Woody debris in gullies Should we clean it out? VFR Research - T. Millard

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

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

Factors to consider n Amount of woody debris n Incorporation of debris into channel n Processes in gully: water transport, debris flow n Consequences of cleaning or not cleaning debris n Downstream resources

Clean debris by hazard potential

Debris cleaning for water transport: recent logging n Remove woody debris which will be transported by water flows n Remove debris which directs water against channel banks or sidewalls n Leave large stable pieces of debris n Return channel to same volume capacity as before logging

Debris cleaning for water transport: old logging n Remove woody debris which is loose and moved by water flows n Remove woody debris which directs water against channel banks n Leave embedded woody debris in place n Cut and remove top end of embedded debris if it will trap more debris

Debris cleaning for debris flows n New logging: if volume of woody debris in channel is >2m 3 /m, then clean debris n yrs after logging: clean if volume of a debris flow will increase by >50% n > 10 years: clean if volume increase from debris is >100% n > 15 years: leave it alone

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

Rehab: jams subject to high or moderate water flows n Remove all wood not buried in sediment n If sidewalls are eroding, cut a notch through the middle of the jam and armour the sidewalls

Rehab: jams subject to debris flows n Consider removing the jam if it has the following characteristics: Steep channel gradients (> 50%?) Erodable sidewalls Primarily composed of small wood

Summary n Don’t remove debris unless you’re confident of the benefits, OR n The potential downstream impacts are so great something needs to be done