Highway Slope Manual

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Table 5.1 Examples of Locations Which May be Considered as ‘Critical' with Regard to the Impact of
Drainage on Stability of Highway Slopes
(a) Road sections similar to that at Nam Long Shan Road as in the Shum Wan Road landslide in 1995 (i.e. a long
and sloping road with adjacent large uphill slopes which can intercept and collect a large volume of surface
runoff and discharge it onto the downhill slopes in the event that nearby catchpits, drainage channels or road
drainage components are blocked during heavy rain).
(b) Road sections traversed by drainage culverts/pipes draining large catchment areas uphill, the blockage of
which or the blockage of the nearby stormwater inlets could lead to severe flooding and adversely affect the
stability of slopes in the adjacent area.
(c) Road sections affected by slopes below a catchwater which could be subject to overflowing due to blockage
of the catchwater, e.g. by landslide debris.
(d) Sag points of roads susceptible to large runoff from adjacent road surfaces and slopes which could be
discharged onto downhill slopes.
(e) Road bends supported by downhill slopes.
(f) Road sections with significant superelevation and large cambering (e.g. greater than 5%) which may lead to
overflowing across the carriageway and onto the downhill slopes.
Note: The examples of locations given in this Table should be considered as ‘critical' if the failure of the ground
at these locations involves slopes in the Consequence-to-life Category 1 or Economic Consequence
Category A.
5.2.2 Design of New Drainage Works
Highway slope drainage systems should be designed based on a two hundred-year
return period rainstorm (GCO, 1984) 勾
The discharge of surface runoff from a slope and groundwater from any subsurface
drainage measures directly onto a road should be avoided as far as possible 勾 Surface runoff
along a road should also be collected and discharged properly 勾 In cases where surface runoff
is carried across the original catchment boundaries, the impact of additional discharge into
existing watercourses and drainage systems should be considered and accounted for (Works
Branch, 1995b) 勾
In the design, critical road sections should be identified (Section 5 勾 2 勾 1) 勾 Figure 5 勾 1
and Plate 5 勾 3 show examples of methods of handling surface water at critical locations along a
road and examples of highway slope drainage and associated road drainage measures 勾
Design detailing of slope drainage and associated road drainage measures should be
given particular attention, especially at critical locations 勾 Also, the drainage design should
incorporate details which require minimal maintenance 勾 Recommended details of slope
drainage and associated road drainage are available in standard drawings (Civil Engineering
Department, 1991; Highways Department, 1998) 勾
A well-designed surface drainage layout for highway slopes (Figure 5 勾 2), comprising
berm channels that drain runoff onto the peripheral down-the-slope channels, would minimise
the provision of stepped channels on a slope face 勾 The velocity of the flow in slope drainage
channels should be within an appropriate range (see Section 8 勾 3 of GCO, 1984) 勾