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

More from our blog:

Dune report

Dune report

To inform our community and stakeholders of activities (Phase 1) undertaken by Ndlambe on the Kenton sand dune...

A word from Alan

A word from Alan

In the ever-changing landscape of the natural world, human interaction has always been a double-edged sword – creating challenges for the environment whilst equally striving to secure a better future for the future.

Annual General Meeting 2022

Annual General Meeting 2022

Our Annual General Meeting will be held on Thursday 22 December at 10:00 at the Kenton Bowling Club, Recreation Road. We are very pleased that Dr Amber-Robyn Childs will address the meeting on her research into tracking the movements of dusky kob both in estuaries...

Brace yourselves swimmers

Brace yourselves swimmers

Eastern Cape climate warning as Agulhas current drifts off Thank you Mike Loewe and DispatchLIVE for kind permission to include this article in our Newsletter. Weird upwellings of frigid water between East London and Port Alfred are affecting local climate, says...

Signage regarding Pipefish

Signage regarding Pipefish

Estuary Care has once again been involved in working alongside Dr Louw Claassens, a Science Officer and Researcher from the Palau National Marine Sanctuary and her team, in designing appropriate signage regarding Pipefish that are found in both our Boesmans and Kariega Estuaries.

Plastic pollution

Plastic pollution

The article below which gives background to the ‘Towards Zero Plastics to the Seas of Africa’ conference is informative and thought provoking. Thank you Dr Tony Ribbink of Sustainable Seas Trust.

AGM 2021

AGM 2021

We are privileged to have Dr Angus Paterson speak at our AGM: Date:  Wednesday 23 DecemberTime:  10:00Venue:  Kenton Tennis ClubTopic:  New technology for a new era – Marine science in SA for the next decade Dr Paterson is the Director of the South African...

Benches

Benches

Estuary Care has organised the placement of a few benches recently and two are on order for December. Ted Gilfillan has helped with the application for another one to be placed at the Kariega Car Park. The benches are made from recycled plastic and are provided...

A threat to the ocean

A threat to the ocean

In 2020 alone, 1.6 billion disposable masks entered the ocean. This is equivalent to 7% of the size of the Great Garbage Plastic Patch and may take over 450 years to biodegrade.  At this rate, there risks being more masks than jellyfish in the ocean. COVID-19 has...

Membership subscriptions 2022

Membership subscriptions 2022

The Estuary Care Committee would like to send out a plea to all recipients of the newsletter to pay their annual subscription fees and to also encourage all other people they know who enjoy the Kariega and Boesmans Estuaries to join and pay their annual fee. The...

Update from our Chairman

Update from our Chairman

We begin the June 2021 newsletter with an important update from our Chairman, Stuart Clarkson. Dear Estuary Care Members and Friends In our last newsletter I reported on our ongoing efforts to secure a sustainable and low risk solution to the ongoing accretion of...

Lewis Pugh Foundation

Lewis Pugh Foundation

Our September 2018 Estuary Care Newsletter featured an article on Lewis Pugh. He had completed what he called The Long Swim which was from Land’s End in Cornwall to Dover in Kent to raise awareness about the health of the worlds oceans.  Read it hereA recent...

Channel Markers & Rock Marker

Channel Markers & Rock Marker

Dave Curran (top), with the help of Don Thomson (bottom) repaired and replaced a number of channel markers in the Kariega Estuary earlier this year.They also replaced the rock marker, which Dave had made, in the Kariega Estuary.Dave and his team install rocks...

June 2021 photos

June 2021 photos

Boesmans Estuary Courtesy of Rob BoydGiant Kingfisher photographed at Boesmans Estuary Courtesy Ted MossGiant Kingfisher photographed at Boesmans Estuary Courtesy Ted MossMiddle Beach low tide  Courtesy Colin Milliken Middle Beach low tide Courtesy Colin...

Channel Markers

Channel Markers

In January Dave Curran with the help of Don Thompson and their team set out to replace various channel markers in both estuaries.  Dave also made a replacement rock marker which was installed in the Kariega...

Repair of the Kenton Jetty

Repair of the Kenton Jetty

As was reported in the previous newsletter the Kariega Slipway Jetty broke loose in a storm and was retrieved by an Estuary Care team. After a covid interrupted process Chester Wilmot, Nick Albrightson and Dennis Dallas completed an extensive repair and...

A huge ball of plastic

A huge ball of plastic

A huge ball of plastic was found above the high tide mark between Middle and Main Beaches.  It probably washed ashore but we were determined it would not be washed back into the sea.  Our Chairman Stuart came to the rescue and removed...

Memorial Benches

Memorial Benches

Over the years Estuary Care has assisted members of the public with the purchase and placement of benches in memory of loved people (or pets!) or simply as a replacement for an existing municipal bench which has become worn or broken down. The procedure to be...

Membership subscriptions

Membership subscriptions

The Estuary Care Committee would like to send out a plea to all recipients of the newsletter to pay their annual subscription fees and to also encourage all other people they know who enjoy the Kariega and Boesmans Estuaries to join and pay their annual fee.

Dune report

Dune report

To inform our community and stakeholders of activities (Phase 1) undertaken by Ndlambe on the Kenton sand dune...