Mechanisms of debris supply to steep channels

Erosion and Sedimentation
in the Pacific Rim (Proceedings of
the C o r v a l l i s Symposium, August, 1987).
IAHS P u b l . no. 165.
Mechanisms of debris supply to steep channels
along Howe Sound, southwest British Columbia
Michael J.
Department
University
Vancouver,
Bovis & Bruce R. Dagg
of Geography
of British Columbia
B.C., V6T 1W5, Canada
ABSTRACT
The mountain wall which forms the east slope
of Howe Sound fjord, near Vancouver, B.C., is deeply
dissected by steep creeks, many of which have produced
destructive debris torrents over the past twenty years.
The fact that high-magnitude torrents have occurred
sporadically during rainstorms with only 2-3 year recurrence intervals suggests that, in addition to rainfall
runoff, the quantity and stability of debris in the
channels, together with large impulsive loads from
adjacent slopes, must be taken into account when examining torrent occurrence. Accordingly, research has
focussed on hillslope processes supplying debris and
impulsive loads to the channels, and on the character of
the materials involved. Debris supply is examined for
selected basins classed as either 'rockslope-dominated'
or 'drift-dominated'. Detailed studies of individual
basins should provide more insight into the causes of
debris torrents here than would rainfall- or runoff-based
models derived in other regions.
INTRODUCTION
Since late 1981, some fourteen debris torrents have occurred along
steep mountain creeks which drain the southeast flank of Howe Sound
fjord, north of Vancouver (Fig. 1). The resulting damage to
transportation and property, and the attendant loss of life, have
led to a number of detailed studies, notably those of Thurber
Consultants (1983), Church & Desloges (1984), Hungr et al. (1984),
and VanDine (1985). An earlier study by Miles & Kellerhals (1981)
predated the most recent spate of torrents and so makes little
reference to Howe Sound, but provided a context for later work. The
first reference to torrent-type events in the Howe Sound area was
made by Russell (1972).
These studies have found that, even within such a small group of
basins, the magnitude and frequency of debris torrents has been
highly variable. Major torrents have been triggered in some basins
by rainstorms with return periods of less than five years. In
adjacent basins, no known torrents have occurred over a 20 to 30
year period. The greatly differing response of the basins is not
explained readily by climatic or hydrologie factors, but is more
likely related to the varying availability and stability of debris
in steep channels. This point has been raised in the above
191
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Michael J.Bovis & Bruce R.Dagg
FIG. 1 Topography and simplified bedrock geology of the
Howe Sound study area. Stippled areas are Gambier Group
rocks. Other areas underlain mainly by dioritic rocks.
references, and conforms broadly with the experience of other
researchers in the Pacific Rim steeplands, namely that debris
torrents are produced by a complex interaction of climatic, geologic
and geomorphic controls (e.g., Swanston & Swanson, 1976; Takahashi,
1981).
In the study area, debris supply and debris storage in steep
channels have received fairly detailed treatment in Thurber Consultants (1983) as a first step to identifying potentially hazardous
channel reaches, and estimating the magnitude of the 'design' debris
torrent. Subsequently, Hungr et al. (1984) provided estimates of
average channel-debris-yield rates (m /m of channel) for selected
catchments in southwest British Columbia. This was based on a 'back
analysis' of recent torrents of roughly known volume, and assumed
that material entrained from the channel had been supplied primarily
by 'line' source processes such as small-scale slumping and ravelling of stream-bank materials. 'Point' sources, such as major
rockfalls, rockslides, and debris slides were considered to account
Mechanisms of debris supply
193
for about 10 percent or less of total torrent volume, though their
potential for triggering debris torrents was discussed in this
paper, and in Church & Desloges (1984), and VanDine (1985). This
rough apportionment of total torrent volume between pre-existing
bedload materials and new point sources conforms with the reported
tendency for debris torrents to increase greatly in volume downstream, as entrainment of bedload takes place (Swanston & Swanson,
1976; Miles & Kellerhals, 1981; Takahashi, 1981).
Recently, emphasis in the Howe Sound problem has shifted to the
design and construction of mitigation works to protect transportation and housing. These public works address the local engineering problem, but the geomorphic problem of understanding the
occurrence of debris torrents in this area requires much further
work. Given the unpredictable behaviour of the basins under nonextreme hydrologie conditions, there seemed little prospect of using
a quantitative, modelling approach as exemplified by Takahashi
(1981). Accordingly, this paper examines the processes of debris
supply to steep channels as a first step to an improved understanding of the debris torrent phenomenon in this region.
STUDY AREA
The study area lies on the southeast flank of Howe. Sound, where the
Pacific Ranges of the Coast Mountains rise steeply to peaks over
1600 m within 2 to 5 km of the shoreline (Figs. 1 & 2). In the 16
km stretch of coast between Brunswick Point and Horseshoe Bay, the
mountain wall is dissected deeply by 16 steep creeks, all of which
are crossed by Highway 99 and the British Columbia Railway.
Most of the area is underlain by late Cretaceous diorite to
granodiorite, intruded into Lower Cretaceous rocks of the Gambier
Group, which comprise intermediate to acidic volcanics, with some
sediments and low-grade metamorphics (Roddick & Woodsworth, 1979).
Gambier Group rocks occur mainly below 900 m, and in the upper parts
of some of the basins (Fig. 1). Where massive, the plutonic rocks
tend to form high cliffs - for example the northwest face of Mt.
Harvey (Fig. 2); elsewhere they possess well developed jointing,
which strikes roughly north, and dips steeply to the east.
The region was deeply scoured by Cordilleran ice sheets during
several Pleistocene glaciations. Glacial drift in the region
consists typically of a dense basal till, laid down during the
Fraser Glaciation, capped by up to several meters of much looser
ablation till. Boulder-sized rock fragments occur throughout the
drift layers. Glacial drift tends to be thicker and more extensive
at the lower elevations and discontinuous in the upper parts of many
basins. Here, extensive boulder-sized rockfall and rockslide
materials are found at the base of many fractured diorite cliffs.
The region has a typical west coast climate, with generally mild
temperatures and a strong winter maximum of precipitation, much of
which falls as rain. Annual precipitation at sea level is about
1800 mm, though areas at about 1000 m elevation receive over twice
that amount as a result of strong orographic enhancement. There is
some evidence that orography may also produce very intense,
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Michael J.Bovis & Bruce R.Dagg
FIG. 2 The M Creek and Magnesia Creek study basins.
localized cells of precipitation, and both Church & Desloges (1984)
and VanDine (1985) cite this as a probable trigger for debris
torrents. This idea derives some support from the fact that floods
derived from heavy rainstorms often occur in specific coastal
basins, rather than in many basins simultaneously (Church &
Desloges, 1984). Since the entire east wall of Howe Sound could be
affected by such storm cells over a twenty to thirty year period, it
is unlikely that the contrasting torrent histories of the basins can
be explained by this precipitation effect alone.
None of the basins in the study area is gauged, but records from
the nearby Stawamus and Capilano rivers show a snowmelt peak in MayJune, and one produced by heavy rains in October-March (Thurber
Consultants, 1983). Because heavy winter rains are usually accompanied by a rapid rise in freezing level, the associated runoff may
contain a significant component of snowmelt. Virtually all torrents
have been triggered by winter storms rather than by spring snowmelt.
Of the 18 torrents recorded since 1960, 16 took place in OctoberFebruary at: Charles Creek (5 torrents); Newman Creek (3); Magnesia
Creek (2); Alberta Creek (2); and one each at M Creek, Turpin,
Montizambert, and Sclufield creeks (Thurber Consultants, 1983;
Church & Desloges, 1984). Additional torrents occurred at Charles
and Newman creeks in September 1969. Twelve fatalities have
resulted, the majority caused by the destruction of a trestle bridge
at M Creek in October 1981.
Mechanisms of debris supply
195
DEBRIS SUPPLY PROCESSES
The erratic response of basins to hydrologie influences, noted
above, stresses the importance of two geomorphic factors in
torrent initiation: total storage of mobilizable debris along a
channel system, and the presence of miscellaneous torrent triggers.
Since rockfall, rockslides, debris slides and snow avalanches can
act as both debris supply mechanisms and torrent triggers, debris
supply and torrent triggering clearly are not independent of each
other. Therefore, while this paper deals mainly with debris supply
processes, torrent triggers are discussed also.
Although most of the study basins display more than one of the
above processes, it is convenient to discuss debris supply under the
headings 'rockslope-dominated basins' and 'drift-dominated basins'.
While some reference will be made to several basins in the study
area, the discussion will focus on Alberta, Magnesia, M, and Loggers
creeks (Fig. 1). These basins were selected for detailed study
because they are contiguous, accessible, and display a wide range of
debris supply processes.
Rockslope-dominated basins
Rockslope-dominated basins are those of Charles, M, and Loggers
creeks, as well as parts of Newman Creek (Fig. 1). A detailed
inventory of features has been conducted in M Creek (Fig. 2).
Rockfall is widespread also in Magnesia Creek basin; however, unlike
in the above-named basins, the source cliffs are not close enough to
the creek to supply coarse debris or impulsive loads.
Slope failures in rockslope-dominated basins are true hillslope
events in that they are usually decoupled from any streamflow
influences. For example, the upper two-thirds of Charles Creek
flows through a bedrock canyon developed in fractured diorite which
yields abundant rockfall debris. Coarse blocks contributed by this
process are not entrainable by running water, and thus tend to
accumulate until mobilized as a debris torrent. It is likely that
rockfall serves also as a torrent trigger in this steep catchment,
since the average slope of the upper, right-bank tributary is 32°.
Six torrents have occurred along this creek since 1960, often during
rainstorms which were heavy, but not unusual by local standards. It
appears that debris supply and stability, not runoff, are the most
critical factors in debris torrent initiation along Charles Creek.
The combination of very steep channels and rapid rock weathering in
this basin is conducive to the occurrence of frequent torrents of
about 10,000 m or less.
Unlike Charles Creek, M Creek has experienced only one torrent in
the past 25 years, in October 1981. Both Church & Desloges (1984)
and Hungr (personal communication, 1986) have inferred that this
event was triggered by a relatively small debris slide from site
M 1, a colluvium-filled depression in a logged area (Fig. 2). The
large volume of this torrent (20,000 m 3 ) can be accounted for by
entrainment of boulders and logs, which had built up in a long reach
of the channel as a result of minor slumps and debris slides over a
196 Michael J.Bovis & Bruce R.Dagg
period of several decades (Hungr, personal communication, 1987).
This process of torrent growth by entrainment has.been documented
elsewhere (e.g., Swanston and Swanson, 1976; Takahashi, 1981).
Since 1981, a prominent rockslope failure at site M 2 (Fig. 2,
Fig. 3) has been delivering substantial quantities of coarse debris
to the creek bed. Air photo records indicate that site M 2 developed sometime between 1968 and 1979, since when its area has
increased considerably. Figure 4 shows that this feature produces
rock failures capable of snapping large Douglas firs. Impact scars
on trees along the slide margins are found up to 10 m above ground
level. Ground photography taken immediately after the 1981 torrent
shows that surprisingly little debris from M 2 had reached the creek
by that time, so it is clear that it had little influence on this
event (Hungr personal communication, 1987). However, given the
present supply rate of rock and large organic debris from M 2
(estimated at 300-500 nrYyear), and the potential for shock loading
by large boulders from the headscarp, this site could control both
FIG. 3 Aerial oblique view of upper M Creek basin,
showing locations of debris sources M 1-M 3.
Mechanisms of debris supply
197
the magnitude and initiation of future torrents along M Creek. Such
events would be almost impossible to predict from precipitation and
runoff alone.
Loggers Creek is somewhat different from the other rockslopedominated basins in the study area in that much of the channel is
choked with very coarse rockfall material. The average gradient in
this reach is 30°, well below the angle of shearing resistance for
this blocky rubble, so it has accumulated close to the source
cliffs. The debris is so thick and porous that the creek flows
subsurface throughout the year. Therefore it is likely that only a
very large impulsive load could trigger a torrent from the middle
reaches of this creek.
FIG. 4
Site M 2, view upslope toward headscarp. Note
destruction of trees along slide margins.
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Michael J.Bovis & Bruce R.Dagg
Drift-dominated basins
Drift-dominated basins are, principally, Magnesia Creek and parts of
Alberta Creek (Fig. 1). A detailed study has been made of features
in Magnesia Creek (Fig. 2). In areas dominated by colluvium or
glacial drift, slope failures tend to be linked more directly to
undercutting by streamflow. At site Mg 3 (Fig. 2), unconsolidated
drift over compact basal till favors the development of a perched
aquifer during prolonged rainfall or snowmelt recharge. This
condition, combined with erosion of the slope foot, has produced a
series of shallow slides supplying debris directly to Magnesia
Creek. Here the average stream gradient is about 22°, but because
of frequent log jams and large boulders, the stream profile is
stepped, and most of the debris from these slides is stored where it
enters the channel.
Another important debris source is seepage-controlled failures
from colluvium-filled depressions in bedrock or basal till, similar
to the soil wedges described by Dietrich and Dunne (1978). The
largest of these, Mg 1, enters Magnesia Creek at 690 m elevation
(Fig. 2), where the channel slope is only 17°. Consequently, the
larger fragments have accumulated at this stream junction. A soil
wedge failure into M Creek at site M 1 (Fig. 2) may have triggered
the 1981 torrent, as noted above.
Alberta Creek is the steepest of all the study basins, but it has
few major point sources of debris. Below 650 m it is deeply incised
in unconsolidated colluvial materials, which serve as an important
line source of material along both banks. The major torrent of
February 1983 apparently was triggered by a wet snow avalanche which
surged over a prominent waterfall at 650 m elevation. This destabilized loose material at the point of impact, following which the
torrent grew in volume appreciably by scouring the bed and banks of
the creek below this point (Church & Desloges, 1984).
DISCUSSION
Despite the diversity of debris supply processes within this small
sample of basins, torrent-prone channels in this area have three
common characteristics. First, the plutonic rocks usually weather
to large angular fragments and blocks, ranging from a few centimeters to more than two meters in diameter. Colluvium and ablation
till derived from these rocks also produce coarse-grained lag
deposits, after reworking by streamflow. Since these deposits
possess high frictional strength, and are very free-draining, they
are able to accumulate in steep channels.
Secondly, much of the coarse material above about 50 cm in
diameter is not readily transported by average streamflow events,
and so is prone to accumulation in the channel as 'bedload', particularly where average channel gradient is below 20°, such as the
central parts of Magnesia and Harvey creeks. An extreme case of
this is the upper part of Loggers Creek, where coarse rockfall
debris has been stable for a long time. As the debris layer
thickens, so the discharge per unit channel width required to
Mechanisms of debris supply
199
destabilize the layer must increase. Channels are able to store
material on slopes steeper than 30°, since as gradient increases,
basin area and peak discharge both decrease. For example, site M 3
in M Creek basin is a talus-filled channel inclined at 30-35°.
Talus shift and rockfall are active at the upper end of this
channel, but the lowermost 100 m show no signs of recent movement.
Thirdly, many Coast Mountain creeks flow below unstable rockslopes capable of delivering impulsive loads from rockfall, or have
tributaries which discharge debris slides or snow avalanches to the
main channel. The potential for rockfall and debris slides is
greater during prolonged rainfall because of the progressive buildup of joint- and pore-water pressures. Wet snow avalanches also are
favored by these conditions, since rain on snow causes a decrease of
shear strength and an increase in shear stress in the snowpack.
The three attributes of debris texture, debris storage, and the
potential for impulsive loading, coupled with heavy orographic
precipitation, are thought to account for the high incidence of
debris torrents in the plutonic rocks of the Coast Mountains.
Preliminary observations suggest that other rock types in this area,
such as Mesozoic sediments and Quaternary volcanics, break down more
rapidly than the plutonic rocks. The result often is a finergrained weathering product, less prone to accumulation in steep
channels because it is readily transported by abundant runoff.
These conditions should produce a lower torrent potential than
exists in plutonic rock areas.
Comparisons can be drawn also between the debris torrent activity
described here, and that documented elsewhere in the Pacific Rim
mountains. VanDine (1985) suggests that the concept of 'critical
discharge', developed in Japan by Takahashi (1981), may be
applicable to torrent prediction in southwest British Columbia.
However, the implied similarity between torrent controls in both
regions is still untested. In Japan, many active torrent channels
are cut in unstable volcaniclastic materials, which provide very
active line sources of material. Although point failures such as
debris slides may be important locally, there seems to be a lower
potential for channel destabilization by rockfall, and certainly by
snow avalanche, in comparison with near-coastal areas in British
Columbia. Therefore, the relevance and workability of the critical
discharge approach in Japan may reflect the lesser importance of
extraneous geomorphic events in torrent triggering.
CONCLUSION
Debris supply processes to steep creeks along the east shore of Howe
Sound, British Columbia, exhibit considerable spatial variability,
and are considered here as a prime cause of the contrasting debris
torrent histories of the selected basins examined here. Important
debris supply processes are rockfall, rockslide, and debris slide.
Together with snow avalanches, these may serve also as debris
torrent triggers. The influence of such hillslope events
complicates the use of a hydro-climatic approach to debris torrent
prediction.
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Michael J.Bovis & Bruce R.Dagg
Preliminary observations in the southwestern Coast Mountains
suggest that the coarse-textured rock detritus yielded by steep
basins cut into intrusive rocks probably renders them more prone to
destructive debris torrent activity than similar basins cut in
volcanic and sedimentary rock types in this region. This is because
coarse detritus is more prone to build-up in steep channels since it
is not readily removed by average discharge events.
ACKNOWLEDGEMENTS
This work is funded by a grant from the Natural Sciences and
Engineering Research Council of Canada. The paper was greatly
improved by comments from Dr. Oldrich Hungr and Prof. S.O. Russell.
Dr. Hungr also provided unpublished information on M Creek basin.
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