Wave barriers, unlike bank reinforcements, are not built into the bank, but lie slightly out from the shoreline to form a breakwater.
Wave barriers may be used for the following reasons:
- Substituting bank reinforcement, to protect exposed shorelines or newly-created islands and promontories.
- Protecting new plantings of emergent vegetation.
- Providing a substitute for emergent vegetation where this has been lost through trampling, water pollution or other reasons.
- Providing calm areas of water for young wildfowl, in otherwise exposed conditions.
Factors to consider
- What local wind and wave conditions must the barrier withstand? The sandbag barrier described below is more robust than the stake and batten barrier.
- Where it is hoped to encourage breeding of wildfowl, openings must be left between sections of the barrier, or ramps included, to provide access beween the open and protected areas.
- It is usually desirable to encourage stands of emergent vegetation in the calm water behind the barrier, although some areas of open lagoon should be kept. Eventually it may be possible to remove the barriers and reuse the materials.
Sandbag barrier
Sand, clay or concrete filled bags or sacks make a durable barrier. After a year or two, willow cuttings or other vegetation can be planted in the silt trapped by the barrier, to provide additional shelter. Like filled-sack bankworks and weirs, they make up in simplicity what they lack in attractiveness. Points to consider:
- Polypropylene woven bags are best for barriers constantly in water. All bags will tend to weaken and tear where they are subject to temporary drying. Smooth fertiliser sacks are no good as they slip off one another. Look in the Yellow Pages under ‘Sack and Bag Manufacturers and Suppliers’ for local sources of supply.
- Sand is normally the cheapest filling, and may cost you nothing if you can dig it on site, but ensure this doesn’t itself cause erosion problems. Also check that the sand cannot wash through the weave. About 20 tonnes of sand are needed to fill 800 bags.
- Concrete or clay can be substituted for sand if a more permanent structure is required. When deciding, balance the availability and cost of materials against the intended life of the barrier. It may only be required until vegetation is established.
- When using clay, puddle it and force it well into the bags. Concrete can be mixed and placed dry into porous bags.
Lay the bags in a double line, as shown, for the strongest structure.
Wooden stake and batten barrier
This is an economical design, especially if you can use scrap materials.
Log barrier
This design uses logs from local felling operations, or old telegraph poles. On large water bodies, 75-90m (80-100yd) sections can be made up as a unit. When fringe vegetation has become established in one location, the barrier can be unhooked from both ends and towed to the next site.
The logs or poles should be fastened together with hooks and eyes or with toggle bolts, and then anchored about 1-2m (4-6′) from shore. Ramps should be nailed to the logs to give ducklings access to and from open water.
