basic hydrology water quality: sediment production and transport
TRANSCRIPT
Basic HydrologyBasic Hydrology
Water Quality:
Sediment production and transport
Sediment in streamsSediment in streams The significance of sediment in streams has two
aspects– the water quality aspect is related to suspended
sediment• unwanted suspended sediment can cause domestic water
quality problems, and can cause physical damage to fish
– supply and movement of coarse sediment in streams is a channel morphology issue
• excessive coarse sediment in streams can adversely affect fish habitat and fill in water control structures
Sediment movement in streamsSediment movement in streams Sediment movement in stream channels can be
categorized by grain size or by the process that transports it– suspended sediment consists of fine particles of silt
and clay that are carried by the water column• suspended sediment movement is independent of flow rate
– wash load is a term that refers to sediment that is carried by the water column, that has grain size distribution smaller than the channel bed
– saltating bed load• consists of fine to medium sand • movement is dependent on the velocity of flow: at low flow it
may not move, but at high flow it may bounce high up into the water column
– behaviour may be similar to suspended sediment for fine sand at high flow
– bed load consists of coarse sand, gravel and larger sized particles that are transported by rolling along the channel bed.
• bed load transport is also dependent on flow rate• there is a threshold flow velocity needed to get it moving
Grain size classesGrain size classessize class diameter range (mm) micronsgravel 2.00 +v. coarse sand 1.0 - 2.0coarse sand 0.5 - 1.0medium sand 0.25 - 0.5 250 - 500fine sand 0.125 - 0.25 125 - 250v. fine sand 0.0625 - 0.125 63 - 125coarse silt 0.031 - 0.0625 31 - 63silt 0.0039 - 0.031 3.9 - 31clay < 0.0039
Sediment production vs. Sediment production vs. transporttransport
Sediment production refers to the addition of new sediment to the channel system– landslides– surface erosion– channel bank erosion
Sediment transport refers to the movement of sediment that is already in the channel system
Types of sediment transport systemsTypes of sediment transport systems Supply limited
– most rivers are supply limited in terms of sediment production
– the stream has the ability to move all the sediment that is supplied to it, hence sediment transport is limited by the supply
– sediment transport tends to exhibit pulse behaviour, sediment movement occurs mostly on the hydrograph rising limb
– hysteresis occurs where sediment concentration for a given flow is greater for rising than falling streamflow
Suspended sediment pulse, Suspended sediment pulse, Russell CreekRussell Creek
11/19/91
11/20/91
11/21/91
11/22/91
11/23/91
11/24/91
0
10
20
30
40
Dis
char
ge
(m3/
s)
0
100
200
300
Su
spen
ded
Sed
imen
t (m
g/l)
SS concentration vs. dischargeSS concentration vs. discharge
0 10 20 30 40Discharge (m 3/s)
0
100
200
300S
usp
end
ed S
edim
ent
(mg
/l) 19 - 20 November 1991Russell CreekSupply limited
Rising
Falli
ng
Spray River near Banff, 1973Spray River near Banff, 1973
4/30/73 5/31/73 7/1/73 8/1/73 9/1/73
0
5
10
15
Str
eam
flo
w (
m3/
s)
0
100
200
300
400
Su
spen
ded
Sed
imen
t (m
g/l)
Supply limited
Sediment transport typesSediment transport types Transport limited
– transport limited sediment transport is far less common than supply limited
– occurs when the sediment supply to the stream is in excess of the ability of the stream to transport it
• occurs in braided and anastamosed rivers where sediment supply is high
– as a result, more sediment moves on the falling limb than the rising limb of the hydrograph
– hysteresis occurs where sediment concentration for a given flow is greater for falling than rising streamflow
– large rivers can exhibit both types of transport behaviour
Dual behaviour of large riversDual behaviour of large rivers Large rivers tend to be transport limited during freshet
periods– sediment concentrations tend to be high during
spring freshet because • flow is low - sediment is “concentrated”• sediment production occurs as a result of supply of
sediment accumulated over winter that enters the system all at once
As flows increase, they become supply limited– sediment transport capability increases
Fraser River at Mission, 1983Fraser River at Mission, 1983
3/31/83 4/30/83 5/30/83 6/29/83 7/29/83
0
2000
4000
6000
8000
10000
Str
eam
flo
w (
m3/
s)
0
100
200
300
400
Su
spen
ded
Sed
imen
t (m
g/l)
Transport vs. supply limitedTransport vs. supply limitedFraser River 1983Fraser River 1983
0 2000 4000 6000 8000 10000Stream flow (m 3/s)
0
100
200
300
400
Su
spen
ded
Sed
imen
t (m
g/l)
transport limitedApril
supply limitedMay - June
risi
ng
fall
ing
rising
falli
ng
Columbia River 1976Columbia River 1976somewhere south of Goldensomewhere south of Golden
3/31/76 4/30/76 5/30/76 6/29/76 7/29/76 8/28/76
0
1000
2000
3000
Str
eam
flo
w (
m3/
s)
0
200
400
600
800
Su
spen
ded
Sed
imen
t (m
g/l)transport limited
event
May 1976 Columbia RiverMay 1976 Columbia River
200 400 600 800Streamflow (m 3/s)
0
100
200
300
400
Su
spen
ded
Sed
imen
t (m
g/l)
Ris
ing
Falli
ng
Measurement of suspended sed.Measurement of suspended sed. Direct method: sample analysis
– a sample of stream water is collected using a manual DH48 depth integrated sampler or an automatic sampler such as ISCO 3700
– the sample is filtered through a 2 m filter to determine sediment concentration in mg/l
Indirect method: turbidity is measured and interpreted as SS conc. by field and lab calibration
Sediment production processesSediment production processes rain splash erosion
– sheet wash erosion - surface runoff
freeze-thaw erosion– expansion and contraction of erodible rock surfaces
– frost expansion
– needle ice development
animal movement ravelling of loose sediment surfaces undercutting and slumping of loose surfaces mass wasting
Factors controlling sediment Factors controlling sediment productionproduction
Cause of runoff– rainfall produces more sediment than an
equivalent volume of snowmelt• rain erosion on exposed sediment sources vs. water
yield from snowpack that blankets the exposed sed.
• rainfall produces larger peaks than streamflow, hence greater sediment transport and channel scour
Size of storm– larger storms generally produce more sediment
• larger yield, and greater peak concentration
Controlling factorsControlling factors Watershed morphology
– lithology: softer more erodible bedrock (e.g., basalt - Catherine Cr.) produces more sediment than harder more resistant bedrock (e.g., granite - Russell Creek)
– type and connectivity of sediment sources• debris flow deposits are big producers
• hillside sediment sources– landslide scars, road cuts
– slope stability
Controlling factorsControlling factors Land use, e.g. logging and related activities
– related to other factors such as slope stability and surface erosion from road surfaces and cut and fill slopes
• increase in mass wasting processes following logging
• yarding disturbance
– potential increase in streamflow after logging • increase in sediment transport capability
• increase in stream channel and bank erosion
Sediment Production at Russell Cr.Sediment Production at Russell Cr.
0 5 10 15 20 25M axim um 3-hour R ain In tensity (m m )
0
1
10
100
1000
Pea
k S
uspe
nded
S
edim
ent C
onc.
(m
g/l)
0 20 40 60M axim um 12-hour R ain In tensity
1 0
1 0 0
1 0 0 0
1 0 0 0 0
1 0 0 0 0 0
1 0 0 0 0 0 0
Tot
al S
edim
ent Y
ield
(kg/
stor
m)
How does logging affect sediment How does logging affect sediment production?production?
Studies show that increased sediment after logging is related to logging roads– surface erosion from road surfaces
– road related landslides
Open slope failures increase after logging If soil disturbance is minimized, clear-cuts are not
sediment sources– burning
– ground skidding