KOUGA MUNICIPALITY
ST FRANCIS BAY BEACH EROSION
ENVIRONMENTAL IMPACT ASSESSMENT

SPECIALIST REPORT

ECOLOGICAL IMPACT ASSESSMENT OF THE PREFERRED TECHNICAL ALTERNATIVES

Prepared by

Norbert Klages, Eileen Campbell,
Dave Schoeman,Tris Wooldridge
 

IECM REPORT Number C69

JULY 2002
 

EXECUTIVE SUMMARY
An assessment is provided of the environmental impact of the construction of the different engineering options to address coastal erosion problems as presented by Entech as well as the impact of increased sand deposition on the dune environment of the St. Francis Bay Beach.

The foredune complex backed by the town development of St. Francis Bay has been severely eroded to the extent that the functional dune system has been destroyed and its natural fluctuations have ceased. With the loss of dune malleability, the protective role of the foredune complex has been lost. The inland side of the Marina Glades dune is largely stable and will remain so provided the erosive action of canal water is kept low. The stability of this dune spit is extremely tenuous because it is surrounded by water and adjacent to an estuary mouth. Measures taken to replenish the sand volume of the St. Francis Bay Beach address only one of the impacts currently acting on the foredune complex. Unless action is taken to address the other erosive forces, the problems are likely to persist and continual action will be required to maintain beachfront integrity.

Beach functioning is already compromised. With the current rate of erosion, it may be concluded that failure to intervene will eventually result in the loss of the local beach as a functional ecological system.

Freshwater inflow into the Kromme River is significantly attenuated by two large storage reservoirs in the catchment, so that the estuary becomes an extension of the sea most of the time. Effective removal of accumulated sediment by occasional floods in the estuary is diminished. Species composition and distribution as well as biological processes in the estuary differ from other estuarine systems that receive an adequate supply of freshwater.

The risks to the dune environment all relate to interference with dune malleability. Developments close to sandy shores are only sustainable if the natural sand transport patterns are maintained. These patterns have been so seriously altered by developments in St. Francis Bay that the foredune dune system along much of the beach has ceased to function.

Most risks to the beach arise from the compaction of sand during construction activity as well as the addition of poorly matched sediments containing silt, clay and organic components. Erosion downstream from groynes presents a major risk.

Most impacts on the estuary relate to the removal of sand, leading to changes in mouth and river channel configuration. A further risk arises from the flooding of the Kromme River.

The sand banks near the mouth of the Kromme River estuary and the open (unvegetated) dunes around the Sand River seem best suited to providing the sand for beach restoration and nourishment. Mining of 600 000 m3 and 1 000 000 m3 of sand, respectively, is feasible. Most of the vegetation found on the Sand River dune system is exotic making its conservation value minimal. Sand in these vegetated areas, however, will contain plant litter (organic detritus). If sand is taken from the vegetated areas and deposited on the St. Francis Bay Beach, it will cause an undesirable black colour on the beach and the decomposition of litter will affect the beach ecosystem. Sand removal from the Sand River dune system should therefore be confined to unvegetated areas. This will result in loss of the only positive impact, namely the removal of alien trees, but will completely mitigate for the negative impacts. Seasonal vleis and pans should not be mined to protect the resident small vertebrate fauna. No negative environmental impacts are predicted if sand was taken from the municipal dump site next to the R330 towards Cape St Francis although the volume is small and may contain rubbish. The grey colour of the sand excavated from the canal extension is likely a result of its high sulphur content deriving from anoxic conditions thus making it the least desirable option.

Mitigation of the most important impacts is possible. Fatal flaws preventing the proposed development to go ahead were not discovered. Construction of a set of three relatively short groynes, combined with initial beach nourishment, presents the environmentally preferred alternative.

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