Construction overview

The Tweed River Entrance Sand Bypassing Project is an unobtrusive installation designed for unattended operation, low operating costs and minimal maintenance requirements. The Tweed River Entrance Sand Bypassing Project is similar in design to the successful Nerang River Entrance Sand Bypassing System, which was also designed and constructed by McConnell Dowell.

The sand collection system is the key to successful operation of the Tweed River Entrance Sand Bypassing Project. This comprises of a sand collection jetty with an overall length of 450 metres constructed perpendicular to the Letitia Spit beach some 250 metres south of the tip of the southern breakwater.

The jetty extends out to the location of the September 1997, RL-5.00 contour. A sand trap is formed under this jetty by the operation of a series of ten submerged jet pumps. The sand trap is maintained as a permanent depression under the sand collection jetty and natural wave driven currents feed sand into it. In the early stages of operation, the accumulated sand reserve on Letitia Spit is also used to supply the sand traps.

The system is operated with up to five jet pumps operating together at one time and the sand slurry is discharged into a flume and flows under gravity to a slurry pit located on shore.

The discharge of each jet pump is monitored for slurry density to enable the operating jet pumps to be switched when the sand supply at each jet pump is exhausted. The use of jet pumps to collect and transport the sand from the sand trap eliminates the need for major plant items on the jetty, thereby reducing the risk of environmental damage.This simplifies the functions of the sand collection jetty to: providing access over the sand trap area; and supporting the required pipework in such a way as to ensure the safety of jetty users during operations.

Maintenance requirements are also reduced, as all large plant items are located in protected environments. Platforms are provided at the jet pump locations for maintenance purposes.

The jet pumps are operated using high-pressure water drawn from the Tweed River upstream of the entrance. Here, water free from sand, can be reliably obtained to reduce wear in the water pumping equipment. Clean salt water is collected at the low-pressure pump station, which comprises of one submersible pump supported in the Tweed River on a steel piled structure. Screens at the pump inlet prevent large items from being drawn into the pump. The low-pressure water is delivered to the control building via an underground 600 mm diameter medium density polyethylene pipeline to supply the high-pressure pump, which in turn supplies the jet pumps on the jetty.

A single high-pressure pump driven by a variable speed motor supplies the high-pressure motive water for the jet pumps. The pump speed is adjustable and limits the maximum volume of sand delivered by the jet pumps to match the capacity of the sand transfer system. The control system automatically cycles combinations of the jet pumps to maintain the sand trap and optimise the sand delivery rate.

Sand slurry discharge from the jet pumps is returned to shore via an inclined flume pipe to a slurry pit. A screening device is provided to prevent large items (shells, bricks, stones etc) picked up by the jet pumps from reaching and blocking the sand transfer system. These items are collected in a separate waste bin and disposed of appropriately.

The sand transfer system draws sand slurry from the pit and pumps it through a 400 mm diameter polyurethane lined steel slurry pipeline under the Tweed River to one of the two fixed or two temporary outlets that exist along the southern Gold Coast beaches or at Duranbah in New South Wales. The main fixed outlet is at Snapper Rocks East, and a secondary fixed outlet is located at Snapper Rocks West. A temporary outlet could not be assembled safely at Snapper Rocks West due to wave and tide action so a fixed outlet has been constructed by imbedding the pipeline in the rock platform. An outlet at Kirra is assembled as necessary, and similarly at Duranbah the outlet can be located where it provides the best outcome for the area. The system is designed to discharge to only one outlet at a time - the operating outlet is selected by operating and locking valves, which are located at branches along the slurry pipelines.

Sand slurry is pumped from one of two slurry pumps, or two pumps in series depending on which outlet is selected. One variable frequency power supply is provided and normally connected to the first pump motor. The second pump is driven by a fixed speed drive. The variable speed drive will be used to vary the discharge pressure to control the pipeline operating velocity within safe limits and control the maximum slurry density in the pipeline to eliminate the possibility of blockages. The slurry pumps are configured using interchangeable spools to enable either pump to be operated alone for the closer outlets including the main operating outlet at Snapper Rocks East. The slurry pumps are configured to operate in series for the more distant outlets (Snapper Rocks West and Kirra Point).

The sand transfer system from the control building is designed to operate at slurry densities up to 50% by weight. The maximum slurry density would apply for sand transfers to the main outlet at Snapper Rocks East and also Snapper Rocks West. Lower slurry densities may be utilised when pumping to the remote temporary outlet at Kirra Point in order to reduce the maximum pumping head.

A slurry pit receives the sand slurry from the jet pumps and concentrates the sand slurry to the required density to suit the outlet point. Surplus water from the slurry pit overflows through an underground medium density polyethylene 750mm diameter combined stormwater drain to the Letitia Spit beach.

All large plant except for the low-pressure water supply pumps is located within the controlled environment of the control building. This building also houses all electrical and control equipment, control room, maintenance facilities and staff amenities. A secure compound around the control building is provided for the safe storage of supplies including spare jet pumps and pipework, including the medium density polyethylene (MDPE) pipe to be used for the temporary outlets. A comprehensive computerised control system is provided to operate and control the entire system (except outlet selection which is manual). The system maintains full records of all operations including the number of hours pumped, and sand quantities pumped to each outlet.

Scope of works

The Scope of Works included the following major work components:

  • Design of the entire sand bypassing system.
  • Supply and construction of a sand collection jetty.
  • Supply and construction of a slurry pipeline under the Tweed River by horizontal directional drilling.
  • Supply and construction of slurry transfer pipelines on Letitia Spit and through Tweed Heads and Coolangatta townships.
  • Construction of slurry outlet structures.
  • Supply and installation of all process and services mechanical equipment and associated piping.
  • Supply and installation of electrical works.
  • Cathodic protection of jetty structure and pipelines.
  • Supply and construction of a low-pressure pump station.
  • Supply and construction of a control building and compound.
  • Commissioning of the entire system.
  • Dredging and beach nourishment works as instructed by Governments.
  • Hydrographic survey works.

 

Page last updated/reviewed: 28 Feb 2014