Short course on gullies and streams

This learning resource provides an overview of two slash in streams studies conducted to predict channel disturbance resulting from woody debris left in the channel and provide an update on gully assessment procedure (GAP) changes.

On this page:

• Debris definitions
• Water transport of wood debris
• Debris flow
• History of GAP

Debris definitions

• Woody debris: “slash”
• Landslide debris: includes woody debris, rocks, soil, mud

Water transport of woody debris

Characteristics of water transport of woody debris:

• Woody debris floats downstream during floods
• Small woody debris is easier to float than large slash
• Floods may also transport small or moderate amounts of sediment
• Channel bed and banks remain relatively unchanged
• Occurs in any gradient channel
• Debris flows and debris torrents: types of landslides

Debris flow (or debris torrent)

Characteristics of debris flow:

• Debris flow 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

Background on the development of GAP:

• The GAP started with a district request to identify gullies that would respond badly to logging
• A guidebook was developed in 1995, the first formal procedure for identifying different gully hazards and risks to downstream resources
• It was recognized in 1995 that the procedures were preliminary and needed to be tested
• Studies undertaken from 1996 to 2000 have resulted in changes to the GAP

Water transport potential