Watershed Response to FireWatershed Response to FireWatershed Response to FireWatershed Response to Fire
Christine MayChristine May
Earth & Planetary ScienceEarth & Planetary Science
Severe firesSevere fires can result in accelerated can result in accelerated erosion by:erosion by:
1.) removing the forest canopy and litter layer, exposing mineral soil to the direct impact of rainfall
2.) heating and combusting soil organic matter3.) burning logs that trap soil on steep slopes or store
sediment in stream channels 4.) reducing the root strength of the soil
Rain SplashRain Splash
• Fine sediment or ash dislodged by rain splash can clog soil pores causing surface sealing.
Severe firesSevere fires can result in accelerated can result in accelerated erosion by:erosion by:
1.) removing the forest canopy and litter layer, exposing mineral soil to the direct impact of rainfall
2.) heating and combusting soil organic matter3.) burning logs that trap soil on steep slopes or store
sediment in stream channels 4.) reducing the root strength of the soil
Rainfall
Infiltration
Subsurface flow
Overlandflow
Infiltration RatesInfiltration Rates
• Infiltration = the movement of water across the soil surface
• Influenced by• Soil texture
• Ambient moisture
Time
Infi
ltra
tion
Rat
e (m
m/m
in)
clay
Time
Infi
ltra
tion
Rat
e (m
m/m
in)
clay
sand
Overland FlowOverland Flow
Surface erosion requires overland flow, which occurs when
1.) the rainfall rate exceeds the infiltration rate of the soil surface, or
2.) the soil is saturated
0.1
1.0
10.0
100.0
0 20 40 60 80 100
% postfire herbaceous vegetation cover
av
era
ge
slo
pe
ero
sio
n (
mm
) NE YNP
Gibbon Canyon
>40 15-40 <15Runoff power index, slope (m/m) x relief:
POSTFIRE REVEGETATION and RILL EROSION IN DEBRIS-FLOW BASINS
Hydrophobic SoilsHydrophobic Soils
Hydrophobic SoilsHydrophobic Soils• A water-repellent layer of soil that prevents infiltration
below that layer.
• Derived from plant material burned during a hot fire. The hydrophobic compounds (hydrocarbons) penetrate the soil surface as a gas and solidify after cooling, forming a waxy coating.
• Sandy soils with large pore spaces and areas with thick litter accumulations that experience very hot fires are especially susceptible.
Hydrophobic Soils (con’t)Hydrophobic Soils (con’t)
• The thickness and continuity of hydrophobic layers varies, as does their persistence.
• Recovery: plant roots, soil microorganisms, and soil fauna help break up the hydrophobic layer.
• Negative feedback: reduced infiltration will decrease the amount of water available for plant growth and biological activity in the soil.
Field TestsField Tests
• Sprinkler experiments
• Infiltrometer
• Analysis: compare infiltration rates with rainfall rates from local raingages
Rehabilitation MeasuresRehabilitation Measures
• On-site: revegetation– grass seeding – pitfalls?– straw mulch
• Off-site: sediment retention devices
– straw bale check dams– directional log felling– sediment retention ponds
Photos by F.J. Swanson
[debris flow video]
Debris FlowsDebris Flows
Two initiation mechanisms:
1.) runoff-initiated - driven by low soil infiltration rates
and the bulking of sediment detached by overland flow and surface erosion
2.) landslide-initiated- driven by soil saturation (requires
high infiltration rates and low rooting strength)
Rill and channel erosion (1988 fire, 1989 storm, Yellowstone)
Loss of root strength, saturation-failureof colluvium (1989 fire, 1997 storm, Idaho)
Initiation of events through runoff and sediment bulking, early post-fire
Copyright © Tom Black 2002
1989 debris flow-dominated event, NE Yellowstone1989 debris flow-dominated event, NE Yellowstone
Initiation of events through loss of root strength, saturation and
failure of colluvium (1989 fire, 1997 storm, central Idaho)
Years after forest removal / fire
Rel
ativ
e ro
ot r
einf
orce
men
t
From Ziemer 1981
Decay of dead roots
Years after forest removal / fire
Rel
ativ
e ro
ot r
einf
orce
men
t
From Ziemer 1981
Decay of dead roots
Live roots
Years after forest removal / fire
Rel
ativ
e ro
ot r
einf
orce
men
t
From Ziemer 1981
Decay of dead roots
Live roots
5 – 15 yrs
Probability of OccurrenceProbability of Occurrence
• Depends upon post-fire storm characteristics and the spatial pattern of high severity fire patches.
Temporal PatternsTemporal Patterns
Time since fire
Lik
elih
ood
of la
rge-
scal
e er
osio
n
Runoff-DominatedSurface Erosion
Two-Phase Erosional Response
Time since fire
Lik
elih
ood
of la
rge-
scal
e er
osio
n
Runoff-DominatedSurface Erosion
Saturation-InducedSlope Failures
Two-Phase Erosional Response
Time since fire
Lik
elih
ood
of la
rge-
scal
e er
osio
n
Runoff-DominatedSurface Erosion
Saturation-InducedSlope Failures
Two-Phase Erosional Response
0 – 5 yrs
5 – 15 yrs
Post-Fire Rehabilitation Efforts for Post-Fire Rehabilitation Efforts for Debris FlowsDebris Flows
Suggestions from the class…
0 to 30 yrs 30 to 60 yrs 60 to 90 yrs > 90 yrs
Time Since the Previous Debris Flow
Salvage LoggingSalvage Logging
• Dead wood can be an important element in sediment storage on steep hillslopes and in stream channels.
• Soil disturbance by logging operations and road construction can further accelerate erosion.
Short-term PatternsShort-term Patterns
LongLong-term Patterns-term Patterns
Event ReconstructionsEvent Reconstructions
• Studies that attempt to decipher long-term correlations among climate, fire, and erosion and their effects on landscape evolution using a variety of dating methods and evidence for past erosional events.
Copyright © Ron Dorn 2002
1989 debris flow
older fan sediments
1988 charred litter layer (burned soil surface)
‘Fire-related debris flows’
Yellowstone lodgepole: large, severe stand-
replacing fires, RI 200-400 yr
Onset of the Little IceAge (1200 AD)
Meyer et al. 1992Meyer et al. 1992
Warmer millennial-scale periods
Cooler millennial-scale periods
• (terraces?)
300112
169
43000
1
10
100
1000
10000
100000
Apatite fissiontracks
(Sweetkind andBlackwell, 1989)
Cosmogenicnuclides
(Kirchner et al.,2001)
7400-6600 cal yrBP rates
Sedimenttrapping and
gauging (Claytonand Megahan,
1986)
1997 debris-flowevents
sedi
men
t yie
ld in
T/k
m2/y
r
10,000,000
Time scale in years
10,000 1,000 100 1 0.01
Idaho batholith estimated mean sediment yields over different timescales
(log scale)
Suspended sediment and bedload transportWatershed 3, HJA
0
4000
8000
12000
1956 1962 1968 1974 1980 1986 1992
t/km
2 /yr
Suspended sediment
Bedload
Roads Patch cut
Floods and debris flows
Questions?Questions?
Questions for the class:Questions for the class:
Is there evidence that fires preferentially travel through or burn hotter in steep, narrow valleys compared to planar hillslopes?
Questions for the class:Questions for the class:
How can information about erosion-prone areas be incorporated into:
1.) planning fuels treatment projects?
2.) wildfire management?
0
20
40
60
0 30 60 90 120 150
Time Since Fire (yrs)
Re
lati
ve
Fre
qu
en
cy
of
De
bri
s
Flo
w O
cc
ure
nc
e
Central Idaho ponderosa: presettlement regime of light surface fires, RI 5-30 yr
Northern Hemisphere tree-ring temperature reconstruction (from Esper et al., 2002)
Mann-Bradley-Hughes (1999) Esper et al. (2002)
multiproxy tree-rings
“Medieval Warm Period” “Little Ice Age”
Severe fire, large debris flows both areas
Frequent light fires Idaho; few fires Yellowstone
Year AD
Adapted from Swanson (1981);additional point G. Meyer (pers. comm).
Sediment yields in fire-induced debris-flow events, western U.S.
basin area (km2)
0.1 1 10
sedi
men
t yie
ld (
Mg)
1000
10000
100000
1000000saturation-failure eventsrunoff-generated eventsrunoff-generated events, YNPsaturation-failure event(?), YNP
ConclusionsConclusions
• Herbaceous revegetation reduces probability of post-fire runoff-generated events (maximum probability of occurrence 1-3+ yr after fire?)
• Later postfire saturation-induced failures –root strength control (maximum probability 4-10 yr after fire?)
• Geomorphic response to fire is transient but produces transient to persistent stream habitat alteration
• Climate is a strong control on fire regimes and associated geomorphic response, both in space and time, therefore…
• Fire-induced sedimentation is strongly episodic and variable at ~1000 yr timescales
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