INSTALLATION GUIDE
HIGHWAY DESIGN CONSIDERATIONS
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 Note: *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 – see table below). 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 125mm kerb-face. If so, capacity can be protected by cutting back the surfacing adjacent to each inlet. 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 the 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 150mm 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 adjacent table). 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.
OUTFALL PIPE CAPACITY
Gradient
Pipe diameter (mm) 150 225 300 375 450
1:20 (5%)
42 29 24 20 19
122 260 468 736
1:40 (2.50%) 1:60 (1.67%) 1:80 (1.25%) 1:100 (1.00%)
86 70 60 54
183 330 534 149 268 436 129 233 376 116 208 337
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TECHNICAL INFORMATION: charcon.technical@aggregate.com
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