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By Norman Cowper (Snr) and Allan Thomas, Slurry Systems Pty Limited

The Nerang Sands Bypass System in Queensland is located immediately south of the Gold Coast Waterway entrance. The system was designed by Slurry Systems Pty Limited and constructed by McConnell Dowell Constructors Ltd. The system began operation in 1986 and was world-first technology then, and still remains very much at the leading edge of sands bypassing technology.

Because of the prevailing wave direction in the area, there is a littoral drift of sand northwards along the coast averaging about 500,000 m3 per year. Without any intervention, the sand would build up against the southern groyne until eventually flowing around the tip of the groyne and forming sand bars in the entrance to the newly created Gold Coast Seaway. The conventional solution would be to periodically dredge the entrance but this method could not be guaranteed to keep the entrance navigable at all times. The Nerang Sands Bypass System was the world’s first fixed sands bypass system capable of operating in all weather conditions.

The system consists of a 500 m long jetty with ten vertical jet pumps spaced along its length and buried in the sand at RL minus 11 m. The jet pumps pump sand/water slurry vertically up into a sloping flume which transfers it by gravity to the land based transfer pump station to feed a conventional centrifugal slurry pump. This pump then pumps the slurry through a pipeline laid under the entrance to the north for discharge on to the beach.

During its 27 years of operation the system has been extremely successful. It has proved capable of continuous operation during the most severe storms and has transported a total of 15 million cubic metres of sand.

Design basis

Sand Properties

Specific Gravity – 2.67

Particle Size – d90=0.45 mm, d50=0.23 mm, d10=0.17 mm

Design Sand Transfer Volumes

Annual average volume – 500,000 m3

Peak monthly volume – 200,000 m3

Peak 5 day volume – 100,000 m3/h

These sand volumes refer to the in-situ volume of settled sand at an approximate volume concentration of 60%. (i.e. one cubic metre of in-situ sand is equivalent to about 1.6 tonnes of dry sand). The system is designed to transfer a maximum 585 m3/h of (in-situ) sand with normal weekly operation being 30 hours to transfer 10,000 m3 of sand.

System description

The sands bypass system consists of a remote sea water supply pump station, a jet pump recovery system, a flume transfer pipe, a transfer pump station and a sand transfer pipeline.

A jet pump operates by providing a high velocity upwards flowing jet of water which entrains sand. Because of the high water velocity in the jet it is essential that the supply water be free of sand. Two 150 kW low pressure sea water supply pumps, installed in a remote pump station inland on the Broadwater, supply water to the suction of two 560 kW high pressure jet water supply pumps located in the main onshore pump station.

Ten Genflo jet lift pumps are installed on the jetty to serve the full 300 m length of the sand trap, with any four or any seven operating at any one time. Under normal operation one low pressure sea water pump and one high pressure jet water supply pump supply four jet pumps to transfer the average sand volume. When peak sand transfer rates are required the second sea water supply and jet water supply pump and the remaining three of the seven jet pumps are operated. Any combination of seven of the ten jet lift pumps covering the sand trap can be operated. Each of the ten jet pumps discharge into an elevated pipe flume transferring sand slurry to the transfer pump station. The outer four jet pumps transfer via separate horizontal pipes before discharging into the end of the flume.

The Genflo jet pumps are designed specifically for sand dredging operations and to be non-clogging under fully buried conditions. Each pump includes integral fluidising jets which expand and fluidise the sand bed, enabling sand to be freely entrained by the jet pump at a controlled concentration. As sand is excavated from the region around the pump, the sand bed collapses to maintain a fluid bed adjacent to the pump. The trap continues to expand until the walls stabilise at the prevailing angle of repose.

The pipe flume provides a non-blocking transfer system which has capacity to handle a wide range of flow rates and solids concentrations. With each jet pump discharging separately into the flume, jet pump performance is not affected by the discharge pressure of other jet pumps, enabling the jet pump units to be properly balanced for equal performance. The jet pump discharge and pipe flume are lined with polyurethane for extended life.

The pipe flume discharges into a cone bottom sump which supplies the 710 kW centrifugal slurry transfer pump. During peak flow operation, excess water overflows the pipeline feed sump and the sand concentration automatically increases to the maximum design value in the transfer pipeline. A single DN400 steel pipeline, lined with polyurethane, transfers sand under the entrance for discharge north of the northern groyne. The total length of the transfer pipeline is 1430 m.

Jet pump details

The photo shows a jet pump being installed and indicates the scale of the pump. Further details of the jet pump are provided in the drawing.

Figure 2 (above right) shows typical jet pump performance curves. The higher the ratio of jet diameter to throat diameter, the higher the discharge head but the lower the flow rate. The Nerang mixer diameter is 102 mm and the pump can pass particles up to 100 mm. Sea weed is easily disintegrated by the high velocity jet. Jet diameters range from 40 mm to 48 mm giving d/D ratios from 0.39 to 0.47. The mass flow ratio is around 1.5. Efficiencies of jet pumps are low, around 35% maximum.

Elevated flume performance

Selection of the optimum pipe diameter and slope of the flume was very important, and to confirm and refine predictions a test rig was built and operated in Sydney. Also of importance were the ingoing and outgoing slopes of the buried transfer pipeline beneath the entrance. At the end of the flume tests the rig was reconfigured to investigate the potential for sliding and blockage in the downslope and upslope during shutdown/restart.

The test rig consisted of two 12 m long clear plastic pipes, one 200 mm diameter (190 ID) and one 100 mm diameter (94 ID), the slope of which could be varied. A centrifugal slurry pump driven by a diesel engine transferred slurry from the 3 m3 collection hopper through 150 mm hose to either of the pipes. Flow exited from the sloping pipes into the hopper via a 1 m length of flexible hose which allowed the flow to be diverted into a 200 litre drum mounted on scales to measure flow rate and solids concentration. Measurements of slurry flow height and the height of any bed of solids, if present, were measured at locations 2, 4, 6, 8 and 10 m from the pipe entrance.

After analysis of the flume test data, a DN600 spiral welded pipe was selected for the Nerang flume.  The flume is 370 m long set at 2.5% slope and is lined with 6.4 mm of polyurethane.

Conclusions

The Nerang Sands Bypassing System has been a resounding success. It was the world’s first fixed sands bypass system capable of operating in all weather conditions and after 27 years operation still represents the gold standard for sands bypassing systems. To date it has transferred overt 15 million cubic metres of in-situ sand.

The Nerang Sands Bypass system requires a jetty structure. More recently Slurry Systems embarked on the development of a significantly cheaper sand bypass concept not requiring a jetty. This concept can be applied more universally to the majority of ocean entrances.

The development resulted in the invention of a fixed submarine sand recovery system, the Slurry Systems Sand Shifter Unit (SSUnitTM ). The SSUnit development was trialled at Noosa Qld. The Noosa system recovers littoral sand which accumulates at the western end of Noosa beach and recycles the sand for beach nourishment at the eastern of Noosa beach. The trialled system was successful and the permanent system was installed a couple of years ago. The technology has been commercially proved and is available for installation for future fixed bypassing of ocean entrances.  ■

 

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