Areas Within a Wetland
Wetland areas are sorted according to their depth, how long they are covered by water and whether the wetland has fresh or salt water. Functionally, wetlands are divided into three areas or zones, the:
- inlet zone
- large plant (macrophyte) zone
- open water zone.
The inlet at a stormwater quality wetland
Inlet zone
This is where the rivers and creeks (draining the catchment) meet the wetland.
The purpose of this zone is to:
- slow the water flowing into the wetland
- enable the larger particles carried in the water to settle and sink.
Aquatic plants can also be grown in the edges of this area. These plants are generally able to withstand the fast flowing water from severe storms.
Large plant (macrophyte) zone
Typical macrophytes within a wetland
Plants that grow wholly or partly underwater usually occupy this area. The plants convert the nutrients and assist in the treatment of stormwater. They are helped by the:
- micro-organisms and algae on their stems
- sediments in which they grow.
To be effective, the wetland needs to be 0.1 to 0.4 metres deep with many plants and micro-organisms to filter the water and take up pollutants.
Wetlands filter suspended solids, nutrients, heavy metals, organic matter (which may break down here too) and even oil.
Large open water zone in a wetland
Open water zone
This is a deeper area that allows time for:
- finer particles to settle and sink to the bottom of the pond (bed)
- sunlight to kill bacteria.
Due to sunlight and high nutrient levels, algal growth occurs from time to time in this zone. Algae traps the dissolved nutrients and allows them to either enter the food chain or settle to the bottom of the pond.
A wetland case study
At Hallam Valley Retarding Basin the Healthy Bay Initiative project involves the construction of 10 wetlands to treat stormwater runoff in the Cities of Casey, Greater Dandenong and Kingston, and will serve as a demonstration of best practice environmental management of stormwater.