Steep Streams:Steep Streams:what’s new, 

what’s problematicwhat s problematic

Ellen WohlEllen WohlDepartment of GeosciencesColorado State University

• characteristics of steep streams

• discharge estimation

What’s new

g

• process domains

d d i• wood dynamics

• emergent properties• spatial distribution & remote prediction of channel

characteristics

What’s problematic• quantitative prediction of sediment dynamics & channel• quantitative prediction of sediment dynamics & channel

change

• climate change

• uncertainty

• management

Characteristics of steep streams

• boundaries: erosionally resistant, hydraulically rough

• hydrology: substantial spatial & temporal variation in discharge

• hydraulics: highly turbulent, repeated sub‐supercritical transitions

• sediment: limited supply of fines, large spatial & temporal variation in movement, higher entrainment thresholds

• channel & valley geometry: narrow valley bottoms; close couplingto hillslopes (water, sediment, organic matter); substantiallongitudinal variationslongitudinal variations

Debris flowsinitiation

scour

Montgomery & Buffington, 1997

scour

deposition

hillslope

Woodlargely immobile; largely mobile;

hollowg y ;

traps sedimentlargely mobile;acts as sediment

colluvial

cascade

pool-riffleplane-bed

step-pool

d i lp

dune-ripple

transport response

What’s new: discharge estimation

Broad crested weir equation Q = C* g1/2 W H3/2Broad‐crested weir equation   Q = C* g1/2 W H3/2

C* is weir‐step coefficientW is crest widthW is crest widthH is head just upstream from step

criticalycy

Hcriticalpool VV WeirStep

Hcriticaly

criticalapproach VV

Step Pool Channel Broad-crested WeirStep-Pool Channel Broad-crested Weir

Dust & Wohl, submitted

a) Planar c) Rounded

CrestCrest--ClastClastG tG t

b) Sub-Rounded

Crest-Clast GeometryGeometry

d) Straight e) Arched f) Oblique Angle

Geometry

PlanformPlanformGeometryGeometry

Wcrest

PlanformGeometry

LongitudinalLongitudinal

i) Staggered Cresth) Tread Slope j) Notched CrestStaggered Clast

LongitudinalGeometryGeometryV

HLongitudinalGeometry

Instream WoodInstream WoodGeometryGeometry

m) Logjam on Crestl) Log on Crestk) Log Step

log

log logInstream Wood Geometry

log

0 60

0.70C* Oscillating

FlowInterstitial Flow

0.40

0.50

0.60 InterstitialFlow

Weir Flow

0.20

0.30Weir Flow

Linear Model: C* = m (h/Dstep) + b Oscillating Flow

0.100.4 0.6 0.8 1.0

Relative Submergence (h/Dstep)

Observed flow regimes Observed flow regimes Typical Experiment ResultsTypical Experiment Results

estimate C* using values in paper:  Q = C*g1/2 W H3/2

1.0

1.1

C*

0.7

0.8

0.9

W1.1: Straight-Log Step (St=8H:1V)

WR3 1 1: Straight Staggered Step (St=70H:1V) with Wood

0.4

0.5

0.6WR3.1.1: Straight-Staggered Step (St=70H:1V) with Wood

WR3.1.2: Straight-Staggered Step (St=70H:1V) with Logjam

R3.1: Straight-Staggered Step (St=70H:1V)

0.1

0.2

0.3

0.10.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4

Relative Submergence (h/Dstep)

What’s new: process domains

Montgomery, 1999

avalanches

debris flows

avalanches

debris flows

flooding

channel migration

fluvial

channel migration

glaciatedglaciatedconfinedconfined

glaciatedglaciatedunconfinedunconfined 23002300--m contourm contour

glaciatedglaciatedpartiallypartiallyconfinedconfined

fluvial confinedfluvial confined

fluvial unconfinedfluvial unconfined

fluvial partially confinedfluvial partially confined

http://warnercnr.colostate.edu/class_info/g692/Front_Range/index.php

What’s new: wood dynamics

Volumetric mass balance of wood in unit length of channelVolumetric mass balance of wood in unit length of channel

ΔSc = [Li – L0 + Qi/Δx – Q0/Δx – D + B]Δt

ΔSc change in storage within a reach of length Δx over time interval Δt

Li lateral wood recruitmentL0 loss of wood to overbank deposition & channel movementQ fluvial transport of wood into segmentQi fluvial transport of wood into segmentQ0 fluvial transport out of segmentD in situ decayB storage in beaver dams

Benda & Sais, 2003

Li = I + If + Ib + I + Iexhumationof buried

dLi Im + If + Ibe + Is + Ie

chronicforest tree topple,

fi &

wood

massbankmortality fire &

windstormmovementserosion

Benda & Sais, 2003

Differences with respect to position in the catchment

glaciated confined

ΔΔSScc = [= [LLii –– LL00 + + QQii//ΔΔx x –– QQ00//ΔΔx x –– D D ++ BB]]ΔΔtt

l i t d fi d

cc [[ ii 00 QQii QQ00 ]]LLii = = IImm + + IIff + + IIbebe + + IIss + + IIee

glaciated unconfined

ΔΔSScc = [= [LLii –– LL00 + + QQii//ΔΔx x –– QQ00//ΔΔx x –– D D ++ BB]]ΔΔtt

fluvial unconfined

cc 00LLii = = IImm + + IIff + + IIbebe + + IIss + + IIee

fluvial unconfinedΔΔSScc = [= [LLii –– LL00 + + QQii//ΔΔx x –– QQ00//ΔΔx x –– D D ++ BB]]ΔΔtt

LLii = = IImm + + IIff + + IIbebe + + IIss + + IIee

What’s new: emergent propertiesoverbank deposition of wood& trapping of fine sediment& trapping of fine sediment

hyporheic exchange &spring-head channels

avulsion/anastomosing

logjam bank erosion

woodwoodrecruitment

threshold based on L/w and D50/d

101–102 m101–102 yr

localizedalluviation

i

L/w and D50/d

100–101 m100–101 yr

transient

Wohl, 2011

)0.8

L/w D50/d

b

L mean length wood

d D

50/d

(m/m

0 4

0.6

a b

a

L   mean length woodw  mean channel widthD50  mean diameter woodD mean flow depth

L/w

(m/m

) and

0.2

0.4

0.295

0.3450.447

D     mean flow depth

L

0.0Single-thread Multi-thread Single-thread Multi-thread

0.235

alternative stable states ofwood‐rich & wood‐poor

What’s new: spatial distribution & remote prediction of channel characteristics

Ch d t1000

10000

Wohl & Merritt (2008)

Chagres data

D84

(mm

)

10

100

step-poolplane-bed

Gradient (m/m)

0.0001 0.001 0.01 0.1 11

ppool-riffle

0.20

nt (m

/m)

0.10

0.15

a

Gra

die

0.00

0.05

n = 177mean: 0.071std dev: 0.039

n = 44mean: 0.020std dev: 0.012

n = 114mean: 0.013std dev: 0.008

bb

Predicted spatial distribution of channel types in theWillapa basin, Washington (Buffington & Tonina, 2009)

step-pool plane-bed pool-riffle

What’s problematic: quantitative prediction of sediment dynamics & channel change

Parameter                            Quantitative Predictiondischarge (Q)                                              XXgradient (S)                                                 XXvalley geometry                                    Xa ey geo et ysediment supplyexternal resistance (f)                                            (X)   total stream power Xtotal stream power                                      Xsuspended sedimentbedload transport                                                 ((X))bedforms                                                         Xsinuosity                                                          (X)channel lateral mobility                                 (X)y ( )bank resistance from riparian vegetation         ((X))

What’s problematic: climate change

air temperatureair temperatureprecipitation

wildfirevegetationslope stability

watersedimentwoodwood

hydrographhabitat disturbance regimet hi t ttrophic structure

What’s problematic: uncertainty

• inherent uncertainty of complex natural systems• inherent uncertainty of complex natural systems

• human‐induced uncertainty (land use, management/humanperceptions, climate change)

• relevance of historical range of variability or reference conditions?

p p , g )

• communicating uncertainty (nonstationarity, probabilistic river)

• importance of conceptualizing river as complex ecosystemwith thresholds feedbacks and nonlinear behaviorwith thresholds, feedbacks, and nonlinear behavior

What’s problematic: management

• what should this river look like?• what should this river look like?

• what can this river look like?

h ill hi i l k lik ?• what will this river look like?

The Gordian knot of steep stream dynamics …

valleygeometry

channel

grainsizesinuosity

lateral

geometry

GRADIENT

channelmorphologymobility

overbank flooding hydraulicg

roughness

instreamaquatic h bit t

riparian habitat sediment

mobilitywood loads

habitat

References Cited

B d & S i 2003 A tit ti f k f l ti th b l f i tBenda & Sais, 2003, A quantitative framework for evaluating the mass balance of in-stream organic debris, Forest Ecology & Management, 172, 1-16.

Dust & Wohl, submitted, A new approach: characterization of the hydraulics in step-pool streamsvia weir flow concepts. Water Resources Research.

Montgomer 1999 Process domains and the ri er contin m J Am Water Reso rcesMontgomery, 1999, Process domains and the river continuum, J. Am. Water ResourcesAssociation, 35, 397-410.

Wohl, 2011, Threshold-induced complex behavior of wood in mountain streams, Geology, 39,587-590.

Wohl & Merritt 2008 Reach scale channel geometry of mountain streams Geomorphology 93Wohl & Merritt, 2008, Reach-scale channel geometry of mountain streams, Geomorphology, 93,168-185.

http://warnercnr.colostate.edu/class_info/g692/Front_Range/index.phpp _ g _ g p p