Hydraulic Design for Multi-Level Car Parks

What You Need to Know

Car park hydraulic design covers stormwater drainage, basement pump-out systems, ramp drainage, vehicle wash-down areas, fire water collection, and sewer connections for any amenities. The scope depends on whether the car park is open-deck, enclosed, or below ground. Basement car parks are the most complex because every litre of water that enters must be pumped out.

For a standard multi-level car park, hydraulic design fees range from $8,000 to $20,000. Basement car parks with pump-out systems, oil/water separators, and stormwater quality treatment sit at the higher end. Projects with multiple basement levels and on-site detention can exceed $25,000. The hydraulic engineer coordinates closely with the structural engineer (for slab penetrations and waterproofing), the architect (for floor grades and drain locations), and the fire engineer (for sprinkler and hydrant drainage).

Getting the drainage wrong in a car park is expensive. A flooded basement shuts down the building. Undersized ramp drainage sends stormwater cascading into lower levels during heavy rain. Missing oil/water separators trigger EPA enforcement. These are not edge cases. They are the default outcome when hydraulic design is treated as an afterthought.

• AS/NZS 3500.3 governs stormwater drainage design for car parks. This includes pipe sizing, grated drain capacity, pump sizing for basement levels, and connection to the council stormwater system. All drainage must be designed by a licensed hydraulic engineer and comply with the relevant authority's requirements. (AS/NZS 3500.3:2021)
• Basement levels below the stormwater invert require a pump-out system. Gravity drainage is not possible. The system must include a sump pit, submersible pumps in duty/standby configuration, high-level alarm, and connection to the gravity stormwater system above ground. (AS/NZS 3500.3:2021, Sydney Water requirements)
• Oil/water separators are required where vehicle washing or fuel spills are possible. Car parks with more than 10 spaces, wash bays, or loading docks typically require a Class 1 separator complying with AS 1940 and council conditions. The separator must be installed upstream of the stormwater connection. (AS 1940, EPA requirements, Council DA conditions)
• Ramp drainage must intercept stormwater before it reaches lower levels. A full-width grated trench drain is required at the top of every ramp connecting to a lower level. The drain must be sized for the tributary catchment area above, including wind-driven rain on open decks. (AS/NZS 3500.3:2021, BCA/NCC 2025)
• Fire hydrant and sprinkler systems require dedicated drainage. Test drains, fire pump test header drains, and sprinkler collection drains must be connected to the stormwater system. Fire water in basement levels connects to the pump-out system. (AS 2118, AS 2419, NCC 2025)
• Sydney Water requires a Section 73 compliance certificate for stormwater connections to their network. The hydraulic design must comply with Sydney Water's requirements for pipe sizing, connection points, and backflow prevention. (Sydney Water Act 1994, Sydney Water Technical Standards)
• Stormwater quality treatment is typically a DA condition. Councils require gross pollutant traps, sediment traps, or proprietary stormwater quality improvement devices (SQIDs) to treat runoff from car park surfaces before discharge. Treatment targets vary by council. (Council DCP/DA conditions, ANZECC Water Quality Guidelines)

Open-deck car parks are the simplest hydraulic scope. Rainwater lands on the deck, flows to grated drains or floor wastes set into the slab, and drains by gravity through downpipes to the stormwater system. The hydraulic engineer sizes the drains and downpipes based on the deck area, the rainfall intensity for the location (Sydney typically uses a 1-in-20 year ARI for roof and deck drainage), and the maximum ponding depth acceptable on the slab. Floor grades of 1:100 to 1:60 direct water to collection points.

Basement car parks are where the complexity increases significantly. Any water that enters the basement, whether from vehicle tyres tracking in rainwater, wash-down hoses, fire sprinkler discharge, or groundwater seepage, must be collected and pumped out. The sump pit is the heart of the system. It collects water from floor drains, grated channels along drive aisles, and ramp drains. Submersible pumps in the sump lift the water to a gravity connection above the stormwater invert level.

Pump sizing is critical. The pumps must handle the peak inflow rate, which includes ramp drainage during a storm event plus any fire sprinkler discharge. Most councils and Sydney Water require duty/standby pump configurations, meaning two pumps are installed but only one operates at a time. If the duty pump fails, the standby pump activates automatically. A high-level float switch triggers an alarm to the building management system (BMS) if water rises above a set level in the sump.

Ramp drainage is the most critical element for preventing basement flooding. A grated trench drain across the full width of the ramp at the top intercepts stormwater before it can flow down. For ramps connecting to basement levels, a second trench drain at the base of the ramp catches any water that bypasses the upper drain or drips off vehicles. Ramp drains connect to the pump-out system if they are below the stormwater invert. The hydraulic engineer must account for the velocity of water flowing down the ramp, which increases with ramp length and gradient.

Vehicle wash-down areas require separate drainage treatment. Wash water from car parks contains oils, grease, heavy metals, and suspended solids. An oil/water separator, also called a coalescing plate separator, removes hydrocarbons before the water enters the stormwater system. The separator is sized based on the expected flow rate from the wash bay or the number of car spaces draining through it. Regular maintenance is essential. A blocked separator renders the entire system non-compliant.

Fire hydrant and sprinkler drainage is often overlooked during design. When a fire hydrant is tested, it discharges 10 to 15 litres per second. In a basement, that water must go somewhere. The hydraulic design must include a test drain connection near the fire hydrant booster and floor drainage capacity to handle sprinkler discharge in a fire event. These flows connect to the basement sump pit and pump-out system.

Waterproofing and hydraulic design are closely linked. In basement car parks, the structural slab has a trafficable waterproof membrane applied to the top surface. The hydraulic engineer must coordinate drain locations with the waterproofing contractor because every penetration through the membrane is a potential leak path. Drains are set into puddle flanges that clamp to the membrane. The floor grading must direct water to drains without creating ponding zones where water sits on the membrane for extended periods.

Sewer connections are required if the car park includes amenities such as toilets, cleaners rooms, or first aid rooms. These connect to the sewer system via a separate drainage network. If the amenities are below the sewer invert, a sewage ejection pump system is needed, similar to stormwater pump-out but handling sewage. This is a separate system from the stormwater pump-out and must not cross-connect.

Stormwater quality treatment devices are sized based on the total impervious catchment area of the car park and the council's pollutant reduction targets. Gross pollutant traps (GPTs) capture litter and debris. Oil/sediment interceptors handle hydrocarbons and fine particles. Some councils require proprietary devices such as StormFilter or Jellyfish units that achieve specific pollutant removal rates. These devices require ongoing maintenance and are typically located at the point of discharge to the council stormwater system.