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