Watershed Torbay: Torbay wetland project

Torbay Wetland Project

Reducing sediment and nutrient inputs to Torbay Inlet

MARSHALL'S CONSTRUCTED WETLAND

This project site demonstrates 'best management practice' aimed at reducing sediment and nutrient transport from a small agricultural catchment to Torbay Inlet.

The wetland is Iocated on the corner of South Coast Highway and Henning's Road in Torbay Catchment. It was constructed in January 2001 on an area of low-lying unproductive pasture. It captures and treats sediment and nutrient - rich runoff from a catchment area of 150 hectares.

THE WETLAND AT A GLANCE

Catchment area	150 hectares
Sediment pond	2,500 square metres
Wetland nutrient filter pond	7000 square metres
Total area of wetland	9,500 square metres
Cost of design	$5,000
Cost of earthworks and drainage	$23,000
Cost of supervision	$5,000
Number of fringing wetland plants	2,500 plants
Cost of fringing wetland plants	$700
Number of sedge and rush plants	17,000 plants
Cost of sedge and rush plants	$14,000

HOW THE WETLAND WORKS

This wetland is designed to mimic the sediment and nutrient removal processes of natural wetlands. Catchment run-off flows into the sediment pond. It is designed to slow flow rates enough for larger sediment particles to drop out. The water then flows through the perforated rock bund to the next "bio-filter" pond.

This pond was densely planted with 17,000 sedges and rushes by the Porongorup Green Corp Team in November 2001 and February 2002. This mass planting functions as the nutrient filter. The treated waterleaves the wetland and continues downstream via drains in the Albany Drainage District.

The rapid growth rate of sedges and rushes requires a large supply of nutrients, recycled from the sediment store. A "bio-film" of bacteria covers the densely planted stems and forms a large surface area of activity which uses up nitrogen from the water column. It takes 2-3 years of growth to reach peak nutrient uptake rates, by 2004 in this pond at present growth rates.

During storm events, the sediment pond rapidly fills to capacity, and some of the flow diverts from the wetland to downstream drains via the stormwater bypass channel.

This system ensures that for most of the year at normal flow rates, most of the run-off is retained in the main pond for sufficient time for treatment. The treated water leaves the wetland and continues downstream via drains in the Albany Drainage District.

WETLAND MAINTENANCE

Sediment eventually builds up in the sediment pond and will need to be removed. The sedge and rush plants also will eventually need to be harvested so that new plant growth can continue to take up nutrients.

IS THE WETLAND WORKING?

Torbay Catchment Group volunteers carry out macroinvertebrate sampling with WRC staff every two months. The variety and number of macroinvertebrates present provides an indicator of wetland health. Over time as the wetland plants improve the water quality, we expect to see colonisation of the wetland with sensitive macroinvertebrate species.

Automatic sampling equipment has been installed at the wetiand to constantly measure inflow, outflow and water quality within the wetiand. Salinity, pH, dissolved oxygen, turbidity, temperature, total nitrogen, nitrate, ammonia, total suspended solids (TSS) and phosphorus levels are measured. By 2004 we would expect to a significant reduction in the sediment and nutrient loads leaving the wetland.

WETLAND PLANTS AND HABITAT

The fringing vegetation surrounding a healthy water body has an important role to play. It supports animal life and maintains the quality of the environment surrounding the wetland.

The wetland fringing plants and sedge and rush species planted at the wetland are all native to the coastal floodplain of the Albany region. The vegetation in and surrounding the waterbody provides shelter and breeding habitat for fauna, as well as the important function of nutrient, pollutant and sediment filtering.

Trees and shrubs fringing the wetland

Agonis linearifolia - Rosa Tea Tree
Agonis parviceps - White Tea Tree
Agonis juniperina - River Cedar
Melaleuca densa - Lemon Honey Myrtle
Melaleuca cuticularis
Melaleuca rhaphiophylla
Melaleuca preissiana
Beaufortia sparsa - Swamp Bottlebrush
Callistemon glauca - Albany Bottlebrush
Calothamnus quadrifidus - One Sided Bottlebrush
Callystachis lanceolatis - Native Willow

Sedges and rushes in the bio-filter pond

Baumea articulata - Jointed Twig Rush
Baumea juncea - Bare Twig Rush
Juncus kraussii - Sea Rush
Eleacharis acuta - Common Spike Rush
Isolepis nodosa - Knotted Club Rush
Baumea rubiginosa - River Twig Sedge

WETLAND PROJECT PARTNERS

Phillip Marshall has allowed access and use of his land for the demonstration site. This commitment by Phillip is much appreciated and has ensured the success of the project.

The wetland's site works and plants were funded by the Natural Heritage Trust's Coast and Clean Seas Program, the Water and Rivers Commission and Water Corporation.

Torbay Catchment Group volunteers are partners in the wetland's management and have been instrumentalmental in assisting with the site works and planting at the wetland.

For more information on Coast and Clean Seas Demonstration Sites contact: