EFFECTS OF RIPARIAN RETENTION (IN WATERSHEDS) ON ALLUVIAL FANS TIM GILES GEOMORPHOLOGIST FLNRO
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
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
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
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,
HUMMINGBIRD CREEK 1951 - recent event on the fan 1970 ongoing development
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
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
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
SICAMOUS CREEK Sicamous Creek channel on upper fan in the 90 s Sicamous Creek Highway bridge washed out in the 1950 s
ROGERS CREEK Sisco, February 1,
ROGERS CREEK Sisco, February 1,
PLANNING DEVELOPMENT ON AN ALLUVIAL FAN Sisco, February 1, 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
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
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
BOULDER CREEK Near Mount Meager, volcano which had massive landslide in 2010 IPP hydro Fire in 2015
SETON PORTAGE Apex of the alluvial fan, ephemeral creek, distributary channel
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
SETON PORTAGE Distal portion of the fan into residential areas
Thank You Tim.Giles@gov.bc.ca Sisco, February 1,