INTERIM REPORT ON THE ARTIFICIAL REEF
CONSTRUCTED IN THE LONG BAY-OKURA MARINE RESERVE, AUCKLAND
22nd November
2001
PERMIT AO1/1 26th APRIL 2001
Preamble:
The aim of the research is
to construct two small patterns of artificial reef comprising different sizes
of pierced concrete hemispherical shells. These will be constructed utilising
the American system called “Reefballs”. Colonisation of these will then be
monitored and the results made widely available. It is anticipated that this
activity will increase conservation awareness from the use of the Reserve as an
educational resource and recreational dive site, and will permit future
informed decisions to be made on artificial reef construction in New Zealand.
Construction:
There are only three “reefball moulds” in New Zealand at present; David Head and Reefballs N.Z own these. As a condition of use, the reefs constructed at Long Bay will be monitored for at least three years with still and video images published on the web.
Firth Certified Concrete and Gibbons Crib Walls very kindly offered logistical support which has proved invaluable. Firth made supplies of sand, cement, aggregates, compression test facilities and technical advice available; Gibbons provided an invaluable site to pour and store the reefballs conveniently adjacent to the Firth Albany plant. In addition, their technical advice and use of their plant such as a forklift truck as well as a pouring platform, a pencil vibrator and labour was provided as needed
Concrete mix design:
After a series of tests of
different aggregate mixes, it was decided to opt for 19 mm basalt aggregate
with a chip and pebble content. This affords a mixed, rough exterior, which is
largely “natural rock” after waterblasting, thus reducing the surface pH increasing
effect of the cement. Standard Builders Mix with 19 mm aggregate and a 1:4
cement : mix ratio and a 0.4 :1 water : cement ratio proved the optimum mix
with no additives. Using a microsilica additive increased both the strength of
the mix (eliminating the need for reinforcing), and significantly reduced the
surface pH and porosity. All of the units poured used a standard Firth recipe
for a 19 mm aggregate; 4;1 mix:cement; 0.4:1 water:cement ratio with 8%
microsilica by mass of cementitious material and a small amount of
polycarboxylate superplasticiser [no toxic effects from this additive have been
found in successive web searches]. The superplasticiser is essential to make
such a strong microsilica concrete workable. The compressive strength of this mix
at 28 days is greater than 60 MPa. The exterior surfaces of the resulting units
are very rough after waterblasting largely made up of exposed aggregate.
Further tests of microsilica content and pH are underway. An extension of the project might consider the inclusion of organic material such as crushed mussel shell, or reinforcements such as polymer fibres. The use of microkaolins rather than microsilicas could also be considered.
Placement:
The reefballs were aged for at least a month before being put out on the beach at morning low tides at Waiake Beach. There they were made buoyant by inserted inflated bladders as well as others attached by chains at the periphery. They were then towed by a 5.22 m Bonito Calais powered by a 120 hp Evinrude VRO outboard to the north end of Long Bay Beach. Here over fine sand and silt in 5+ m of water the two reef patterns were established.
Reef 1 – Zig Zag Reef:
This was started with the
placement of two Bay Balls [mass approximately 300 kg, height 0.7 m, base diameter
1.0 m with about 10 piercings] on 23rd August 2001. There is now a
series of 9 of these units in a N – S line. At the Northern end, there is a
Pallet Ball [mass approximately 1200 kg height 0.9 m; base diameter 1.2m with
about 15 piercings]. At the Southern end, there are two further Pallet Balls;
one with a 30x30 cm slate plate attached to a vertical surface and the other
with a plate attached to close the top hole of the unit; i.e. in a horizontal
position. These slates are the same as the ones utilised by current E. O. S.
intertidal studies. Placement took approximately 6 weeks because of
tide/weather/logistical constraints. All 12 units lie on a N – S line parallel
to the beach with an at angle of approximately 450 between this line
and the zig zags. We were attended by Karl McLeod (Long Bay-Okura Regional Park
Ranger) and he commented that it was good to see the speed of colonisation. The
reef is located at 360 40’ 52” S, 1740 45’ 14.8” E in 5.5
m at minimum spring low tides.
Reef 2 – Octagon Reef.
Placement of this started
with four of the smaller Bay Ball units planted Thursday 25th
October 2001 as the central core. There is now a Pallet Ball at the Eastern
point of the octagon; four Bay Balls around the perimeter and one further
Pallet Ball on the Southern margin. There are slate plates attached to a
vertical surface of both Pallet Balls. It is hoped that two further Pallet
Balls will be placed in the 5th – 11th December tide
window. These will complete the perimeter of the octagon. The reef is located
at 360 40’ 51.1” S, 1740 45’ 19.6” E in 6.2 m at minimum
spring low tides.
Colonisation sequence:
Hard surfaces are
colonised firstly by planktonic organisms, e.g. pelagic phytoplankton. The
first settlement to be observed (after a period of about 6-7 days) was a
microfilamentous brown algae (unidentified, but common in the Gulf). The next
colonising organisms are vagrant (and opportunistic) benthos, such as the
cushion star: Patiriella regularis, and the hermit crab: Pagurus sp.
c.f. P. novaezelandiae and herbivorous gastropods like the whelk: Cominella
adspersa (egg cases of the latter were recorded on a number of the
reefballs). Concurrent with this, nektonic organisms like the
spotty: Notolabrus celidotus and triplefins: Fosterygium varium and
Fosterygium sp. feed on small invertebrates and graze on the algae. The
next wave of colonisation observed was that of sciaphilic invertebrates such as
barnacles and polychaete worms. Two settlements of the rapidly colonising
barnacle Austrominius modestus have been recorded; the first occurring
about ten days after the placement of the reefballs. A second, more numerous
settlement event of A. modestus occurred about four weeks later. Of the
annelids, the first was the spiny tube worm: Spirobranchus cariniferus
which settled about two-three weeks after reefball placement. After a month,
these reached lengths of up to 12 mm.
Overall change in biota:
New organisms are present in the area. These have either migrated
from other parts of the marine reserve, or have settled from the moving water
body. (A combination of both is likely). The latter group probably would not
have colonised the area if the reefballs had not been present.
Bottom Conditions:
Sediment: The sites
selected have bottom sediments classified as grey-brown shelly muddy fine sand.
Sediments around the reefballs contain a larger amount of fine silt than could
be expected in this environment. The origin of this is probably urban run-off.
There are large numbers of empty bivalve shells on/near the surface.
Settlement: After three
months, the reefballs showed no appreciable settlement into the sediment. The
holes in the base of the units were left open and no additional thickening
measures taken in manufacture. There have been storms over the period with
nor-east winds gusting up to 100 kmh-1 however the site selected
appears to be well protected, with the artificial reef remaining stable with no
significant scouring even during storm periods.
Project team:-
|
Supervisor:- Professor John Buckeridge
|
Jonathan Jaffrey
Trent Taylor |
Earth
and Oceanic Science Research Centre Faculty of Science and Engineering
|
Andreas Proesl |
Auckland
University of Technology
|