THE STRAND REDEVELOPMENT MANAGING COASTAL IMPACT – A SUCCESS STORY Author: Dawson Wilkie, Director Engineering Services, Townsville City Council, PO Box 1268 Townsville QLD 4810 DAY 2 – SATURDAY MORNING SESSION WELCOME FROM TOWNSVILLE people. The Strand however had seen many sky shows as well as VP 50 and Townsville is a major regional city in North celebrations for the battle of the Coral Sea. Queensland. It is some 1800km north of In 1993 Council conducted a “design Brisbane so it has a need to be independent competition” to start the process of and self-supportive. We affectionately call it developing and improving the Strand. The the capital of North Queensland. competition attracted wide interest with many submissions received. Gillespie WHAT IS TOWNSVILLE? Peddle Thorp – Brisbane, submitted the winning entry. The designs included a number of interesting design concepts, Townsville is a mixture of light industry and including piles in the sea with replicas of is home to Queensland’s third largest marine animals on them, a lagoon to return industrial port. The port is an important part the area to its pre settled configuration, and of the City and has played a vital role in the many others. Unfortunately there was limited economic increase of the cane, fertiliser and funds and these works were not undertaken. mineral industries. Whilst the port is very In addition the Council over the years had important to Townsville’s economic growth it struggled with the problem of sand loss and also has a down side. The regular dredging what to do. of platypus channel has prevented the regular littoral sand drift, as it now is trapped In 1996 as a result of a further dredging in the channel. This in addition to the application by the Townsville Port Authority construction of a dam and weirs on the Ross Sinclair Knight Mertz were commissioned to River means that the Strand is now starved investigate beach erosion issues in of sand and that any sand lost in extreme Townsville. (1) This report was events is lost forever. In co-operation with comprehensive and forms the basis of the port, a number of other factors caused ongoing investigations. The “problems“ that major disruption to the flow of sand including Townsville faced in sand loss were four fold: the construction of the breakwater marina. - 1. The Townsville Port Authority contributed HISTORY OF THE STRAND to the lack of sand by way of the modification of the beachfront, The Strand was the coastal icon of reclamation dredging and maintenance Townsville however without the natural dredging of the Platypus channel. replenishment of sand it was slowly losing The report estimated that approximately its appeal. Cyclone Althea was a large 48,000 tonnes of suitable beach sand is cyclone that hit the coast in 1971. The lost annually. damage to the City was significant, as was the damage to the sea wall at the Strand. 2. The construction of the Ross River Dam Approximately 55% of the sea wall was and three downstream weirs also replaced with a new larger boulder prevented the flows of sediment by:- revetment with a nominal facing slope of § Trapping sediment behind their walls. 1:1.5 and a raised crest level to 6.1 meters § The flood retarding effect of the weirs, which was approximately 1m higher than the in that they cause water to flow more original wall. The high rock wall however slowly resulting in a lower sediment meant that the beach was lost to the view of carrying capacity. Page 1 of 1 PDF created with FinePrint pdfFactory trial version http://www.fineprint.com § The report estimated that the weirs and night Townsville Airport (a traditionally dry dam trap approximately 68,000 tonnes place) recorded in excess of 474 mm in a per year. period of five hours, other unofficial rain gauges recorded in excess of 700 mm for 3. The Queensland Department of Natural the same time. This translated into a record Resources and Mines allow the rainfall event of anywhere between 1 in 120 extraction of up to 260,000 tonnes per year up to 1 in 233 year event, depending year from the Ross River upper reaches. on which expert you speak to. Sand supply rates into the Ross River system are estimated at 34,000 tonnes Tropical cyclone Sid was around long per year. The net loss is not a simple enough to whip up a strong swell, which subtraction but could be in the order of stripped the remaining sand off the Strand. 200,000 tonnes per year. A severely weakened rock wall was then breached both by ocean swell and storm 4. Storm water flows through urban water runoff. Approximately 120 m of wall catchments is usually of higher velocity was lost along with the road and other however with improved drainage infrastructure. In all about 60% of the wall structures, sediment flows are was damaged. Again financial assistance predominately debris and waste from the was granted in the order of $1.4m urban area. The urban catchment also comprises only a small part of the total THE REPAIR catchment (approximately 42 square kilometers of approximately 750 square The old saying of “good things come out of kilometers). adversity” fell true for Townsville. The Premier of Queensland, standing on a wind The report identified a long-term problem swept rock front after having seen the that had to be resolved. flooding and hardship, agreed that to allow the City to continue, the State Government THE DISASTER No. 1 would allocate significant funds to repair the Strand. In March 1997 the residents of Townsville were warned of the approaching tropical The January 1998 event had started the cyclone Justin. Before crossing the planning. We had the concept for a coastline the cyclone disintegrated into a recreational design and an understanding of tropical rain depression and settled over the the technical problems. It was also decided city. The result was a declared disaster, to design for a much higher level of with many homes flooded and people immunity and 1 in 100 year event became displaced. The Strand experienced a the design criteria. severe wind storm event, most likely in the order of 1 in 50 year event, and suffered a The usual problems were there. What to do major loss to the remaining sand. The with the storm water outlets that don’t look persistent wave action over several weeks attractive flowing across the sand. How to during ex-cyclone Justin resulted in the keep the sand etc. The result was that we undermining of the central portion and had to step in and do what nature once did. subsequent failure of approximately 33% of The saving grace was that in all but extreme the wall. The event was of such magnitude events the sand was held in the Cleveland that Natural Disaster Financial Assistance Bay basin and was just moved around to the Arrangements were triggered. The costing tune of 70% south and 30% north. All we of the repair was $1.8m. need to do was to truck it back! THE DISASTER No. 2 THE PROPOSAL th The evening of Saturday 10 January 1998 The problem was assessed and Council was a land mark day in Townsville. All for decided that whatever solution was adopted the wrong reasons. During the course of the it had to be a long-term solution. It was from Page 2 of 2 PDF created with FinePrint pdfFactory trial version http://www.fineprint.com this that the headland concept was shallow water with a gently shoaling seabed introduced. The idea of compartmentalizing that allows larger waves to break before the beach and increasing the time the sand reaching shore. was trapped solved a number of problems. It remedied the stormwater outlets, gave Other waves considered included those Council some land to do something with as generated by a cyclone in the Coral Sea. well as reduced the ongoing maintenance These waves were potentially the largest but costs for sand replacement. were attenuated greatly as they passed over the shallow Great Barrier Reef. Also waves What we did generated between the Great Barrier Reef and the coastline were considered. These The aims of the redevelopment were: waves set the design criteria. ¨ Secure the coastline with a civil structure designed and constructed to For the Storm Tide Frequency of 100 years withstand a 1:100 year storm event; the Storm Tide level was 3.1m, the water ¨ Restore the foreshore using a technique depth was 4.7m and the maximum wave designed to minimise the need for future was 2.8m. wall maintenance and sand replenishment; All options were considered with the ¨ Redevelop by reinstating a beach optimum protection being a beach that in the alignment slightly offshore, providing long term equilibrium can be maintained with greater opportunities for improved minimal effort. Given that the beach had amenity and passive recreation been relatively stable prior to those external opportunities; sand supply effects a beach system seemed ¨ Redevelop to create stable “sandy to be the answer. It also met some of the beach” compartments and extend active expectations of Council for recreational recreation such as swimming and facilities. fishing; ¨ Provide all of the above and, at the The concept of foreshore protection and same time, minimise the risk of tidal land reclamation is often based on environmental harm. placement of large quantities of sand with the realisation that nature will eventually Council had retained Coastal Engineering create the beach profile. While protection Solutions as their consultant after cyclone through rock armoring is by no means a new Justine.