How to make a floating wetland

Waterbodies on Redlands Coast can have significant problems with excessive nutrients and sediments. These pollutants can result in poor water quality, algal blooms, water weed outbreaks, stress on aquatic life and poor water health in Moreton Bay.

A floating wetland is a raft of vegetation that floats on the surface of waterbodies, acting as a natural biofiltration system to help improve the overall condition of the water body. 

Floating wetlands will improve water quality in lakes, dams, wetlands, and wastewater lagoons. They are particularly useful in established waterbodies where it is not practical or economically feasible to create a constructed wetland.

How does a floating wetland work?

Filtering nutrients:

Floating wetlands help prevent excessive growth of unwanted algae and aquatic weeds by reducing nutrients like nitrogen and phosphorus. The plants on the floating wetlands take in nutrients from the water. However, most of the nutrient absorption happens through bacterial biofilms that grow on the root mass below the floating wetland. Studies have shown that floating wetlands can reduce nitrogen by 40-90% and phosphorus by 20-90%. Nitrogen is converted into gas and released into the air, while phosphorus forms sediment beneath the floating islands. If this sediment lacks oxygen, phosphorus may return to the water and become available to algae and weeds. In such cases, physically removing the phosphorus-rich sediment may be necessary.

Reducing sediments:

Floating wetlands are effective in reducing sediments and other pollutants, such as heavy metals, in water bodies. The key lies in the root systems and sticky biofilm hanging beneath the floating wetlands. The roots and biofilm attract fine sediments, causing them to form larger particles that eventually settle at the bottom of the water body. Research has demonstrated that floating wetlands can achieve a 50-100% reduction in total suspended sediments.

Providing habitat:

Floating wetlands provide a habitat for local wildlife, offering protection from natural predators and feral or domesticated animals. Water birds like the Purple Swamphen and Dusky Moorhen utilize these wetlands for nesting and protection. Eastern Water Dragons, turtles, and frogs seek refuge and basking areas in floating wetlands. The hanging roots of the plants also serve as excellent habitat and cover for native fish, such as the Pacific Blue-eye. Once the plants are established, floating wetlands become visually appealing and enhance the aesthetic value of the water body.

Where can they be used?

Floating wetlands can be used on almost any waterbody where there is potential to remove excess nutrients and sediment. Expansive versions have been used in large lakes in Australia and internationally. Trials of floating wetlands on smaller farm dams have been particularly successful. 

Important considerations

Situation

In the Redlands Coast, there are approximately 2000 waterbodies scattered across the city with ninety percent being privately owned farm dams. These farm dams provide the ideal situation for employing a floating wetland.  

The minimum depth for the location of the floating wetland should be greater than 0.5m to avoid roots growing into the bed of the waterbody. The floating wetlands should be placed in an area that will receive sufficient sunlight which will achieve the healthiest plant growth (i.e. away from the shade of bank vegetation). When placing the floating wetland, consider securing them near areas where nutrient rich waters may flow from.

Securing the raft

The floating wetlands can be secured either by tying them to a stake or tree on the waterbody banks, or by using an anchor (such as besser bricks). Care should be taken to ensure floating wetlands are secured well in flood prone waterbodies.

Quantity

To achieve nutrient reduction targets, floating wetlands should cover 5% of the surface area of the waterbody. You can join and arrange several floating wetlands with synthetic rope to obtain the 5% coverage.

Plant selection

Locally native wetland plants are ideal for floating wetlands, as they have adapted to the climatic growing conditions of your local area, will not become weeds and will provide food and shelter for local wildlife.

The plants should have a maximum growth height of 1-1.5 meters to avoid sinking the floating wetland. Approximately 8-10 tube stock plants per square meter of floating wetland are recommended. 

The following species have been successfully used on floating wetlands:

•   Baloskion tetraphyllus (Tassel Cord Rush)
• Baumea articulate (Jointed Twigrush)
• Carex appressa (Tall Sedge)
• Carex fascicularis (Tassel Sedge)
• Eleocharis cylindrostachys (Dropping Spikerush)
• Ficinia nodusa (Knobbly Club Rush)
• Juncus usitatus (Common Rush)
• Phylidrium lanuginosum (Woolly Frogsmouth)
• Schenoplectus mucronatus (Lake Club Rush)

Caring for your floating wetland

Some long term maintenance of the floating wetland is required. Regular inspections to check for disturbance of panels and plant replacment should be undertaken. Some floating wetlands may become colonised by exotic weeds, and weed control may be need to ensure the desired local native species dominate.

To remove the captured phosphorous and sediments, the root material below the floating wetland can be trimmed periodically. 

As discussed earlier, physical removal of the waterbody’s phosphorous-rich ‘sludge’ layer may be considered. Care needs to be taken when considering this action in terms of damage to aquatic ecology (fish, turtles, water plants etc.) and disposal of the sludge layer.  De-sludging can also be an expensive procedure.

Floating wetlands should be part of a comprehensive waterbody management plan, which includes a suite of ecological and water quality improvement actions and applications such as:

• removing livestock from waterbodies
• land management (fertilizer reductions, manure stockpiles, preventing soil erosion etc.)
• riparian and edge revegetation (including treatment wetlands at waterbody inflows)
• water circulation and aeration systems.

Step-by-step floating wetland instructions

Materials required:

Floating wetland construction

• 2 x 2m 100mm stormwater DWV PVCU PVCU pipe
• 2 x 1m 100mm stormwater DWV PVCU pipe
• 4 x right angle 100mm stormwater joiners
• Plastic mesh (approximately 2 x 2m)
• 1m x 2m Geo-textile (Jute mat, Recover or similar)
• Pliers (suitable for cutting plastic mesh)
• Cable ties (200mm and 450mm lengths)
• PVC pipe cement (blue plumbers glue) and PVC priming fluid (red)
• Scissors (suitable for cutting geo-textile)
• Aquatic wetland plants (locally native species)

Recommended Personal Protective Equipment

• Eye protection – approved safety glasses or goggles
• Gloves - chemical resistant gloves for handling pipe cement and priming fluid and safety gloves for when using hand tools
•  Protective clothing – long sleeve shirt and long pants

Step 1: Building the frame

1.  Apply red primer to internal edge of the joiner and the external edge of the 100mm pipe.
2. Liberally apply glue to the internal edge of the joiner and the external edge of the 100mm pipe. Your aim is to provide a water tight joint so apply the glue liberally!
3. Once you have completed the above, place the pipe into the joiner and press hard for at least 15 seconds to allow the glue to set.
4. Repeat the above process until you have a rectangle that looks like the picture.

Note: When working with the plumbing glue, make sure you are working in a well-ventilated area.

Step 2: Construct the layer to hold the plants in place

Measure and cut the plastic mesh so that it will wrap around the frame and meet with a little overlap.

1. Pull the mesh as tight as possible and where there is overlap use cable ties to secure the mesh in place.
2. To add additional strength and stability you can use cable ties around the edge of the frame to secure the mesh to the frame.

Note:  Adding a material such as shade cloth between the two layers of plastic mesh will provide additional surface area for biofilm growth.   

Step 3: Attach geo-textile layer

1. The geo-textile layer will protect plants roots from direct sunlight, capture falling organic matter and prevent weed colonization.  Measure and cut the geo-textile so that it is slightly larger than your frame.
2.  Lay the geo-textile on top of the frame and use cable ties to secure the recover to the frame.
3. Using a pair of scissors cut a small hole in the geo-textile (slightly larger than the size of a tube stock plant). Repeat this process until you have enough holes to match the number of plants you will be planting.
4. After cutting the holes in the geo-textile you will need to cut holes in the top layer only of the plastic mesh. Using tin snips or pliers cut the mesh so that it is the same size as the holes in the geo-textile.

Step 4: Adding plants to your floating wetland

1. Select locally native plants that grow well in water and will not grow too large for the floating wetland.
2.  Place 8-10 plants per square meter.
3.  Remove tube-stock from plastic pot and insert into slots in geo-textile and plastic mesh.
4. Important: Secure your plants to the floating wetland with wire to ensure animals don't pull them out. 

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