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Mini Highway

General principles

The aim is to ensure prompt and cost-effective removal of surface water during and immediately after rainfall.

Multiple inlet holes and continuous U-shaped channel profile enable system design which minimises underground pipework. This can provide a lower-cost solution than traditional gully systems.

System capacity

For Mini Highway drainage, identify any flow constraints in advance, including:

• vehicular crossings requiring crossing base units
• tight radii*
• junction units creating turbulent flow*
• outlet unit capacity
• outfall chamber/pipe capacity
• linear grates

* Hydraulic properties given in above tables are conservative and so, in most cases, any flow reductions arising from these factors may be ignored in design.

In common with all combined kerb and drainage systems, Mini Highway top and base units will NOT run full due to turbulence from the inlet holes. System flow capacities given in tables below are therefore confined to realistic flow levels rather than theoretical/full bore maximum levels. Capacities vary according to type of base unit installed in the system.

Note Capacity will be reduced over time wherever there is a likelihood of a significant build-up of silt.

Outlet capacity

In practice, capacity requirements can vary according to location of the outlet and the depth of flow within the system. For design purposes, an outlet capacity of 30 litres per second should be adopted. If this capacity is insufficient, more than one outlet unit should be installed to discharge into the outfall chamber.

Outfall chamber/pipe capacity

Outfall discharge capacity should be as large as possible to ensure benefits from Mini Highway system. In large schemes gully pots used as outfall chambers may be insufficient because turbulence can reduce capacity levels below that which a standard 150 mm outfall pipe can sustain. Where possible, purpose-built chambers (brick or precast manhole rings) are recommended instead to help minimise effects of turbulence on capacity. The appropriate outfall pipe can then be selected to suit the specific case (see ‘Outfall pipe capacity’ table below).

Mini Highway system flow capacities

Highway outfall pipe capacity