Bayerisches Landesamt für Umwelt
Bavarian Environment Agency
Woody Debris –
questions and answers
Federal state of Bavaria
Bavarian Environment Agency
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
Contents
1 Overview Woody debris
2 Legal aspects related to the reduction of woody debris
→ Level of awareness in Bavaria
→ Overview of the concept for torrential endangered areas in Bavaria
3 Woody debris in hazard analysis
→ Preliminary work
→ Woody debris – GIS-Application
→ Woody debris – Excel
4 Technical structures to entrap woody debris
→ Case study torrent Arzbach
Bavarian Environment Agency
Overview Woody debris
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
• Also known as driftwood
• Great variability of damages caused by woody debris
• Woody debris management → active & passive protective measures
• Formation of woody debris depend on several parameters, such as (e.g.) forest condition,
geology, hill slope inclination → difficulties in assessment
Bavarian Environment Agency
Woody debris in general // Transport and Deposit
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
• Redistribution of woody debris throughout the entire river system
• Length and shape of the wood and water depth are the key factors for the probability,
distance and mode of the transport
• Woody debris travels in direction of flow and in the stream center
• Continuous reduction in size
• Possible deposit scenarios:
→ Scenario “log jam”
→ Scenario “backwater”
→ Scenario “flooding”
→ Scenario “deposit along the channel”
(Mazzorana et al 2009)
Bavarian Environment Agency
Level of awareness – split of competence in Bavaria
Bavarian State Ministry of the
Environment and Consumer
Protection
→ Bavarian Environment Agency
→ Bavarian Watermanagement
Authorities
• employees have different responsibilities
(waste water, torrents, river maintance …)
→ hydraulic engineers
Bavarian State Ministry of Nutrition,
Agriculture and Forestry
→ Bavarian State Institute for Forestry and
Silviculture
→ Offices for Nutrition, Agriculture and
Forestry
• employees have different responsibilities
(protection forest, thinning …)
→ forester, forestry worker
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
Bavarian Environment Agency
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Torrent endangered areas in Bavaria
• Until now: Not elaborated but target of Bavarian Water Law (art. 46 para. 2)
→ Considering log jams if they occur during design event (100 years)!
• Consequence: Building prohibited – No gradation („yellow and red zone“ in Austria)
→ Exceptions are possible under strict criteria in individual cases
APPROACH:
• Elaboration of technical concept
• Realization according to priorities
WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
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Bavarian Environment Agency
Erosions-
schutz
Ufer-
schutz
Rückhalt Konsoli-
dierung
Schutzwald
Biol. Maßn. Sonstige
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
Assessing torrential endangered areas – overview
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Bavarian Environment Agency
Assessing woody debris
Requirements for all steps
• Based on state of the art
• Applicable for all catchement areas
• Robust, easy to use, efficient (time effort)
• Data compatibility to Bavarian data basis
→ process-based assessment
→ main Tool: ArcGIS
→ side tools: MS Excel
8
Comp. Mayer (2013)
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
Bavarian Environment Agency
Assessment of torrent endangered areas – woody debris
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
Bavarian Environment Agency
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
Preliminary Work
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Bavarian Environment Agency
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Preliminary Work
WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
© LfU / Referat 61 / Dressel / 13.05.2016
Bavarian Environment Agency
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
Preliminary Work
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Bavarian Environment Agency
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
Preliminary Work
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Minimum critical dimensions:
• 7,5 m clearance width,
• 2 m clearance height
• Water level analysis
Gschnitzer et al. (2014)
Bavarian Environment Agency
Preliminary work results → debris wood problem might be given
→ Closer examination necessary
→ Appilication of GIS-based estimation tool
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
Application of GIS-based estimation tool // Basics
Bavarian Environment Agency
Application of the GIS-based assessment method // Theory
15
Basic Concept
Various processes contribute to the intake of debris wood into torrent channels. A certain
portion of this wood, which is dependent on the transport capabilities of the torrent, makes
up the debris wood quantity of the catchment area.
Process area: “effective range” of the process
Potential: process area [m²] * wood volume [m³/m²]
Weighting factor: The portion of the existing process area, mobilized during an event, as
well as the portion of the area that are triggered
Transport: manual or automatic generation of partial catchment areas
WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
© LfU / Referat 61 / Dressel / 13.05.2016
Bavarian Environment Agency
Bank tillering carried off
Bank erosion
Windfall
Landslides
Deposition Point
DW3
DW3+DW2+DW1
DW3+DW2
WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
Application of the GIS-based assessment method // process parameters
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Bavarian Environment Agency
WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
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Bavarian Environment Agency
Excel-Evaluation // Weighting factors in general
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
• Decrease the input values of a process area because (usually):
– not the entire wood volume of an area will be released
– not all areas of a process become “active”
• Values are based on documented events + expert evaluations → Implementation of
scenarios
• Incorporation of qualitative knowledge → transparency and comparability
Scenario frequent moderate infrequent
Bank tillering 0,1 0,5 1
Bank erosion 0,05 0,25 0,5
Windfall 0,002 0,01 0,2
Landslide 0,03 0,15 0,35
According to Mazzorana et al (2011), Asenkerschbaumer (2009), Rimböck (2001)
Bavarian Environment Agency
Excel-Evaluation // Weighting factors & Scenarios
Suggestions for adaptations – qualitative aspects:
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
Frequent Moderate Infrequent
Bank tillerings
being carried
away
Circumstances similar to
those of floods with low
annualities
Only a small portion of the
tillering located on the
embankments is carried
away
hydrological events of
moderate intensities
around half of the vegetation
located near the
embankment is carried
away
highly erosive runoff
the entire bank vegetation is
mobilized
Bank erosion ‘impact banks’ and narrow
curves are only slightly
affected
some of the ‘impact banks’
and embankments are
affected by bank erosion
all of the ‘impact banks’ and
narrow curves are affected
by bank erosions
Windfall
deeply indented, wind
protected areas
deep rooted tree species
regular clearance of
deadwood
both wind exposed and wind
sheltered inclines
deep and shallow rooted
tree species are present
both sections which are
cleared regularly as well as
ones which can not be
cleared
mainly wind exposed areas
lots of deadwood
windfall- prone species in
unfavourable constellations
(e.g. spruce monoculture)
Landslide no large-scale landslides are
triggered
a considerable portion of
known landslides are
reactivated
around half of the tillering
located on it enters the
stream
all known landslides in the
catchment area are
triggered
large volume Landslides:
nearly all off its mass enters
the stream
Bavarian Environment Agency
Excel-Evaluation // Weighting factors & Scenarios
Suggestions for adaptations – qualitative aspects:
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
Frequent Moderate Infrequent
Bank tillerings
being carried
away
Circumstances similar to
those of floods with low
annualities
Only a small portion of the
tillering located on the
embankments is carried
away
hydrological events of
moderate intensities
around half of the vegetation
located near the
embankment is carried
away
highly erosive runoff
the entire bank vegetation is
mobilized
Bank erosion ‘impact banks’ and narrow
curves are only slightly
affected
some of the ‘impact banks’
and embankments are
affected by bank erosion
all of the ‘impact banks’ and
narrow curves are affected
by bank erosions
Windfall
deeply indented, wind
protected areas
deep rooted tree species
regular clearance of
deadwood
both wind exposed and wind
sheltered inclines
deep and shallow rooted
tree species are present
both sections which are
cleared regularly as well as
ones which can not be
cleared
mainly wind exposed areas
lots of deadwood
windfall- prone species in
unfavourable constellations
(e.g. spruce monoculture)
Landslide no large-scale landslides are
triggered
a considerable portion of
known landslides are
reactivated
around half of the tillering
located on it enters the
stream
all known landslides in the
catchment area are
triggered
large volume Landslides:
nearly all off its mass enters
the stream
Frequent Moderate Infrequent
Bank tillerings
being carried
away
Circumstances similar to those
of floods with low annualities
Only a small portion of the
tillering located on the
embankments is carried away
hydrological events of
moderate intensities
around half of the vegetation
located near the embankment
is carried away
highly erosive runoff
the entire bank vegetation is
mobilized
Bavarian Environment Agency
Torrent endangered zone: woody debris – results
• Woody debris at the outlet and each
deposition point along the torrent
→ Rough estimation of potential
→ Detecting of „hotspots“
• Percentage of individual processes in the
total entry
• Rate of transport
→ Plausibility
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
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Bavarian Environment Agency
GIS-based assessment // Plausibility
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
Which factors are included in the
assessment? Uncertainties
• Size of the catchment area
• Stream length (sections capable of transport)
• Forested stream length
• Topography of the catchment area
• River morphology
• Forest structure
• Documented embankment breaches
• Documented debris deposits
• Transport / deposit
• Experience / event documentation
• Dimensions of structures
• Weighting factors
• Probability of transport
• Evaluation of the risk of driftwood jams
Bavarian Environment Agency
Consequences for hydraulic calculations
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
• Establishing driftwood jam scenarios for certain structures based on the risk
assessment
• “Closing” the clearance profile by lowering the lower edge
• Higher closing levels possible (worst case) → depending on sediment transport
• Outlook: higher degrees for small bridges/pipes; combination with bed load
Hazard Assessment Level of Closure
low max. 25%
moderate 50%
high min. 75%
(LfU 2016)
Bavarian Environment Agency
Consequences for strategy planning
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
• Evaluation whether regarded locations are generally at risk of driftwood jams based on the
risk assessment
• Estimate of expected woody debris quantity → guideline for selection/dimension of the
strategies
(Rimböck 2004)
Bavarian Environment Agency
Technical structures to stop debris wood – case study Torrent Arzbach
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
Bavarian Environment Agency
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
Densely
forested,
large
debris wood
potential
Bavarian Environment Agency
Torrent Arzbach
• Frequent events with a great amount of bed load in
the past
• In 1990 log jam on bridges upstream the settlement
• According to the protection system at another torrent
catchment a V-Rake was built in 1990
→ AIM: Permeability of bed load and
retention of woody debris
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
(WWA Weilheim 2003)
(WWA Weilheim 1997)
Bavarian Environment Agency
Woody debris retention rake
• Rake with columns, arranged in a V-shape (plan view)
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
Model test in the hydraulics lab Obernach,
TU Munich
Rake at torrent Lainbach near
Benediktbeuern after Whitsun flood 1999
Bavarian Environment Agency
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
Densely forested,
large woody debris
potential
Wide and natural
active reach
Narrow, many
bridges
V-Rake
Lower reach – torrent Arzbach
Bavarian Environment Agency
Rake Arzbach – Event 2005
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
→ Problem of the permeability of bed load transport!
Bavarian Environment Agency
Arzbach – downstream of the V-Rake after the event in August 2005
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
Channel incision up to 7m
Bavarian Environment Agency
Processes during woody debris entrapment
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Problems:
→ compaction of woody debris
→ large height – overflow ?
→ case of overload ?
→ sodden wood is clogging the
lower zone
WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
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Bavarian Environment Agency
Bedload (after woody debris entrapment)
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
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Bavarian Environment Agency
Conclusion
• Regularely bedload retention accompanied by wood
retention (after beginning of wood retention)
• Partly permeability for bedload:
– (Temporary) before wood retention
– V-shaped rake (under wood carpet)
– Inclined rake (under wood carpet)
• (Spatial) seperation wood and bedload:
– Enforce impoundment → wood at barrage, bedload at
head of reservoir
– First bedload retention
– Second wood retention
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
Bavarian Environment Agency
Thank you for your attention
Dipl.-Ing. Peter Dressel
Bavarian Agency of Environment
86179 Augsburg / Germany
+49 861 9071-5949
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WOODY DEBRIS – QUESTIONS AND ANSWERS Federal state of Bavaria
Stillach 2005, WWA Kempten