Speculations on disturbance frequency through the fluvial system

1. 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

2. 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

3. The fluvial system continued sediment grain size becomes finer, better sorted and more mobile downstream

4. The fluvial system continued headwaters consequently, the frequency of sediment-moving events varies dramatically through the system upper valley downstream

5. The fluvial systema process concept headwaters upper valley downstream Buffington and Montgomery, 1996

6. 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

7. 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

8. 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

9. Concept 3For human society, significant disturbance is channel (or floodplain) reforming disturbance Plum Creek, Colorado; 1965 flood deposit Alberta Creek, B.C., 3 February, 1983

10. 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

11. threshold channel: a boulder cascade labile channel: a wandering gravel-bed channel

12. 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

13. 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.

14. 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

15. 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)

16. 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

17. Proximal labile reachepisodes 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

18. 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

19. 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

20. 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

21. Relative frequency across process domains headwater proximal labile distal labile relative magnitude frequent rare return period

22. 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

23. Thank you Big Thompson Canyon, Colorado, USA