The siltation of the Bushman’s Estuary

Dec 7, 2016

Estuary Care members and concerned members of the public have expressed rising concern at the ever increasing sedimentation occurring in the Bushmans and Kariega estuaries. This is threatening the navigability of the estuaries and could ultimately lead to the closure of the estuary mouths with all of the consequences that would bring.

Thanks to the generosity of a local resident, a firm of Internationally Recognized Harbour Consulting Engineers from Cape Town, PRDW, was engaged to undertake a detailed survey of the Bushman’s Estuary and to build a two dimensional computer model. This was used to test various proposed interventions. The report which follows is a summary of the findings and recommendations.


 

Summary of the report:

BUSHMANS RIVER ESTUARY
STUDY OF INTERVENTIONS TO REDUCE SEDIMENTATION – SUMMARY

Residents and associations such as Estuary Care have for decades been expressing their concern over the progressive marine sedimentation of the Bushmans River Estuary. The concern is that the sedimentation is reducing the navigability of the channels, resulting in a loss of amenity of the estuary which is widely used for recreational boating. There are also concerns that if nothing is done, the mouth of the estuary will effectively close.

PRDW Consulting (Port and Coastal Engineers) were thus commissioned to investigate the progressive sedimentation of Bushmans River Estuary using a calibrated two-dimensional hydrodynamic model and to test intervention options which could return the estuary to its former more functional state.

Fourteen intervention options were conceptually designed and tested in the model. The intervention options are grouped into the following categories:

  • Base case – the do nothing test.
  • Dredging – interventions involving dredging various areas of the river.
  • Tidal flooding – interventions utilising a collapsible dam to hold back the flood tide and release it during low tide to induce scouring of the river bed. An additional temporary dam section and current deflectors are also required.
  • Blocking of channels – based on the results of initial numerical model tests the temporary closure of selected channels was tested for their effectiveness of scouring certain areas.
  • Deflectors – use of deflectors to direct river flow in order to increase current speeds and scour in certain areas.
    Each intervention option was tested in the calibrated model. The performance of each option was assessed against a set of key performance indicators. The interventions were evaluated using a multi criteria analysis (MCA) method, which allows for the relative comparison of numerous alternatives using multiple decision making criteria. The results of the MCA are presented in Table 1.

Table 1: Results of multi-criteria analysis.

table1

Based on the results of this study a dredging option (D1, D2 or D3), suiting the available funding, should be considered as the preferred solution. If funding is constrained, a lower cost alternative which achieves scouring at the marina is available through the application of TC2 (temporary closure of the western opening of the R72 bridge through the installation of stop logs between the bridge piers).

The dredging plant currently being considered includes a small cutter suction dredger and a barge-mounted long-reach excavator. It is currently being considered that organic matter and eel grass will be disposed at a land fill site, while sand could be disposed at the beach to the west of the river mouth, in the slack areas in the sand dunes west of the mouth, at Middle Beach or at a land fill site. It is anticipated that the sand will be pumped to the disposal site via a pipeline consisting of floating and land-based components, with booster pumps required at approximately 800 m intervals.
Success on the Bushmans will lead to a similar operation being proposed for the Kariega which is equally in need of intervention. An extensive Environmental Impact Assessment will precede any action on this proposal.

Click here for the full report

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