EFFECTS OF RIPARIAN RETENTION
(IN WATERSHEDS) ON ALLUVIAL
FANS
TIM GILES
GEOMORPHOLOGIST
FLNRO
Sisco, February 1,
2017
EFFECTS OF RIPARIAN RETENTION (IN
WATERSHEDS) ON ALLUVIAL FANS
• Description of alluvial fans
• Three case studies of alluvial fans along Mara Lake
• Hummingbird Creek
• Sicamous Creek
• Rogers Creek
• Effects of wildfire on riparian areas
Sisco, February 1,
2017
ALLUVIAL FANS
cone-shaped deposits of sediments that form
where a stream channel leaves a confined valley
fans are usually more gently-sloped than the
stream channel
appear to be relatively quiet environments, but
fans are very dynamic landforms - run-out areas
for floods, debris floods or debris flows
(hydrogeomorphic processes)
fans commonly have unconfined stream channels
which can result in rapid avulsion, flooding and
widespread deposition of sediment across the
fan surface
the whole of the fan is active so it can be
difficult to define the riparian zone
Sisco, February 1,
2017
ALLUVIAL FANS
• they are often the site of residential developments,
transportation and utility corridors, as well as they are high
value habitat for fish and high-productivity sites for forests
• risks from hydrogeomorphic events must be carefully considered
prior to watershed development
• changing the water and sediment systems within a watershed has
the potential to lead to changes in timing, magnitude and
frequency of hydrogeomorphic processes which affect the fan
• protection forests
Sisco, February 1,
2017
HUMMINGBIRD CREEK
1997 - A landslide off the Skyline Road initiated a debris flow in
the main channel which ran down the moderately steep channel and
impacted the alluvial fan
• Moderate forestry development
• Good riparian protection
Sisco, February 1,
2017
HUMMINGBIRD CREEK
1951 - recent event on the fan
1970 – ongoing
development
Sisco, February 1,
2017
HUMMINGBIRD CREEK
2012 – Hummingbird and Mara Creek respond to rainfall with
elevated flows and sediment movement, plugged the highway culvert
and avulsed across the fan surface
Sisco, February 1,
2017
SICAMOUS CREEK
2012
• An intense and prolonged rainfall event in June
• Flood or debris flood conditions along the upper tributaries
• Debris flood along the main stem
• Overwhelmed the capacity of
the channel across the fan
• Limited forestry
development
• Good riparian
protection
Sisco, February 1,
2017
SICAMOUS CREEK
Riparian removal for development
on the fan confined the channel,
restricting lateral migration with
roads and residences.
Creek controlled in an armoured
channel and then forced through a
too-small highway bridge and
down into Mara Lake
Sisco, February 1,
2017
SICAMOUS CREEK
Sicamous Creek channel
on upper fan in the 90’s
Sicamous Creek Highway bridge
washed out in the 1950’s
Sisco, February 1,
2017
ROGERS CREEK
Sisco, February 1,
2017
ROGERS CREEK
Sisco, February 1,
2017
PLANNING DEVELOPMENT ON AN
ALLUVIAL FAN
• the whole fan is active, channel avulsion always a possibility,
water and sediment dispersion across the entire fan surface
• dynamic landforms - floods, debris floods or debris flows –
leave enough room for lateral migration of the stream channel
• Do not force infrastructure onto a fan, build the infrastructure
to suit the fan
- road or bridge locations and orientations
- bridge or culvert sizing
- residential or other developments
• Present creek location is not always natural
Sisco, February 1,
2017
WILDFIRE
• McLure Fire in 2003 – burned majority of Fishtrap watershed
• WSC weir since 1972
• Study from the weir
upstream towards Skull
• Riparian zone burned
along the floodplain
• Channel stability
• Sediment movement
• Root strength
Sisco, February 1,
2017
FISHTRAP CREEK
• Salvage harvesting took lots of wood, left steeper ground and
majority of the riparian areas
• Rapid recovery of grasses due to seeding, trees coming back
Sisco, February 1,
2017
BOULDER CREEK
• Near Mount Meager,
volcano which had massive
landslide in 2010
• IPP – hydro
• Fire in 2015
Sisco, February 1,
2017
SETON PORTAGE
• Apex of the alluvial fan, ephemeral creek, distributary channel
Sisco, February 1,
2017
SETON PORTAGE
• Mid-fan deposition where
channel confinement lost
• Forest removal by harvest
and burning
• Lower fan heading towards
townsite of Seton Portage
• Unburned fringe of fan
Sisco, February 1,
2017
SETON PORTAGE
• Distal portion of the fan into residential areas
Sisco, February 1,
2017
Thank You
[email protected]
Sisco, February 1,
2017