E3S Web of Conferences 415, 06014 (2023)
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The Cheekeye Debris-Flow Barrier – unique features of aproposed open check dam in CanadaEmily Mark1*, and Alex Strouth2
1BGC Engineering, Vancouver, BC, Canada
2BGC Engineering, Golden, CO, USA

Abstract. The proposed Cheekeye debris-flow barrier in Squamish, BC, Canada is an open check dam witha very large storage capacity of 2.4 million m3. There are several unique features of the design that deviate
from standard international practice, including the very low probability (1 in 10,000 years) design event, theuse of a single large structure, the selected construction material, the crest design, and the outlet design.
These features were selected for the Cheekeye barrier due to idiosyncrasies of the site specifically, and ofdebris-flow mitigation in Canada, in general.

1 Introduction
The Cheekeye River fan in Squamish, BC, Canada is
subject to debris flows from the Mount Garibaldi
volcano. These debris flows pose intolerable risk toexisting and proposed development. A debris-flow
mitigation system has been proposed to reduce risk to atolerable level. As of summer 2022, the detailed designof the mitigation has been completed and construction isanticipated to begin in the next few years.
There are several unique features of the Cheekeye
mitigation system, particularly compared to how thesystem may have been designed if it was situatedelsewhere in the world. The purpose of this paper is tohighlight these design features and share the designers’
rationale for this deviation from standard international
practice.

2 Project Overview
Existing development on the Cheekeye fan includes twoschools, businesses, residential areas with about 1,400residents and the major highway that connects
Vancouver and Whistler, BC (Fig. 1). Additional
development in the at-risk area is conditional on theconstruction of debris-flow mitigation by the developer,
to reduce risks for both existing and proposeddevelopment.
The principal component of the proposed mitigation
system is a 24 m tall barrier with a 6 m wide vertical slit
outlet and a design storage capacity of 2.4 million m3 ofdebris (Fig. 2). The proposed barrier is located at anatural bedrock constriction near the upper apex of theCheekeye Fan. This location provides a naturally
confined basin and is efficient for sediment and debris
retention (Fig. 1).

The barrier is sized to manage a 10,000-year return
period debris flow with 2.8 million m3 volume. It is asymmetrical gravity dam with 45-degree inclined slope
faces composed of roller compacted concrete. A 6 m
wide, vertical slit outlet that extends from the riverbed
to the barrier crest allows routine Cheekeye River flows
to pass, but restricts debris-flow discharge to a value thatcan be conveyed downstream with tolerable risk. Sevenhorizontal steel beams, each 1.3 m square, span acrossthe slit outlet in the upper 19 m of its height to encourage
blockage of the slit by boulders during debris flows. If
future changes to the barrier performance are desired(e.g., to adjust the of balance residual risk and sedimentremoval cost), the spacing of the beams is adjustable,
allowing beams to be added or removed to promote
blockage or passage of sediment.

3 Design Event Selection
The use of a 1 in 10,000-year design event for theCheekeye barrier is a particularly unique characteristicof the design, compared to debris-flow mitigation
systems designed elsewhere in the world. In othercountries, design events for debris-flow mitigation
typically range from about 100 to 300 years, with up to
1,000-year events being considered in circumstances
involving high-density development or criticalinfrastructure [1].
For the Cheekeye project, the design event wasselected through a comprehensive life-loss risk
assessment process. The process included input from thelocal government, the provincial government, and anindependent expert review panel, as well as decades ofhazard assessment on the fan and in the watershed.

* Corresponding author: emark@bgcengineering.ca© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

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NFIRSTNATIONSRESERVECAT
LAKE

PROPOSED MAIN
BARRIER SITE

LOWER FAN APEX

NATURAL
BASIN

ALICE
LAKE

ALICE LAKEPROVINCIALPARKSCHOOLSPROPOSEDPROPOSEDBARRIERBARRIERSITESITEPARK BOUNDARYCHEEKEYE FANBRACKENDALEBC HYDRORIGHT OF WAY
WATER BODY
ROAD0500

1000 mTRAIL

RIVER(follows northPark Boundary)

Figure 1: Map of the proposed barrier location and the Cheekeye fan.

Figure 2: A rendering of the proposed Cheekeye barrier, looking upstream (provided by Sqomish Sea to Sky Developments LP).

2

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event, with a return period of approximately 10,000years, because it is the largest magnitude event thatposes intolerable risk. The barrier has a retention
capacity of 2.4 Mm3, and up to an additional 0.4 Mm3
can be passed through the outlet with tolerable risk.
To our knowledge, the selection of such a low
probability/high magnitude design event is unique indebris-flow mitigation. This exceedingly rare design
event is the primary reason for several of Cheekeye’s
unique design features. Design norms that make sense
for a 100- or 200-year return period design event
become less practical for a 10,000-year return period
event.

In 2015, the expert review panel determined that themitigation design needed to account for (but notnecessarily directly mitigate) the largest credible debris
flow that could occur: a 5.5 Mm3 rock-avalanchegenerated debris flow from Mount Garibaldi. Table 1
shows the frequency-magnitude relationship for
Cheekeye River debris flows that was the basis fordesign [2]. The expert panel were also involved in theselection of risk tolerance criteria for the project, whichwere equivalent to those adopted in Hong Kong forlandslides, and were adopted by the local government in
the absence of regional or federal criteria in Canada [3].
Risk greater than 0.1 lives lost in 100-years wasconsidered intolerable.

4 Unique Features of the Design

Table 1: Cheekeye debris-flow frequency and magnitude
Scenario Name
VolumeReturn period
Class(years)
(VC)
130
2
100
3300
4
1000
5
3000
10,0006(best estimate)
10,0007(upper bound)

Scenario Magnitude
EventPeakvolumedischarge(Mm3)(m3/s)
0.2
800
0.4
1,600
0.8
3,400
1.4
6,600
2.2
11,0002.8

15,0005.5

33,0004.1 Single Large Structure
Many debris-flow mitigation systems use a “functionalchain” of structures to manage risk at multiple points inthe watershed or channel. For example, a debris
regulation barrier might be combined with a series ofclosed check dams to limit channel erosion, a parallel
sediment basin, or even watershed scale management
including hundreds of structures (see Tateyama Caldera,Japan).
At Cheekeye, debris-flow management relies on asingle large debris-flow barrier for several reasons. The
barrier is located at the downstream end of a naturalbasin, which is efficient and sufficient for retaining therequired sediment volume. The land downstream of thebarrier to the fan apex is within a Provincial Park, whereconstruction and development are prohibited (Fig. 1).
Finally, construction of additional structures at the fan
apex or on the lower fan was ruled out due thedistribution of elements at risk, and potential for a
mitigation structure to cause risk transfer by diverting
flows to other areas of the fan.

Using the frequency-magnitude relationship as astarting point, extensive numerical run-out modelling
and risk analysis were completed to assess the life loss
risk to existing and proposed development. Figure 3shows the results of the assessment in terms of the risk
contribution from each debris-flow volume class (VC).
2.50

Life Loss in 100 years

2.00

4.2 Lack of Excavated Basin

1.50

The proposed design does not include basin excavationin the area upstream of the barrier, because excavation
would change the natural channel gradient and inhibit
the passage of sediment during clearwater floods orsmall debris flows. Retention of this sediment would
unnecessarily increase maintenance costs and may also
cause sediment starvation downstream. In the current
condition, tens of thousands of cubic meters of sediment
are transported annually by the Cheekeye River byclearwater flows. This sediment is beneficial for themajor fish-bearing rivers downstream, and the design
seeks to maintain connectivity of this sediment.

1.00
0.50
0.00

Intolerable risk
>0.1 lives lost per 100 years
VC 1

VC 2

VC 3

VC 4

VC 5

VC 6

VC 7

Figure 3: Risk contribution by volume class (dashed line)
compared to intolerable risk threshold (solid line).

The only tolerable debris-flow scenarios are VC1and VC7. VC1 is high probability, but low consequence
because the event is not expected to cause fatalities.
VC7 is high consequence, but the risk is tolerable
because the probability of occurrence is so low.
The mitigation system is required to reduce
intolerable risks to a tolerable level: in other words, any
scenarios that plot above the solid red line on Figure 3
need to be reduced to below the red line. Therefore, thesystem was designed to manage the VC6 (2.8 Mm3)

4.3 Construction Material
The proposed Cheekeye barrier is a gravity dam
composed of roller compacted concrete (RCC). RCC isa concrete mixture (cement, sand, and gravel) that isplaced in horizontal lifts and compacted. RCC is acommon material for water dam construction, and a3

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similar construction material is commonly used fordebris-flow barriers in Japan, known as Sabo Soil
Cement. RCC was selected for Cheekeye because it ismore cost effective than alternative construction
materials (e.g., earthfill with an erosion protectioncover) that result in similar performance.
The main body of the gravity barrier does notinclude reinforced concrete; instead, the cohesivestrength of the horizontal joint interfaces of the RCC issufficient to resist the debris-flow impact. The dynamic
impact force of the design event is estimated to be in therange of 300 to 1,000 kPa, with a flow depth of 2 to 12 m
and a velocity of 6 to 20 m/s.
The barrier will have 45-degree (1H:1V) side slopes,
unlike the near vertical slopes often seen on reinforced
concrete structures. This side slope angle allows theRCC to be placed and compacted without using
formwork, and provides the mass needed for the gravity
structure to resist the debris-flow impact.
RCC is more resistant to erosion and overtopping
than earthworks, but less resistant than the structural orreinforced concrete that is used elsewhere in the world.
However, unlike most other debris-flow barriers, theCheekeye barrier will not be overtopped, except by very
low probability events.

to manage downstream erosion. Instead, a reinforced
concrete parapet wall will be constructed on the crest to
direct overtopping flows toward the portion of thebarrier that is founded on bedrock. In addition, the outlet
structure is located on bedrock.
4.5 Outlet Design
The Cheekeye barrier has a 6-m wide vertical slit outlet
with horizontal steel bars. The outlet size was selected
to restrict discharge to the value that can be managed
downstream, according to the methods outlined in [4].
The steel bars are necessary because there is someuncertainty about whether there is sufficient large
woody debris to block the outlet.
Aside from size, the main unique feature of the outlet
is the upstream funnel. The outlet is not a consistentwidth throughout the barrier; instead, it flares open to
the upstream and downstream. Physical modellingsuggested that an upstream funnel can promote thepassage of large wood during floods because it
encourages logs to align with the flow direction.
Avoiding blockage of the barrier during routine and low
risk flows is beneficial to passing sediment and reducing
maintenance costs. The flared opening also facilitatesaccess to outlet constriction for debris removal andmaintenance.

4.4 Crest Design and Overtopping Management
The Cheekeye barrier’s unique crest design is also
informed by the high magnitude, low probability design
event that was selected through the risk assessment
process. Rather than designing to withstand regular
overtopping, the Cheekeye design reduces the frequency
of overtopping by providing more debris storage. Thisapproach resulted in changes to the barrier crest profileand downstream erosion management compared to otherstructures.

5 Conclusions
At Cheekeye, the selection of a very low probability,
high magnitude design event has led to the adoption ofunique design features. Hopefully, it is informative forother professionals to understand the justification forthese choices. We welcome further questions andcomments, particularly in this period before the barrier
is constructed.
Design of debris-flow mitigation involves
considerable uncertainty due to the variability of debris
flows. The design decisions were made to balance these
uncertainties with the many political and geotechnicalconstraints to achieve the local government’s risk
reduction objectives.

4.4.1 Crest profile
The crests of typical debris-flow barriers have an
overflow section or spillway, with high abutments(wingwalls) to direct flow and prevent overtopping.
These structures may be designed to retain the 100- or
200-year return period event, and pass an event of asimilar magnitude and discharge over the spillway.
The Cheekeye barrier abutments are higher than thespillway, but not as high as they would be elsewhere.
The estimated spillway discharge capacity (250500 m3/s) is only about 2-3% of the estimated peakdischarge of the design event (15,000 m3/s). The lowdischarge capacity of the spillway is acceptable because
only small portions at the tail of very exceptional events
will overtop the structure. The abutments of theCheekeye barrier are protected against erosion andoutflanking if the structure is overtopped.

References
1.
2.
3.
4.

4.4.2 Downstream erosion management
The Cheekeye barrier foundation and downstream area
includes both soil and bedrock zones, but the design
does not include additional structures such as a sub-dam

4E. Moase (Mark). Guidance for debris-flow
mitigation design in Canada. M.Sc. thesis, Simon
Fraser University (2017).
M. Jakob & P. Friele, Geomorphology, 114(3),382-395, (2009).
District of Squamish, Official community planbylaw no. 2500 (2017).
G. Piton, S.R. Goodwin, E. Mark., & A. Strouth, J.
Geophys. Research: Earth Surface, 127(5), (2022).