Stormwater detention and bioretention basin design.

Project overview

ASTCAD designed the on-site stormwater detention basin and a 1,200 m² bioretention basin for a new 4.8 ha commercial industrial estate in Western Sydney. Design included MUSIC modelling for water quality targets and detailed civil documentation for council submission and Sydney Water approval.

The challenge

The estate’s runoff coefficient post-development jumped from 0.35 to 0.85. Council required on-site detention to pre-development discharge plus 90% TSS and 60% TP load reduction. The available basin footprint was constrained by setback requirements and an underground sewer easement.

Our approach

• DRAINS modelling for storm event design (Q5, Q20, Q100)
• Basin sizing optimised for available footprint with multi-stage outlet structure
• MUSIC modelling targeting 90% TSS / 60% TP load reduction
• Bioretention media specification per FAWB guidelines
• Outlet structure detailing for low-flow and overflow events

Deliverables

• Civil drawings — basin, outlet structure, pipework
• DRAINS model and stormwater report
• MUSIC model and water quality compliance report
• Sydney Water approval documentation
• Vegetation specification and maintenance plan

Outcome

Both council and Sydney Water approvals issued without revision request. The basin was constructed during dry-weather and operational at handover. Monitoring after first 12 months of operation confirmed the bioretention is meeting water quality targets.

How we approach stormwater detention basin design

Stormwater detention basin design starts with the catchment, not the basin. We model pre- and post-development flows for the council’s nominated storm events, size the basin volume to hold post-development discharge at or below pre-development rates, and design the outlet structure — orifice, weir and spillway — so the basin performs across the full range of storms rather than just the design event. Embankment design considers geotechnical stability and maintenance access, and we detail scour protection at inlets and outlets because that is where basins actually fail. Safety in design is documented, including batter slopes and egress provisions.

Detention basin documentation

The package typically includes catchment plans and hydrologic calculations, basin layout and sections, outlet structure details, spillway design, and a maintenance schedule the asset owner can actually follow. Drawings are prepared to satisfy the council’s development conditions and the relevant Australian Rainfall and Runoff methodology, and we respond to referral comments as part of the service so approval does not stall on drainage.

Maintenance and lifecycle design

A stormwater detention basin design is a thirty-year asset decision disguised as a drainage calculation. Sediment accumulation, mowing access, outlet blockage and public safety dominate the whole-of-life cost, so we design for them explicitly: access ramps sized for maintenance plant, low-flow channels that keep the basin floor dry and mowable, trash racks that can be cleared without confined-space entry, and planting zones that stabilise batters without hiding the outlet works. Councils accepting these assets into their maintenance portfolio review exactly these details, and a basin designed for maintainability moves through the acceptance process with far less friction.

We prepare detention basin designs as standalone packages or as part of full civil documentation for development sites. If council has conditioned your approval on detention and you need the drainage response prepared quickly, we can typically turn around a compliant basin design within the referral response window.