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Some General Sediment ConceptsJuly 29, 2019
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: incorporating sediment in channel assessment & designadd geomorphic & design concepts channel/floodplain design exercise
Friday: present & evaluate designs, visit field site
Sediment transport application to urban streamsapplication to non‐urban streams
Monitoring and modeling sediment transport Sediment Transport in stable channel designWhen and how to apply sediment transport analysisI have lots of RAS ‐ how do I bring in sediment?How to deal with ST uncertainty in assessment & design
Agency 5
Industry 9
Student 4
Geoscience 9
Ecology 1
Civil Engineering 9
What is the supply of water and sediment to a stream & what do you want to do with it?
I. What flow moves the bed material? How often does it occur?
II. What is the sediment balance?
Discharge QBed material DChannel Geometry
HydraulicsIncipient Motion
Discharge QBed material DChannel GeometrySediment Supply Qs & D
HydraulicsTransport Capacity Qs & D
Input ‐ Output = StorageSurplus or Deficit?
Stored sediment is the real geomorphic and restoration topic!
Wha
t is the
supp
ly of w
ater and
sedimen
t?What do you w
ant to do with it?
Flow Competence
Transport Capacity vs Sediment Supply
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?
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
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?
Both !
…
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
o = hS (h in cm, S in %, o in Pa)
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 goes the depth. Bed shear incrases also.Will sediment move? Is > ? Incipient motion - critical discharge - flow competenceHow fast will sediment
o
o
QghS
cmove? does exceed ? The transport rate is transport capacity - match against rate of sediment supply
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 depth of flow is neededto move the bed material? What discharge produces this flow depth?
What is grain size of the sedimentsupply compared to the bed?
* 0.03( )
cc
s gD
in a wide channel
o
o
gRSghS
0.5c D