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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.

The volume of liquid a channel system needs to collect and remove in a given time period determines its size. Slab restrictions may limit the depth of the channel leaving its width as the usual variable for correct sizing.

Typical factors that affect the size of a channel:

  • number of, and discharge rate (generally measured in litres per second) of wash down hoses in a room
  • volume of spill containment
  • hydraulic capacity of waste pipe beneath the floor connecting to the channel (for liquid evacuation)
  • quantity, size(s) and location (along its length) of trough outlet(s)
  • desired trough invert fall along the length of the system
  • speed of liquid across the floor during service and/or wash down operations. In this instance the selection of the grate must also be considered
  • for external areas: rainfall intensity,size of catchment, ground falls etc

ACO offers different sized channels and a broad selection of grates to meet most hydraulic requirements and offers technical support to customers to help ensure correct system specification.


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 vulnerableareas.

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