Highway Slope Manual

50
Cascades, with cover where necessary, and properly designed catchpits at the bottom,
should be used on steep soilrock slopes to carry water safely down the slope with minimum risk
of overspill or excessive splash 勾 They are much more effective at controlling velocity of water
than stepped channels, and are preferred especially for channels larger than 400 mm wide 勾
Guidance on the design methodology of cascades is given by Essery & Horner (CIRIA, 1978) 勾
In some situations, down pipes may be used, but appropriate detailing of the inlet is required to
avoid large volumes of debris from entering the pipe and provision should be made for
maintenance to clear blockages if required 勾 Recommended details are available in standard
drawings (Civil Engineering Department, 1991) 勾
At critical locations, gullies and buried drainage facilities including pipes and cross
road drainsculverts should be generously provided 勾 The capacity to be provided should take
into account the possible consequences of overflow that may arise in exceptionally heavy
rainstorms or if there is partial blockage 勾 The headwater level at the inlets of cross road
drainsculverts should also be checked, taking into account the outlet headflow conditions, to
ensure that water will not overflow onto the road level in the case of these drainage facilities
being partially blocked (with say only 50 of the design capacity remaining) 勾 Guidance on
the hydraulics and design of cross road drainsculverts is given in various design manuals (e 勾 g 勾
Drainage Services Department, 2000; Federal Highway Administration, 1985; Texas Highway
Department, 1962) 勾
The containment of the road surface runoff within the limits of the road carriageway
should be enhanced at critical locations 勾 This can be achieved, for example, by the provision
of an upstand wall or by the provision of channels with an upstand along the crest of the
downhill slopes (see Figure 5 勾 3), to contain and divert any surface runoff overflow to an
appropriate discharge point 勾 As an additional contingency measure, the slopes on the
downhill side of the road may be provided with a robust surface protection (e 勾 g 勾 vegetation
established using an erosion protection mat or stone pitching) to enhance their erosion
resistance 勾 Under no circumstances should the slopes be used as a runoff surface to cater for
surface water overflow from the road above 勾
For roadside drainage, roadside channels with removable, perforated covers may be
used as they are easy to maintain and do not become blocked so easily as buried drainage pipe
systems with gullies at wide spacings 勾 They can collect runoff from uphill slopes as well as
from the carriageway and can also form a continuous cut-off to prevent surface runoff from
overflowing onto the downhill area (see Figure 5 勾 3) 勾 Where they are provided, attention
should be given to the design detailing and construction aspects so that they do not obstruct
the subsoil drainage flow in the pavement sub-base 勾