The 3 zones of wetland design

Sealed Air global headquarters water quality management system

Valued for their social, cultural and environmental benefits, Australia’s waterways are an integral element within the rural landscape. Wetlands play a key role in creating biodiversity amongst urban environments, promoting habitat and naturally filtering stormwater. As a result of urban development, an increase in contaminated runoff, sediment and other pollutants are fed into the surrounding water bodies, having an adverse effect on the fragile ecosystems. As greenfield development continues to occur on the periphery of our cities, considering how we integrate our homes and streets into the natural processes of the land is integral. 

Recently, I have been working on some residential estates around Central NSW and the coast. These large scale projects often require contour land forming, retaining walls and road networks which reduced water permeability and potentially impacting local habitats. The provision of basins has a three-fold requirement; water quality management, interaction for residents & most importantly, providing habitat for wildlife.

In addition to their visual and aesthetic importance, the selection of plants and macrophytes has an integral role in the residence time of stormwater, and must take into consideration both the consistency of their inundation and the frequency of dry periods. Plant species are sensitive to different levels of water inundation and must be positioned in detail, with careful consideration of the water depths before, during and after rainfall.

The vegetation used within a wetland design can typically be broken into three zones:

1. The Dry Embankment Zone or, Top of Embankment refers to the consistent level around the perimeter of the wetland, and is usually located 300mm above the maximum water level

• This portion of the wetland features little to no inundation, and consists of plant species suited to having little or no wet feet.

2. The Ephemeral Zone Maximum Water Level refers to water level during rain and storm events. It should not exceed the limits of the embankment.

• As this is an ephemeral batter, planting should be suited to both inundation and also tolerant of extended periods of drought.

3. The Macrophyte Zone is in line with the consistent level of water held by the basin outside of rainfall events (ie, the level of the drainage outlet). Macrophyte plant species should only occur below this level, as they require a constant supply of water to survive.

• Position of macrophyte species should be concentrated close to the discharge point in order to promote a reduced water flow and increased filtration efficiency.

The macrophyte zone typically includes densely planted species in shallow depth water, dispersed by deep water pools at the inlet and outlet pipes. The length of the macrophyte zone should be a minimum of four times its width, to provide ample filtration of water. The proper positioning of a macrophyte zone can help to control water flows and meet the required stormwater residence time. As the macrophyte zone slows water, it also reduces the risk of erosion and the need to use rock scour solutions. 

In addition to macrophytes, the inclusion of groundcovers along batters provide stability through their fast growing habit, and fibrous root systems. Groundcovers often include a mixture of native grasses, sedges and rushes. This is a great alternative to turf, which has a shallow root system and often requires more maintenance.

Shrubs, while relatively slower growing, provide additional deep reaching roots and are great for long term absorption of stormwater pollutants. Similarly, the inclusion of trees can provide even stronger, deep roots for batter stabilisation. They also enhance the amenity value, create additional shade and microclimates suitable for habitat and suppress weed growth.

I have created a simplified table which lists out a number of potential plants and how they fit into the three zones. Part of the table is shown below and can be made available upon request.

This table serves as a starting point when selecting plants for a basin or pond design. However - it is always important to select species that are suited to the specific site, and careful consideration of plant availability, density and position should be undertaken in addition to using this table. 

Wetlands (and other WSUD interventions) are not just a singular approach to mitigate the negative effects of urban growth; they provide an opportunity to celebrate water, it’s conservation and the relationship between it and the wildlife it harbours. In addition to improving water quality, reducing runoff and minimising erosion, wetlands also:

• Provide amenity and aesthetic value to the area

• Celebrate the native flora species of the area

• Create new habitats for local fauna

• Educate the community about their waterways