Hydrogeomorphic Hazards in the British Columbia

Coast Mountains: Floods to Flows

Hydrogeomorphic events are especially hazardous for two reasons:

1. They are the product of extreme water inputs to a channel that, locally,

occurs only rarely. As the result, the channel may remain stable and

apparently innocuous for many years before delivering an unexpected

flood flow capable of doing major damage.

2. Mountain streams flow down steep hillsides that are the source of much

of the rock debris and sediment that enters and moves through the

stream system. Mountain channels typically are lined with rock cobbles

and boulders. Their size makes them difficult to move—an additional

reason for the typically long period between damaging floods.

Coastal mountain drainage basins in British Columbia tend to:

• have low sediment storage capability and thus allow for less buffering of changes

in hillslope morphology or process.

• have strong structural control and high rates of incision.

• have experienced pronounced elevational shifts in vegetation, weathering

regimes, and major geomorphic processes due to Quaternary climatic change.

• have hydrological regimes commonly controlled by melt of snow and glacier ice.

• experience hydrological events due to orography, barotrophic, baroclinic, and

monsoonal conditions.

• rarely achieve graded or stable

conditions, but instead consist of step

pools, plane beds, and pool riffles in

which differential velocities, shear

stresses and sediment movement can

cause velocity reversals at bank full

flow.

Charles Creek and the village of

Strachan Creek. The upper parts of

the drainage system is

characterized by extreme slopes

and sparse vegetation. The houses

are built on an alluvial fan from

Charles Creek. The British

Columbia Railway (below) and the

Sea to Sky Highway (99) are visible.

• Steep mountain creeks are typically subject to a spectrum of events,

ranging from clear water floods to debris floods to debris flows.

• Central to hazard recognition is the need to identify the specific

hydrogeomorphic process because each process has different

associated hazard characteristics.

For example, debris flows can have peak discharges 5 to 40

times greater than floods, while debris floods have relative

peak discharges of only up to twice those of flood discharges.

Classification of flow and landslide processes by sediment

concentration, velocity, and flow behaviour.

1. Clearwater floods

• mountainous terrain of coastal British Columbia has strong influence

on climate and flood characteristics.

• moist air from the Pacific Ocean is forced by the prevailing westerly

winds up over coastal mountain ranges where it drops its moisture

load as rain or snow.

• in coastal regions, the most common cause of flooding is rainfall,

sometimes in association with melting snow.

Squamish, 2003

In-SHUCK-ch

Flood Story

In the UCWALMICWTS (lower

Lillooet dialect), the name of this

mountain is pronounced In-SHUCK-

ch, meaning "split like a crutch"

(referring to the split precipice at its

peak.)

All the Lillooet people lived together around Green Lake. At the time

there came a great and continuous rain, which made all the lakes and

rivers overflow their banks, and deluge the surrounding country. When

the people saw the waters rise far above the ordinary high-water mark,

they became afraid.

A man called Ntci'nemkin had a very large canoe in which he took

refuge with his family. The other people ascended the mountains for

safety; but the water soon covered them too. When they saw that they

would probably be drowned they begged Ntci'nemkin to save their

children. As for themselves, they did not care. The canoe was too

small, however, to hold all their children: So Ntci'nemkin took one child

from each family, -a male from one, a female from the next, and so on.

The rain continued falling and the water rising, until all the land was

submerged except the peak of the high mountain called Split (Nci'kata).

The canoe drifted about until the water receded, and it grounded on

Smimelc Mountain. When the ground was dry again, the people settled

just opposite the present site of Pemberton. Ntci'nemkin with his wives

and children settled there, and he made the young people marry one

another. He sent out pairs to settle at all the good food places though

the country. Some were sent back to Green Lake and Green River;

others were sent down to Little Lillooet Lake and along the Lower

Lillooet River; and some were sent up to Anderson and Seton Lakes.

Thus was the country peopled by the offspring of the Green Lake

People.

(Teit, James. 1912. Traditions of the Lillooet Indians of British

Columbia. Journal of American Folklore 25:287-371)

B. C. Rail line destroyed by flooding Cheakamus River

Sea to Sky

Highway

washed out by

flooding of the

Cheakamus

River

17th - 24th October 2003

Record heavy rains deluged the Whistler -

Pemberton - Squamish region as major

storm systems collided over British

Columbia's Coast Mountains. Two people

drowned and two more are missing, after a

highway bridge and a rail bridge collapsed

into a rain -swollen creek. Nearly 800

people were forced from their homes as

heavy rains caused rapidly rising rivers in

the towns of Squamish, Pemberton and

Mount Currie, along British Columbia's Sea

to Sky highway.

Logging is ongoing around Rutherford Creek, and

much of the valley is now covered by second-

growth forests.

Appa Glacier, headwaters of

Rutherford Creek

A major Pacific frontal system arrived on the coast on the evening of October

15th, developing into a full “Pineapple Express” as circulation became well

established and a series of moist impulses surged along the frontal stream as

the system stalled over southwestern BC for the next several days.

The Operational polar orbiting

microwave composite of total

moisture in the atmosphere for

October 16, 2003 over the North

Pacific Ocean. The plume of

moisture (deep red band in top

panel) entering the Pacific Northwest

is the Pineapple Express. [Black

areas are regions of non-coverage

during passes.]

The bottom panel is the GOES 6.7

micron water vapour channel images

from GOES-10 and GOES-9

satellites, showing moisture levels

mainly above 10,000 ft.

(Satellite Analysis Branch, NOAA.)

HOWE SOUND: 11:26 PM PDT FRIDAY 17 OCTOBER 2003

HEAVY RAINFALL WARNING FOR HOWE SOUND CONTINUED. FURTHER RAINFALL AMOUNTS OF

100 TO 150 MM FOR HOWE SOUND AND 50 TO 80 MILLIMETRES FOR THE REST OF THE REGIONS

TONIGHT AND SATURDAY. THIS IS A WARNING THAT HEAVY RAIN IS IMMINENT OR OCCURRING

IN THESE REGIONS. MONITOR WEATHER CONDITIONS..LISTEN FOR UPDATED STATEMENTS. A

NEARLY STATIONARY FRONT OVER THE SOUTH COAST WILL CONTINUE TO BRING HEAVY RAIN

TO MOST OF THE SOUTH COAST. PRECIPITATION AMOUNTS OF 60 TO 140 MM HAVE BEEN

RECORDED IN THE LAST 24 HOURS AT SOME STATIONS ON THE INNER COAST. FURTHER

AMOUNTS OF 50 TO 80 MM FOR THE INNER SOUTH COAST AND AS MUCH AS 100 TO 150 MM

FOR HOWE SOUND CAN BE EXPECTED TONIGHT AND SATURDAY. MEANWHILE RAIN HAVE

EASED OFF TONIGHT OVER THE EXTREME SOUTH COAST...BUT MORE RAIN IS EXPECTED WITH

AMOUNTS NEAR 30 TO 40 MM FOR VANCOUVER AND VICTORIA AND THE LOWER FRASER

VALLEY ON SATURDAY. RAIN WILL TAPER OFF IN ALL AREAS LATE SATURDAY AS THE FRONT

MOVES SOUTH AND WEAKENS.

The severe rainfall resulted in record breaking water levels and discharge

flow according to hydrometric station 08MG005 on the Lillooet River.

Water Level (m)

Oct. 16 - Oct. 22, 2003

Hydrometric Station: 08MG005

Flow Discharge (m3/s)

Oct. 16 - Oct. 22, 2003

Hydrometric Station: 08MG055

Flooding initiated a new cycle of

sediment deposition and

redistribution, which has resulted in

overbank flooding and bank erosion,

damaging property and threatening

structures

2. Debris floods are a very rapid, surging flow of water, heavily charged with

debris, in a steep channel. The sediment may, furthermore, be transported in

the form of massive surges, leaving sheets of poorly sorted debris ranging

from sand to cobbles or small boulders.

Sediment surges in debris floods are propelled by the tractive forces of water

overlying the debris, and flow velocities are comparable to those of water

floods. The discharge of debris floods is commonly 2 to 5 times higher than

that of 200-year return period water floods.

A debris flood deposit that

overwhelmed a channel

Britannia before and after flood of 1921. Thirty-seven people were killed and

about half the 170 houses in the town were destroyed.

The mining community of Britannia was devastated by a sudden flood on

October 28, 1921. This flood occurred when a blocked culvert caused a railway

embankment to temporarily dam Britannia Creek and then fail, releasing a

catastrophic flood.

Debris flood occurrence:

• debris floods are a poorly understood process because they are rarely directly

observed.

• can be caused by a variety of processes, the most commonly observed

processes are breaches of temporary stream blockages caused by tributary

debris flows or other landslide types.

• debris flood discharge depends strongly on the composition and geometry of

the landslide dam and the geometry of the floodplain downstream.

• the typical range for debris flood discharge is 2 to 5 times the 200-year return

period peak water flood (Q200).

• in extreme cases, debris flood discharge may be more than five times the

Q200.

Debris flood deposits include bars, fans, sheets, and splays, and stream

channels with large width-to-depth ratio.

The differentiation of floods and debris floods involves assessing the volume

of sediment deposits relative to the size of stream channel, and determining

the orientation of clasts.

• debris floods have sediment concentrations of 20 to 47% by volume and

characteristically have significant sediment deposits beyond the channel on the fan

(e.g., where the sediment load overwhelms the channel on the fan). Floods have

sediment concentrations of less than 20% by volume and commonly have limited or

localized sediment deposits beyond the channel on the fan (the channel can generally

contain the sediment load).

• A-axes of all clasts in flood deposits are oriented perpendicular to flow. Sediments in

flood deposits are well sorted and the clasts are usually well imbricated. Clast

orientation in debris-flood deposits are mixed, with the A-axes of large cobble to

boulder clasts usually perpendicular to the flow and pebbles to small cobbles usually

parallel to flow. Debris-flood deposits commonly have weak imbrication and collapse

packing.

Case Study 1:

Debris floods in Bertram Creek, Lebanon Creek, Rembler Creek and

around Jack Smith Lake.

• an intense storm event deposited 12 to 20 mm of rain on several of the

burned watersheds in Kelowna during a 20 to 45 minute period.

• flood flows and debris floods were triggered.

• drainage structures and stream channels were overtopped on Rembler

and Lebanon Creeks by these hazards, impacting residences, orchards

and infrastructure.

Case Study 2:

Debris Floods in Ostler Creek and Allan Creek, District of North

Vancouver.

Debris Flood – A flood that carries an unusually high amount of sediment

and/or debris, but not to the extent that the event character is transformed

from a flood to a debris flow. Debris floods may be visualized as an extension

of the flood process, whereas debris flows behave very differently than floods.

Mitigation of debris flow hazards requires a considerably different approach.

Lower reaches of Ostler Creek

and Allan Creek

Ostler Creek has a total watershed area of 1.2 km2. The watershed ranges in

elevation from the Mount Seymour Mountain ski area at 1,000 m to sea level.

The mainstem channel extends to an elevation of 675 m. Above this elevation,

the channel is not sufficiently defined to delineate on air photographs. The total

mainstem channel length is about 2.2 km with an average gradient of 17o. The

fan of Ostler Creek is relatively small with the creek flowing over steep slopes

within 50 m of Indian Arm. The creek is well incised into the steep slopes

before it reaches gentle terrain on the fan and flows between two houses.

Allan Creek is located immediately to the north of Ostler Creek and its upper

watershed also extends to the Mount Seymour Mountain ski area. The

watershed has an area of 1.0 km2 and its mainstem channel can be delineated

on air photographs to an elevation of approximately 900 m. The total length of

the mainstem channel is 3.2 km and its average gradient is 16o.

Geomorphology

There is little geomorphic activity in either watershed, evidenced by an

absence of debris slides, rockfall, and snow avalanche tracks. The upper

mainstem channels are completely overshadowed by a closed tree canopy.

In addition, field investigations showed little evidence of significant coarse

sediment movements. Except for the extreme upper slopes, a majority of the

area was logged in the early 1900’s and is now covered by second growth

forest. Soils in the area consist of glacial till overlain by organic matter.

A debris flow would likely only result from a large sideslope failure during an

extreme hydroclimatic event.

In contrast, debris floods are likely to occur on both creeks. Debris flood

magnitude on steep mountain creeks may be estimated reliably if there is

geomorphic or botanical evidence that allows the reconstruction of peak

discharge. Otherwise, magnitude may be assessed empirically using judgement

and experience. Because of a lack of suitable geomorphic or botanical

evidence, the latter approach was used for Ostler Creek and Allan Creek.

Estimated debris flow and debris flood magnitudes for various probability

classifications on Ostler Creek and Allan Creek (the estimates are the same for the

two creeks).

Assessing debris flow probability and magnitude:

1) debris supply sources:

- are no major debris supply sources along the mainstem channel of either

creek. Although debris slides could occur along some of the steeper

sideslopes, there is no evidence of previous debris slides.

2) debris flood initiation:

- debris floods would most likely be initiated by a debris slide impacting the

mainstem channel at an oblique angle. Landslide dam outbreak floods are

possible but channel blockages would likely be short in duration due to the

relatively narrow width of the mainstem channels. As such, only small

amounts of water could be impounded behind a landslide dam. Log jam

collapses are also considered to be potential sources of debris flood initiation

but large woody debris jams are generally lacking along both mainstem

channels.

3) debris flow initiation:

- a tributary to Allan Creek has the potential for debris flow initiation. If

significant flows from Ostler Creek were diverted into the channel that feeds

the gully, the concentrated drainage could scour significant volumes of

sediment (up to 1,000 m3) and thereby initiate a debris flow.

Assessing debris flow probability and magnitude: (continued)

4) Debris flow hazards are defined by a combination of probability and

magnitude. The probability or frequency of debris flows and are defined

as per the following table:

Dating Methods

Three dating methods can be used to determine the frequency of past creek

events: historic air photograph analysis, dendrochronology, and documented

events from newspapers and other reports.

In this case:

- historic air photographs dating back to the 1940s were reviewed. None of these photos

showed evidence of debris floods on the fan or along the channel of either creek. No large

sideslope failures were identified that would indicate a possible initiation of a debris flood.

From this evidence, it was concluded that neither creek had experienced a significant

debris flood in decades.

-debris floods can impact trees and leave scars, which subsequently overgrow. Cutting a

wedge from this scar tissue allows the reconstruction of the year of damage. Coring a tree

and counting back to a ring sequence that is very narrow enables a researcher to

determine the date of the event. Unfortunately, no suitable trees were found for analysis

along either creek.

- review of newspapers and other reports. Apart from the small debris flow in the North

Gully of Allan Creek in 1992/93, no other events appear to have been documented

Conclusion - there is an incomplete record for debris floods at both creeks. Even

though no significant debris floods appear to have occurred for several decades,

the potential for debris floods persists.

Debris Flood Magnitude

Determination of debris flood magnitude involves consideration of both the total volume of

a debris flood and the peak discharge. Total volume is important for those mitigative

structures that contain the debris, whereas peak discharge estimates are required for

mitigative structures that channelize the debris and for the design of creek crossings.

The design debris flood is defined as having a return period of approximately 500

years, which corresponds roughly to a 10% probability of occurrence in 50 years.

a) design debris flood volume - There are no reliable methods available to calculate

debris flood volumes. Even complex sediment transport equations can only provide

very rough estimates of the type and amount of gravel mobilized. The second unknown

is the distance to which gravel will be transported in a creek channel. Unlike debris

flows, where the majority of mobilized debris is deposited downstream of the fan apex,

debris floods entrain and deposit debris intermittently, depending on stream gradient,

degree of confinement and grain size. The distance of sediment transport is also

dependent on the duration and discharge of the debris flood, which in turn is a function

of the type and duration of the event that triggered the debris flood.

The order-of-magnitude estimate for the volume associated with a medium probability

debris flood is 200 m3. The volume is very approximate and depends on the peak flow

and duration of the flood, as well as whether and for how long debris jams and log jams

develop.

Debris Flood Magnitude (continued)

b) debris flood discharge volume - The second step in assessing debris flood

magnitude is an estimate of peak discharge. There are no commonly accepted

methods available to estimate the peak discharge of debris floods. Estimates are

possible where trees have been impacted by debris, enabling a reconstruction of the

cross-sectional flow area.

In B.C., it is often practical to double the Q200 to obtain an approximate discharge

estimate for debris floods. Jakob and Jordan (2001) have shown that this value may be

too low for debris floods of similar return intervals. However, higher ratios are more

often associated with landslide dam outbreak floods, which appear unlikely at Ostler

Creek or Allan Creek.

In light of the available information, a factor of three was adopted to extrapolate for

debris floods with a 500-year return period. The results are summarized below for this

case:

Debris flood volume and peak discharge at Ostler Creek and Allan Creek

Osler Creek looking down

Summary of findings:

• The mainstem channels of Ostler Creek and Allan Creek are subject to debris

floods. The design debris flood volume is estimated at 200 m3, with an associated

peak discharge of up to 24 m3/s.

• For the 500-year return period, larger debris flows may occur on these gullies,

yielding volumes of up to 1,000 m3 and peak discharges of 25 m3/s. Indian River

Drive and other downstream development would also be subject to damage during

the design events.

Mitigation:

The hazard at Ostler Creek can be mitigated by constructing a debris

barrier upstream and replacing a culvert. Check dams downstream of

could also be constructed to stabilize the channel.

Mitigation of the risk at Allan Creek would involve building upstream

debris barriers at the mainstem channel and two gully crossings,

installing check dams, replacing culverts at road crossings, and

performing minor channelization in the lower reaches.

The total cost of risk mitigation is

estimated at roughly $0.6 million for

Ostler Creek and $2.5 million at

Allan Creek, for a total of about $3.1

million.

Existng debris barrier at Ostler Creek

culvert inlet, Indian River Drive.

3. Debris flows

A form of rapid water-saturated channelized landslide. Debris flow velocity

typically ranges between 5 and 10 m/s, but some fine-grained debris flows

have been known to travel up to 20 m/s. They are most likely to occur on

small, steep creeks that have abundant sources of debris.

Debris flows are sometimes alternatively referred to as debris torrents where

they are particularly coarse in nature and carry large amounts of organic

debris or mudflows where they are particularly fine in nature.

1999

1. Torrential rainfall swells streams

along the mountain crest.

2. Sediment slumps into a raging

stream, forming a slurry (debris flow)

that surges down the channel.

3. The debris flow swells in volume

as it picks up additional sediment and

trees from the channel and canyon

walls.

4. The debris flow emerges from the

canyon onto a fan where it damages

houses, roads, bridges, and a rail

line.

Debris-torrented gullies. a) In old-growth forest. Old-growth gully scoured to

bedrock in previous winter (1991–92) Gregory Creek, Queen Charlotte

Islands. b) In clearcut. Clearcut gully scoured to bedrock approximately

1984. Riley Creek, Queen Charlotte Islands.

Sediment output is greatest from torrented and slash cleared gullies;

in the former by mass wasting and in the latter by fluvial channel

erosion. The rate of sediment recharge drops off with time since the

previous debris flow-torrent and the clearcut recharge rate drops off

more rapidly than that of the old-growth gullies.

The February 11, 1983, debris flow at Lions Bay, near Vancouver. Triggered by a snow

avalanche, this flow entrained more than 10,000 m3 of debris and killed two people.

• Damage in creek fan areas during debris flow deposition can be catastrophic.

The nature of the deposited material is highly variable, but typically covers a

wide range from mud to boulders, and usually also includes a significant wood

debris component. Debris flow deposition may also result in flooding of adjacent

areas as a result of subsequent relocation of the creek channel.

Debris flow characteristics

In general, the frequency of debris flows on a particular creek is a function of:

• availability of debris supply sources that contribute materials to the main creek

channel and its tributaries (necessity to differentiate drainage basins between

material supply-limited vs. material supply-unlimited)

• degree of instability and level of activity of the debris supply sources

• characteristics of the debris supply source (fine vs. coarse material,

consolidated vs. unconsolidated)

• existence of potential triggers of debris flows (debris slides, rockfall,

avalanches)

• capability of a creek channel to transport a debris flow (gradient, channel

crosssection, longitudinal profile, channel roughness)

• frequency of hydroclimatic events that have the capability of triggering debris

flows. As debris accumulates, a system gradually becomes "ripe" for a debris

flow. The rate at which debris accumulates in a channel is a function of basin

type.

Debris flow probability:

• While significant floods occur virtually every year on a creek system, debris

flows are usually an intermittent occurrence.

• Typical debris flow recurrence intervals range from 5 to 50 years; however, this

is highly variable.

• Debris flow occurrence can be put into perspective by considering

geomorphological processes since the most recent glaciation about 10,000

years ago. In the centuries following glaciation, the landscape was unforested

and littered with glacial debris. Debris flow activity is believed to have been

considerably higher than today during this period. As the landscape became

forested and watersheds stabilized, debris production and debris flow activity

gradually decreased on a regional basis.

• However, debris flow activity may increase for any particular watershed as a

result of natural or anthropogenic watershed instability. There is also reason to

believe that if the present trend of increasingly wetter conditions in coastal areas

continues, debris flow occurrence will increase in frequency and possibly

magnitude.

Debris flow forms

a) Hillslope (Open-Slope) Debris Flows

These form their own path down valley slopes as tracks or sheets before

depositing material on lower areas with lower slope gradients or where flow

rates are reduced: e.g. obstructions, changes in topography. The deposition

area may contain channels and levees.

Debris flow forms

b) Channelised Debris Flows

These follow existing channel type features: e.g. valleys, gullies, depressions,

and hollows. The flows are often of high density, 80% solids by weight and

have a consistency equivalent to that of wet concrete. Hence, they can

transport boulders that are some metres in diameter.

DEBRIS FLOW OCCURRENCE

Debris flows in the B.C. Coast Mountains tend to occur in wet weather, but are

not necessarily coincident with record rainfall or flood events. Debris flow

occurrence can be described by three consecutive processes as follows:

• Initiation where a mass movement is triggered at the source area in the

creek headwaters. Possible trigger mechanisms include debris slides, log

jam release, flood surges, and creek bed instability.

• Transport of the debris flow down the creek channel. The transport

zone is typically scoured as the debris flow grows in size. A straight and

uniformly steep gradient channel represents the most favourable

transport condition.

• Deposition where either the channel becomes laterally unconfined, or

the creek gradient flattens to the point that there is insufficient energy for

continued movement. Depositional landforms are known as creek fans.

a) Debris flow initiation

Debris flow triggering factors

- the most significant triggering factor is likely to be the development of

transient high pore water pressures along pre-existing or potential rupture

surfaces. High pore water pressures are typically generated as a result of

extreme antecedent (long-duration) rainfall conditions and intense rainstorms,

both of which can result in high groundwater levels and perched groundwater

conditions. If the soil becomes fully saturated surface water flow may occur

which can result in erosion and triggering of hillside debris flows.

- the permeability of soils and the speed by which surface water can be

transmitted to potential rupture surfaces is a key factor in the initiation of

upland landslides. The interface between permeable soils and relatively

impermeable substrate can lead to the development of cleft water pressures

along soil and rock discontinuities and artesian pore water pressures along

potential rupture surfaces. Certain geological situations are particularly prone

to the effects of water infiltration, for example where permeable soil overlies

less permeable bedrock. In such circumstances rapid increases in pore water

pressures can trigger slope failure and mobilisation of landslides.

a) Debris flow initiation

Surficial deposits prone to debris flows:

Colluvium/hillwash: unconsolidated, heterogeneous soil mass deposited by

water run-off or slow down slope creep.

Talus: accumulation of angular rock fragments at the base of a cliff or steep rock

slope due to weathering, spalling/ravelling and rockfalls.

Fluvial: unconsolidated detrital material laid down by a stream, river or other

body of water

Glacial: unconsolidated heterogeneous soil mass (clay, silt, sand, gravel,

cobbles and boulders) in Proglacial ( e.g. glaciofluvial, glacio lacustrine and

glaciomarine) and Glacial (glaciofluvial, morainic and some till) deposits.

Regolith: mantle of unconsolidated rock fragments (gravel, cobble and boulder

sized), sand, silt and clay covering bedrock, and formed by the in situ, or nearly

in situ, weathering of bedrock.

Role of basin type:

Recent research has identified two distinctly different basin types.

A) One type, referred to as weathering-limited or supply-limited, is

characterized by those basins that have a limited source of sediment and

thus require recharge after a debris flow event for the next one to occur. In

other words, even an exceptionally intensive storm will not trigger a debris

flow if not enough sediment has accumulated to produce a debris flow.

B) The other basin type is referred to as transport-limited or supply-unlimited.

In those basins, there is a quasi-infinite amount of sediment available for

transport and a debris flow can be triggered as soon as a critical climatic

threshold (rainfall, rain-on-snow) is exceeded.

From the above descriptions, it is clear that transport-limited basins experience

a higher frequency of debris flows than weathering-limited basins. Examples

for transport-limited basins are young volcanic complexes that rapidly shed

material into the channel system, or basins with massive Quaternary

deposits in the source area of debris flows. Weathering-limited basins are

found primarily in slow weathering plutonic rock of the Coast Mountains.

a) Debris flow initiation

Causes of debris flows:

a) Hillside debris flows typically start as a sliding detachment of material

(upland debris slide, peat slide, rock slide etc.), usually initiated during

heavy rainfall, which subsequently breaks down into a disaggregated

mass in which shear surfaces are short-lived and usually not preserved.

The failure mass usually combines with surface water flow, which

typically results in high mobility and run-out.

- the prevailing theory proposed to explain debris flow mobilization

traces its roots to work done by Casagrande (1936) who found that,

if a drained soil is continually sheared, it will eventually attain a

critical-state (i.e., steady-state) porosity. Loose soils will contract to

reach the critical-state porosity and dense soils will dilate.

Sediment dilation

a) Debris flow initiation

1) debris flows that begin as rigid translational slides that liquefy.

a) Debris flow initiation

1) debris flows that begin as rigid translational slides that liquefy.

Longitudinal profiles of a

slump and debris flow

scar.

Inset: Sequence leading to

slumps and debris flows:

(A) Incipient landslide

(shaded) and failure

planes (dashed line).

(B) Failure begins at the upper

scarp.

(C) Mobilized as debris flows

completely evacuates the

scar.

A. During shear, both contractive

and dilative soils eventually

reach a constant critical porosity,

nc.

B. Under drained conditions, loose

soils collapse during shear,

whereas dense soils must dilate

to overcome the resistance from

the interlocking of grains.

Causes of debris flows: (continued)

b) channelised debris flows may develop as a result of the mobilisation and

entrainment of sediments by extreme flows confined within stream valleys,

which may include the collapse of natural landslide dams that may have partly

or completely blocked channels and stream valleys for some period prior to the

event. For this reason, it is particularly important to investigate entire

catchments in respect of channelised debris flow hazard and risk assessment.

Deroche Creek basin showing

locations of potential damming of the

main creek.

The basin has been clearcut and

dissected by logging roads during

several logging cycles. Failures from

logging road fills increased the

frequency of landslides by a factor of

10. Several landslides reached and

temporarily blocked the creek

causing outburst floods in some

cases with discharges greater than

those associated with 200-year flood

flows.

Causes of debris flows: (continued)

From the above, it may be concluded there are two principal causes of

debris flows:

1. The initiation of a source upland landslide that develops into a hillside

debris flow.

2. The mobilisation and entrainment of sediments by extreme flows within

stream valleys.

___________________________________________________________

Debris flow propagation:

Once the fall or slide is in motion and depending on the coherency of

the displaced mass, the failure breaks up on impact and as the slide

avalanches downslope. The failure may develop into a debris flow

when the debris comes into contact with surface water and stream

flow, dramatically decreasing the viscosity of the debris-water mix. As

a general rule, where the constituent particles of the slide debris cease

to be in contact and become supported by fluids, a change in

mechanism from debris slide to debris flow takes place. This transition

may be very rapid once the slide debris makes contact with surface

water or stream flow.

Debris flow propagating factors:

a) Debris Dams: formed when vegetation, landslide debris or previous flows

create "dams" behind which further debris can build up. Eventually these

dams become unstable, due to their size or the state of the vegetation,

and will fail catastrophically during debris flows. This additional sediment

charge increases the debris flow mass, erosive power and may create

flow pulses. Tree trunks and branches entrained in debris flows form

debris "dams" which are likely to trap large quantities of debris then fail

catastrophically releasing highly erosive debris flow pulse.

b) Rockmass: Rockhead inclined down slope tends to shed superficial

deposits relatively easily and does not tend to hold retain debris flow

material. Discontinuities within bedrock may be exploited by debris flow,

providing more rock debris and concentrating the erosive force.

Discontinuities dipping into the slope may form steps on rockhead where

debris can become trapped and lead to the formation of debris “

Debris flow propagating factors:

c) Convex Slopes: may form zone of tension within superficial deposits may

increase water infiltration leading to increased pore pressures, a

decrease in shear strength and the potential for further landsliding. At the

change in slope a "waterfall" like feature may form leading to scour and

the supply of more debris to the flow, further increasing its mass and

erosive power.

Debris flow propagating factors:

d) Drainage: drainage culverts may become blocked forming debris "dams".

Inadequate drainage designs may lead to erosion and scour: e.g.

inadequate wing walls, erosion down stream of culverts and bridges due

to venturi effect. Tracks and drainage may concentrate surface water

run-off.

b) Debris flow transport

Once the fall or slide is in motion and depending on the coherency of the

displaced mass, the failure breaks up on impact and as the slide

avalanches downslope. The failure may develop into a debris flow when

the debris comes into contact with surface water and stream flow,

dramatically decreasing the viscosity of the debris-water mix.

As a general rule, where the constituent particles of the slide debris cease

to be in contact and become supported by fluids, a change in mechanism

from debris slide to debris flow takes place. This transition may be very

rapid once the slide debris makes contact with surface water or stream

flow.

Debris flows usually comprise a

mixture of fine (clay, silt and

sand) and coarse (gravel,

cobbles and boulders) materials

with a variable quantity of water.

The resulting mixtures often

behave like viscous "slurries" as

they flow down slope. They are

often of high density, 60% to

80% by weight solids may be

described as being analogous to

"wet concrete".

Describing the physics of debris flows remains an active research topic. One of

the fundamental questions is the characterization of the material that composes

the flow. In other words, what is the rheology of debris flow material.

The problem: Debris flows are complex phenomena, involving highly unsteady

motion of heterogeneous material ranging from water and slurries to boulders and

timber remains. It is therefore very difficult to find a unique constitutive

relationship applicable to all parts of the flow.

The volume solids concentration in

the front part of such a flow varies

between about 30 and 65%, and

generally decreases toward the rear.

The flow depth is of the order of 1 to

several meters, mean velocities may

be as high as 15 m/s, and channel

gradients vary from about 40° in the

starting zones to about 3° in the

deposition zone.

Debris flows consist of grains which differ in

size, shape, etc. When such a material is

agitated or deformed in the presence of a

gravitational field, segregation or grading of the

particles can occur and particles having the

same or similar properties tend to collect

together. In a tear-drop like debris flow starting

from a uniform distribution of particle size,

shape etc. large particles tend with time to

accumulate at the front and at the free surface.

a) Debris flow depostion

After the onset of flow, debris masses are often

constrained to move down gullies or narrow

valleys in a channel-type flow whose thickness

may be comparable to its width, and both are

much smaller than its length. Upon reaching the

mouth of the gully or valley, however, this

constraint is removed, allowing the debris flow to

spread out into a fan whose thickness is

substantially smaller than its width or length.

Aftermath of a debris flow that

swept through Lions Bay in

February 1983.

Port Alice debris flow

Case Study:

Debris flow at Hummingbird Creek 11 July 1997, Mara Lake, BC

On July 11, 1997 a debris flow entered

Hummingbird Creek impacting Highway

97A and the downstream community of

Swansea Point. The debris flow caused

extensive damage to homes and cabins

on the upper fan east of Highway 97A. As

well, homes and businesses downstream

of the highway were inundated with water,

sand and gravel.

Approximately 92 000 m3 of sediment

was deposited during this event, which

makes it the largest nonvolcanic debris

flow recorded in British Columbia to date.

A 25 000 m3 debris

avalanche triggered a

debris flow that

destroyed several

homes, scoured roads,

and caused extensive

damage through

inundation by silts and

sands. Approximately

92 000 m3 of solid

material were

deposited during the

event.

Hummingbird Creek debris flow, July 11, 1997

Hummingbird Creek basin is a moderately steep forested watershed on the east side of Mara

Lake on the western slopes of the Monashee Mountains in south-central British Columbia.

• The creek follows a fault line that drains a 23

km2 basin that joins Hummingbird Creek

upstream of the developed area on the fan.

• Much of the basin of Hummingbird Creek

has moderately steep slopes ranging between

16° and 26°. Local slopes, particularly those

facing northwest and southeast in the vicinity

of the creek, are steeper than 35°. These

slopes are susceptible to landsliding because

of their steepness and shallow cover of

surficial material that can become saturated

during extended periods of high precipitation.

• Apart from the channel and its side slopes,

the basin is covered by a veneer of soil and

morainal material varying inthickness between

0.3 and 1.0 m. In the upper basin, several

small debris chutes leading into Hummingbird

Creek are lined with colluvium derived from

fractured bedrock along these gullies.

At around 19:00, a debris avalanche was triggered on

the forested northwest-facing slope of Hummingbird

Creek basin. The debris avalanche was initiated by

shallow (approximately 0.75 m) overburden sliding over

bedrock. The overburden is classified as a poorly drained

weathered till. The bedrock, which is smoothed by glacial

erosion, dips at 32–35°, supporting debris slide initiation

on steep slopes overlain by shallow overburden. The

debris avalanche scar has a triangular shape.

Looking upstream along creek channel near apex

of the fan.

The apex of the triangle located 35 m downslope

of the outfall of a 400 mm corrugated, metal pipe

culvert. This culvert discharged onto coarse road

fill. From there it infiltrated into the soil below. A

second 500 mm culvert is located 65 m northeast

of the first culvert and drains a 37.5 ha area

Estimated peak runoff at both culverts indicate

that the first culvert was flowing at about 40%

capacity at the time of debris-flow initiation. The

capacity of the second culvert was slightly less

than full capacity on 11 July. Although the road

was apparently not overtopped during the runoff

event of 11 July, piping through road fill was noted

adjacent to the culvert. Forest road construction

above the landslide initiation point increased

groundwater interception rates, concentrated

surface runoff, and caused an increase in the

drainage area to the first culvert by a factor of 3.3,

from 1.6 ha to 5.3 ha. Logging slash consisting of

branches, tree bark, and smaller trees found in

the stream draining to the second culvert resulted

in a partial diversion of flow to the basin draining

the first culvert.

Culvert 2

Surface water in stream

below culvert 2

Debris flow initiation:

• 1997 was an abnormally wet year; snowpack was on average 25% higher in April-May.

Ttotal precipitation values were also above any previously recorded spring, summer, or

early fall maxima for a data record of 104 years. These observations indicate that

substantially higher than average antecedent moisture conditions prevailed before 11

July.

• In the period between 5 and 12 July, a strong low-pressure system, characterized by

two separate fronts, brought heavy precipitation on 5 July and frequent showers of

increasing intensity and duration between 7 July and the early hours of 12 July.

Rainfall intensity–duration curve showing the return intervals of the

5 July and 11 July storms.

• results show that the amount of rainfall preceding this event was rare, but

not extreme on a 4 or 7 day basis. However, the duration of the rainfall

period may be more significant than the amount, as this would have

provided more time to increase the degree of saturation of soils within the

failure zone.

Channelized debris flow transport:

Debris from the initiation zone flowed northwest and entered Hummingbird Creek

before running up the opposite bank about 9 m. It appears that the debris

spontaneously assumed true flow in the confined channel reach, as no sign of

abnormal ponding of creek flow was apparent. No evidence of ponding in the creek

also suggests that the debris slide was quite fluid as it entered Hummingbird Creek.

The channel of Hummingbird Creek from the slide entry point down to the fan apex,

a distance of approximately 2.5 km, has had its bedload scoured out. Creek banks

were scoured of forest soils, debris and trees up to a height of 5 to 7 m. It is

estimated that about 50,000 m3 of bedload, soil, rock, trees and forest debris was

scoured from the channel.

Debris flow depostion:

The volume of material transported to the

deposition zone is estimated at between 70,000

and 80,000 m3(slope debris at 20,000 to 30,000

m3 and channel scour at 50,000 m3). A wedge of

very coarse material raised and relocated the

creek in the levee zone upstream of Highway 97A.

The gradients in this zone range between 14 and

18 percent (8 to 10 degrees). The maximum

thickness of the wedge is difficult to estimate, but

appears to be in excess of 2 m. Boulders to 2 m

diameter were deposited in the levee zone.

Cabins along the north side of the creek

upstream of Highway 97A were directly

impacted, being pushed downslope

about 5 to 7 m and inundated with

coarse gravel, cobbles, boulders and

water to a depth of 1 to 1.5 m. Damage

to homes and property downslope of the

highway was primarily due to inundation

of water, sand and gravel

Factors contributing to debris flow initiation:

The failure is believed to have been debris flow initiation at the headscarp. This

interpretation is supported by:

a) removal of all surficial soil (stripped to bedrock) within the upper failure zone

b) splatter on the trees at the margins of the failure zone which suggests that

the upper slope was fully saturated at the time of initiation

c) the apparent excess water on the slope prior to initiation of the debris flow

Peak discharge is estimated to approximately 1000 m3/s. This value is 50 times

greater than the estimated 200 year flood of 20.1 m3/s, which emphasizes that

channel restoration aimed at flood hazard reduction is clearly inadequate to

convey debris flows of even low magnitude to Mara Lake

Debris-flow frequency:

Three methods were applied to date previous debris flows at the site:

a) Air photographs: 1928 air photographs indicated a debris flood or debris flow that had

recently occurred. An older deposit estimated between 1930 and 1940 was visible

north of the intersection of the highway and Hummingbird Creek.

b) Dendrochronology: dates were obtained for 1925–1926, 1948–1949, 1964–1965,

1971–1972, 1976–1977, and 1980–1981.

c) Anecdotal information: One person residing in Sicamous recalled that it occurred on

30 June in the early 1930s. Anewspaper article of 2 July 1935 reports a downpour in

Kamloops and several washed-out bridges, including four south of Sicamous. This

evidence indicates that the event most likely occurred in 1935.

Three different methods were used to reconstruct debris-flow occurrence on the

Hummingbird Creek fan. There is evidence that a debris flood occurred in the 1920s,

1935, 1946, 1964 or 1965, and 1976 or 1977. Air photograph evidence and witness

accounts are insufficient to estimate the volume of those events. The only known

debris flow occurred on 11 July 1997.

Conclusions:

• The slope failure which occurred on the northwest slope of Hummingbird Creek on July

11, 1997 was initiated by the saturation of shallow colluvium overlying bedrock on steep

terrain which has been classified as potentially unstable. It originated as a debris

avalanche below a forest road culvert. Drainage area above the culvert had been

artificially tripled.

• The slope failed as a debris flow which stripped an estimated 20,000 to 30,000 m3 of

surficial soils from the slope. Debris entered Hummingbird Creek where it became a

channelized debris flow and was transported to the alluvial fan on Mara Lake, where the

community of Swansea Point is located. It is estimated that 50,000 m3 of material was

scoured from the channel and transported downstream.

• The factors cited which contributed to initiation of the July 11, 1997 debris flow on

Hummingbird Creek are: steep slope gradient, adversely dipping bedrock, surficial soil

(colluvium) susceptible to shear strength lose when wet, long duration and at times

intense rainfall, high antecedent soil moisture, increased groundwater flow, and the

concentration of runoff onto the headscarp from culvert 1.

• The time at which the debris flow occurred is linked to the duration of the rainfall, its

increased intensity during the hours immediately preceding the slope failure, and the time

required for the buildup of porewater pressure to critical levels in shallow soils near the

headscarp. Rainfall during the week preceding the debris flow is characterized as rare

with a return period of about 60 years. This rainfall caused prolonged storm runoff from

the culvert (culvert 1) above the headscarp, and dispersal of runoff as a plume on the

forest floor below the culvert outfall.

Conclusions (continued):

• Bedrock geology and surficial geology both play a part in slope failures in the

Hummingbird Creek valley. Although the extent to which groundwater contributed to

porewater pressure rise in soil within the initiation zone during the period July 5 to 11,

1997 has not been evaluated, its contribution to porewater pressure rise within the failure

zone may be small in comparison to the contribution of water concentrated on the slope

from culvert 1 discharge.

• This event caused substantial damage to property within the community of Swansea

Point. This event also impacted and will continue to impact downstream environmental

values. These include the fishery, water quality and loss of productive land base. The

destabilized channel of Hummingbird Creek between the point of entry of the debris and

the fan will continue to produce excess sediment for a considerable time.

• The Hummingbird Creek alluvial fan has also been subjected to debris flow events in the

past. Similar adverse meteorological conditions coupled with site characteristics similar to

those in this study area could initiate similar events in the future at other locations.

• On this basis the factors contributing to initiation of the July 11, 1997 debris flow on

Hummingbird Creek are: concentration of discharge onto the steep slope from culvert 1;

long duration and, at times, high intensity rainfall; high antecedent soil moisture; increased

groundwater flow; thin surficial soil (colluvium) susceptible to shear strength loss when

wet; steep slope gradient; and adversely dipping bedrock;

Debris flow and flood risk mitigation:

There are two strategies for mitigating debris flow and debris flood hazards:

1. active measures to mitigate the hazard occurrence; or

2. passive measures to avoid the hazard, such as land use planning.

Active measures are usually needed when a debris flow or debris flood affects

a developed area. Passive measures can be used to preclude development

in high hazard areas, or can complement active measures to maximize

safety in high risk areas.

_______________________________________________________________

a) Landuse planning: zoning, land acquisition.

b) Warning systems: advance warning, post-warning systems

c) Watershed management actions: watershed stabilization, check dams etc.

d) Mitigation structures: debris basins, barriers, berms channelization.

a) Landuse planning: zoning, land acquisition:

Land use planning is the primary form of passive measure for natural hazard mitigation.

In a case where a fan is already developed, such measures are generally limited in

application, and are best considered for implementation in conjunction with active

measures to minimize risks.

i) ZONING In some cases, it is possible to delineate a fan into zones of varying

hazards, either with or without mitigative measures.

ii) LAND ACQUISITION. Acquisition of property along creek corridors may be beneficial

for future mitigation structures, so development activity in this area should be closely

scrutinized. In addition, the need for building setbacks, erosion protection works and

floodproofing measures should be considered on a site-specific basis. Provision of

maintenance access routes or easements along creekd should also be considered at

such time.

b) Warning systems: advance warning, post-warning systems

Systems can be installed to provide warning of an impending debris flow (advance

warning system), a debris flow occurring (event warning system), or after a debris

flow has occurred (post-event warning system).

i) ADVANCE WARNING SYSTEMS: Advance warning systems can involve real-time

monitoring of precipitation and creek flow data to determine when hydrological

conditions approach a threshold for regional landslide occurrence and debris flow

activity. Activities in high-risk areas may then be restricted and public notification

considered. The period of notice may range from a few hours to a day or two.

Warnings will typically apply to all creeks in a regional area as opposed to any

specific creek. False warnings may occur relatively often. An advance warning

system could also warn downstream residents and road users of high risk periods.

But given the high likelihood of false warnings, it would not provide an effective

means for temporarily relocating residents.

ii) POST-EVENT WARNING SYSTEMS: Post-event warning systems may be useful in

providing notice of a service disruption of critical infrastructure, such as bridges.

Such a system would have been effective in preventing multiple deaths in 1981 at M

Creek on the Squamish Highway, resulting from several vehicles driving

unknowingly into a gorge following a bridge washout, as occurred on Rutherford

Creek near Pemberton in October 2003.

c) Watershed management actions: watershed stabilization, check dams etc.

Some general and site-specific watershed actions that should be considered in

mitigating the risks associated with debris flows and debris floods: .

i) WATERSHED STABILIZATION: Watershed stabilization activities can be

considered to reduce the level of debris flow and debris flood hazard. Such

measures attempt to tackle the problem at the source area where debris is

generated and point source failures are most likely. Stabilization efforts in

upstrea gullies could involve bioengineering techniques forsideslopes, partial

removal of fill material and/or stabilization at toe of the potential failure slopes.

ii) CHECK DAMS: Check dams are weirs, typically constructed of concrete,

about 2 m to 3 m high that can be constructed in series along a creek channel.

Their primary purpose is to reduce debris production along creek channels.

This is accomplished by storing in-channel material, stabilizing sideslopes, and

directing the creek flow toward the centre of the channel. Check dams will not

significantly reduce the debris flood or debris flow risk.

d) Flood Mitigation Structures

i) Debris basins:

A debris basin is a constructed storage area

in which a debris flow is contained above a

critical area. A debris basin includes an outlet

structure that can be designed to allow

passage of debris below a certain size.

ii) Debris barriers:

A debris barrier typically consists of an open steel grillage or concrete slot

structure that is anchored to bedrock in a confined section of the creek. Its

function is to “filter” boulders and trees or root wads, while allowing smaller

debris to pass.

ii) Debris flow net:

Debris flow nets are a form of debris barrier that have been recently

developed for creeks where the expected debris volumes are small (less

than 1,000 m3). Debris flow nets are a derivative of rockfall nets, which

have been widely used to prevent rockfall from reaching development or

infrastructure. The nets consist of interconnected steel rings that are

positioned such that the net can expand dynamically to withstand impact

forces.

ii) Deflection berm:

A deflection berm (or training berm) can be used to deflect a debris flow or

debris flood away from a development area and allow it to deposit in an

area where it will cause minimal damage. Construction of a deflection

berm depends on the availability of a suitable runout area on the fan.

ii) Channelization works:

Channelization works can funnel a debris flow or debris flood through a

critical reach to a downstream area where deposition will result in minimal

damage. Implementation of channelization works depends on such a

downstream area being present.

Concrete flume on Alberta Creek, and BC Railway crossing

in the lower part of Lions Bay

Concrete flume on Alberta Creek in

the upper part of Lions Bay. The

flume is designed to allow water, mud

and debris to pass unimpeded during

major flooding events.

Mountain Flood Hazards: Summary comments

Mountain streams present significant hazards to human settlement and

activities.

• Floods that mobilise sediments as debris flows, or in sufficient quantity to

create a ‘debris flood,’ usually block mountain roads and railways at the

point of crossing.

• In confined valleys, structures are sometimes placed on debris cones.

Such sites are dangerous in the long term because of the high probability

that a debris flow or flood eventually will escape the current channel and

inundate the site.

• Structural defensive measures include isolation of an adequate

conveyance channel within high berms and provision of high-clearance

road and rail crossings. In extreme cases catch basins may be constructed

to contain expected debris flows. All these measures are expensive, the

last exorbitantly so. Communication routes are compelled to cross

mountain streams, but the most effective defensive strategy for settlement

or infrastructure is recognition and avoidance of exposed sites.

• In a distressing number of cases, the possibility for significant stream-

related damage has been ignored. In fact, it is easy to ignore the hazard

because of the possibly long inter-event period.