16-001014-02A (Ultimate SCMMP)

                                                                    Chapter 6 – Constructed Stormwater Wetlands




        Plant species for the wetland area will be selected based on the hydrologic regime, microclimate and soil
        types of the region, and the life histories, physiological and structural characteristics, natural distribution,
        and community groups of the wetland plants. The reader is referred to the Appendix A (Plant Selection
        for WSUD Systems) for a list of suggested plant species suitable for constructed wetland systems in
        SEQ. The planting densities recommended in the list should ensure that 70 - 80 % cover is achieved
        within two growing seasons (2 years). The distribution of the species within the wetland will relate to
        their structure, function, relationship and compatibility with other species.

6.2.8   Designing to Avoid Mosquitoes
        To reduce the risk of high numbers of mosquitoes, there are a number of design features that can be
        considered. Not all of these will be feasible in any one situation, but they include:

        ƒ Providing access for mosquito predators, such as fish and predatory insects, to all parts of the water
          body (avoid stagnant isolated areas of water).
        ƒ Providing a deep sump of permanent water (for long dry periods or for when water levels are artificially
          lowered) so that mosquito predators can seek refuge and maintain a presence in the wetland.
        ƒ Maintaining natural water level fluctuations that disturb the breeding cycle of some mosquito species,
          but be aware that this may suit other mosquito species.
        ƒ Where possible, incorporating a steep slope into the water, preferably greater than 30° or 3:1
            horizontal to vertical. Note that steep edges may be unacceptable for public safety reasons, and a
            slope of up to 8:1 horizontal to vertical is generally used.
        ƒ   Wave action from wind over open water will discourage mosquito egg laying and disrupt the ability of
            larvae to breathe.
        ƒ   Providing a bathymetry such that regular wetting and drying is achieved and water draws down evenly
            so isolated pools are avoided.
        ƒ   Providing sufficient gross pollutant control at the inlet such that human derived litter does not
            accumulate and provide breeding habitat.
        ƒ   Providing ready access for field operators to monitor and treat mosquito larvae.
        ƒ   Ensuring maintenance procedures do not result in wheel rut and other localised depressions that
            create isolated pools when water levels fall.
        ƒ   Ensuring overflow channels don’t have depressions that will hold water after a storm event.
        ƒ   Water weeds such as Water Hyacinth and Salvinia can provide a breeding medium for some mosquito
            species whose larvae attach to these plants under water. These weeds should be removed
            immediately if encountered.

        Each case has to be considered on its own merits. It may be possible that a well established constructed
        wetland will have no significant mosquito breeding associated with it; however, changes in climatic and
        vegetation conditions could change that situation rapidly. Maintaining awareness for mosquito problems
        and regular monitoring for mosquito activity should be considered as a component of the management of
        these sites. Effective and environmentally sound control products are available for control of mosquito
        larvae in these situations.

6.2.9   Designing for Maintenance Access
        Access to all areas of a constructed wetland is required for maintenance. In particular inlet zones and
        gross pollutant traps require a track suitable for heavy machinery for removal of debris and desilting as
        well as an area for dewatering removed sediments (refer to Chapter 4). If sediment removal requires
        earthmoving equipment to enter the basin, then a stable ramp suitable for heavy plant will be required
        into the base of the inlet zone (maximum slope 1:10).

        To aid maintenance, it is recommended that the inlet zone is constructed with a hard (i.e. rock) bottom.
        This is important if maintenance is performed by driving into the basin. It also serves an important role by
        allowing excavator operators to detect when they have reached the base of the inlet zone during desilting
        operations.

        Macrophyte zones require access to the areas for weeding and replanting as well as regular inspections.
        Commonly, these access tracks can be incorporated with walking paths around a wetland system.
        Maintenance access to constructed wetland needs to be considered when determining the layout of a
        wetland system.




            WSUD Technical Design Guidelines for South East Queensland – Version 1 June 2006                        6-6