Large Woody Debris in the Snowy River Estuary, Australia
Jon Hinwood & Errol McLean
Department of Mechanical Engineering
Monash University, Melbourne
Australia
School of Earth & Environmental Sciences
University of Wollongong, Wollongong
Australia
Australian Geographic Magazine
moving down the Snowy river
the estuary – wider & deeper than the river, with wind, waves and tides
logs lifted by
tides & waves
stranded
by floods
logs lifted by
tides & waves
Brown Stringybark (Eucalyptus baxteri)
Coast Banksia (Banksia integrifolia)
Red Stringybark (Eucalyptus macrorhyncha)
Mountain Ash (Eucalyptus regnans)
Frequency of occurrence of tree species – Snowy river Estuary 2006
Tree type - Section 2
TreeEstuary
type - Section 2
Lower Snowy
Tree Estuary
type - Section 1
Upper Snowy
Unknown
red stringybark
Unknown
brown stringybark
Unknown
red stringybark
red Stringybark
stringybark
Unknown
brown stringybark
acacia wattle
Unknown Stringybark
brown stringybark
Unknown Stringybark
acacia wattle
acacia
acacia wattle
acacia
pine
acacia
pine
pine
sallow wattle
sallow wattle
sallow wattle
messmate stringybark
messmate stringybark
messmate
stringybark
Forest red gum
Forest red gum
eucalyptus unknown
Forest red
gum unknown
eucalyptus
Mountain ash
Mountain ash
eucalyptus
unknown
Mountain ash
Data collected
Census each September 2003-2008
Source area scan 2006
Tracking experiments 2007-2010
Census data:
Quantitative data:
• Location of the LWD (GPS).
• Angle of the LWD to the flow direction.
• Length of the LWD.
• Largest diameter of the LWD.
• Mean diameter of the LWD.
• Depth and location of scour/deposition.
Data collected
Census each September 2003-2008
Source area scan 2006
Tracking experiments 2007-2010
Census data:
Quantitative data:
• Location of the LWD (GPS).
• Angle of the LWD to the flow direction.
• Length of the LWD.
• Largest diameter of the LWD.
• Mean diameter of the LWD.
• Depth and location of scour/deposition.
Qualitative Data:
• Origin of the LWD
• Type (root ball, whole tree, etc)
• Log jams
• Degree of impregnation & submergence
• State of decay & weathering
• Morphological effects
2004
2006
2007 (not Brodribb)
Lake
Corringle
Brodribb River
Marlo
Drought 2001-2010
Entrance nearly closed 2004
Bushfire prior to 2006
Major flood June 2007
Lagoon
Pacific Ocean
LWD density distribution September 2008
Brodribb not surveyed
Brodribb River
Changes in LWD density distribution
Scale for weathering and decay (part)
Weathering & decay as indicators of passage through the estuary
semiweathered
fresh
weathered
Input from catchment
Input by man
after wind storm
after wind storm
Fresh
Semi-weathered
Input via river
Input by man
Weathered
Geomorphic effects – local but durable
Bank accretion
Bank scour
Conclusions
Sources and travel:
•
•
•
•
•
•
LWD is much sparser in the estuary than the river
Riparian trees are a major source of estuarine LWD, falling naturally or cut down
Strong winds and bushfires increase supply significantly
LWD stranded by floods may remain for years, large logs for decades
LWD is lost to sea from the estuary by freshes and floods
Wave , wind and tide are effective in stranding LWD mainly along banks
Conclusions
Sources and travel:
•
•
•
•
•
•
LWD is much sparser in the estuary than the river
Riparian trees are a major source of estuarine LWD, falling naturally or cut down
Strong winds and bushfires increase supply significantly
LWD stranded by floods may remain for years, large logs for decades
LWD is lost to sea from the estuary by freshes and floods
Wave , wind and tide are effective in stranding LWD mainly along banks
Effects of LWD in the estuary:
•
•
•
•
LWD is predominantly parallel to the shore and in contact with the shore
LWD generally promotes minor bank accretion
LWD does not have a significant effect on the hydraulics or sediment transport
LWD, particularly if not shore-parallel, promotes localised scour under and at its
offshore end, these holes and shelter are potential fish and crustacean habitat
• LWD provides roosting and nesting sites for birds
Conclusions
Sources and travel:
•
•
•
•
•
•
LWD is much sparser in the estuary than the river
Riparian trees are a major source of estuarine LWD, falling naturally or cut down
Strong winds and bushfires increase supply significantly
LWD stranded by floods may remain for years, large logs for decades
LWD is lost to sea from the estuary by freshes and floods
Wave , wind and tide are effective in stranding LWD mainly along banks
Effects of LWD in the estuary:
•
•
•
•
LWD is predominantly parallel to the shore and in contact with the shore
LWD generally promotes minor bank accretion
LWD does not have a significant effect on the hydraulics or sediment transport
LWD, particularly if not shore-parallel, promotes localised scour under and at its
offshore end, these holes and shelter are potential fish and crustacean habitat
• LWD provides roosting and nesting sites for birds
Sampling LWD in the estuary:
• The sparse but clumped distribution required intensive sampling
• Use of a photo guide and training session are essential to ensure consistency