Summary 10: ARTIFICIAL REEFS

Appropriate locations Exposed dunes of high ecological and landscape value.
Costs Moderate to high, and may need some shoreline maintenance (£20,000 to £60,000/100m of structure, plus minor works for unprotected areas).
Effectiveness Causes lee side accretion, but least effective during storm surge conditions. Unlimited structure life.
Benefits Natural processes are only partly disrupted, allowing dunes to stabilise. Rocks create new intertidal habitat.
Problems May cause navigation hazard. Visually intrusive at low tide. Disrupt amenity use of beach.

General description

Artificial reefs are shore parallel rock mound structures set part way down the beach face. They may be long single structures or form a series of reefs extending for some distance alongshore. They are distinguished from Nearshore Breakwaters (Summary 11) by being submerged for at least part of the tidal cycle, and are therefore less intrusive on the coastal landscape, have less impact on upper beach longshore processes and add a new intertidal habitat to sandy foreshores.

Function

Reefs dissipate part of the incident wave energy before it reaches the dune face, protecting the upper beach from erosion and encouraging deposition. Long structures (sills) reduce wave energy over an extended frontage, resulting in a more stable upper beach and dune face. Shorter, segmented reefs protect short lengths of the shore, allowing erosion to continue elsewhere. The result is an embayed shoreline with upper beach deposits (salients) forming behind the reefs.

Salients will allow new foredunes to develop, but this accretion may be at the expense of continued erosion elsewhere. Recycling or nourishment, followed by fencing, thatching and transplanting may address this problem, and will enhance the rate of dune-beach recovery. Reefs have less impact on upper beach transport processes than nearshore breakwaters, and can be used on open beaches. In particular, tombolos will not form behind low level reefs, but can form with higher breakwaters; tombolos would significantly disrupt longshore drift, potentially causing downdrift erosion.

Reefs are of little use within estuaries where currents, rather than waves, are the main erosive force.

Methods

Small reef schemes can be implemented without specialist assistance, but normally the services of a competent coastal consultant and contractors are required. Information on the design of rock structures is available from the CIRIA/CUR “Manual on the use of rock in coastal and shoreline engineering”. The accompanying figures provide initial guidance but this should be confirmed for each site.

As with all rock structures on the shoreline the rock size, face slopes, crest elevation and crest width must be designed with care. Rock size is dependent on incident wave height, period and direction, structure slope, acceptance of risk, cross-sectional design, and the availability/cost of armour rock from quarries. In general 3-6 tonne rock will suffice, provided that it is placed as at least a double layer, with a 1:1.5 to 1:2.5 face slope, and there is an acceptance of some risk of failure. This assumes a structure location close to the mid-tide level of the beach. Rock size or side slopes may need to increase if the reefs are built further down the beach face where wave action is stronger.

Randomly dumped rock with a high void to solid ratio is hydraulically more efficient than placed and packed rock. However, rock structures on recreational beaches should be built with a view to minimising the potential for accidents involving beach users slipping between rocks.

The structures should be constructed within a shallow trench and a geotextile filter should be laid under the rocks to prevent the migration of sand upwards and the settlement of the rocks into the beach. The geotextile should be wrapped around the base layer of rocks, and the rock toe should be set below the lowest expected beach level.

fig-a10

Reef cross-section

fig-a11

Planview of a series of reefs

The crest elevation will have a major impact on the extent of wave energy dissipation. Some erosion protection will be achieved if the crest is as much as 2m below the water level during storms. Energy dissipation will increase as the crest rises up, until the point at which the structure is rarely submerged. An elevation close to the mean high tide level should be acceptable as a first approximation. A crest much above this level effectively converts a reef to a detached breakwater, introducing the potential for the formation of harmful tombolos and disruption to longshore processes.

Wave energy dissipation will increase with crest width, but significant dissipation is not achieved until the width equals about half of the wave period (say 30m – 50m). Widths of this magnitude are simply not practical, so structure crests are usually restricted to 2-3 rock diameters.

Reefs may actually increase shoreline problems if they are used in areas subject to strong nearshore tidal currents. Scour along the seaward face and around the ends of reefs should be monitored, and structure maintenance undertaken prior to failure where beach levels drop.

The ends of each reef should be formed into a roundhead with shallower side slopes, particularly along the landward face. The roundhead reduces the tendency for local scour and improves the stability of the structure.

Concrete armour units of various types can be used instead of rock, but are normally considerably more expensive. The potentially greater hydraulic efficiency of some units is of no importance to dune defence structures and the units are normally considered to be more unattractive than rock armour.

There is little guidance available for the length and spacing of reefs. Numerical modelling should be undertaken by a competent coastal consultant during preliminary studies. Longer or higher structures will provide greater protection but will also increase potential long shore impacts. Wider gaps will allow greater shoreline erosion, which may threaten assets. Structure dimensions can vary along the length of a scheme to provide varying levels of backshore protection.

The area behind the structures can be built up by recycling or nourishment (Summaries 5 and 7), and then grassed, thatched and fenced (Summaries 2, 3, and 4) to stabilise the dunes and possibly encourage regeneration.

Costs for reef schemes depend on structure dimensions and spacings. They can be heavily influenced by the availability of suitable rock (or other material), transport and the costs of any recycling or nourishment. Work windows are limited to low tide periods and may be influenced by stormy seas. Rock structures can be assumed to have an unlimited life with respect to economic assessments.

Impacts

Even though this form of defence is intended to give only partial protection to the shoreline the impacts on shoreline processes, intertidal habitats and landscape will still be high, and may be unacceptable in environmentally sensitive areas. Erosion in the lee of the gaps may well continue for several years after construction while a new beach planshape develops. Long, sill type, reefs with no gaps may suffer from a build up of fine sediment, seaweed or other debris along the inshore side - gaps provide a flushing mechanism.

As the structures are separated from the shore as the tide rises, and then become submerged, they are potentially hazardous to anyone using them as a perch. Rocks below the level of Spring tides will tend to be covered with marine growth including slippery algae, again forming a public hazard.

The submerged reefs will form a hazard for water sports and navigation, and must be clearly marked with appropriate beacons. Wave induced currents around the ends of reefs can be locally strong and a danger to swimmers.

Best practice and environmental opportunities

The width of the upper beach along the embayed shoreline may increase, providing improved recreation. New foredunes may develop in the lee of the reefs. The structures allow natural beach-dune processes to continue, albeit along a modified shoreline.

The reefs will form a new intertidal habitat, bringing rocky shore communities to a sandy beach. The structures may well prove to be popular with beach users.

All dune management schemes should observe the following guidelines to maximise the probability of success and minimise impacts on the natural and human environment:

In addition to these general guidelines, the following are of specific importance to artificial reefs: