Woody debris in gullies

1
Woody debris in gullies

• Should we clean it out?
• VFR Research - T. Millard

2
Gully Processes

• Water transport transport of sediment and woody
  debris by high water flows - gt channel size, gt
  transport capability
• Debris flow (or debris torrent) Slurry-like mix
  of sediment and wood, generally cleans channel out

3
Problems with woody debris

• Erosion of channel banks and sidewalls
• Increased debris flow volumes, debris jams, and
  channel avulsions
• Woody debris delivered to fish streams
• Site loss

4
Factors to consider

• Amount of woody debris
• Incorporation of debris into channel
• Processes in gully water transport, debris flow
• Consequences of cleaning or not cleaning debris
• Downstream resources

5
Clean debris by hazard potential
6
Debris cleaning for water transport recent
logging

• Remove woody debris which will be transported by
  water flows
• Remove debris which directs water against channel
  banks or sidewalls
• Leave large stable pieces of debris
• Return channel to same volume
  capacity as before logging

7
Debris cleaning for water transport old logging

• Remove woody debris which is loose and moved by
  water flows
• Remove woody debris which directs water against
  channel banks
• Leave embedded woody debris in place
• Cut and remove top end of embedded debris if it
  will trap more debris

8
Debris cleaning for debris flows

• New logging if volume of woody debris in channel
  is gt2m3/m, then clean debris
• 2 - 10 yrs after logging clean if volume of a
  debris flow will increase by gt50
• gt 10 years clean if volume increase
  from debris is gt100
• gt 15 years leave it alone

9
Debris jams

• Water transported jams channel develops stepped
  profile
• Debris flow jams usually one large jam
• Location sidewall materials, channel gradient,
  and confinement
• Jam structure volume, height, size and integrity

10
Rehab jams subject to high or moderate water
flows

• Remove all wood not buried in sediment
• If sidewalls are eroding, cut a notch through the
  middle of the jam and armour the sidewalls

11
Rehab jams subject to debris flows

• Consider removing the jam if it has the following
  characteristics Steep channel gradients (gt
  50?) Erodable sidewalls Primarily composed of
  small wood

12
Summary

• Dont remove debris unless youre confident of
  the benefits, OR
• The potential downstream impacts are so great
  something needs to be done