Part 2 Download

Transcript
IV – Large grains and large floods
Influence of boulders on sediment export during floods
 “Bedrock aggradation”
Only extreme events can mobilize a
significant fraction of the boulders
Marsyandi
River,
Himalayas
Feather River,
California
1m
IV – Large grains and large floods
Influence of boulders on sediment export during floods
Only extreme events can mobilize a
significant fraction of the boulders
 long term channel adjustment
Elevation (m)
 “Bedrock aggradation”
Distance (km)
French Alps (Brocard and van der Beek, 2006)
Utah (Johnson et al., 2009)
IV – Large grains and large floods
Influence of extreme flood events on sediment export during floods
-
Exceptional floods (recurrence interval > 100
yrs)
-
Produce major landscape change (sediment
transport, deposition, bedrock erosion)
-
Caused by powerful storms / typhoons,
landslide dam break, glacial lake outburst,
subglacial volcanic eruptions
Humla, Nepal, 2011
Uttarakhand, 2013 (https://sursangeetblog.wordpress.com/
(https://floodinhumla.wordpress.com/)
2013/06/24/uttarakhand-flood-2013/)
IV – Large grains and large floods
Influence of extreme flood events on sediment export during floods
-
Exceptional floods (recurrence interval > 100
yrs)
-
Produce major landscape change (sediment
transport, deposition, bedrock erosion)
-
Caused by powerful storms / typhoons,
landslide dam break, glacial lake outburst,
subglacial volcanic eruptions
-
Full mobility
-
Net sediment export from mountain to
“basin”
-
Can have a flood-cleaning or flooddepositing effect
Paradise Creek,
East Australia
(Baggs Sargood
et al., 2014)
IV – Large grains and large floods
Influence of extreme flood events on sediment export during floods
 Extreme flood inhibits subsequent sediment export
- Cleaning effect
500 m
Jökulsá a
Fjöllum, NE
Iceland
(Baynes et al.,
2015a, b)
IV – Large grains and large floods
Influence of extreme flood events on sediment export during floods
 Extreme flood inhibits subsequent sediment export
- Cleaning effect
Limited by transport capacity
Limited by bedrock weathering
Limited by tools availability
IV – Large grains and large floods
Influence of extreme flood events on sediment export during floods
 Extreme flood inhibits subsequent sediment export
- Cleaning effect
- Formation of stable boulder bars
Paradise Creek, East Australia
(Baggs Sargood et al., 2014)
IV – Large grains and large floods
Influence of extreme flood events on sediment export during floods
 Extreme flood inhibits subsequent sediment export
- Cleaning effect
- Formation of stable boulder bars
 Extreme flood enhances subsequent sediment export
- Hillslope undercutting
- Destruction of resistant landforms
Partnach River, Bavarian Alps, 2005
(Morche and Schmidt, 2012)
https://gpsrepublic.files.wordpress.com/2011/02/gorges-de-daluis-4.jpg
Conclusions
Boulders are common in mountain rivers;
they can inhibit sediment transport and
bedrock erosion when abundant
Extreme floods are crucial in mobilising
the whole grain population. They can be
flood-cleaning or flood-depositing; they
can inhibit or enhance sediment export
by subsequent floods
Taking episodic supply and transport into
account is essential for predicting
sediment dynamics and bedrock erosion
in mountain rivers, as well as the
development of stratigraphy
Amount and grain size of sediment exported
during a flood = f (flow competence, transport
capacity AND characteristics of sediment available
for transport)
Vary in space and time due to episodic
nature of sediment supply + differences in
sources
Similar