The volume of liquid a trough system needs to collect and remove in a given time period determines its size.

Slab restrictions may limit the depth of the trough system leaving its width as the usual variable for correct sizing.

Typical factors that affect the size of a trough system:

• 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 trough system (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

Modeling Surface Drainage Flow
There are two methods used to try and size trough system to account for all surface water collected - we also have to account for any discharge into the drain such as discharges from other sources.

Non-uniform flow (Spatially Varied Flow)

Non-uniform flow accounts for liquid being carried in a trough plus the constant addition of liquid collected through the grates along the trench run - lateral intake. A characteristic of non-uniform flow is that liquid velocity and height, change at successive cross sections along the trench. To correctly model this situation, differential calculus is required; usually computer modeling is used. As a result of empirical testing, ACO has developed a computer program, ‘Hydro’, to model trench hydraulics.

Steady, uniform flow (Manning’s Theory)

A traditionally used method of calculating trench hydraulics involves the use of equations for steady uniform flow. This approach is better suited for pipe design where liquid velocity and height remain constant along the pipe. No allowance is made for lateral intake. Steady, uniform flow can be used in culvert (open top) design for spillways or irrigation trenches. These culverts are used to carry liquids from one area to another and do not account for lateral intake of liquids.