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Wetlands Research
Floating Wetlands
Floating Wetlands in the Las Vegas Bay
The Bureau of Reclamation implemented a floating wetland project in Las Vegas Bay from 2000 to 2003.
The project goals included:
- Developing a floating platform and islands that were structurally durable.
- Determining how effective floating islands were at removing nutrients, and improving water quality within Las Vegas Bay at the mouth of Las Vegas Wash.
- Evaluating the riparian vegetation growth compared to the nutrient uptake.
- Evaluating the plant establishment success and different planting techniques.
Floating wetlands have been proven to be effective in ponds, lakes and reservoirs of Japan. They are an innovative way to improve water quality and clarity, provide shoreline protection from waves and to enhance habitat for fish and wildlife. The installation of the floating wetland structures (view the platforms) occurred in November 2000. The platforms were first planted in December of that year.
Design
The floating wetlands were made up of a series of 4-foot by 4-foot plastic pallets filled with coconut fiber to support wetland vegetation such as cattail, bulrush and sedge. A series of the vegetated pallets were attached together and secured to the floating platform superstructure. The floating platform superstructure was a large rectangular dock composed of a series of boat slips within which the vegetated pallets were inserted. The entire superstructure was secured in place by anchors from the bottom of Las Vegas Bay. Floating wetland platforms were more than 2,000 square feet in size.
Research and Monitoring
The wetland vegetation spread its roots and rhizomes through the coconut fiber substrate and penetrated into the water column, providing the potential for the vegetation to improve the quality of the water by removing nutrients and other compounds. Research and monitoring programs were established to monitor the wetlands effect on nutrients, salinity, trace elements, fecal coliform, perchlorate and cryptosporidium. In addition to water quality, research included monitoring aquatic invertebrates, zooplankton and phytoplankton use of the area below the floating platforms in comparison to open water areas and monitoring wildlife and fishery habitat evaluations both above and below the vegetated platform.
Results
The project was concluded in 2003. Due to lowering water levels in Lake Mead and increased sedimentation in Las Vegas Bay from the Las Vegas Wash, the platforms were relocated three times during the study period. Silt deposition and loss of lake elevation became so extreme that one of the platforms became so laden with silt that it sank on May 20, 2002. Despite the many challenges that were faced during the study, observations made at the platforms over the course of the three-year period did provide some practical information.
Water Quality
A nutrient reduction was identified under the cattail and bulrush platforms. Unfortunately, it was not possible to determine how well the floating islands removed nutrients compared to natural wetlands because they had to be moved several times during the course of the study.
Vegetation and Establishment
At least 18 species of plants grew on the floating islands. Common reed proved to be an aggressive weed on the platforms and competed with the cattail and bulrush species. The aerial coverage of the plants was relatively low ranging from 25 to 60 percent. However, the height of the plants reached 11 feet for cattails and 8 feet for the Olney's bulrush.
Wildlife and Fishery Habitat
An additional benefit of the floating wetland was increased habitat. Isolated from the shore, birds were protected from their natural terrestrial predators like coyotes and kit foxes. The water column around the wetland platform was a highly productive area that attracted fish and many other aquatic species. The floating wetlands improved this productive area by providing shade, nesting and hiding areas for many species.
Floating Island Durability
The plastic pallets used to form the islands worked well, although sedimentation caused by the fluctuating lake level created challenges. Some of the islands developed a silt load of more than 500 pounds, causing them to become swaybacked. Removing the silt that accumulated proved to be a maintenance issue throughout the life of the project. Using a soft connecting system for fastening the pallets together rather than the stainless steel hardware that was originally used could have improved the durability of the structure.
For more information on this project you can read the full report, Technical Memorandum No. 8220-03-09: Water Quality and Plant Growth Evaluation of the Floating islands in the Las Vegas Bay.
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