Third Chinese-German Joint Symposium on Coastal and Ocean Engineering
National Cheng Kung University, Tainan
November 8-16, 2006
Artificial Reef Nienhagen / Baltic Sea
Hans Nickels*, Dirk Lesemann
Knabe Beratende Ingenieure GmbH, Hamburg
*[email protected]
Thomas Mohr
Landesforschungsanstalt für Landwirtschaft und Fischerei Mecklenburg-Vorpommern,
Institut für Fischerei, Rostock
Abstract
An article about the construction of an artificial underwater reef in the Baltic Sea
for the purpose of fishing research.
1 Background
The Landesforschungsanstalt für Landwirtschaft und Fischerei (Agriculture and
Fishing Research Institute) for the German Federal State of MecklenburgVorpommern, in particular the Institut für Fischerei (Institute for Fishing) in Rostock
proposed to construct an artificial underwater reef in the Baltic Sea for the purpose
of carrying out fishing research. The artificial reef was constructed out of precast
concrete units of various shapes with flexible elements integrated into these
structures. The establishment and development of fish populations in the artificial
reef structures of this artificial habitat would then be the subject of research.
In accordance with the findings of earlier investigations and studies by the client,
concrete tetrapods of various sizes, concrete rings and Reef Balls™ or similar
concrete elements were considered as suitable for the construction of reef
structures. The requirements for concrete quality and the environmental
compatibility of the concrete were particularly important in this project.
Similar high requirements were placed on the stability of the precast concrete units
in terms of their structural stability under hydrodynamic effects, and the safety of
the research divers during their work.
2 Objective of the Research Proposal
The project with the full title “Erhöhung der fischereilichen Wertigkeit von
Seegebieten vor der Küste Mecklenburg-Vorpommerns durch die Errichtung
künstlicher Unterwasserhabitate - Aufbau eines Großriffs im Fischereischutzgebiet
Nienhagen” (Increasing the fishing value of the Mecklenburg-Vorpommern coast
by the construction of artificial underwater habitats - Construction of a large reef in
the Nienhagen fishery protection zone) was supported as part of the FIAF
operational programme for the promotion of fishing and the fishing economy by the
EU (75%) and the Ministerium für Ernährung, Landwirtschaft, Forsten und
Fischerei (Ministry of Food, Agriculture, Forestry and Fishing) for MecklenburgVorpommern (25%). The project duration covers the period from 01.09.2002 to
31.12.2006. The coordinator is the Landesforschungsanstalt für Landwirtschaft
und Fischerei - Institut für Fischerei.
In addition to conventional management measures such as quotas, minimum
landed lengths, minimum mesh sizes, seasonal catching bans (e.g. summer
catching ban on cod) and temporary fishing area bans, the intention is, along with
the creation of extensive artificial underwater habitats for the gathering, growing
and refuge zones for juvenile and small fish populations, to develop alternative
possible means for the regulation and stabilisation of the population sizes of the
economically important and endangered fish types.
The objective is to obtain proof that artificial structures bring with them an increase
in the fishing value in this area and thus ensure the conservation of fishing
resources in the off-shore waters of the state of Mecklenburg-Vorpommern.
In other words, to determine and evaluate the effects of these types of structures
on the fish communities in this part of the sea and thus answer the question as
what extent these types of structures influence the conservation of local fish
populations and whether the concentration and protective mechanisms discovered
in preliminary research on smaller reef structures have a significant effect on
existing population structures and sizes of economically important fish populations.
3 The Fishery Protection Zone off Nienhagen / Baltic Sea
The artificial underwater reef was placed in the fishery protection zone off the
coast near the settlement of Nienhagen on Baltic Sea, approximately six sea miles
west of Rostock-Warnemünde. It was constructed in September 2003, after a
construction contract was awarded on the basis for the design drawn up by
consulting engineers Knabe Beratende Ingenieure GmbH, Hamburg. Contractor
Heinrich Hirdes GmbH, Niederlassung Rostock built various reef structures at a
distance of one sea mile off the coast on an area of approximately four hectares.
This part of the sea has water depths of approx. 12.00 m.
The first small reef structures were constructed in this area seven years ago out of
concrete pipes as part of a preliminary project. These artificial underwater
structures were extended by importing approx. 2000 t of natural rock that became
surplus during the clearing of the middle jetty at Warnemünde and were kindly
made available by the waterway and shipping authority, WSA Stralsund.
Following on from this first collection of experiences, it was possible during a
subsequent monitoring programme to record the observations of fish occurrences
in the area of this small artificial reef. These results gave the scientists an
optimistic look into the future and were the basis of further planning and the
proposal for the major project described here.
Figure 1 Small reef structures of concrete pipes
4 Special Local Conditions
The sea bed in the fishery protection zone consists of a thin sand layer of 10 to 20
cm thickness with localised rock outcrops and boulders underlied by the bedrock
(marly till). The precast concrete units could therefore be placed directly on the
sea bed.
During construction, storm warnings with wind directions from the N, NNE and NE
were to be taken particular notice of as wind fetches are longest in these directions.
The fetches were as follows:
NE = 200 km NNE = 96 km
N = 56 km
NNW = 50 km
NW = 40 km
For the execution of the works at sea in September 2003, good weather conditions
set in for an adequate period to enable the works to progress almost without
interruption.
However, the most unfavourable weather conditions are of considerable
importance to the long-term stability of the reef elements. Calculations showed
that, e.g. during a sea event that could be expected to occur once in 50 years, in
this part of the sea the design should take into account the significant wave height
of 3.70 m. The maximum expected wave height is larger. These largest waves
occur when the wind is from one of these northerly directions.
The calculated current at the sea bed of the site is up to 1 m/s.
5 The Various Reef Structures Constructed of Precast Concrete Units
A special constraint in the execution of the works was that only precast concrete
units made in a precast concrete plant were to be used. In the forming of the
precast concrete units it was to be ensured that the coarse solid reef structures
have great porosity, that they are stable and that further flexible reef structures on
floating bodies could be attached to these elements. In addition, it was also to be
considered whether “reef ball” elements (Reef Balls™), which had been installed
in artificial submerged reefs all over the world, were suitable.
Figure 2 Placing plan for the works
5-1 Reef Structures of Precast Concrete Units
5-1-1 Concrete Rings
Two reef areas each on approx. 21 m x 10 m sites to the west of the planned
location of the telemetry mast were constructed of concrete manhole rings. The
manhole rings, with an internal diameter of 2.50 m and a construction depth of
0.75 m, were placed in layers one on top of the other in a pyramid shape. The
manhole rings were divided with internal partitioning walls into 4 (3) individual
chambers (see figure). Irregularly arranged holes with diameters of 10 to 25 cm
were made in the internal and external walls.
These manhole rings were seen as being very suitable because they offered large
protective zones for juvenile fish through their shape and placing arrangement.
Figure 3 Concrete manhole rings
5-1-2 Tetrapods Weighing between 2 and 6 Tonnes Each
High self-weight, the excellent stability provided by their interlocking arrangement
and relatively large pore volumes mean tetrapods weighing upwards of 2 tonnes
placed in three layers are suitable for reef fill. The feet of the concrete tetrapods
were fitted with several threaded sleeves for attaching flexible floating bodies. A
reef height of approx. 2.5 to 3.0 m could be achieved with a two layer placement of
tetrapods.
Tetrapods with a self-weight of 6 tonnes were chosen in order to be able to attach
extensive flexible elements, such as nets, ropes and floating bodies. These
tetrapods were placed individually in a grid of approx. 10.0 x 12.5 m on the sea
bed.
The flexible structures fastened to these tetrapods provide a place for colonisation
by sea grass, seaweed and shellfish.
Figure 4 2-tonne tetrapods
5-1-3Reef Cones
In considering the construction of an artificial reef, the client thought about the
placement of Reef Balls™, which had been installed in and used for the
construction of submerged reefs in other countries. These reef balls certainly did
not fulfil the requirements for adequately high strength and stability. Manufacturing
these reef elements in the US-American patented shapes would have greatly
exceeded the planned time and cost frames for the project, therefore the reef balls
were redesigned to have superior technical and economic properties. The reef
ballsTM where replaced by alike so called reef cones.
The reef cones were individually placed in groups of up to 30 on the sea bed.
Figure 5 Picture of reef ball UltraTM
6 Requirements for the Material
6-1 General
The following special requirements applied to the quality of the concrete of the
precast concrete units:
1.
1. The concrete strength class had at least to fulfil the requirements of
concrete class B 45 WU. The concrete used had to have a high frost and
thaw resistance. Round aggregate only was permitted. Special requirements
for tensile bending strength and water-cement ratio were set.
2.
The recipe for the concrete mix and quality of the surface of the precast units
were designed to provide ideal conditions for colonisation by algae, shellfish
etc. The pH-value of the cured concrete had to correspond with that of the
surrounding seawater. The pH-value at the surface of the precast concrete
units had to be between 8 and 8.5 at the time of installation.
3.
The surrounding seawater was to suffer neither significant nor permanent
detrimental affects from the cured concrete or from its raw or associated
materials.
4.
Any additives or additions had to be substances with environmentally
protective properties (readily biologically degradable). This applied in
particular to the use of release agents. Environmentally compatible
deactivators were to be used as set retarders.
5.
The external concrete surfaces were not to have a smooth, polished, asformed surface texture. The surfaces were required to be rough textured so
that marine biological life would have a near-natural substrate for colonisation.
6.
All the threaded attachment elements that were required to be cast in or
remain permanently on the site (transport anchors, threaded sleeves etc.)
were to be manufactured from stainless steel. Reinforcement elements that
were completely encased in concrete were not affected by this requirement.
7.
The above-detailed concrete properties were to be proved before production
started by extended suitability tests.
8.
All precast concrete units were to be labelled so that production was fully
traceable, in particular in relation to the checking of the pH-value. In this
respect the post-stripping handling and treatment were to be specifically
recorded.
9.
The manufacturer had to conclude a special project-related inspection
contract with a materials testing consultant or with a quality mark awarding
organisation and record the results of the quality testing.
10. In addition the requirements of EAK 2004 Section 5.4 “Requirements and
composition” were to be fulfilled.
6-2 Concrete Mix Design
The precast concrete units were manufactured at three different precasting plants.
Therefore, as could be expected, three concrete mix designs were used.
The manhole rings (Hass + Hatje) were manufactured using quick-setting cement
CEM I-42,5-R-NA. The precast units were stripped of their formwork and treated
fully automatically immediately after concreting. The manhole rings were fitted with
a ring and stirrup strengthening arrangement for moving them immediately after
stripping.
The very solid 2- and 6-tonne tetrapods were manufactured using CEM I-52,5-RFT. Before stripping, the precast units had to cure in the formwork sufficiently for
the concrete to reach a strength to allow the tetrapods to be lifted. The tetrapods
were fitted with a strengthening arrangement for moving them. In addition, 3 - 6
threaded sleeves were provided in each tetrapod for transport and to allow the soft
structures (nets, floating bodies and the like) to be attached.
The reef cones were manufactured using CEM I-52,5-R-NA. The external
formwork was treated with an easily degradable retarding agent before concreting.
After a short period of curing the balls were stripped and the desired rough surface
texture was created (see 6.3). The reef balls had a ring and stirrup strengthening
arrangement and transport eyes for moving them.
6-3 Surface Quality of the Precast Concrete Units
In the fabrication of the precast concrete units, as well as the concrete mix the
concrete surface was also to be designed to be favourable to colonisation by
marine organisms (e.g. algae, shellfish etc.). In order to form a suitably coarse /
porous surface texture, it was required to use biologically degradable retarding
agents, preferably sugar solution, and treat the concrete surface later by various
methods.
In the case of the concrete rings, they were stripped of their formwork immediately
after concreting and roughened with a nail board. This was done in such a way
that the stone particles (aggregate) were not exposed. After this process the
surface consisted almost wholly of cement, even after roughening.
After a curing period the surfaces of the tetrapods were sand blasted. This was
done in such a way that the grain structure of the stone particles (aggregate) was
exposed. This technique results in the cementitious (alkaline) part of the surface
being greatly reduced. Over several weeks of production a total of three different
schedules for carrying out the surface roughening treatment were tried.
The surfaces of the reef balls were treated with an application of a deactivator as a
retarding agent and were washed with pressurised water immediately after
stripping to create an exposed stone particle structure with a rough, almost
cement-free surface that would be available for colonisation.
To reduce the pH-value of the concrete surface, the precast concrete units were
exposed to weathering, in particular solar radiation, immediately after stripping or
roughening in order to accelerate carbonation of the surface.
6-4 PH-Value at the Concrete Surface
Proof of compliance of the pH-value of the surface of the precast concrete units
was obtained using a number of techniques. For each manufacturer, the youngest
precast unit of each production week was tested as a representative sample in
good time before installation.
First the unit was tested with phenolphthalein to prove that the pH-value was
below 10. As the faint colour change below a pH-value of approx. 9 to 10 meant
that the violet colour on the grey concrete could no longer be seen, this test alone
was not recognised as sufficient to detect a pH-value below 8.5.
Therefore the surface of the concrete was scratched with a wire brush to remove a
sample of the cement mortar, which was dissolved in distilled water and the pHvalue of this solution determined using pH-electrodes. As there was a tendency to
scratch off some of the uncarbonated layer as well in doing this some of the
results were too negative (too alkaline).
The concrete bodies that, according to the test using pH-electrodes, had a slightly
too high pH-value were lightly moistened and then retested with pH-paper.
By an appropriate organisation of the production processes and installation
procedure it was possible to ensure that an adequate waiting period would be
maintained to allow carbonisation of the top layer of concrete to take place.
7 Transport and Installation of the Precast Concrete Units
The Client’s decision that the reef was to be constructed of precast concrete units
meant it was possible to construct all the reef elements in a precast concrete
plants under ideal conditions. The harbour in Rostock-Warnemünde provided the
opportunity to surround each concrete unit, which had been delivered by lorry, with
a floating transportation device to carry it to the installation site some six sea miles
west of Rostock.
The computer-controlled installation device used GPS navigation to place the
concrete unit exactly in the planned location in twelve metres of water. The
placement works under water were supervised and escorted by construction
divers.
Figure 6 Temporary storage of
reef cones in Rostock harbour
Figure 7 Installation of oncrete ings
Monitoring since completion of the artificial reef has shown that the installed
concrete elements have not moved in their position and hence are safe for
scientific and leisure diver activities under the water.
Figure 8 Navigation for installation
Figure 9 Installation of 2-tonne
tetrapods
7 Monitoring Programme and Initial Experiences
Scientific investigations will be carried out until 31.12.2006 coordinated by the
Institut für Fischerei and include the following main tasks:
。
Fish biology investigations
。
Underwater - long-term video observations
。
Fishing investigations
。
Possibilities of commercial use of macrophytes
。
Growth investigations
。
Effects of the artificial reef on natural habitats
。
Further possible uses, such as aquaculture, angling and underwater
tourism
。
Economic considerations
The fish biology and ecological investigations began in 2002 with an existing
condition survey of the investigation site off Nienhagen and of a similarly sized
reference area some 4 km west off Börgerende in the same depth of water.
Commercial fishing gear (Fig. 10) and special nets for catching juvenile and small
fish were used for this and in the continued programme. The fishing investigations
commenced with the trialling of selected fishing gear directly in and around the
structures from which the local fishery operators are seeking to develop new
working technology.
Parallel to the fishery monitoring programme, the rate of colonisation was
determined from concrete panels with the same surface texture as the reef
elements, which were removed every month from a test panel frame (Fig. 11 ).
Sediment sampling, special investigations of macrophytes, hydrological
measurements and a photographic record and analysis were also undertaken. A
key aspect of all these investigations was the use of a telemetry mast to transmit
data, to which up to nine underwater cameras and one aerial camera can be
attached.
Figure 10 Eel basket
Figure 11 Test panel frame
With the completion of the extension to the reef, the telemetry mast along with the
weather station, sensor system and the multiple camera system was installed in
the planned position on 01.10.2003 (Fig. 12). In parallel to this, the land station, in
which data collection and processing takes place, was erected by the maritime
weather service in Warnemünde. Until 05.11.2003, the day that the various
devices were removed from the mast, the station supplied uninterrupted long-term
videos of the structures and data about the major hydrological and technical
parameters. This information should enable conclusions to be drawn about fish
behaviour and the effects of this sort of artificial habitat on the fish species and
also on possible sediment transport in this part of the sea. The mast is attached by
a universal joint to a 3-tonne suction anchor and can flooded and be supported on
the sea bed in the event of a threatening ice drift.
Figure 12 Telemetry mast
The first results of the fish biology
investigations, the long-term observations
from the underwater video systems (Fig.
13 + 14) and visual observations allow
researchers to form an optimistic view of
the future. Large schools of juvenile cod
were observed, which could not be found
in similar quantities during concurrent
fishing in the reference area. The reef elements not only offer juvenile and small
fish protected zones; the solid structures also create 11,000 square metres of
additional artificial growth areas for colonisation by micro-organisms. This growth
will integrate itself as a biomass measure naturally and positively into the food
chain and form the basis of a self-contained functional ecosystem. The scientific
investigations are concentrated on questions of growth development, where the
term growth relates in a wider sense to macroalgae, sessile (permanently attached)
and vagile (freely motile) invertebrates, and the influence of the growth on the fish
fauna, among other things.
Figures 13 and 14
Cod in the artificial reef
In addition to the monitoring programme and the objective of conserving fishery
resources in near-coastal waters of the state of Mecklenburg-Vorpommern, an
economic evaluation is being carried out and investigations are ongoing into the
multiple use of artificial habitats, in order to open up further fields of business to
fishery operators, such as aquaculture, angling or underwater tourism. It should be
noted here that considerable material and scientific damage has been caused by
third parties (anchors in the fishery protection zone in the direct vicinity of the
telemetry mast) and especially to the panel frame. A more forceful declaration of
the necessity for undisturbed scientific work and advice indicating the high level of
danger above all from the flexible structures, would appeal to the sense of reason
of everyone involved to keep clear of the fishery protection zone identified in the
nautical bulletins issued by WSA Stralsund, in which fishing, angling and anchors
are prohibited. Otherwise for anglers the consequences might include the loss of a
spinner; but for divers it could mean loss of life. All interested leisure divers will
therefore be given the opportunity by the Institut für Fischerei and the local dive
centres to undertake a guided dive on the artificial reef.
8 Outlook
This artificial reef, constructed out of precast concrete units in the Baltic Sea off
the coast of Mecklenburg-Vorpommern to the west of Rostock, is the first of its
kind. The construction of this large reef was prompted by observations of fish and
plant colonisation of a small reef composed of stacked of concrete pipes. The
observations since the completion of this artificial reef in 2003 have so far
exceeded all expectations. The artificial underwater habitat is particularly suitable
for offering juvenile cod and other small fish a protected area of refuge and thus
functions as a local maritime protection zone.
The observations of the occurrence and breeding behaviour of many fish species
have shown that population protection and the possibility of commercial farming of
interesting algae (e.g. red algae for the cosmetics industry) and the use for tourism
of the artificial reef by leisure anglers and divers can be brought together in
harmony.
With the results of the scientific investigations and the associated economic
considerations not only possible compensatory measures but also stand-alone
projects in the context of multiple use as a protective and commercially
sustainable concept can be assessed.
In this way these types of artificial underwater habitats could offer future
opportunities for compensatory and replacement measures to make up for the loss
of habitat for underwater flora and fauna caused by new construction works, such
as harbours, offshore windparks and other offshore construction works, navigable
channel deepening or extensions.
Figure 15 Illustration of an area of the artificial reef (Christian Gschweng,
Uwe Friedri