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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 the 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 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 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, Highway top and base units will NOT run full due to turbulence from the inlet holes. System flow capacities, given in the table 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, (standard, shallow, deep or crossing base units).

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

Inlet capacity

The capacity of each inlet, maximised when first laid, can be affected by future re-surfacing which may raise carriageway levels above the normal 125 mm kerb-face. If so, capacity can be protected by cutting back the surfacing adjacent to each inlet (special tool available).

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 62 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 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 table below).

Linear grates

Highway can be used for long runs of linear drainage where high volumes of surface water drainage is required or high wheel loads are imposed.

Highway outfall pipe capacity