5.2 Drainage

On-road cycle lanes and mixed use streets are frequently the preferred retro-fit option given the limited changes to existing infrastructure, including drainage, that are required.

However, while the design of drainage is an important consideration, drainage convenience is never the primary or sole rationale for determining which facility should be chosen.

The various types of cycle facility and their appropriateness are dealt with in Link Types and Integration or Segregation.

The cycle route surface should be as smooth as possible to ensure efficient surface water run-off. A rough texture will provide for increased grip and reduced wheel spray compared to a smooth texture.

On cycle surfaces, there is no requirement for significant macro-texture. Therefore the wearing course should consist primarily of smaller aggregates, e.g. 10mm or less.

Surfacing material choice is significant when allowing for effective drainage solutions, and the materials commonly used on sealed or impermeable surfaces include:

• 45/6F or 45/10F hot rolled asphalt wearing course to EN13108: 4 (BS 594:1)
• 0/6 or 0/10 Dense bitumen macadam surface course to EN13108:1 (BS 4987:1)
• Close graded SMA (10mm or 6mm aggregate) to EN 13108-5

While high-friction (anti-skid) surfacing, in a range of colours, is often used to provide grip as well as delineation, the cost can be excessive, and its use should be limited to high-risk locations.

Drainage gullies, channels and manhole covers can present hazards to cyclists and should be located away from travelling surface used by cyclists. This is particularly important on bends and sharp curves as wet ironmongery is quite likely to cause cyclists to skid, slip or fall off. Gullies with slots running parallel to bicycle wheels are also a serious hazard to cyclists.

Cycle friendly design solutions include:

• Offline positioning for manholes etc.
• Side-entry gullies
• Continuous kerb drainage

If ironmongery cannot be located offline, it must be finished flush (typically +/-5mm) and recessed manhole covers should be considered to avoid slippery metal surfaces.

If gullies cannot be located offline, their slots must be at right angles to the cycling direction.

Cross falls and long falls are be used to drain road and cycling surfaces. Adequate drainage will generally be provided within the following gradient ranges:

Cross Fall:	1.25% to 2.5% (1:80 to 1:40)
Long Fall:	0.5% to 5.0% (1:200 to 1:20)

Depending on the type of cycle link, the surface can either:

• Drain to both sides. This might be used on cycleways and on certain segregated cycle tracks.
• Drain to one side only. This is applicable to on-road cycle facilities and certain cycle tracks.

On bends, a cycle track should always drain towards the shorter radius. A right hand bend should drain to the right, and left hand bend should drain to the left. If as track were to drain the other way, there would be a negative camber, increasing the risk of skidding.

The extra thickness of road markings and coloured surfacing can block cross-flow and cause ponding. Detailed design should ensure that the finished levels of all surfaces are flush. This may require milling of existing surfaces to achieve a smooth joint of the new material with the adjoining surface.

Gaps may need to be provided in continuous line markings (mandatory cycle lanes and bus lanes) to ensure effective cross-flow.

On adjacent cycle tracks, cross-fall away from the main carriageway is more comfortable for cyclists. However, this requires an independent gully or channel system. In such instances, ensure that the cycle track gully spacings match the main road gully locations to reduce pipework.

The provision of footpaths and cycle facilities can generate as much run-off as a standard 7.3 metre wide carriageway. This must be taken into account in the design of the overall drainage network.

For new roads, this can be managed using sustainable drainage solutions such as permeable road surfaces.

Where new cycle facilities are retrofitted into verges or other permeable surfaces along existing roads, there is the potential for overloading the existing drainage system. Options for managing this include:

• Creating a permeable cycle route surface
• Maintaining a small verge with linear filter trench
• Upgrading the existing drainage network

Unbound surfaces are appropriate for certain cycle routes such as cycleways and cycle trails.

Where an unbound surface is used, it is important that the cycle track is constructed so that surface water is shed to the sides, notwithstanding the fact that the surface is permeable. This is to prevent the sub-surface from becoming saturated.

Unbound surfaces are normally formed in a well-graded granular material such as CL 804 Sub Base material (sometimes referred to as dry bound macadam) or 20mm single sized stone pea gravel.

In locations with a high water table, a french drain should be considered in order to:

• Allow surface runoff from the track to be directed away quickly
• Reduce the effects of ground saturation locally by lowering the water table in the vicinity of the edge, and
• Limit erosion or rutting along the surface of the facility (exacerbated if the verge edge is higher than the track)