SHOALHAVEN COASTAL EROSION REMEDIATION: THE HOLY GRAIL P Lawless 1, G Blumberg 1, I Ghetti 2, R Massie 2 1Haskoning Australia (a company of Royal HaskoningDHV), Sydney 2Shoalhaven City Council, Nowra Introduction Coastal erosion threatens significant public and private assets at Mollymook, Collingwood and Callala Beaches on the NSW South Coast, which are listed as “authorised locations” for emergency protective works under the Coastal Protection Act 1979 . Shoalhaven City Council has recently prepared its draft Coastal Zone Management Plan (CZMP) and draft Emergency Action Plan (EAP) for the Local Government Area (LGA). Strategies and actions identified in the draft CZMP for the “authorised locations” beaches mainly focus on relocation of public assets, building dune resilience, and beach scraping. For some existing development exposed to coastal hazard risks, Council and the local communities now face the challenge of deciding how to mitigate the risks in the short-term, and manage the coastal impacts as they occur across a city where 35 beach compartments are managed by Council. This is reflected in a range of “make-safe/make- good” provisions in the draft EAP that can be introduced in the event of storms. In the last two years, Haskoning Australia (a company of Royal HaskoningDHV), has developed for Council prefeasibility schemes and conceptual designs for “end-state” protection of public and private assets at the three “authorised locations” beaches to determine the extent of mitigation costs (Royal HaskoningDHV, 2012, 2013a and 2013b). Measures considered included revetments, beach nourishment and nearshore breakwaters of suitable scale to deliver comprehensive protection to threatened assets. These schemes were costed in the tens of millions of dollars for each beach, and would face a number of other constraints to implement. Clearly, if asset protection is to be achieved, a more practical approach is required. This must offer affordable and feasible coastal protection that adequately mitigates some or all of the erosion risk. In light of this, Council is currently preparing an Adaptive Works Strategy for all of its exposed beaches, but which focusses on a number of high risk beaches in making the transition from “make-safe/make-good” to “end-state” (Royal HaskoningDHV, 2014). Options being considered for the “authorised locations” beaches include construction of a buried toe as an initial stage for possible future revetment construction, and sand backpassing from local creek entrances. 1 Assets at Risk The Shoalhaven Public Asset Coastal Risk Management Review (BMT WBM, 2012) was undertaken to prioritise public assets and infrastructure at threat from coastal hazards in the Shoalhaven LGA for the 2050 or 2100 planning periods. Several public assets at Mollymook Beach were identified as having ‘Extreme’ or ‘High’ risk ratings, most notably Mollymook Surf Club, sewage pump stations and the Golf Club. At Collingwood Beach, ‘Extreme’ or ‘High’ risk ratings were applied to a sewage pump station, wastewater infrastructure along Elizabeth Drive, the Illfracombe Avenue roadway and adjacent water infrastructure, and the public cycleway. At Callala Beach, ‘High’ risk ratings were identified for the Tennis Club (Callala Beach Community Centre), the Greenway Road roadway and adjacent water infrastructure. Of all beaches in the Shoalhaven LGA, Mollymook Beach has the highest value of assets at risk (Table 1), heavily weighted by the Golf Club ($10 million) and Surf Club ($3 million). Wastewater assets are also significant, with sewage pump stations valued at $1 million each. Furthermore, it is noted that damage or destruction of these assets would disrupt essential community services and create health risks. Table 1: Values of public assets at risk at subject beaches (BMT WBM, 2012) Planning Buil dings / Beach Wastewater Water Roads TOTAL Period Infrastructure 2050 $2.7 million $80,000 $13 million $300,000 $16 million Mollymook 2100 $3.1 million $170,000 $13.6 million $485,000 $17.5 million 2050 $1.8 million $40,000 -- $340,000 $2.2 million Collingwood 2100 $1.8 million $100,000 -- $390,000 $2.3 million 2050 -- -- $170,000 $60,000 $230,000 Callala 2100 -- $170,000 $665,000 $245,000 $1.1 million Coastline hazard lines for the Shoalhaven LGA were defined for the immediate, 2025, 2050 and 2100 planning periods. The location of these lines was at the landward edge of the Zone of Reduced Foundation Capacity (ZRFC), which takes account of the reduced bearing capacity of the sand adjacent to the storm erosion escarpment. Hazard mapping illustrating the immediate, 2025, 2050 and 2100 landward limits of the ZRFC is shown in Figure 1 (Mollymook Beach), Figure 2 (Collingwood Beach) and Figure 3 (Callala Beach). Private assets at risk can be summarised as follows: • At Mollymook Beach, 15 residential properties are located in the immediate coastal erosion risk area, while 46 residences are projected to be at risk in 2050, with 55 residences at risk by 2100. In addition, some 30 properties (highlighted in yellow) are at risk due to coastal inundation. • At Collingwood Beach, 17 residential properties are located in the immediate coastal erosion risk area, while more than 100 properties are projected to be at risk in 2050. • At Callala Beach, 12 residential properties are located in the immediate coastal erosion risk area, while more than 80 residences are projected to be at risk in 2050. 2 Figure 1: Coastal hazard zones for Mollymook Beach (Umwelt, 2012) 3 Figure 2: Coastal hazard zones for Collingwood Beach (Umwelt, 2012) 4 Figure 3: Coastal hazard zones for Callala Beach (Source: SMEC reported in Umwelt, 2012) 5 Existing Management Provisions Draft CZMP The management strategies and actions identified for the subject beaches in the draft CZMP include (Umwelt, 2012): • building dune resilience by maintaining minimum dune crest levels of 5.5 m AHD (Mollymook Beach), 5.0 m AHD (Collingwood Beach), and 6.0 m AHD (Callala Beach) and vegetation management; • beach scraping, particularly after storms; • dune planting and rationalisation of private (and mostly illegal) beach access tracks at Mollymook Beach; • reducing the number of pedestrian tracks on the frontal dune at Callala Beach to reduce pressures on the vegetation and encourage a resilient landform; • auditing site constraints and structure foundation capacities (e.g. Mollymook Surf Club and sewage pump stations); • review the design of the revetment that provides protection to Mollymook Golf Club; • medium-term relocation of sewage infrastructure along the beachfront reserve between Argyle Street and Berry Street (Collingwood Beach); • coastal condition and morphological monitoring; and, • delivery of information to the community. The actions and strategies in the draft CZMP could not be expected to fully mitigate the threat to public and private assets at the subject beaches, in particular that from design storm erosion. Emergency Action Plan Council has in place a number of emergency “make-safe/make-good” provisions, which are mainly enunciated in the draft EAP (Umwelt, 2011). In accordance with these Plans, Council will take the following generic actions during and after a coastal emergency event: • temporarily close public accessways damaged by storm waves; • monitor seawalls; • implement beach scraping if necessary. Preparatory nourishment to raise dune crest levels is also a proposed emergency action for Collingwood and Callala Beaches. Installation of sand-filled geotextile bags is also recommended at the toe of constructed accessways at Callala Beach, and to form a “tripper wall” at Blackwater Creek (Mollymook Beach). However, it must be recognised that it may not be feasible to implement these actions quickly before a storm hits. 6 “End-State” Solutions for Coastal Protection Mollymook Beach For the Mollymook Beach investigation (Royal HaskoningDHV, 2012), the beach was divided into five precincts (A, B, C, D and E) to identify protective works options, see Figure 4. Seawall Protection The existing 160 m seawall fronting the Golf Club and adjacent sewage pump station comprises a gabion revetment but is of insufficient capacity to provide 50 year protection due to a low and unacceptable Factor of Safety (FOS) against downslope failure, and because the fabric of the mattress could not be guaranteed for a further 50 years (Maunsell, 2008). The structure could be enhanced to meet the 50 year criterion by using it as an underlayer for a more robust cover protection. A total modified revetment cost in the order of $2.3 million ($14,500/m) is estimated. Little is known about the design and construction of the three other seawalls in Precinct A, which extend approximately 175 m in length. Disregarding these existing structures and allowing for the construction of new upgraded walls throughout could be expected to cost an additional $2.6 million ($14,900/m) in Precinct A. This includes a 50 m stepped seawall section in the vicinity of the Surf Club to improve beach access and amenity in this high usage area. The seawall upgrade works in Precinct A must be attended to immediately if the immediate threat to the infrastructure in the area is to be addressed. The revetment concept design for Precinct B, shown in Figure 5, comprises a conventional basalt rock revetment with a minimum of two layers of armour rock over two layers of underlayer rock. The structure toe would be set at -2 m AHD, and a crest level of 6.2 m AHD has been adopted. A total cost in the order of $8.0 million ($23,000/m) is estimated to provide seawall protection along the entire 350 m length of Precinct B. 7 Figure 4: Mollymook Beach precincts (Royal HaskoningDHV, 2012) 8 Figure 5: Concept section for proposed revetment section for Precinct B, Mollymook Beach (Royal HaskoningDHV, 2012) At the present time, the five southern-most properties in Precinct B are at immediate risk from coastal hazards. Creek training and dune enhancement works proposed in Royal HaskoningDHV (2012) would effectively translate the coastline hazard lines in the southern portion of the precinct such that the degree of risk to all 14 of the private properties in Precinct B is equalised.