Speculations on disturbance frequency
through the fluvial system
Michael Church
Department of Geography
The University of British Columbia
Seminar at CEMAGREF
3 November, 2010
Rio Cordon, September, 2005
Concept 1
The fluvial system
a zonal concept
the upland system is directly
coupled to hillslopes; in
montane regions, channels
are steep; water flows modest
and highly variable
the valley system is uncoupled
from hillslopes; it receives
fluvially sorted sediments from
upstream; channels are less
steep; water flows larger and
less variable
The fluvial system
continued
sediment grain size
becomes finer, better
sorted and more
mobile downstream
headwaters
The fluvial system
continued
upper valley
downstream
consequently,
the frequency of
sedimentmoving events
varies
dramatically
through the
system
The fluvial system
a process concept
headwaters
upper valley
downstream
Buffington and Montgomery, 1996
Concept 2
Disturbance
“disturbance” is different for different organisms and
circumstances;
• for aquatic animals, disturbance occurs when they are
not able to withstand flows in the channel;
• for the channel, disturbance occurs when the flow moves
sufficient sediment to reform the channel, in whole or in
part
disturbance of the channel may involve bed scour and
fill, extraordinary bank erosion and bar construction,
avulsion or sediment mass flow (debris flow) that change
the form of the channel
Channel disturbance
brief, severe flood:
little channel
disturbance
extended, severe
flood: significant
channel disturbance
long, moderate
flood: little channel
disturbance
Costa and O’Connor, 2003
Channel disturbance
the ‘normal’ process of progressive
lateral channel migration – an integral
part of the staging of sediment through
the fluvial system – is not considered
to be ‘channel disturbance’;
. . . it is part of ordinary, pattern-stable
channel process
Concept 3
For human society, significant disturbance is
channel (or floodplain) reforming disturbance
Plum Creek, Colorado; 1965 flood
deposit
Alberta Creek, B.C., 3 February, 1983
The fluvial system
revisited
upland channels are threshold
channels: normal sediment
transport occurs rarely and at
low rates
distal channels are labile
channels: normal sediment
transport occurs frequently and
may increase to moderately
high rates
normal sediment transport is
transport that does not disturb
the system; i.e., does not
reform the channel
threshold channel: a boulder cascade
labile channel: a wandering gravel-bed channel
In threshold channels the critical Shields number (or
‘mobility number’) is high
the reason is the structured
arrangement of individual
stones, requiring additional
force to mobilise them
Shatford Creek, B.C, a boulder step-pool channel
Shields numbers may vary
from 0.06 to > 0.1
Harris Creek, B.C., a cobble-boulder rapid
Headwater disturbance
. . . therefore, disturbance is rare in most headwater
channels but, when it occurs, it is severe
Left: debris flow track, Mosquito Creek; below: toe of
debris flow, Deena River, both Haida Gwaii, B.C.
In headwaters, disturbance event frequency is
influenced by sediment supply (geology) and by
weather
erodible rocks
regenerate
headwater channel
fills quickly after an
erosional event
Franciscan mélange, northern California
though temporally rare at any one
location, severe weather, able to ignite
debris flows, may be common regionally
Headwater flood frequencies
• effective floods in central Texas result from intense convective
showers with return period of order 100 years (Baker)
• hurricane-induced runoff with long return period (Kochel) and rare,
extreme convective storms (Andrews) dominate Appalachian stream
response
• recurrence interval for debris flow on Haida Gwaii, British Columbia,
is of order 100 years, controlled by debris recharge time (Bovis and
Oden)
• the recurrence interval for debris flow in the Oregon Cascades may
be > 103 years (Dunne and Dietrich)
• a 100-year storm created exceptional sedimentation on upland fells
in northern England (Harvey)
• In Howe Sound, British Columbia, there were two episodes of debris
flow in the 20th century
• debris flow occurrence in the southern Coast Mountains of British
Columbia may be as frequent as 10 years on susceptible lithologies
(Jakob and Bovis)
Proximal labile reach
integrates upland drainage, hence
stability is critically affected by
drainage structure
braided channels and
avulsion are common
but do not constitute a
‘disturbance’ so long
as they remain within
the established
channel zone
Upper Scar Creek, Coast Mountains, British Columbia
Proximal labile reach
episodes of rapid aggradation and degradation are more common than
elsewhere in the fluvial system
Upper Liard River, northern British Columbia
Bull Creek, California after significant floods in
1964 and 1965
Proximal labile reach: alluvial fans
are depositional features notoriously subject to avulsion
Post Creek, Chilliwack valley,
British Columbia
Rakaia valley, New Zealand
a specially sensitive zone
for channel disturbance
Distal labile channels
are mainly subject to progressive erosoion and lateral or
vertical sedimentation; often controlled by human agency
the channel is adapted to
relatively frequently recurring
events
Distal labile channels
significant disturbance is restricted to events that defeat
mitigative or protective measures
since flood and erosion
protection are often built
to century scale, such
events are rare but
particularly serious
Relative frequency across process
domains
relative magnitude
headwater
proximal labile
distal labile
frequent
rare
return period
Summary
human disturbance events are relatively rare in most
headwater systems, but depend critically on sediment
sources
they are more frequent in proximal labile systems
they are naturally frequent in distal labile systems but have
been suppressed by dykes and other human defensive
measures
characteristic time scale is of order decades to a century in
most circumstances in headwater and protected reaches
this timescale is outside community reactive memory
Big Thompson Canyon, Colorado, USA
Thank you