Some General Sediment ConceptsAugust 1, 2016

DEPARTMENT OF WATERSHED SCIENCES

Monday: estimating sediment sources, sinks, transport rates

Tuesday: channel, sediment & flow  RAS model and critical discharge

Wednesday: estimating sediment transport rates: rating curves, budget, transport frequency

Thursday: geomorphic & design concepts  channel/floodplain design exercise

Friday: present & evaluate designs, visit field site

SEDIMENT SUPPLY TRANSPORT CAPACITY

Design a stream channel that is in equilibrium withthe present regime of water and sediment supply

Sediment Supply Transport Capacity

This de

fines th

e controls for an alluvial cha

nnel

Lane/Borland Balance (USBR 1955‐1960)

Does the sediment balance matter in this stream?

This was designed as a threshold channel

What is the flow that is competent to move the bed material?

Does the sediment balance matter in this stream?

What is the transport capacity compared to the 

sediment supply?

This is trying to be an alluvial channel

• Flow competenceWill a flow move the grains on the bed?

(focus on bed material)

• Transport CapacityAt what rate can the flow transport sediment?

(focus on sediment supply)

There are two basic transport problems

These are different problems!!!

Define Qc the water discharge at which grains on the bed begin to move

For Q < Qc … no transport, right?

For Q Qc … all the sediment supply will be transported?

Wash load is not just for mud !

NO! consider a pipe

NO! consider a toll booth

Competence v. CapacityFlow CompetenceCan a flow entrain the grains on the bed?

Applied to the channel bed

Leads to a threshold channel

Transport CapacityAt what rate can a flow 

transport sediment?

Compare to sediment supply

Leads to a mobile channel

Example channel problems – competence or capacity?( ) Channel design: will a channel be able to transport the imposed

sediment load with the available flow without substantial aggradationand degradation?

( ) Channel design: will the bed and banks of a canal remain intact at a design flow? What is the chance that the channel will need ‘repair’ in the next 10 yrs? 25 yrs?

( ) How much sediment do we need to add to restore streams below dams? ( ) How much flow is needed to mobilize >90% of the sediment on the

bed surface, in order to flush fines from the subsurface?( ) Can we mine sediment from the stream w/o causing downstream

problems?( ) Will the frequency of bed disturbance change with alterations to the

flood regime? (changes in climate, land use, reservoir operation, fire)( ) How will a channel respond to changes in water and sediment supply?

How far downstream will changes occur? How long will it take? Can it be reversed or managed? (changes in climate, land use, or reservoir operation, fire, tributary floods)

Objectivesediment & nutrients

property & infrastructurebiological recovery

aestheticpenance

Whatneeds

fixing?

Stormwater control

Nothing

Channel change

Introduced species Disturbance Internal orexternal?

InternalExternal

Fence out the cows!Remove the concrete!

Template approachcan work

SmallChannel Design

Sediment supply large

or small?Large

Estimate flood frequency

Design threshold channel

Estimate sediment supply & flow

durationDesign alluvial

channel Alluvial Channel?Threshold Channel?

Other ???

…

Sediment Transport in Channel DesignE

nvir

onm

enta

l D

rive

rs

A few slides about sediment

MUD

SAND

PEAGRA

VEL

22(mm) D

0.01 0.1 1 10 100

Grain Size (mm)

1/16 mm 2 mm 64 mm

silt gravelsand

suspended load, wash load

bed load, bed material load

We can classify transport byMechanism — bedload vs suspended load, or Source — bed-material load vs. wash load

The boundary between each subcategory is diffuse

Size Fraction Mechanism Source Notes

Washloadclay, silt, fine sand

Suspended load ?? uplands, banks, backwaters, …

Washload:(a) Transport not ‘predictable’(b) too little in bed to affect transport of other fractions

Bed Material –Fine

med-crs sand, pea gravel

bed load or suspended load

Bed matrix: interstices, stripes dunes, subsurface

grain path in near-bed region dominated by larger grains; hard to sample & model

Bed Material –Coarse

med-crs gravel, cobble

bed load Bed framework displacements generally rare and hard to capture

Bed Material –Huge

boulder

immobile at typical high flows

Bed surface Requires decision regarding grains to exclude from the transportable population

We can classify transport by mechanism or source

Background: Calculus, differential eq., elementary mechanics, fluid mechanics, numerical methodsOpen Channel HydraulicsHenderson, F., 1966.  Open channel Flow, Macmillan.Sturm, T., 2001.  Open channel hydraulics.  SedimentationWilcock, P; Pitlick, J; Cui, Y. 2009. Sediment Transport Primer: Estimating Bed‐Material Transport in Gravel‐

bed Rivers. RMRS‐GTR‐226. USDA Forest Service, http://www.stream.fs.fed.usGarcia, M. (ed.) 2008.  Sedimentation Engineering: Theory, Measurement, Modeling, and Practice, ASC 

Manual 110Hydrology, Geomorphology, EcologyDingman, L., Physical Hydrology, Culinary and Hospitality Industry Publications Services?Freeze, A. and Cherry, J., 1979, Groundwater, Prentice HallKnighton, D., 1998.  Fluvial Forms and Processes.  ArnoldKondolf, G.M. and H. Piegay (eds.), 2003.  Tools in Fluvial Geomorphology.  John Wiley.Allan, J.D., 1995.  Stream Ecology: structure and function of running waters.  Chapman Hall.Falk, D., M. Palmer and J. Zedler, 2006, Foundations of Restorations Ecology, Island Press.Naiman, R.J., H. Decamps, M.E. McClain, 2005.  Riparia: Ecology, conservation, and management of 

streamside communities. Elsevier Academic Press, 430 p.Restoration Design & PracticeCopeland, R., D.N. McComas, C.R. Thorne, P.J. Soar, M.M. Jonas, J.B. Fripp, 2001.  Hydraulic Design of Stream 

Restoration Projects, ERDC/CHL TR‐01‐28.   ERDC/CHL TR‐01‐28Shields, F D, R R Copeland, P C Klingeman, M W Doyle, and A Simon; 2003 (August); Design for Stream 

Restoration, Journal of Hydraulic Engineering; 129, 8: 575‐584Federal Interagency Stream Restoration Working Group (FISRWG). 1998. Stream Corridor Restoration: 

Principles, Processes and Practices. Springfield, Va: National Technical Information Service.   http://www.nrcs.usda.gov/technical/stream_restoration/newgra.html.

NRCS, 2007. Stream Restoration Design, Part 654 National Engineering Handbookdownload: http://policy.nrcs.usda.gov/index.aspx, request CD: http://landcare.nrcs.usda.gov/ 

A few slides about sediment transport &incipient grain motion

o

3

2

*

(Flow force on bed)/(bed area):

(Grain weight): ( 1)6

Number of grains/area 1/Flow ForceShields Number:

Grain weight ( 1)This is THE most important variable in sediment transport

o

s g D

D

s gD

The Shields Number

s grain size; acceleration of gravity; , water, sediment density; /sD g s

What drives grain motion?

Quite generally, to estimate sediment transport, we face a (i) sediment problem – specifying the model and parameter values to estimate

incipient motion and transport rate as a function of boundary stress 

(ii) hydraulics problem – estimating boundary stress  in terms of channel geometry, bed roughness, and flow

How do we find the critical stress     at incipient grain motion?

Shields Curve: relates critical Shields Number     and Dimensionless viscosity S*

c

*c 0.1

1

10

100

0.1 1 10 100Grain Size D (mm)

She

ar s

tress

(P

a)

0.01

0.1

1

0.1 1 10 100 1000 10000

*

*S

0.5c D

For c in Pa & D in mm

quartz in water

c

No Motion

Motion

*( )

os gD

3/ 2* ( 1)DS s g

* 0.03

( )c

cs gD

For D > 4 mm

s grain size; , water, sediment density; acceleration of gravity; water viscosity

Dg

No Motion

Motion

When does transport begin?

slope top width depth wetted perimeter x/s area

SBhPA

oBoundary stress: (stresss is force/area)

Boundary force: PL

Volume of water: Weight of water: Downslope component of weight of water:

ALgAL

gALS

L

in a wide channel

o

o

o

PL gALSgRSghS The ‘depth‐slope’ product

Figure adapted from WINXSPro manual, USFS

How do we get this boundary shear o? Normal flowno accelerations, so F = 0

is the boundary shear stress - the flow force per unit area - it drives the sediment transport

o

o balances the downslope component of the weight of the water

0.0

0.5

1.0

1.5

2.0

2.5

0 2 4 6 8 10 12 14 16

Lazy River Section 2

Station (m)

Elev

atio

n (m

)

We have to calculate the flow – solve the flow problem –to find o as a function of water discharge,channel geometry, and roughness. Sediment properties – and a transport model – gives us c , the stress that initiates motion.

c

As discharge increases, up go the depth. Bed shear incrases with flow depthWill sediment move? Is > ? Incipient motion problem - determines critical dischargeHow fast will

o

o

QghS

csediment move? does exceed ? Transport problem - determines transport rate

oBy how much

Competence v. CapacityFlow CompetenceCan a flow entrain the grains on the bed?

Applied to the channel bed

Leads to a threshold channel

Transport CapacityAt what rate can a flow 

transport sediment?

Compare to sediment supply

Leads to a mobile channel

*

Does exceed ?

Tranpsort model:

Specified

o c

c

By how muchdoes exceed ?

Transport rate = ( )o c

o cf

Incipient Motiondoes a grain move?

Transport RateHow fast do grains move?

Big Deal: does c refer to the BED or the SUPPLY ???

*

o

The Shields Number ( 1)

is a ratio of stress to grain size Both of which vary spatially ...

os gD

D

Spatial variability: im

plications fo

r sam

plingan

d mod

eling

Entrainm

ent a

nd tran

sport a

re lo

caland

 non

linea

rprocesses

In a qua

si lin

ear system

Some questions to ponder in the field today, at each channel reach.

Think Shields Number.What is critical stress of grains in the reach? Use median size D50 of the gravel bed materialand c = 0.5 D50 (D50 in mm)

What is the approximate shear stress from flowat different flow depths?Use o = hS (h in cm, S in %)

What kind of flows might move the bed material?

What is grain size of the sediment supply compared to the bed?

* 0.03( )

cc

s gD

in a wide channel

o

o

gRSghS

0.5c D