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