As an experienced flood control specialist, I’ve witnessed firsthand the growing need for innovative solutions to address the challenges posed by urban stormwater runoff and aging flood infrastructure. One promising approach that has gained traction in recent years is the use of floating treatment wetlands (FTWs) – a versatile technology that can significantly enhance the performance of stormwater retention ponds while providing additional environmental benefits.
Now, this might seem counterintuitive…
Stormwater retention ponds, or “wet ponds,” are one of the most common stormwater control measures (SCMs) used to manage urban runoff. While initially designed for flood management, these ponds have evolved to also address water quality concerns by promoting the settling of pollutants. However, many older wet ponds have struggled to meet current water quality standards, prompting the development of various retrofit options – and FTWs have emerged as a particularly effective solution.
The Science Behind Floating Wetlands
FTWs are essentially artificial floating islands, composed of a buoyant mat planted with wetland vegetation. These mats float on the surface of the stormwater pond, allowing the plant roots to extend down into the water column. This root system, combined with the microbial communities that colonize it, forms a root-biofilm network that enhances water quality treatment through a variety of mechanisms.
As stormwater runoff flows through the FTW root system, pollutants can be physically trapped, transformed by microbial processes, or directly absorbed by the plants. The dense, fibrous roots provide a large surface area for these interactions to occur, effectively filtering the water and promoting the settling of particulate matter. Additionally, the plants can directly uptake dissolved nutrients, such as nitrogen and phosphorus, transforming them into plant biomass that can be periodically harvested, permanently removing these pollutants from the system.
Compared to traditional constructed wetlands, FTWs offer several advantages. Because they float on the water’s surface, they do not diminish the storage capacity or peak flow attenuation of the pond. Furthermore, the floating mats can rise and fall with changing water levels, eliminating the risk of plant loss due to prolonged inundation. FTWs also provide diverse wildlife habitat, support aquatic ecosystems, and can be designed to enhance the aesthetic value of the pond.
Designing Effective Floating Wetlands
When retrofitting a wet pond with FTWs, it is essential to double-check that the pond is easily accessible for installation and maintenance. The depth of the pond should be between 3 and 6 feet to avoid the risks of plants rooting into the benthic layer or runoff bypassing the treatment zone.
The FTW itself consists of several key components:
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Mat Material: The buoyant mat might want to be durable, nontoxic, and capable of supporting the weight of mature wetland plants. Common materials include polyethylene or polystyrene foam, plastic pipes or tubes, bamboo, and stainless steel.
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Potting Media: While soil or other potting media can facilitate rapid plant growth, they may also become a source of pollution if nutrients leach into the water. An alternative is to use plug plants without soil, allowing the roots to take up nutrients directly from the water column.
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Wetland Plants: A diverse mix of native wetland species with dense, fibrous roots that can grow 2-3 feet long is ideal. This maximizes the surface area for biofilm growth and nutrient uptake. Some recommended species for FTWs in North Carolina include cattails (Typha spp.), bulrushes (Schoenoplectus spp.), and spikerushes (Eleocharis spp.).
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Planting Density: The optimal plant density is around one plant per 2 square feet, with a minimum of 85% plant coverage maintained on the FTWs.
The placement and coverage of FTWs within the pond are crucial design parameters. Randomly distributing FTWs across the pond’s surface is not recommended, as it allows runoff to bypass the treatment zone. Instead, the FTWs should be positioned to span the full width of the pond, either directly downstream of the forebay or just upstream of the outlet structure. This strategic placement ensures that runoff might want to flow through the FTW root network before exiting the pond, maximizing treatment potential even at relatively low coverage rates (as low as 3-5% of the pond’s surface area).
Installation and Maintenance
FTW installation is typically conducted in the spring or early summer to facilitate successful plant establishment before the growing season. Careful anchoring is essential to prevent the mats from shifting or becoming dislodged, with options such as tethered cinder blocks, boat anchors, or helical anchors.
During the initial growing season, protecting the FTWs from predation by wildlife (e.g., geese, turtles) is crucial. Fencing, reflective pinwheels, and predator decoys can all be effective deterrents. Once the plants are well-established, these protective measures can be removed.
Ongoing maintenance of FTWs is relatively straightforward but vital to double-check that sustained water quality treatment and aesthetic appeal. Quarterly inspections should be conducted to check the integrity of the anchoring system, remove any invasive plants or debris, and monitor the overall health of the vegetation. If plant coverage drops below 85%, replanting may be necessary.
Periodic harvesting of the FTW vegetation can also improve long-term performance by permanently removing accumulated nutrients and pollutants from the system. Cutting back the above-mat plant biomass is recommended, particularly in nutrient-rich ponds where the shoots can store a significant amount of the captured pollutants.
The Benefits of Floating Wetlands
By retrofitting aging or underperforming stormwater ponds with FTWs, communities can significantly enhance their water quality treatment capabilities. Case studies in North Carolina have demonstrated that the introduction of FTWs can improve the average removal of total nitrogen by 22%, total phosphorus by 15%, and total suspended solids by 7% compared to conventional wet ponds.
In addition to water quality improvements, FTWs offer a range of other benefits:
- Flood Attenuation: The floating nature of the wetlands does not diminish the pond’s storage capacity or peak flow reduction, maintaining its effectiveness for flood management.
- Habitat Enhancement: FTWs provide diverse habitat for a variety of aquatic and terrestrial species, supporting local biodiversity.
- Aesthetics: Carefully designed FTW installations can significantly improve the visual appeal of stormwater ponds, transforming them into attractive community amenities.
- Cost-Effectiveness: As a retrofit solution, FTWs can be a relatively low-cost way to upgrade the performance of existing stormwater infrastructure.
With their versatility, proven effectiveness, and broad range of co-benefits, floating treatment wetlands are a valuable tool in the flood control specialist’s toolkit. By strategically incorporating FTWs into stormwater management strategies, communities can enhance water quality, strengthen resilience to flooding, and promote more sustainable and ecologically-integrated urban water systems.
To learn more about the latest innovations in flood control and water management, I encourage you to explore the resources available on the Flood Control 2015 website.
Tip: Regularly inspect and maintain flood barriers and drainage systems
