The Tweed River Entrance Sand Bypassing Project – Ten Years Of
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THE TWEED RIVER ENTRANCE SAND BYPASSING PROJECT TEN YEARS OF MANAGING OPERATIONS IN A HIGHLY VARIABLE COASTAL SYSTEM C Acworth 1 and S Lawson 2 1Department of Environment and Resource Management, QLD 2Department of Primary Industries, NSW Introduction The Tweed River Entrance Sand Bypassing Project (TRESBP) is a joint coastal engineering project of the New South Wales and Queensland State Governments. The Tweed River entrance training walls were extended to improve the navigability of the river entrance in the 1960s. This substantially reduced the net northerly transport of sand moving to Queensland and resulted in severe erosion, recession and vulnerability of the southern Gold Coast beaches to large swell events. By the early 1990’s, North Letitia Beach, located immediately to the south of the southern training wall, had accreted so significantly that a sub-tidal delta had once again formed at the Tweed River entrance, creating a navigational hazard for vessels. To address the coastal management issues in both NSW and Qld, the TRESBP was developed by the State governments. The objectives of the project are to establish and maintain a navigable entrance to the Tweed River; and to provide an ongoing supply of sand to the beaches that is consistent with the natural net rate of longshore drift. The permanent bypass system was commissioned in 2001 and is operated by the Tweed River Entrance Sand Bypassing Company (TRESBCo), a subsidiary of McConnell Dowell. The system consists of a sand pumping jetty that intercepts northward moving sand at Letitia Spit, pumping the sand via a buried pipeline primarily to the east snapper rocks outlet. Dredging is periodically required to maintain the navigation channel when the sand transport rate exceeds the capacity of the bypass system (Figure 1). The project area is a complex high energy coastal/estuarine environment with various coastal processes operating over a range of temporal and spatial scales. Managing sand delivery in such a highly variable coastal system has proven to be challenging, particular when taking into consideration the needs of a wide range of stakeholders. This paper will describe some of the natural coastal processes that present particular management challenges in the TRESBP operational environment and provide examples of methodologies and tools that have been adopted for more effective coastal management. Location The project area is located on the border of NSW and Qld, approximately 100km south of Brisbane and 900km north of Sydney, extending from Coolangatta Creek in Qld, to Northern Letitia Beach in NSW (Figure 2). The project area falls into the jurisdiction of both Gold Coast City Council (GCCC) and Tweed Shire Council and is an area of natural beauty offering world class beaches and surfing breaks to both local residents and domestic and international tourists. 1 Figure 1 Sand bypassing jetty and discharge pipelin C ur r u mb in C r e ek C u r r u m b i n r o c k C u E l e p h a r n t r r o c k u m T E E M A N G U M S T . b i n T u g u n S E A S T . SH OR T S T . JO HN S T . S UR B F I S L T. I N G A S . L .S . C . M A T T E RS S T. E n d B o u l d e r W a l l G I B S O N S T . B MI LL S ST . i l GE ORG i E S T. n g GR AH AM ST. B o a u l d e r W a l l CA HI LL ST. e layout JO HNST ON S T. ARCH ER ST . N T H . K I R R A S . L .S . C L A . NG S T. CREEK ST. W I N S T O N K S T . CO YNE ST . i r Figure 2 Location of the Tweed River Entrance Sand HAI G ST. r a DOUGLAS ST . LORD ST. QLD CHURCH ILL ST . ST . C 0 MIL ES M U S G R A V E ST. M o A R P I TL DGE NE R E U O W S E T L . L S T . P AR A o D E ST REET GARRICK 500m l AN ST. MCLE a G R N.S.W. I F F I T H S T . DUT TON ST . n M A R I N E g 1000m T ARNER S . a W P A R A D E t t NAL D ST. MCDO a S T N . G R E E N M O U N C L T A R H K I S L T L . HILL ST. HILL MARI NE S T N . R BOUNDARY STREET B 5 WARD ST. WARD S T PARADE N .R B 1 N S 1 T 8 N 0 .R 0 B EDEN AV. 2 0 . 0 . ST PETRIE T w e e d P R i v e D r o a i n n t g e r L e t i t i a S p i t Bypassing Project 2 Coastal Processes The state of the coastal system depends on the underlying geology, the nature and abundance of the coastal sediment and the degree to which these controls are acted on by marine and atmospheric forces. Feedback loops are inherent in coastal systems, with the beach state at any particular time depending on the incident and preceding interactions between the sediments and coastal processes. The nature of these feedback loops results in the coastal zone being highly stochastic, dynamic and very challenging to predict (Figure 3). Figure 3 Primary components involved in coastal morphodynamics (Masselink and Hughes, 2003) These components of coastal systems apply directly to the project area where a large range of coastal processes operate over various temporal and spatial scales. The following will briefly outline these processes thus providing a background into the operational environment of the Tweed River Entrance Sand Bypassing Project. Coastal Geomorphology The project area forms part of a Holocene dune barrier system that was formed when rising sea levels submerged valley mouths and coastal lowlands that subsequently began filling with sediment from the land and sea during the most recent cycle of seal-level rise (Roy et al , 2001). This took place during the Holocene Marine Transgression, which was the most recent world wide rise in sea level that accompanied global warming and de-glaciation, bringing the oceans to their present level some 6,500 years ago (Bird, 1965). Coastal bedrock features of Fingal Head, Cook Island and Point Danger, have provided controlling influences on the movements of sand and the coastline shape throughout the past 6,500 years of shoreline evolution. The Tweed River is naturally a wave dominated estuary and the morphology of the area has changed significantly due to the construction and subsequent extension of the training walls (Figures 4-6). 3 Figure 4 Point Danger and the Tweed River Entrance in 1935, before the Tweed River training walls were extended Figure 5 Point Danger and the Tweed River Entrance in 1967 shortly after the Tweed River training walls were extended Figure 6 Point Danger and the Tweed River Entrance in 2004 shortly after the bypass system was commissioned 4 Sediment Transport The sediment reserves that were deposited during the Holocene Marine Transgression now forms the supply of sand that originates just north of the Clarence River in mid-northern NSW and flows parallel along to coast, slipping over the continental shelf, just north of Fraser Island. Each year, approximately 500,000 cubic meters of sand moves along Letitia Beach as a result of this longshore sediment transport process. Although the northward net longshore transport of sand along the project beaches is continuous, it varies both spatially and temporarily due to the effects of the predominant wave climate, currents, geomorphological control features such as heads and man-made structures such as the Tweed River entrance training walls. Superimposed on the predominant process of longshore transport is cross-shore sediment transport. Within the TRESBP project area, significant cross shore sediment transport occurs during large swell events when sand is removed from the upper beach face and transported offshore to form a subaqueous bar. This process can be particularly damaging if exacerbated by storm surge and spring tides. Conversely, during low swell periods, the sand previously deposited in offshore bar formations migrates onshore, aided by aeolian sand transport to re-build the upper beach face. A particularly important sand transport process in the project area is the movement of large quantities of longshore sediment transport around heads in ‘slug formations’. Slug movements can operate over various time scales, however significant movements are generally associated with periods of high wave energy, aided by accelerated longshore currents. During these conditions, episodic ‘slugs’ of sand are moved past heads, whereas longshore transport at adjacent beaches tends to be more continuous at lower rates. These movements of sand result in a temporarily imbalance in the sediment budget with a loss of sand from the updrift beach and an accumulation of sand to the immediate downdrift of the head (Patterson, 1999). Wave Climate The beaches of northern NSW are wave dominated with a micro-tidal current regime and an average deepwater wave height of 1-2m (Short and Woodroffe 2009). The median significant wave height is 1.3m, with severe ocean storm waves in excess of 6m (the maximum recorded at Tweed Heads is 7.5m).