Long Term Average Sand Delivery Rate
Re-Assessment of the Long Term Average Sand Delivery Rate for the Tweed River Entrance Sand Bypassing Project
The Tweed River Entrance Sand bypassing Project (TRESBP) is a joint scheme of the New South Wales and Queensland Governments. The primary objectives of the project are to establish and maintain a navigable entrance to the Tweed River and to restore and maintain the coastal sand supply along the southern Gold Coast beaches (General Information).
The sand bypassing jetty, which commenced operation in 2001, is the primary means for achieving these objectives by capturing the majority of the northwards moving coastal sand drift before it reaches the river entrance. The jetty system pumps the sand through a pipeline under the Tweed River to beaches north of the river entrance. Access diagram showing Basis of Outlet Usage.
Sand that is not intercepted by the jetty, particularly during high sand transport storm events, is moved by waves and currents into the river entrance area. The jetty operation is also supplemented as required by occasional dredging at the river entrance, to remove sand that has gradually accumulated in the entrance over time, as part of the sand bypassing operations.
A minor portion of the sand drift that is not intercepted by the jetty does not remain in the entrance area but is naturally reworked across the entrance area towards the Queensland Beaches (ie 'natural bypassing' of the river entrance).
The TRESBP commissioned BMT-WBM consultants to undertake an investigation of the long term average sand transport rates through the project area. The purpose of the study was to re-assess the 'Long Term Average' (LTA) sand delivery target for the sand bypassing operations.
The LTA sand delivery target is formally defined as:
"the long term average annual net littoral transport of sand that would, in the absence of any artificial actions to influence it, cross a line perpendicular to the coastline, situated one kilometre south of the southern training wall at the Tweed River entrance and extending to the 20 metre depth contour, less the annual net quantity of sand which, after the commissioning of the System, crosses that line and reaches Queensland", (ie 'natural bypassing').
From this definition, it can be seen that the LTA is not the same as the average natural sand drift. It is the sum of the average natural bypassing quantity plus the LTA quantity of sand (ie artificial sand bypassing delivery target), which together make up the average natural sand drift supply.
The initially adopted LTA sand quantity was 500,000 m3/yr.
BMT-WBM re-assessed the LTA by carrying out numerical modelling of wave propagation and longshore sand transport at the project site, and analysed coastal survey data and measurements of dredging sand and sand pumping quantities over the period from 1993 to 2015 (after about 15 years of system operation).
The study evaluated the 'natural bypassing' of sand to Queensland beaches, which has varied significantly over this period as it depends on the variability of the natural sand transport processes near the jetty and at the river entrance, and the sand bypassing pumping and entrance dredging operations which have also varied over time.
The investigation also assessed the study results derived from the period of available data (1993 to 2015), against the longer-term context.
Access a copy of BMT-WBM's LTA Re-assessment Report 2015 (PDF 3.0MB).
The study reported a re-assessed LTA rate of 490,000 m3/yr, which is very close to the original estimate of 500,000 m3/yr (within 2%).
It is noted that the LTA quantity is a longer-term average yearly sand delivery target and the natural coastal processes are highly variable resulting in natural sand supply conditions that are likely to be significantly different from year to year, to which the sand bypassing operations must respond.