woody debris – questions and answers · geology, hill slope inclination → difficulties in...

Bayerisches Landesamt für Umwelt

Bavarian Environment Agency

Woody Debris –

questions and answers

Federal state of Bavaria


© LfU / Referat 61 / Dressel / 13.05.2016 2

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


Overview Woody debris



• 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


Woody debris in general // Transport and Deposit



• 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)


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




6

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


© LfU / Referat 61 / Dressel / 13.05.2016


Erosions-

schutz

Ufer-

schutz

Rückhalt Konsoli-

dierung

Schutzwald

Biol. Maßn. Sonstige



Assessing torrential endangered areas – overview

7


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)




Assessment of torrent endangered areas – woody debris




10


Preliminary Work



11

Preliminary Work




12


Preliminary Work



13


Preliminary Work


Minimum critical dimensions:

• 7,5 m clearance width,

• 2 m clearance height

• Water level analysis

Gschnitzer et al. (2014)


Preliminary work results → debris wood problem might be given

→ Closer examination necessary

→ Appilication of GIS-based estimation tool

14 © LfU / Referat 61 / Dressel / 13.05.2016


Application of GIS-based estimation tool // Basics


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




Bank tillering carried off

Bank erosion

Windfall

Landslides

Deposition Point

DW3

DW3+DW2+DW1

DW3+DW2


Application of the GIS-based assessment method // process parameters



Excel-Evaluation // Weighting factors in general



• 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)


Excel-Evaluation // Weighting factors & Scenarios

Suggestions for adaptations – qualitative aspects:



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


Excel-Evaluation // Weighting factors & Scenarios

Suggestions for adaptations – qualitative aspects:




Bank tillerings

being carried

away

Circumstances similar to

those of floods with low

annualities



embankments is carried

away




located near the

embankment is carried

away



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


Bank tillerings

being carried

away

Circumstances similar to those

of floods with low annualities



embankments is carried away




located near the embankment

is carried away



mobilized


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

21




GIS-based assessment // Plausibility



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


Consequences for hydraulic calculations



• 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)


Consequences for strategy planning



• 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)


Technical structures to stop debris wood – case study Torrent Arzbach






Densely

forested,

large

debris wood

potential


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



(WWA Weilheim 2003)

(WWA Weilheim 1997)


Woody debris retention rake

• Rake with columns, arranged in a V-shape (plan view)



Model test in the hydraulics lab Obernach,

TU Munich

Rake at torrent Lainbach near

Benediktbeuern after Whitsun flood 1999




Densely forested,

large woody debris

potential

Wide and natural

active reach

Narrow, many

bridges

V-Rake

Lower reach – torrent Arzbach


Rake Arzbach – Event 2005



→ Problem of the permeability of bed load transport!


Arzbach – downstream of the V-Rake after the event in August 2005



Channel incision up to 7m


Processes during woody debris entrapment

32

Problems:

→ compaction of woody debris

→ large height – overflow ?

→ case of overload ?

→ sodden wood is clogging the

lower zone




Bedload (after woody debris entrapment)

33




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




Thank you for your attention

Dipl.-Ing. Peter Dressel

Bavarian Agency of Environment

86179 Augsburg / Germany

[email protected]

+49 861 9071-5949



Stillach 2005, WWA Kempten

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