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Hydraulic considerations

ACO has an established Technical Services Department, with many years experience advising on surface drainage. Services include advice at the initial design stage through to on-site support, where required.

If you’d like assistance with trench and grate hydraulics to help select the correct size drain for your application, please contact us here.

Channel hydraulics

A channel’s hydraulic capacity is calculated by the amount of water the channel is able to collect and drain in a given time period. This determines the size of channel required.

One way to alter a channels capacity is by changing its physical cross sectional size (width x depth). The other is by changing its hydraulic run length. This is defined as the distance water needs to travel before being discharged through an outlet. Changing this can dramatically alter a channel’s run capacity. With all other factors equal, the shorter a hydraulic run length, the higher a channels capacity to drain.

Grate hydraulics

A grate fails hydraulically when water bypass occurs. Consideration must also be given to the inlet size of the grate to ensure it adequately removes water, but not at the expense of introducing litter into the drainage system, or compromise the safety of users.

A grate’s hydraulic capacity is calculated by the amount of water it will allow to pass through it in a given time period. Failure to allow passage of water into its underlying channel will result in bypass, regardless of how much capacity the channel has.

There are three factors that affect the hydraulic capacity of a grate, its size, its collective intake area and the design of its inlets.
For instance, a longitudinal bar grate can have a large cumulative intake area.
Between the bars, each slot acts as an individual elongated orifice and the grate will only reach its capacity once each orifice has flooded. The bars also have the effect of slowing down the speed of water, ensuring gravity has enough time to maximise the evacuation of the water  between the bars.

In comparison, the intake areas (and therefore capacity) of Tile and Brickslot grates are much smaller than a longitudinal bar design. A Tile grate has only two openings along the length of the drain. There are fewer opportunities for water to be evacuated through the design.



Furthermore, Brickslot grate styles are the most compromised design as there is only one slot for water to be interrupted and slowed.


During heavy storms, water bridging is common due to the increased flow rate. This can result in bypass, so longitudinal bar grates are preferred in vulnerable areas.

pavement catchment design

Balconies and terraces

Flat and level pavement design reduces risks to pedestrian safety, but requires an efficient drainage system to prevent hazards caused by ponding and standing water, damage to buildings and to preserve the life of the pavement.

Balconies are popular in multi-residential buildings to ensure occupants have access to outdoor space. AS 3500.3 specifies that balconies and terraces should be designed with two drainage methods. One designed for a 20 year storm event (to drain rain from walls, windows and wind driven rain collecting on the door frame and prevent the aluminium sub sill from flooding), and an alternative, contingency method designed for a 100 year storm event.

It is imperative that the exposed area falls away from the building, so any threshold drainage is designed to remove only wind driven rain and not surface runoff from the pavement.

Level Thresholds

Level thresholds

To make the most of the Australian climate, open air living and working areas are popular. This has meant that level thresholds have become a sought after design feature in many residential and commercial buildings leaving sleek, distinct architectural lines for the designer to integrate. These lines are often the regularly trafficked access zones of buildings and become the barriers between wet and dry areas.

Stormwater drainage must be effective as water damage repairs are not only disruptive and extremely costly, but can have legal ramifications in the event of structural damage or injury from standing water.

With the increasing regularity and intensity of storm events, linear drainage is an effective barrier to stop water transgression into buildings and other sensitive areas. Drainage must be correctly specified to ensure performance even during peak storms. Factors to consider are:

  • The speed and volume of run off from the exposed pavement, or facade,
  • Relative position of the drain,
  • The hydraulic performance of the drainage channel
  • The inlet performance of the grate
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