Treatment MeasuresBio-Retention Systems : Design Details

Needs following statement at beginning of design section: Comprehensive guidelines for the design of infiltration measures can be found in Chapter 11 "Infiltration Measures" in the WSUD Engineering Procedures available for purchase from CSIRO publishing.

Bio-retention systems combine various Water Sensitive Urban Design (WSUD) treatment types in one system. The system is designed to carry out primary and/or secondary treatment processes of stormwaters and retard flows.

The current types of treatments used include grassed swales (primary treatment) in combination with infiltration trenches (secondary treatment). Reducing velocities and retarding water reduces the flow of stormwater during the infiltration process.

The Operation of a Bio-Retention System

Normal operations and flow of stormwater through the infiltration trench of the bio-retention system

Normal operations and flow of stormwater through the infiltration trench of the bio-retention system.
Diagrams Courtesy of Tony Wong., Ecological Engineering Pty Ltd

Operations and flow of stormwater through the infiltration trench of the bio-retention system exhibiting the use of the overflow pits

Operations and flow of stormwater through the infiltration trench of the bio-retention system exhibiting the use of the overflow pits.
Diagrams Courtesy of Tony Wong., Ecological Engineering Pty Ltd

Excerpt taken from Preliminary Report: "Assessment of pollutant removal performance in a bio-filtration system - preliminary results". Sara D. Lloyd, Tim D. Fletcher, Tony H.F. Wong and Richard M. Wootton. Department of Civil Engineering, Monash University and Cooperative Research Centre for Catchment Hydrology.

Case Study: Lynbrook Estate, Melbourne, Victoria.

Figure 1 illustrates the design attributes of the bio-filtration system used for local residential streets in the precinct. The system consists of an underground gravel trench of 0.8 m depth and 0.45m width overlain with a grass swale. The gravels range from 2mm to 7mm in diameter.

Figure 1

Figure 1. Cross Section Design and Construction Images of the Bio-filtration System at the Lynbrook Estate

Roof runoff from adjoining dwellings is connected directly to the gravel trench, whereas road runoff is diverted into the swale using kerb chutes located at the downstream side of each driveway crossover. Road runoff is conveyed as surface flow along the length of each 'cell' of the swale and subjected to infiltration into the gravel trench. At the end of each cell, any runoff remaining as surface flow is discharged directly into the underlying trench via an inlet pit located on each driveway crossover. Each cell is thus subjected to a hydraulic loading from runoff generated from their individual catchment, until the discharge capacity of the subsurface gravel trench system is reached, at which time the swale of the bio-filtration cells operate as a continuous system.

Stormwater infiltrated through the gravel trench is collected by a 150 mm diameter perforated pipe, which conveys the treated water to an underground concrete pipe and then into the receiving waters. Stormwater runoff exceeding the discharge capacity of the 150 mm diameter perforated pipe/gravel trench system is conveyed as open channel flow along the grass swale to a grated entry pit and then directly into the underground pipe to the receiving waters.