Koor, Nick

Thorn, Mark

Portsmouth University Research Portal (Pure)

  
 
1 INTRODUCTION 
Scott Wilson Ltd. was commissioned in 1995 by the Territory Development Department of the 
Hong Kong SAR Government to investigate, design and supervise the construction of the 
Foothills Bypass (now re-named as Lung Fu Road). This scheme was to provide a bypass road 
along the Castle Peak Foothills, aimed at relieving the traffic burden along Lung Mun Road by 
Debris Flow Mitigation Works, Foothills Bypass, Tuen Man 
Mark Thorn & Nick Koor1 
Scott Wilson Ltd., Hong Kong, China 
1WSP (formerly Scott Wilson Ltd.) 
 
ABSTRACT: Foothills Bypass (now re-named as Lung Fu Road) was officially opened in 
March 2002. This scheme provides a bypass road along the base of the Castle Peak Foothills to 
relieve the traffic burden along Lung Mun Road.  
 The large man-made cut slope immediately above the Castle Peak section of the new road 
(Tuen Mun Area 19) has experienced continuous displacement for more than 20 years. 
Channelised debris flows from the hills above the proposed route also present a significant 
geotechnical hazard, with the largest flow in 1990 involving some 10,000 m3. 
 In response to these hazards the road was formed on an embankment that acted as a toe 
weight to stabilise the cut slope, with the road embankment also forming a dam to trap major 
debris flows from the hills above that might otherwise traverse the road in the future. 
 Although this arrangement served to address the direct hazards to the road at the design stage, 
it was considered prudent to construct check dams in the natural valleys above the man-made 
cut slope and surface drainage system in order to provide a degree of protection to the cut slope 
from smaller-scale, more frequent debris flow events. It was also anticipated that such an 
arrangement would encourage deposition of debris in large debris flow events, thereby reducing 
the potential for damage to the cut slope in such cases.  Nevertheless it was accepted that in the 
event of a major debris flow event, such as that in 1990, the check dams could be irreparably 
damaged and might need to be replaced. 
 During construction it was established that rockhead was deeper at the proposed check dam 
locations than had been anticipated, requiring that piled foundations be introduced. In the light 
of this finding the opportunity was taken to conduct a cost-benefit assessment in order to 
determine the way forward. This assessment indicated that the construction of the check dams 
was not justified economically and, as a result, they were deleted from the works. In their place, 
less visually-intrusive debris flow deposition basins were constructed in each valley as a 
prescriptive measure. 
 This paper broadly describes the approach taken in the cost-benefit assessment, as well as 
outlining the debris flow mitigation measures that were finally implemented. 
 Whilst such an approach is not applicable to all cases, for example where there is a clear life-
threatening risk, it is recommended that in future cost-benefit assessment form a more routine 
element of the design of mitigation measures against natural terrain failures. 
  PLATE 1: 1990 debris flow 
diverting most of the heavy vehicles coming from Tuen Mun Area 38 and Tuen Mun southwest. 
Of its overall 2.6km length, 1.7km was to be below the foothills of Castle Peak (Tsing Shan).  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
FIGURE 1: Plan of the route of Foothills Bypass (Lung Fu Road) 
 
The large man-made cut slope immediately above the Castle Peak section of the proposed 
road (Area 19, as shown on Figure 1) has experienced continuous displacement for more than 
20 years. Debris flows from the hills above the proposed route also present a significant 
geotechnical hazard, with the largest flow in 1990 involving some 10,000 cubic metres (see 
Plate 1).  This event, with a return period of perhaps 100 to 1000 years, as well as the debris 
flood of 1992 served to focus attention on natural slope failures in this area and their potential to 
develop into destructive debris flows, floods and torrents at lower levels (Langford and Hadley 
1990, Chan et al 1991, King 1996). 
The upper very steep slopes of the Tsing Shan 
massif (see Plate 2) dominate the landscape of the 
study area and comprise megacrystic granites. The 
slopes are essentially devoid of any superficial surface 
cover other than local and generally shallow talus and 
colluvial deposits. The granite massif gives way 
downslope to a distinct set of rounded spurs and 
associated less steep slopes, typically between 30 and 
40o, which are underlain by meta-sediments of the 
Tsing Shan Formation. These moderately steep slopes 
are covered by a relatively thin mantle of immature 
hillwash, largely derived from the meta-sediments. 
Further downhill, and marked by another change in 
gradient, the footslopes of Tsing Shan form a subdued 
and gently sloping landform underlain by volcanics of 
the Tuen Mun Formation. These continue to the shore 
and recent reclamations. Large areas of the footslopes 
(ie Area 19 on Figure 1) are prone to landslip, despite 
the shallow angles. These slopes are also covered by 
large tracts of colluvium containing a high proportion 
of granite clasts, with both older, weathered and locally 
cemented colluvium and a younger looser colluvium 
being present. These materials have found their way 
onto the footslopes both by debris flow and by less 
intense transportation processes. Although the more 
recent events that can be identified from field mapping 
and aerial photographs have not been dated, most are 
considered to be old on an engineering timescale. 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
PLATE 2: Area 19 cut slope in the foreground and Tsing Shan massif above 
(before construction) 
2 GEOTECHNICAL CHALLENGES 
Key geotechnical objectives of the Foothills Bypass Project were to: 
 Stabilise the man-made cut slope along the foothills above the proposed route 
(Area 19) 
 Protect the new road from natural terrain hazards i.e. debris flows emanating 
from Castle Peak (Tsing Shan) 
This paper concentrates on the latter. 
Hazard assessment conducted during 1996 identified that large-scale debris flows emanating 
from Castle Peak could be anticipated and these would probably traverse the proposed route 
(SWK, 1996 and Hadley et al, 1998). It was also concluded, however, that such large-scale 
debris flows could not practically be prevented from traversing the man-made cut slope above 
the proposed road and that, if such an event occurred, there would be some associated damage 
to that slope.  
The natural terrain hazards, and the marginal stability condition of the man-made cut slope 
immediately above the proposed route, were key components that dictated the final form of the 
road below Castle Peak.  
In response to these geotechnical challenges it was determined that below Castle Peak the 
road should be formed on an embankment. This served two purposes: 
 In addition to the embankment forming a toe weight that served to improve the 
stability of the cut slope,  
 The road embankment would also form a dam that would trap the major debris 
flows from the hills above that would have otherwise traversed the road in the 
future.  
Other components of the slope stabilisation included reducing the gradient of the man-made 
slope by placing fill over the lower levels, constructing 6m deep trench drains and installing a 
comprehensive surface drainage system 
The road embankment was engineered so as to provide a retention volume that was more than 
sufficient to capture any debris flow that could be envisaged, including worst credible flows of a 
larger scale than the Tsing Shan debris flow of 1990 (Hadley et al, 1998). Consequently, by 
 
 placing the bypass on an appropriately sized embankment, it was determined that the road itself 
would not be at risk. 
Nevertheless, as there was to be significant investment in the slopeworks, during design it 
was considered prudent to include pragmatically-sized check dams as prescriptive elements in 
the four natural valleys above the man-made cut slope. The benefits of this decision were that: 
 The check dams would serve to protect the cut slope below from the more 
frequent small-scale debris flow events;  
 They would tend to promote deposition of debris flow material and, through this 
process, serve to inhibit the development of larger-scale events that might flow 
down on to the man-made slope below and, in the extreme, would eventually be 
trapped behind the road embankment at the foot of that slope.  This would 
minimise the scope and cost of future slope maintenance. 
It was accepted, however, that in the event of a major debris flow, such as that in 1990, the 
check dams could well be irreparably damaged and might need to be replaced. 
The sizes of the check dams were constrained by the topography in each valley, with 
potential storage volumes ranging up to about 500m3.  These dams were designed to be founded 
on rock, which was anticipated to be at reasonably shallow depth. 
The response to the natural terrain hazards at this site can therefore be seen as a two-tier 
system, with differing criteria applying to the design of the road embankment (primary defence) 
and check dams (secondary defence). 
3 FINDINGS DURING CONSTRUCTION 
Trial pits and boreholes were implemented to confirm the foundation requirements before 
construction of the dams commenced. This work proved that rockhead was considerably deeper 
at the check dam locations than had been anticipated.  
In the light of this finding the design of the check dams was reviewed; the recommendations 
of SPR 1/2000 (GEO, 2000) coincidentally promulgated at this time were also taken into 
account in this review. This design review indicated that piled foundations would have to be 
introduced, considerably increasing the cost of the check dams. Given this conclusion the 
opportunity was taken to conduct a cost-benefit assessment in order to determine the way 
forward.  This is outlined in the following section. 
Before a conclusion had been reached on the way forward, a debris flow was triggered in 
Valley 4 on 14 April 2000 (see Plate 3). The debris flowed down onto the slopeworks being 
implemented below. Whilst such an event was well within contemplation (amounting to some 
300m3 in volume at the location of the proposed check dam, with a total volume of some 
1500m3), it provided further data on the likely return periods of channelised debris flows and 
also the damage that such events could cause. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
PLATE 3: Debris flow of 14 April 2000 (left of centre) 
 4 COST-BENEFIT ASSESSMENT 
The key assumptions made when conducting the cost-benefit assessment for the mitigation 
measures to be constructed in the valleys above the Area 19 slope were that: 
 The small-scale debris flow event of April 2000 was likely to have a return period 
of the order of 10 years; 
 The design event applicable to the debris flow mitigation measures in the four 
valleys (after SPR 1/2000) was therefore taken as 300 m3 (ie comparable to the 
April 2000 event and anticipated to have a return period of 10 years); 
 The mitigation measures (whether a check dam or other form of mitigation) 
would not be expected to cater for larger debris flows that would overtop the 
dams, traverse the Area 19 slope and be trapped behind the road embankment 
below. 
Costs were estimated for a number of scenarios, covering: 
 Check dam construction (ie non-recurrent cost) 
 Regular inspection and routine annual maintenance (ie recurrent cost) 
 Repair following a 1 in 10 year event (either with check dam in place, or 
otherwise as appropriate for each scenario) 
Costs were then compared over the design life of the Bypass (120 years) in order to ascertain 
the most cost-effective solution. 
The main elements of cost are summarized in Table 1. 
 
Cost Element Components 
Check dam construction 
(ie non-recurrent cost) 
Construction cost of the check dams, including provision of 
maintenance access to the areas behind the dams. 
 
Regular inspection and routine 
maintenance 
(ie recurrent cost) 
Where check dams built: 
 Annual inspections (routine) of check dams by IOW 
 Annual maintenance of check dams 
 Valley inspections by geotechnical engineer 
 
 Where check dams not built: 
 More frequent inspections by IOW 
 Annual inspections by geotechnical engineer 
 Annual maintenance 
 
Repair following a 1 in 10 year 
event 
(ie the design event for check dams) 
Where check dams built: 
 Clearing of debris from behind check dam  
 Reinstatement 
 
 Where check dams not built: 
 Removal of debris 
 Repair of damage in valley and on slope below 
 
Repair following a major debris 
flow event that traverses onto Area 
19 slope 
(ie greater than the design event for 
the check dams) 
Not directly considered, as the cost of repairing such a major 
event would be the same for all scenarios.  However, broad-
brush estimates indicated that the cost of repairing a worst 
credible event (circa. 20,000m3) might amount to $70 to $100 
million. 
 
TABLE 1: Main components of cost 
 These principal scenarios were considered in the cost-benefit assessment, with the results 
being presented in Table 2.  The initial comparison made was between a scenario whereby a 
check dam was constructed in each valley (Scenario 1) as opposed to the case where check 
dams were not constructed (Scenario 2). This very clearly demonstrated that it was not cost-
effective to construct check dams in all valleys i.e. it would be cheaper to repair the damage 
resulting from a 1 in 10 year event.  However, as the highest hazard was considered to be posed 
by Valley 2 (the valley in which the large-scale 1990 event occurred), a further scenario was 
introduced whereby a check dam was only constructed in that valley (Scenario 3).  This 
indicated that the decision was very finely balanced, with this third scenario being only 
marginally more expensive than the case where no check dams were constructed. 
 
Scenario 
Non-
Recurrent 
Cost 
 
 
(HK$ M) 
Recurrent 
Costs over 
10 years 
 
 
(HK$ M) 
Repair for a 1:10 
year event 
 
 
 
(HK$ M) 
Total 
Recurrent 
Cost over 
10 years 
 
(HK$ M) 
Total Cost 
over 120-year 
design life 
 
 
(HK$ M) 
1 Check dam constructed 
in all four valleys 
 
41.8 4.0 1.5 5.5 107.8 
2 No check dams 
 0 1.0 
2.5 
(i.e. an event in 
any one valley) 
 
3.5 42.0 
3 Check dam constructed 
in Valley 2, but not in 
Valleys 1, 3 and 4 
 
5.2 1.25 
2.5 
(i.e. an event 
in Valleys 1, 3 
or 4) 
 
3.75 50.2 
TABLE 2: Estimated Costs for Various Scenarios 
 
Given the magnitude of the associated costs compared to the benefits it was decided to delete 
the check dams.  However, the opportunity was taken to explore more economically-justifiable 
forms of mitigation measures that would serve to constrain and reduce the damage that would 
otherwise result from small-scale channelised debris flows.  The final form of mitigation 
measure that was adopted is outlined in the following section. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
PLATE 4: Later Stages of Construction at Area 19, before construction of mitigation works 
 
  
 
 PLATE 5: View of debris flow depositional 
basin looking upslope 
PLATE 6: Downslop view of debris flow 
depositional basin 
 
5 MITIGATION MEASURES FINALLY IMPLEMENTED  
As a result of this review it was determined that the most appropriate form of mitigation would 
be to construct basins at the foot of each valley that would:  
 retain debris flows up to the design volume; 
 encourage deposition of debris in large events before traversing onto the cut slope 
below.  
The prescriptive mitigation measures were designed to be as economical as possible utilising 
second-hand materials wherever possible and making use of the existing topography to 
maximise the depositional area above  the drainage intakes at the head of the Area 19 cut slope. 
The main elements of the debris flow deposition basins were to: 
 Form each debris flow depositional basin a short distance above the intakes to the 
drainage culverts that run down the cut slope below; 
 Encourage deposition by constructing 1m diameter, 0.8m high baffles staggered 
at a 2m spacing across the basin, thereby reducing the energy of the debris and 
promoting deposition; 
 Implement as little cutting and filling as possible; 
 Protect the floor of the basins with rubble stone pitching, tieing in with the 
headwall of the culvert intakes below; 
 Protect the side slopes of the basins with grasscrete or gabion baskets depending 
on the gradient, with the stone used for the baskets won from excavation 
elsewhere on site; 
 Construct boulder-straining structures in front of the drainage intakes to prevent 
blockage by large boulders that do not deposit in the basins. To reduce costs, 
these structures were formed using second-hand galvanised “I” beams, spaced 
1.5m apart; 
 Install trash screens on all intakes to retain cobbles and smaller boulders; 
 Trim the natural valley sides upstream of the basins, together with the provision 
of erosion protection, in order to prevent local distress of the valley sides that 
could trigger failures and infilling of the basins. 
The final form of the basins is illustrated in Plates 5 and 6. 
6 CONCLUSIONS 
Cost-benefit assessment incorporating both 
construction costs and whole-life costs, has proved 
itself to be a valuable tool in the decision-making 
process for this Project. Whilst such an approach is 
necessarily broad-brush in nature (given the 
intrinsic uncertainties about the size and return 
period of debris flows and other assumptions that 
have to be made) it was an essential component of 
the decision to delete the check dams and to 
introduce debris flow depositional basins at this 
site. The decisions that were made as a result were 
defensible, with the long – term maintenance and 
repair implications and costs being clearly 
identified.  
Whilst such an approach is not applicable to all 
cases, for example where there is a clear life-
threatening risk, it is recommended that in future 
cost-benefit assessment form a more routine 
element of the design of mitigation measures 
against natural terrain failures. 
 7 ACKNOWLEDGEMENT 
The authors would like to thank the Director of the Territory Development Department of the 
Hong Kong SAR Government for his kind permission for this paper to be published. 
8 REFERENCES 
Chan, Y. C., C. H. Lam and W. L. Shum (1991). The September 1990 Tsing Shan Landslide : A Factual 
report, Volume 1, Technical Note TN 4/91. Geotechnical Engineering Office, February 1991. 
Hadley et al (1998). Debris Flow Assessments for the Foothills Bypass, Hong Kong. Slope Engineering 
in Hong Kong. Proc. of the Annual Seminar on Slope Engineering in Hong Kong, HKIE. 
King, J. P. (1996). The Tsing Shan Debris Flow, Volumes 1-4, Special Project Report SPR 6/96. 
Geotechnical Engineering Office, May 1996. 
King, J. P. (1996). Tsing Shan Debris Flood of June 1992. Special Project Report SPR 7/96. Geotechnical 
Engineering Office, October 1996. 
Langford, R. L and Hadley, D. (1990). New Debris Floods on the Flanks of the Tsing Shan. Geological 
Society of Hong Kong Newsletter, Volume 8, Part 3, 2-12. 
Lo, D. O. K. (2000).  Review of Natural Terrain Landslide Debris-resisting Barrier Design.  Special 
Project Report SPR 1/2000.  Geotechnical Engineering Office, January 2000. 
Scott Wilson Kirkpatrick (1996). Foothills Bypass Tuen Mun Road / Wong Chu Road Interchange and 
Other Road Junction Improvement Works – Final Geotechnical Appraisal Report 
 
 
 


Debris flow mitigation works, Foothills Bypass, Tuen Mun

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Debris flow mitigation works, Foothills Bypass, Tuen Mun

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