Case study: Gatton Correctional Centre Biobasin

Total Environmental Concepts is a specialised environmental landscaping and maintenance company with over 40 years experience. We use our expertise to solve environmental concerns, and did this when designing and implementing a bioretention basin at Gatton Correctional Centre in Queensland.

Our team is comprised of professionals that provide knowledgeable advice, skilled services, and valuable assistance across a varied range of rehabilitation, construction, and maintenance operations. When contacted to provide our services for the bioretention basin at the Gatton Correction facility, the initial inspection revealed that the basin had several major issues that would require further analysis and solutions.

To begin, there were extensive weeds blocking the batter, which meant either spraying the weeds or removing by hand. We also used a forestry mulcher to create maintenance access down the batter to the bioretention. This involved:

• Clearing regrowth trees on access alignment 
• Clearing vegetation and removing and disposing of sediment and debris from the surface of the bioretention system, including ground covers
• Removal of vegetation on the batters in order to remove of lantana and other weedy material

On completion, batters were mulched and replanted with jute netting to high water level, to protect the embankment during the establishment phase.

On completion, batters were mulched and replanted with jute netting to high water level, to protect the embankment during the establishment phase.

The downstream end of the bioretention had a wall of gabions through which the underdrainage had been placed. The gabions form the end of the bioretention and a bentonite liner had been installed between the filter media and gabion. The was a poor design and resulted in washout of the filter media through the gabions and the creation of a number of large holes.

The solution was to create a barrier upstream of the outlet gabions. A range of options were available with the potential cheapest solution as per below:

• Excavate the area upstream of the gabions for a distance of 2m. Excavation to extent 500mm into in-situ soils below filter media. Care to preserve underdrainage pipes.
• Place well grade suitable clay fill and compact including around underdrain pipes.
• Line the upstream surface of the fill material with Geofabric (Bidim A24) and install bioretention drainage layer, transition layer and filter media and compact.
• Install 100-230mm rock mattress to the surface of fill material over Geofabric (Bidim A24) and tie into existing gabions.

The gabion weir was not properly extended up the batter to the natural ground (the weir had not been properly keyed in). The result is an erosion scour around both ends of the gabion weir.

The solution:

• Repair scour at the ends of the gabion weir and place rock scour protection (100-230m rock mattress) to up the batter to tie in with existing levels.
• The gabion wall was extended to a solid embankment than an additional gabion basket was added on top of the existing wall on each end to control and divert the water to the centre of the overflow of the gabion wall.
• Compact additional clay on the embankment, place Geofabric (Bidim A24) and rock guard with 100- 230mm rock mattress to surface to protect embankment.

Following extensive storm damage from rain in 2020, there were large washouts in the basin’s drainage media, requiring the removal of all existing vegetation and the importing of 100 cubic meters of filter media to repair basin.

We undertook mulching, jute netting and replanting of the basin in accordance with the Construction & Establishment Guidelines for Vegetation Stormwater Systems, including the use of fertiliser and water crystals in planting holes. Planting numbers are based on planting 50 percent of a 1700 m2 area.

The solution is to install a temporary irrigation system from the site irrigation system or directly from the pond. At the end of three months, a review will be undertaken to consider whether further establishment is required. The bioretention will need to achieve the requirements (i.e. plant coverage, maturity) outlined by the measurement of success in the Construction & Establishment Guidelines for Vegetation Stormwater Systems.

Contact Total Environmental Concepts today to discuss how our rebuilding and bioretention services can help you.