Highway Slope Manual

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5. DRAINAGE AND WATER-CARRYING SERVICES
5.1 GENERAL
Slope drainage, road drainage and water-carrying services should be engineered in an
integrated manner to ensure that water is channelled safely without impairing ground stability 勾
General guidance on the provision of subsurface drainage measures and design of slope
surface drainage is given in Chapters 4 and 8 respectively of the Geotechnical Manual for
Slopes (GCO, 1984) 勾 Guidance on prescriptive surface and subsurface drainage measures
applicable to existing slopes is given in GEO Report No 勾 56 (Wong et al, 1999) 勾 Methods
for the design of road drainage and road pavement subsoil drainage are covered in Road Note
6 “Road Pavement Drainage" (Highways Department, 1994) and Road Note 8 “Subsoil
Drainage for Road Pavement" (Highways Department, 2000) respectively 勾 Guidance on the
planning, design and management of stormwater drainage systems and facilities is given in the
Stormwater Drainage Manual (Drainage Services Department, 2000) 勾
Leakage of water from buried water-carrying services (including sewers, stormwater
drains and watermains) can trigger large-scale landslides 勾 The fatal landslide which occurred
in 1994 at Kwun Lung Lau (GEO, 1994) is a well documented example 勾 Reference should
be made to Section 9 勾 7 of the Geotechnical Manual for Slope (GCO, 1984), Section 6 勾 2 勾 4 of
Chapter 7 of the Hong Kong Planning Standards and Guidelines (Planning Department, 1994)
and the Code of Practice on Inspection & Maintenance of Water Carrying Services Affecting
Slopes (Works Branch, 1996) for guidance on the standards of good practice to prevent
leakage from water-carrying services affecting the stability of slopes 勾
5.2 SLOPE DRAINAGE AND ASSOCIATED ROAD DRAINAGE
5.2.1 General Considerations
Unless the highway slope drainage and associated road drainage systems are properly
designed and maintained, surface runoff will overflow onto and channelise along roads 勾 If
this surface runoff is not drained in time, water may overflow onto the downhill area and
trigger serious landslides (Plates 5 勾 1 and 5 勾 2) 勾
Landslides can occur at a road section where:
(a) the upslope drainage or the associated road drainage is
partially or fully blocked or has an inadequate design
capacity; during rainstorms, the road section will collect a
large volume of surface runoff from uphill areas, which may
discharge onto the downhill slope, and
(b) the slope below the road onto which the surface water may
overflow is a substandard man-made feature, or steep
natural terrain of marginal stability, or ground susceptible to
erosion or washout by concentrated surface water flow 勾