding wetlands.

WHAT THE PUBLIC SHOULD SEE BEFORE MAJOR DREDGING

1. The proposed dredging depth and footprint
2. Sediment-core results
3. The depth and composition of the Port Dunford Formation
4. Groundwater-level and salinity data
5. A groundwater–surface-water impact assessment
6. Independent monitoring before, during and after dredging

THE CENTRAL ISSUE

The key issue is whether dredging will cut through hydraulically important sediment layers and change the water balance of the St Lucia system. The exact geology varies from place to place.

RELEVANT RESEARCH AND RESOURCES

Geohydrological studies: Research link

Maputaland coastal aquifer research: Research link

WRC report: WRC report

NOTE: These are practical screening values, not official St Lucia dredging limits. Actual risk must be determined through site-specific investigations.

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