The Fluvial Geomorphic System

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• The Fluvial Geomorphic System
• Definition
• Variables of Stream Flow
• Hydrologic cycle
• Discharge
• Floods
• Effect of Slope, Hydraulic Radius
• Equilibrium in Streams
• Graded Stream
• Degradation
• Aggradation

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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 lt 1
• ? Volcanoes lt 1

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• The fluvial system encompasses
• ? Drainage divides,
• ? Source areas of water and sediment,
• ? Channels and valleys of the drainage basin,
• ? Depositional Areas

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Example watershed--sketch
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Example watershedon shaded relief map
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Example watershedtwo-dimensional
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Hydrologic cycle
Water budget/balance Inputs Outputs /-
Storage
Inputs?
precipitation
Outputs?
evapotranspiration
runoff
GW discharge
Storage?
Soil moisture Flooding aquifer storage
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Inputs Outputs /- Storage PCIP - (ET RO
GW) ?S PCIP - ET - RO - GW ?S PCIP ET
RO GW ?S
100
25-40
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Hydrologic cycle
Interception INT ET Evaporation
Infiltration PCIP RO INT ?S
100 25-40 60-75 0
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Discharge
Cross-sectional area and wetted perimeter
d
w
Area w x d Wetted perimeter w 2d
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Discharge
Cross-sectional area and wetted perimeter
2d
2w
Area 2w x 2d 4wd Wetted perimeter 2w
2(2d) 2w 4d
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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
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Discharge
Cross-sectional area and wetted perimeter

• Small increase in wetted perimeter (relative to
  increase in area) means less frictional
  resistance, water can flow faster (increased
  velocity)

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Discharge
Cross-sectional area and wetted perimeter
Result increased discharge (Q) is caused by
increases in width, depth and velocity
Q w x d x v
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Discharge
Q aQb x cQf x kQm a x c x k 1 b f m 1
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Floods
James River in Richmond, Virginia at flood stage,
November 1985. Photo by Rick Berquist, used
with permission.
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Floods
Hydrograph a plot of river level (or discharge)
versus time
Note equivalence of river elevation (stage) and
discharge
River Elevation
Time
Start of rainstorm
End of rainstorm
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Floods
Different watersheds display different hydrograph
characteristics
small stream
larger river in large watershed
River Elevation
Time
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Prior to urbanization
River Elevation
Time
Start of rainstorm
End of rainstorm
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Prior to urbanization
River Elevation
Time
Start of rainstorm
End of rainstorm
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prior to urbanization
Precipitation
after urbanization
Increased Runoff
Impervious Ground
Little Infiltration
River Elevation
Time
Start of rainstorm
End of rainstorm
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1993 Mississippi River Flood (500-year flood)
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1993 Mississippi River Flood (500-year flood)
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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_galler
y/great_floods/wetness.html
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dry soils
Precipitation
saturated soils
Increased Runoff
Impervious Ground
Little Infiltration
River Elevation
Time
Start of rainstorm
End of rainstorm
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Floods
Constructing a rating curve
Note equivalence of stage and discharge
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Example rating curve
Note that rating curve allows estimation of
discharge for extreme floods.
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Estimating stage level of past floods can then
use rating curve to estimate discharge
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Wayne Co. flood case
STEEP VALLEY WALL
WATTS HOME
WATER LEVEL, 11/12/03
RR TRACKS
FLOOD PLAIN
6-8 ft
5-7 ft
OLD CULVERT
BASE OF DITCH
17 ft
10-12 ft
Normal water level
TWELVEPOLE CREEK
NOT TO SCALE
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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
  (N59), and a stage
• level of 52 ft was the third highest level
  (m3) reached during this
• period, then a flood of this magnitude would
  be categorized as a 20-year
• flood (RI 60/3).

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Example of data used to calculate RI
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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
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Floods
James River in Richmond, Virginia at flood stage,
November 1985. Photo by Rick Berquist, used
with permission.
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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)
• August 21, 1969, 24.95 ft (gage height), 222,000
  cfs (discharge)
• 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

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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 Q752,000,000 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.

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Floods Paleofloods Example use of
paleoflood records to discern mid- Holocene
climates
Hirschboeck, 2003
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Q w x d x v
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

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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.

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Floods Paleofloods
Hirschboeck, 2003
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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

Wetted perimeter (WP) 2d w Area (A) wd R
A / WP wd / (2d w)
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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

Wetted perimeter (WP) 2d w Area (A) wd R
A / WP wd / (2d w)
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Relationships among channel shape, velocity and
erosional energy
Wetted perimeter (WP) 2d w Area (A) wd R
A / WP wd / (2d w)
WP 14 A 20 R 1.4
WP 22 A 20 R 0.9
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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

• 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
  !

WP 14 A 20 R 1.4
WP 22 A 20 R 0.9
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Relationships among channel shape, velocity,
slope and erosional energy
Hydraulic shear
WP 14 A 20 R 1.4
WP 22 A 20 R 0.9
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• low hydraulic radius
• high friction/turbulence
• high scour
• coarse bedload

• high hydraulic radius
• low friction/turbulence
• low scour
• fine bedload

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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 decreasescauses
  more dramatic increase in erosion.
• Scenario might occur as a result of uplift

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Sediment loadmass 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

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Controls on sediment load

• topographic relief
• geology of watershed
• climate
• vegetation
• other processes in watershed (glaciers,mass
  wasting, etc.)

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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?

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Total sediment load dissolved load (50?)
flotation suspended bed load-----------------
--------------------------suspended load
particles supported by water columnbedload
particles suspended by channel bed
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Mississippi River sediment
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As discharge increases, suspended load increases
at more rapid rate than discharge.
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• 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

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• 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.

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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 materialbraiding 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!

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