Transcript The Fluvial Geomorphic System Variables of Stream Flow Equilibrium in Streams

The Fluvial Geomorphic System
• Definition
• Variables of Stream Flow
•
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Hydrologic cycle
Discharge
Floods
Effect of Slope, Hydraulic Radius
• Equilibrium in Streams
• Graded Stream
• Degradation
• Aggradation
The Fluvial Geomorphic System
How is sediment transported and removed from continents?
(i.e., what mechanisms are most important
in shaping landscapes?)
► Rivers: 85-90%
► Glaciers: 7%
► Groundwater & Waves: 1-2%
► Wind: < 1%
► Volcanoes: < 1%
The fluvial system encompasses:
► Drainage divides,
► Source areas of water and sediment,
► Channels and valleys of the drainage basin,
► Depositional Areas
Example watershed--sketch
Example watershed—on shaded relief map
Example watershed—two-dimensional
Hydrologic cycle
Water budget/balance:
Inputs – Outputs = +/- Storage
Inputs?
precipitation
Outputs?
evapotranspiration
runoff
GW discharge
Storage?
Soil moisture
Flooding
aquifer storage
Inputs – Outputs = +/- Storage
PCIP - (ET + RO + GW) = ΔS
PCIP - ET - RO - GW = ΔS
100%
25-40%
PCIP = ET + RO + GW + ΔS
Hydrologic cycle
Interception = INT = ET + Evaporation + Infiltration
100% 25-40% 60-75% 0%
PCIP = RO + INT + ΔS
Discharge
Cross-sectional area and wetted perimeter
d
w
Area = w x d
Wetted perimeter = w + 2d
Discharge
Cross-sectional area and wetted perimeter
d
w
2d
2w
Area = 2w x 2d = 4wd
Wetted perimeter = 2w + 2(2d) = 2w + 4d
Discharge
Area A = wd
Area B = 2w x 2d = 4wd
Area B / Area A = 4wd / wd = 4
----------------------------------------------------------Wetted perimeter A = w + 2d
Wetted perimeter B = 2w + 2(2d) = 2w + 4d
Wetted perimeter B = 2(w + 2d)
Wetted perimeter B / Wetted perimeter A =
2(w + 2d) / (w + 2d) = 2
Discharge
Cross-sectional area and wetted perimeter
d
w
• Small increase in wetted perimeter (relative to increase in area) means
less frictional resistance, water can flow faster (increased velocity)
Discharge
Cross-sectional area and wetted perimeter
Result: increased discharge (Q) is caused by increases in width,
depth and velocity
Q=wxdxv
W
D
V
Discharge
Q = aQb x cQf x kQm
axcxk=1
b + f + m =1
Floods
James River in Richmond, Virginia at flood stage,
November 1985. Photo by Rick Berquist,
used with permission.
Floods
River Elevation
Hydrograph: a plot of river level (or discharge) versus time
Note equivalence of river elevation (stage) and discharge
Time
Start of rainstorm
End of rainstorm
Floods
River Elevation
Different watersheds display different hydrograph characteristics
small stream
larger river in large watershed
Time
Prior to urbanization
Precipitation
Runoff
Ground
River Elevation
Infiltration
Time
Start of rainstorm
End of rainstorm
Prior to urbanization
Precipitation
Runoff
Ground
River Elevation
Infiltration
Time
Start of rainstorm
End of rainstorm
prior to urbanization
Precipitation
after urbanization
Increased Runoff
Impervious
Ground
River Elevation
Little Infiltration
Time
Start of rainstorm
End of rainstorm
1993 Mississippi River Flood (500-year flood)
1993 Mississippi River Flood (500-year flood)
1993 Mississippi River Flood (500-year flood)
Soil Moisture
(brighter = wetter)
June 6, 1993
July 29, 1993
July 15, 1993
http://www.cgrer.uiowa.edu/research/exhibit_gallery/great_floods/wetness.html
dry soils
Precipitation
saturated soils
Increased Runoff
Impervious
Ground
River Elevation
Little Infiltration
Time
Start of rainstorm
End of rainstorm
Floods
Constructing a rating curve
Note equivalence of stage and discharge
Example rating curve
Chester IL Rating Curve
( based on annual peak flows )
1.0E+07
Q (cfs)
1.0E+06
1.0E+05
1.0E+04
0
10
20
30
40
50
60
Stage (ft)
Note that rating curve allows estimation of discharge for extreme floods.
Estimating stage
level of past floods—
can then use
rating curve to
estimate discharge
Wayne Co. flood case
STEEP VALLEY WALL
WATTS HOME
WATER LEVEL,
11/12/03
FLOOD PLAIN
RR TRACKS
~6-8 ft
~5-7 ft
BASE OF DITCH OLD CULVERT
~10-12 ft
~17 ft
Normal water level
TWELVEPOLE CREEK
NOT TO SCALE
Flood Intensity
100.00
Q / Area
10.00
1.00
0.10
0.01
1.E+00
1.E+01 1.E+02
1.E+03
1.E+04
Area
1.E+05 1.E+06
1.E+07
Floods
Recurrence interval (RI) is the average number of years between
a flood of a given magnitude.
• For example: the 100-year flood is the stage or discharge that occurs
on average every 100 years.
• Different for every river.
• Data less reliable for larger RI. Why?
• RI = (N +1) / m
• N = # of years of record , m = rank
• Example: If records were kept for 59 years (N=59), and a stage
level of 52 ft was the third highest level (m=3) reached during this
period, then a flood of this magnitude would be categorized as a 20-year
flood (RI = 60/3).
Example of data used to calculate RI
Chester IL -- Rank versus Year
Year
1930
0
10
20
Rank
30
40
50
60
70
80
1940
1950
1960
1970
1980
1990
2000
2010
Miss. River, Chester, Il – 1993
Note that the probability of a flood of a given
magnitude is 1/RI.
Example: In any year, the chance of a100-year flood is 1/100 = 1%
The mean annual flood is the average of the maximum
annual floods over a period of years.
RImean = 2.33
Chester IL Rating Curve
( based on annual peak flows )
1.0E+07
Q (cfs)
1.0E+06
1.0E+05
1.0E+04
0
10
20
30
Stage (ft)
40
50
60
Floods
James River in Richmond, Virginia at flood stage,
November 1985. Photo by Rick Berquist,
used with permission.
Flood Exercise
James River, Richmond VA
Three largest floods recorded from 1935 to present.
1. June 23, 1972, 28.62 ft (gage height), 313,000 cfs (discharge)
2. August 21, 1969, 24.95 ft (gage height), 222,000 cfs (discharge)
3. November 7, 1985, 24.77 ft (gage height), 218,000 cfs (discharge)
• From the picture of the river at normal flow, estimate the stage
at these conditions.
• Calculate RI and probability for each of these flood events
Floods
Paleofloods
• Causes: dam outbursts, glacial outbursts, extreme precipitation events.
• ice dam collapsed during last Ice Age in eastern Washington, emptying
lake about half size of Lake Michigan; floodwaters had Q~752,000,000 cfs.
• Provide direct evidence of extreme hydrologic events that may shed light
back to mid-Holocene (~5,000 years)
• Flood deposits and flood erosional effects are primary sources of
information about the magnitude and frequency of these extreme events.
Floods
Paleofloods
Example use of paleoflood records to discern midHolocene climates
Hirschboeck, 2003
Q=wxdxv
Floods
Paleoflood
“reconstruction”
• What is needed to estimate discharge, Q, during a modern flood?
•Rating curve allows Q to be estimated from stage
• What is needed to estimate discharge during a paleoflood?
• flood stage may not be known
• If flood stage is known, no rating curve for extreme stage
• velocity must be estimated and ancient valley shape must be estimated
Floods
Paleoflood
“reconstruction”
Methods for estimating stage of paleofloods
• depositional: slack-water deposits in tributary valleys, caves, etc.
• slack-water deposits formed during sudden velocity decreases following peak
discharge
• only preserved in protected areas above elevation of modern floods
(“non-exceedance level”
• erosional: terrace benches, markings on paleosols, bedrock walls, etc.
• vegetation: damaged trees, etc.
Floods
Paleofloods
Hirschboeck, 2003
Floods
Paleoflood
“reconstruction”
Methods for estimating velocity of paleofloods
•quantitative empirical or theoretical relationships
• Chezy formula: uses hydraulic radius and slope to estimate velocity
• Use sizes of boulders transported in flood to estimate velocity
• Manning equation: uses hydraulic radius and slope to estimate velocity:
v = 1.49/n x R2/3 x S1/2
n = roughness factor
R = hydraulic radius
S = slope
d
d
w
Wetted perimeter (WP) = 2d + w
Area (A) = wd
R = A / WP = wd / (2d + w)
Relationships among channel shape,
velocity, slope and erosional energy
Manning equation: relates hydraulic radius and slope to velocity
v = 1.49/n x R2/3 x S1/2
n = roughness factor
R = hydraulic radius
S = slope
d
d
w
Wetted perimeter (WP) = 2d + w
Area (A) = wd
R = A / WP = wd / (2d + w)
Relationships among channel shape,
velocity and erosional energy
Wetted perimeter (WP) = 2d + w
Area (A) = wd
R = A / WP = wd / (2d + w)
2
1
10
20
WP = 14
A = 20
R = 1.4
WP = 22
A = 20
R = 0.9
Relationships among channel shape,
velocity and erosional energy
What does Manning equation say about flow in these two different
channel shapes if slope and roughness are equal?
v = 1.49/n x R2/3 x S1/2
n = roughness factor
R = hydraulic radius
S = slope
2
•larger radius means greater velocity.
• smaller radius means less velocity.
• Tendency of smaller radius to restrict velocity is
result of turbulence and friction as water contacts
the channel margins. This causes erosion !
1
10
WP = 14
A = 20
R = 1.4
20
WP = 22
A = 20
R = 0.9
Relationships among channel shape,
velocity, slope and erosional energy
Hydraulic shear
2
1
10
20
WP = 14
A = 20
R = 1.4
WP = 22
A = 20
R = 0.9
• low hydraulic radius
• high friction/turbulence
• high scour
• coarse bedload
• high hydraulic radius
• low friction/turbulence
• low scour
• fine bedload
Relationships among slope, velocity and
erosional energy
•Increased discharge causes increase in depth,
width and velocity--causes moderate increase
in erosion.
•Scenario might occur as a result of climate
change
•Increased slope at constant discharge means
velocity increases, but depth decreases—
causes more dramatic increase in erosion.
• Scenario might occur as a result of uplift
Sediment load:
mass of sediment transported in
a stream or river per unit time
example: pounds per year
Related concepts:
• denudation rates (example: ft/1000 yrs)
• sediment yield = sediment load / area
Controls on sediment load
•topographic relief
•geology of watershed
•climate
•vegetation
•other processes in watershed
(glaciers,mass wasting, etc.)
Sediment load depends on:
• relief – denudation
rates increase
exponentially with relief
of watershed.
• vegetation – sediment
yield is at maximum for
about 10 in/yr of
precipitation. Why?
Total sediment load =
dissolved load (50%?) + flotation +
suspended + bed load
------------------------------------------suspended load: particles supported by water column
bedload: particles suspended by channel bed
Mississippi River sediment
As discharge increases,
suspended load increases at
more rapid rate than discharge.
Channel patterns
• straight (rare)
• meandering (most common)
• single channel
• sinuous (Ls / Lv)
• few islands
• deep, narrow channels
• meander size proportional to Q,
maybe load
• braided
• low sinuosity
• multiple, shifting channels
• islands
• wide, shallow channels
Causes of meandering
• laminar flow tends not to be
maintained, so water is
deflected, energy is distributed
unequally in channel.
•cut banks
•point bars
• positive feedback system
• more meandering results in
wider valleys, bigger
floodplains.
Braided Streams
• temporary, shifting channels have prompted conclusion
that braided streams are overloaded with sediment and,
in response, are aggrading.
• In fact, braiding is related to erodabilty of bank
material—braiding seems to develop in easily- erodable
(non-cohesive) sediments (i.e., sand & gravel). See
Figures 5-36 & 5-38
• Higher silt/clay ratios of load mean lower W/D ratios,
development of helical flow, resistance of banks to erosion, and
meandering channel patterns.
• Lower silt/clay ratios of load mean higher W/D ratios, absence of
helical flow, erosion of banks, and braided channel patterns.
• Change in silt/clay to sand/gravel bank materials may result in a
change in channel shape from meandering to braided-- will
mean an increase in slope.
– Why?
– But change in slope is a response to change in channel shape,
not a cause of braiding!