Speculations on disturbance frequency through the fluvial system

Michael ChurchDepartment of GeographyThe University of British Columbia

Seminar at CEMAGREF3 November, 2010

Rio Cordon, September, 2005

Concept 1

The fluvial systema 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 systemcontinued

sediment grain size becomes finer, better sorted and more mobile downstream

The fluvial systemcontinued

consequently, the frequency of sediment-moving events varies dramatically through the system

headwaters

upper valley

downstream

The fluvial systema process concept

headwaters

upper valley

downstream

Buffington and Montgomery, 1996

Concept 2Disturbance

“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

Costa and O’Connor, 2003

long, moderate flood: little channel disturbance

brief, severe flood: little channel disturbance

extended, severe flood: significant 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

Channel disturbance

Concept 3For human society, significant disturbance is channel (or floodplain) reforming disturbance

Alberta Creek, B.C., 3 February, 1983

Plum Creek, Colorado; 1965 flood deposit

The fluvial systemrevisited

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

labile channel: a wandering gravel-bed channel

threshold channel: a boulder cascade

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

Shields numbers may vary from 0.06 to > 0.1

Shatford Creek, B.C, a boulder step-pool channel

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

though temporally rare at any one location, severe weather, able to ignite debris flows, may be common regionally

Franciscan mélange, northern California

erodible rocks regenerate headwater channel fills quickly after an erosional event

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 reachepisodes of rapid aggradation and degradation are more common than elsewhere in the fluvial system

Bull Creek, California after significant floods in 1964 and 1965

Upper Liard River, northern British Columbia

Proximal labile reach: alluvial fansare depositional features notoriously subject to avulsion

Rakaia valley, New Zealand

Post Creek, Chilliwack valley, British Columbia

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

rarefrequent

return period

rela

tive

ma

gnitu

de

headwater

proximal labile

distal labile

Relative frequency across process domains

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, USAThank you