Floating breakwater system to save mangroves at the Port of Richards Bay

Abstract

The construction of Berth 208, a new additional bulk liquid berth, in the Port of Richards Bay, has been completed. This berth shares a border with the eChwebeni Natural Heritage Site (NHS), which is part of an original mangrove site that existed prior to the development of the port. The environmental impact assessment (EIA) for Berth 208 identified potential impacts in respect of this site, with specific concern for the expected increased shoreline erosion due to greater ship movement.

INTRODUCTION

The construction of Berth 208, a new additional bulk liquid berth, in the Port of Richards Bay, has been completed. This berth shares a border with the eChwebeni Natural Heritage Site (NHS), which is part of an original mangrove site that existed prior to the development of the port. The environmental impact assessment (EIA) for Berth 208 identified potential impacts in respect of this site, with specific concern for the expected increased shoreline erosion due to greater ship movement. The record of decision (RoD) recommended that a solution be found to address the ongoing erosion between the existing Berth 209 (and the newly commissioned adjacent Berth 208) and Spinach Point.

The floating breakwater system (or floating pontoons) option was preferred from an environmental perspective.

As noted above, the mangroves (woody plants that grow at the interface between land and sea in sheltered and lagoon waters at tropical and sub-tropical latitudes) in the natural heritage site represent part of an original mangrove stand which existed prior to the development of the port. The area is thus particularly worthy of conservation, and its designation by Transnet SOC Limited, under authority of the Transnet National Ports Authority (TNPA), underscores the conservation significance of the site.

Three species of mangrove trees occur in the NHS, namely the white mangrove Avicennia marina), the black mangrove (Briguiera gymnorhiza) and the red mangrove (Rhizophora mucronata). The eChwebeni NHS is one of the few areas in the country where all three species co-exist.

eCHWEBENI NATURAL HERITAGE SITESHOREUNEEROSION

The study identified two major impacts, namely the erosion of the shoreline, and paradoxically, the deposition of sand by high tidal waters immediately landward of the eroded zone along the shoreline. The erosion of the shoreline has resulted in the loss of white mangrove, which is a primary coloniser of the NHS ecotonal habitat.

The sand deposited by high tidal waters onto the mangrove shelf of the eChwebeni NHS forms a levee that interferes with drainage from the land to the harbour, thus reducing the tidal interchange, causing the death of mangroves and a change in the composition of the vegetation cover at the site. All the channels that allow tidal interchange are threatened by sediment deposition, and some are completely blocked. The cause of the problem is the wake wave generated by the movement of vessels in and out of the harbour close to the site. Tugboat waves are considered to be the most significant in terms of erosive potential.

Any solution sought must be aimed at preventing or reducing the generation of wake waves, either through restricting the speed of the tugboats or through a floating breakwater system, The latter was preferred when both technical and environmental performance criteria were considered.

DESCRIPTION OF THE FLOATING BREAKWATER SYSTEM

For the design of the pontoons the CSIR (Council for Scientific and Industrial Research) captured true data from the site, which was used to build a model to test the design criteria. This model test confirmed the effectiveness of the floating breakwater system.

The pontoons are placed away from the shoreline in open water, and since the pontoons float, water flow will not be obstructed. A total of 44 concrete pontoons, each weighing approximately 63 tons and being 15 m long ? 5 m wide ? 1.5 m deep, were precast and lifted onto the water moored into position. The pontoons are coupled together with specialised connectors and then moored to the seabed. The pontoons will eventually extend over a distance of approximately 700 m from the existing walkway of Berth 208 to Spinach Point. They act as a floating attenuating structure.

The floating breakwater system and the mooring system were designed by WSP (Pty) Ltd and Seaflex (a Swedish supplier of the Seaflex mooring system) respectively.

ENVIRONMENTAL PERFORMANCE OF THE FLOATING BREAKWATER SYSTEM

The floating breakwater system is designed to achieve 70-80% wave reduction efficiency. Three environmental performance criteria were assessed, and the conclusions were as follows:

* The floating breakwater system (pontoons) allows sufficient tidal exchange between the mangroves and open port waters.

* The system allows faunal migration between mangroves and open tidal waters.

* As the floating pontoons are in open water away from the shoreline, they do not obstruct water flow, while the helix anchors on the seabed create artificial habitat likely to be used by invertebrates and fish. Similarly, the submerged sections of the pontoons will create artificial habitat likely to be colonised by invertebrates and fish. The floating pontoons could also become a significant roost area for birds.

TOTAL COST OF THE PROJECT

A budget of R50 million was allocated to the project, which is expected to take a year to complete. Stefanutti Stocks (Pty) Ltd is the main contractor for the construction of the floating breakwater system (floating pontoons).

TRANSNET ENVIRONMENTAL GOVERNANCE FRAMEWORK

As with all projects within Transnet Capital Projects, the project is governed by strict environmental controls. The following documents have been developed to ensure that the project is implemented in an environmentally sound manner and in line with the NEMA (National Environmental Management Act) principles for sustainable development. These documents include the following:

* TCP Construction Environmental Management Plan, which sets out roles and responsibilities for environmental management for the employer and the contractor.

* TCP Standard Environmental Specifications, which outline the minimum environmental standards for projects under implementation or in the construction phase.

* TCP Project Environmental Specifications, of which the following documents form a part: (1) Berth 208 Phase 1 - Environmental Authorisation and Associated EMP, (2) TNPA EMP, and (3) the eChwebeni NHS EMP.

As per recommendation, Transnet Capital Projects has appointed an Environmental Control Officer to monitor compliance in respect of all of the above. The construction works will adhere to the eChwebeni NHS Environmental Management Plan aimed at the preservation of this natural heritage site.

CONCLUSION

From the environmental perspective the floating breakwater system is considered the best overall solution. This project clearly demonstrates Transnet SOC Limited's commitment to ensure that environmentally responsible and innovative solutions are found to potential impacts that its operations and infrastructure investments might have on the environment.

INFO

Deirdre Tholet

Communications Manager

Transnet Capital Projects

[phone]

[email protected]