Short Course on Gullies and Streams Tom Millard, Vancouver Forest Region

Objectives Report on recent studies Provide an update on GAP changes

Debris definitions Woody debris - “slash” Landslide debris - includes woody debris, rocks, soil, mud Debris flows and debris torrents - types of landslides

Water transport of woody debris Woody debris floats downstream during floods Small WD easier to float than large slash Flood may also transport small or moderate amounts of sediment Channel bed and banks remain relatively unchanged Occurs in any gradient channel

Debris flow (or debris torrent) Debris flows almost always starts as a landslide into a gully channel Needs a steep channel to start in (>30%) Almost all wood and sediment is eroded - channel often scoured to bedrock Often no channel bed or banks remain Woody debris does not start a debris torrent

History of GAP Started with a district request to identify gullies that would respond badly to logging Became a guidebook in 1995, first formal procedure for identifying different gully hazards and risks to downstream resources Recognized in 1995 that the procedures were preliminary and needed to be tested Studies undertaken from have resulted in changes to the GAP

Section 2: Water Transport Potential

Water transport potential assessment Objective: to identify channels that will be disturbed if logged, or if woody debris is left in the channel. Disturbance: Erosion of channel banks and bed, woody debris jams, avulsions (breakouts), transport of woody debris into fish reaches

No slash movement

Little slash movement

Little - moderate slash movement

Moderate slash movement

Extensive slash movement

Severe slash movement

Old assessment method

Van. Island & Boston Bar Slash in Streams Studies Objective - to identify channels that become disturbed if slash is left in the channel after yarding is completed Channel disturbance = high water transport potential

Vancouver Island study location Near Nitinat Lake, Southwest Vancouver Island Winter floods from rain storms Mostly rocky terrain, coarse grained soils “Nitinat”

Boston Bar study location All streams located with Anderson River basin, east of Boston Bar Almost Interior conditions Snowmelt dominated floods Deep, fine-grained till soils “Anderson”

Slash in Streams Study Methods Select streams logged years ago, with the slash left in the stream Inventory amount and types of disturbance Collect data on width, depth, gradient, and other variables that may be used to predict disturbance Streams <5 m wide

Disturbance Large sizes of woody debris transported Large amounts of woody debris transported Frequent and large woody debris jams Channel and bank erosion

Severe disturbance

Predictor variables Channel width and depth Gradient Channel location Channel and bank types Size of sediment moving

Results Nitinat (Vancouver Island) study - 99 stream reaches Anderson River (Boston Bar) study - 42 stream reaches

Significant predictors of channel disturbance Channel width, depth, and area - width is best Size of sediment transported in the stream

Channel width and size of woody debris moved

Jam volume and width: Nitinat

Jam volume and width: Anderson

Sediment size and woody debris size moving

Jam volume and sediment size: Nitinat

Jam volume and sediment size: Anderson

What about gradient? Gradient has no real effect Steeper streams have more channel roughness Lower gradient streams are more able to float woody debris

Useful predictors of water transport potential Channel width (could also use channel area, but no better) Size of sediment moving in channel In these two studies, size of woody debris moving in channel was used as an response variable, but - can also be used as a predictor prior to harvesting

Water transport assessment and cleaning strategies