The design of aquaculture developments should consider the characteristics of the site and surrounding environment to minimise the risk of environmental harm from unsatisfactory performance of containment structures.
Evaluate a number of configurations to achieve the layout with the lowest risk (in conjunction with other design considerations, such as water flow, noise, odour, flooding, stormwater runoff and acid sulfate soils).
For example, site topography and the relationship to surrounding land use is an important consideration in the risk assessment. Where practical, design of containment structures should account for the risk of horizontal flow or overflow into adjacent properties.
Containment structures:
Foundations:
A number of design solutions are available for the formation of linings with low permeability,
including:
- ripping and re-compaction of in situ clays
- engineered imported clay liners
- mixing with bentonite
- synthetic geo-membranes
- composite liner and leakage detection systems.
The type of low permeability lining required will depend on site properties, the sensitivity of the underlying aquifer and surrounding environment, and the level of risk reduction required to meet acceptable risk levels.
Liners should be designed by a suitably qualified and experienced engineer, taking into account the seepage risks associated with the particular aquaculture and the site characteristics. In large or complex sites, a range of design solutions may be required for different areas. Further detail on these design solutions are provided below.
Ripping and re-compaction of in situ clays:
In situ clays should be ploughed and ripped to the required depth, and moisture conditioned and compacted in accordance with the design specifications. Suitable lining soils should extend deeply enough to ensure that construction, harvest activity or routine pond maintenance will not cut into a water permeable layer, resulting in leakage.
It is critical to establish a positive cut-off between the embankment walls and the floor of the ponds. Typical design solutions involve ‘keying’ the walls into the compacted floor.
Clay lining:
Containment structures can be built in soils that have high percolation rates over the full area or part of the area, provided that some type of modification is undertaken, such as a layer of compacted clay, to reduce seepage.
The liner thickness needs to have sufficient depth and impermeability to achieve the required performance standard. Provide sufficient depth to allow for possible desiccation cracking, which can significantly increase permeability.
The ongoing maintenance of the liner in growout ponds must also be considered, particularly when the ponds need to be drained, tilled and cleaned. This clay should be carefully chosen as some clays contain heavy metals, which can be toxic to cultured organisms.
Mixing with bentonite:
Where containment structures are to be built in soils with high percolation rates, permeability may be reduced by mixing bentonite into the soil. The criteria for this approach would be similar to that required for a clay liner, i.e. the depth and permeability of the conditioned material should be sufficient to achieve the required performance standard.
Building a containment structure where properly engineered bentonite modified soils are used is generally expensive. The ongoing maintenance of these in growout ponds should also be considered, particularly when the ponds need to be drained, tilled and cleaned. Take care not to damage the bentonite lining.
Impervious geotextile liners:
Containment structures can also be built in soils with high percolation rates if impervious geotextile lining is used. Building containment structures where properly engineered impervious liners are used is generally expensive. Consider the following characteristics of geotextiles prior to use in an aquaculture setting.
- Those containment structures with thin lining material should be protected against birds that may puncture the liner with their bills while pursuing their prey. To avoid punctures, liners should be covered with soil.
- Thin liners may be punctured by plant sprouts along edges and embankment walls.
- Liners are slippery particularly when wet. This can create a safety hazard particularly where embankment slopes are steep.
- Pond liners may not work in some situations. Water pressure under the liner can force it completely or even partially away from the bottom or banks of lined structures. This problem can occur in areas where watertables rise near the surface or if springs are located near containment areas.
- Burrowing animals may tear plastic liners. Liners may also tear through normal activities within growout ponds (e.g. harvesting and aerator placement). Consequently, they may need regular repair or replacement.
- Some geotextile liners contain chemical additives that are easily leached and are toxic to cultured organisms and nitrifying bacteria. When selecting a liner for aquaculture facilities, it is important to evaluate its toxicity prior to use. If it appears to be toxic, washing methods can usually be applied to remove toxins from the liner.
- Liners also reduce soil respiration so they can affect nutrient dynamics and can lead to the creation of anaerobic zones underneath liners, causing the build up of toxic gases and reduced compounds.
- Rusting of anchors within liners can pose a safety issue.
Where geotextile liners are used, they should be assessed for their longevity in accordance with the requirements of AS HB 154 – Geosynthetics guideline on durability. Periodic electrical testing of synthetic liners may be required after liners have been installed if your farm threatens sensitive aquifers or shallow groundwaters, or if acid sulfate soils are present.
Embankments:
Aquaculture containment structures are usually trafficable surfaces, and it is recommended that walls are wide enough to ensure strength, stability and safe vehicular access. Walls should be surfaced with an appropriate material to reduce erosion and dust, scour, and improve trafficability during wet weather.
Protect external batters against erosion by establishing vegetation (native or crop species) and/or natural regeneration. This can be encouraged through the application of topsoil (which may be stockpiled for this purpose at the commencement of construction) and irrigation.
The only plant suitable for helping to hold banks together is grass or small succulents because of their small size and shallow root systems. Deep-rooted vegetation on banks should not be planted or encouraged as this destroys structure and increases the potential for leakage. In the case of salt-water aquaculture projects, select appropriate species of salt-tolerant grasses.
Internal batters are typically not steeper than 1H:2V and are not actively revegetated. Erosion of pond walls can be minimised by using rock lining (rip-rap), synthetic liners or other materials such as geotextile fabric.
Erosion control systems such as geotextile fabric or synthetic lining that may degrade and/or ?oat free need to be securely fastened and routinely inspected to minimise the risk of entanglement in equipment.
If erosion control systems are likely to prevent a person from safely exiting the containment structure (in the event that someone accidentally enters it), ensure that safe exit points are provided.
Where rock armouring is undertaken some problematic rock types are known including shales and mudstones – which may break down with time and not perform as expected – and rocks containing harmful minerals or that generate acid. It is advised that these types of armouring materials should not be used.
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