T&T Ref: 28050 07 July 2011 Auckland Council 35 Graham St Auckland Attention: Ian McCormick Dear Ian, Emergency Geotechnical Inspection of Rockfall Rothesay Bay Cliffs, Auckland Introduction As requested, Tonkin & Taylor inspected the subject property on 2nd July 2011 to assess the safety of the natural rockfall event associated with fatality at Rothesay Bay, on Auckland’s North Shore. In particular the visit was undertaken to: • • • Assess the extent and cause(s) of the rockfall and geotechnical issues related to the fatality Determine whether there is an imminent threat to public safety on the adjacent foreshore Make engineering suggestions for immediate safety and longer term actions for public safety. The following is our report on the Rothesay Bay site inspection and rockfall assessment. Site Description The site is part of the coastal cliff extending 1 km from Rothesay Bay in the north to Murrays Bay in the south and is approximately located on the seaward side of 131 Churchill Road, Rothesay Bay. This section of near vertical cliff is 30-40m high, and comprises an interbedded sequence of subhorizontally bedded sandstones and siltstones with a shallow (2-3m) mantle of soil and weathered rock (refer photograph 1). At the base of the cliff is a relatively level wave-cut platform within sandstone. Numerous sandstone boulders (0.3m to 1.0m in size) are common within a distance of about 10m from the base of the cliff (refer to photographs 3 & 4). There appear to be local concentrations of boulders in places within the northern half of this stretch of cliff line from Rothesay Bay beach to the Gum Digger Track outfall at the Churchill Rd Reserve, although boulders are distributed to varying degrees all along the base of this cliff line. The local concentrations of boulders on this northern stretch appear to be associated with a prevalence of thicker sandstone beds within the cliff. The wave cut platform in this area is oriented at an oblique angle to the foreshore. Where exposed to tidal action, the wave cut platform often has a slimy brown coating and is very slippery underfoot. We observed during our initial visit that there were at least three locations where minor rockfalls had recently occurred between Rothesay Bay and the Churchill Ave Reserve culvert outfall. The volume of this fresh rockfall debris at each location was relatively small (estimated at < ¼ to ½ m3). Tonkin & Taylor Ltd - Environmental and Engineering Consultants, 105 Carlton Gore Rd, Newmarket, Auckland, New Zealand PO Box 5271, Wellesley St, Auckland 1141, Ph: 64-9-355 6000, Fax: 64-9-307 0265, Email: auck@tonkin.co.nz, Website: www.tonkin.co.nz 2 We inspected the specific location of the related rockfall on the following day, Sunday, with the NZ Police. The location, shown in photograph 1, was measured at 7m distance from the base of the cliff and is immediately adjacent to a slippery sandstone ledge (about 1m high) within the wave cut platform. There is an accumulation of many sub-rounded boulders at this location along with a few sub-angular (fresh) boulders - refer to photograph 4. The quantity of fresh rockfall material was relatively small (estimated at < ¼ m3). This material comprised fresh angular boulders with freshly coloured iron staining on the surfaces. Based on these observations and similar iron staining on a fresh scarp in the cliff face and other boulders and debris at the foot of the cliff below the scarp, it was estimated that the source of the rock fall material was about 20-25m above the base of the cliff – see photograph 2. The location of the rockfall is shown on the attached Figure 1 and photographs. Site Geology The geology of the site comprises East Coast Bays Formation (ECBF) weak rock, which is part of the Waitemata Group. This sedimentary rock formation is prone to preferential weathering, with the siltstones weathering and frittering at a faster rate than the more durable interbedded sandstones. This process leaves the sandstone beds progressively undermined to the point where the sandstone breaks off and falls as boulders typically 0.3m to 1m across. This is a natural process of coastal weathering of the ECBF cliffs, which are prevalent in the Auckland region. Rates of cliff line regression are typically 2-10m per 100 years for ECBF coastal cliffs. At this particular location, the rate is expected to be closer to the lower end of the range, because of the horizontal bedding (which is favourable for stability), lack of gross defects or faults affecting the rockmass and the near vertical face of the cliff. The nature of the cliff face erosion at this location is one of attrition with progressive generally minor rockfalls, rather than large scale instability. The local accumulation of boulders at the site may be a combination of falls from thicker sandstone beds in the cliff above and a local trap formed where the wave cut platform meets the base of the cliff. It is likely that some boulders are washed into this local boulder trap by (storm) wave action. This feature is also observed in other local areas within the northern stretch of the cliff from Rothesay Bay beach to the Gum Digger Track outfall and is typical of most of the East Coast Bays Formation cliffs throughout the Auckland region. Rockfall Risk & Mitigation The risk of public danger due to rockfall has always existed at the site and the likelihood is considered to be very low. The expected rockfall volumes are likely to be relatively small as a result of the attrition of the cliff face. The boulder risk is primarily expected to occur within 10-15m of the foot of the cliff. This is consistent with published rockfall protection design charts used in the design of rock cuttings. The recent accident is indeed tragic and to our knowledge is the first known fatality of its type in Auckland. The coastline is used by people at mid to low tide, and the measures required to secure the public from injury from rockfall right up to the foot of the cliff would be extensive, expensive and difficult to construct. These measures would be significant and could include measures such as large scale rock nets or drapes (or possibly shotcrete) with significant and frequent anchors, and would require significant ongoing maintenance with associated high risk to construction personnel. Similarly, prevention/control of access into the boulder fall zone would involve fencing and/or barriers within the harsh coastal environment of the tidal zone. This would restrict foreshore access Auckland Council Auckland Council Ref: T&T Ref: 28050 07 July 2011 Rockfall at Rothesay Bay Copyright 2002-2005 Terralink International Limited and its licensors. Some materials sourced from Stats, LINZ and MfE. Crown copyright reserved. 20 40 Rothesay Bay Road Miri Road n Road Masterto n R oa d S an d ow 20 Av e nu e Beulah FIGURE 1: Rockfall Location Plan Rothesay Bay Rockfall area on 2nd July 2011 R oa d Garadice Gulf View Road Br a em ar R oa d d h Roa Be a c d e R oa Jellico ue ve n A y er om nt g o M Churchill Rd Reserve Outfall Accessway d Ro a t on S ea e nu ve sA on Ly ent C re s c n a h Vaug 20 errac e roa T Aot ea 20 Scale: 1:5389 500 metres oad i ll R rc h C hu 40 P o rt al P l ac e 40 4 Likely rockfall source Recent rockfall debris amongst older debris Isolated angular fresh boulders Wave cut sandstone platform & ledge Photograph 1: Area of 2nd July rockfall at Rothesay Bay Cliff, showing the interbedded sandstone/siltstone. Rockfall source Photograph 2: Closer view of rockfall source and iron stained scarp Auckland Council Auckland Council Ref: T&T Ref: 28050 07 July 2011 5 Approximate rockfall debris zone 10-15m from base of cliff Photograph 3: View along beach showing approximate extent of boulder fall zone Photograph 4: Range of boulder sizes within the boulder fall zone, near the location of the fatal accident Auckland Council Auckland Council Ref: T&T Ref: 28050 07 July 2011