LAYER 01
Surface
Open-graded concrete, 100 to 150 mm. Fifteen to twenty-five per cent voids. Water leaves the surface the moment it lands.
THE OBLIGATION
Your permit doesn't care that it rained.
The State Planning Policy sets stormwater design objectives your development has to meet, and up here the drainage itself is built to the FNQROC Design Manual. Conventional pavement sheds every drop to the kerb, and everything the kerb collects is on its way to the lagoon. Meeting the target with a basin or a buried tank costs you land, excavation, and a maintenance obligation that outlives everyone who approved it.
100%
of rainfall runs off a conventional pavement, straight to the kerb
80 / 60 / 40
per cent cut in suspended solids, phosphorus and nitrogen the Wet Tropics objectives ask of your site
2,000 mm
of rain in an average year on the wet tropical coast, most of it arriving in one season
THE SYSTEM
Three layers. One direction: down.
LAYER 02
Base
Washed 20 mm aggregate, 150 to 300 mm deep. The storm is stored in the voids between the rock, not in a tank beside your building.
LAYER 03
Subgrade
Water returns to the soil it fell on, filtered by the trip down. Where the clay says no, an underdrain takes it to your legal point of discharge.
THE TRADE
The best detention tank is the one you never build.
A Hydrax permeable surface is a treatment measure your consultant can put straight into a MUSIC node. The storage sits in the aggregate under the car park you were always going to build. Move the dials and watch what that does to the tank you would otherwise dig.
Conventional pavement
100 L/s
leaves the site from the first minute, heading for the kerb
A 60 kL tank fills in 10 min
Hydrax permeable
0 L/s
nothing leaves while the base still has room
Holds this storm for 1 hr 7 min
384 kL of storage already sitting in the sub-base. That is the tank you do not build.
- Rain onto the area
- 100 L/s
- Surface can take
- 80× this storm
- Storage in the sub-base
- 384,000 L
- Subgrade takes away
- 4.8 L/s
- Base fills in
- 1 hr 7 min
- Runoff after that
- 95 L/s
Base empty
Indicative only. This is a simple balance of what falls, what the sub-base holds and what the subgrade takes, at one steady intensity. The soil rates are textbook values for each texture, and a real design halves them. It is not a design storm, not a MUSIC model, and not a substitute for the FNQROC Design Manual or your geotechnical report. Use it to see the shape of the trade, then bring the real numbers to a spec review.
THE QUESTIONS ENGINEERS ASK
The doubts, answered.
Does permeable concrete clog?
It can, and the fix is routine. Sediment sits in the top 25 mm or so. Scheduled vacuum sweeping restores most of the original infiltration rate. We hand every owner a maintenance schedule at handover, and it is shorter than the one that comes with a detention tank.
What happens in the wet season?
It is the question that matters most up here, and it is a design input rather than a problem. Sustained wet season rain means the subgrade is already wet, so we size the base for storage rather than assuming the ground will keep taking water. Where the soil will not drain fast enough between events, the system is lined and drained: the storm is still captured, still treated and still slowed in the aggregate, then released to your legal point of discharge at the rate the council allows. You give up the infiltration. You keep the treatment and the attenuation.
Is it strong enough for traffic?
For car parks, driveways, footpaths, courts and local access lanes, yes. We design the mix and the depth to the traffic, and the wearing surface is specified above 25 MPa. It is not a highway pavement, and we will tell you when your job is not a fit for it.
Will it satisfy the council?
That is what a spec review is for. We give your civil consultant the infiltration rate, void ratio and layer build they need to put the surface into a MUSIC node against the State Planning Policy objectives, plus the drainage detail your council will check against the FNQROC Design Manual. We would rather find the problem before you lodge than after.
What ruins an installation?
The crew. Water content, compaction and curing each have a narrow window, and permeable concrete is far less forgiving than a normal slab. Our pours are run by crews who place this every week, not once a year. Ask any of our references what pour day looked like.
ON THE GROUND
Heard after the first storm.
It took ninety millimetres in an hour in the February wet. There was not a puddle on it, and the drainage meeting took eleven minutes.
DEPOT UPGRADE, FAR NORTH QUEENSLAND
We got the corner of the site back. That corner is now nine more bays and the substation.
COMMERCIAL DEVELOPMENT, CAIRNS
THE NEXT STEP
Send us the site. We'll talk drainage.
A spec review is thirty minutes with an engineer, not a sales call. Bring the drainage plan, the geotechnical report if you have one, and the number your council reviewer flagged.
RECEIVED
Got it. An engineer reads every request. You'll hear from us within two business days.