BAE 579: Stream Restoration
Lesson 3: Stream Stability Assessment
Greg Jennings, PhD, PE
Professor, Biological & Agricultural Engineering
North Carolina State University
jennings@ncsu.edu
Stream Stability
1. What is Stability?
2. Causes of Instability
3. Stability Assessment
What is a “Stable” Stream?
“Graded” Stream: Condition of “balance between erosion and
deposition attained by mature rivers” (Davis, 1902)
“Dynamic Equilibrium”: Stream form & character unchanged
while continuous inflow of water/sediment (Strahler, 1957)
“Equilibrium Controlling Factors”: Width, depth, velocity, slope,
discharge, sediment size, sediment concentration, channel
roughness (Leopold, 1964)
“Regime Channels”: Some erosion and deposition but no net
change in dimension, pattern, and profile (Hey, 1997)
“Stream Channel Stability”: ability of a stream, over time, in the
present climate, to transport the sediment and flows
produced by its watershed in such a manner that the
stream maintains its dimension, pattern, and profile without
either aggrading or degrading (Rosgen, 1996)
Lane’s Stream Balance Relationship
Causes of Instability
•
•
•
•
•
•
Increase runoff
Increase slope
Changes in sediment load
Loss of riparian buffer
Floodplain filling
Instream modification
Increase Runoff: Land Use Changes
Development Impacts on the
Water Cycle
10%
50%
Courtesy NEMO, Univ. of CT
55%
15%
Impervious Surfaces Across the Landscape
Center for Watershed Protection
Sidewalks
Roads
Driveways
Parking
Buildings
The Science of Stormwater…
More Runoff
Arriving Faster
NEMO
Hydrologic Responses
to Urbanization
1. Increased discharge
2. Increased peak discharge
3. Increased velocities
4. Shorter time to peak flow
5. More frequent bankfull events
6. Increased flooding
7. Lower baseflow
8. Less ground water recharge
Increase Slope
• Channelization
• Lower Reservoir
Water Surface
• Dam Break
• Geologic Uplift
Changes in
Sediment Load
•
•
•
•
Development
Agriculture
Bank Erosion
Impoundments
Aggradation
Mid-channel bar and transverse bar directing flow
into a high bank causing erosion and slumping
Debris Occurrence
 Large Woody
Debris
 Depends on
Riparian Stability
 Beaver Dams
Impoundments
Upstream:
• Decrease
velocities
• Deposition of fine
material
• Loss of habitat
Downstream:
• “Hungry water”
• Change in flow
Riparian Condition (Buffer)
 Composition
 Density
 Potential
Values of Vegetation
•
•
•
•
Habitats
Water Quality
Bank Stability
Shade & Food
Stream Corridor Restoration: Principles, Processes, and Practices, 10/98, by the
Federal Interagency Stream Restoration Working Group (FISRWG)."
Streamside Forests
(also known as riparian buffers)
• Trees, shrubs, herbs, and
grasses are critical to the
health of streams
• Buffers are the first line of
defense against the
impacts of polluted runoff
• natural vegetation buffers
are especially critical in
urban areas
Altered States Due to Disturbance





Channelization
Straightening
Levees
Hardening
Mining
Floodplain
Filling
Simon
Channel
Evolution
Model
Source:
Simon, 1989,
USACE 1990
Head Cut
I
I
I
I
II
Original Nickpoint
VI
I
III
IV
V
I
III
IV
I
II
I
I
I
III
I
G4c Alabama
Stream Channel Succession (WARSSS)
Stream Channel
Succession
(WARSSS)
http://www.epa.gov/WARSSS/
sedsource/successn.htm
Stability Assessment
http://www.epa.gov/warsss/index.htm
Entrenchment Ratio
Lateral Stability
 Meander Width Ratio
 Bank Erosion (BEHI)
Meander Width Ratio
MWR = Wblt / Wbkf
Wblt = Belt Width
Wbkf = Bankfull Width of Riffle Cross-Section
Wbkf
Wblt
Bank Erosion
Monitoring
Harris Reach, South Fork Mitchell River
X-Sec 7+65
Ht above Toe Pin (ft)
10
8
6
1/8/97
4
1/8/97
6/18/97
8/15/97
7/9/98
2
0
0
4
8
12
16
20
24
28
Distance from Toe Pin (ft)
32
36
Bank Erosion Hazard Index (BEHI)
Bank Erosion Hazard Index (BEHI)
Bank Erosion Hazard Index (BEHI)
Bankfull Stage:
Water fills the active channel
and begins to spread onto the floodplain
Stream Corridor Restoration: Principles, Processes, and Practices. 1998. Federal
Interagency Stream Restoration Working Group.
Bank Height Ratio
Very Low
Bank Height Ratio
Moderate
Bank Height Ratio
Very High
Root Depth Ratio
Very Low BEHI
Root Depth Ratio
Moderate BEHI
Root Depth Ratio
Very High BEHI
Root Density
Very Low BEHI
Root Density
Moderate
Root Density
Very High BEHI
Bank Angle
(from horizontal)
Bank Angle
Very Low
Bank Angle
Moderate - High
Bank Angle
Very High
Surface Protection
Very Low BEHI
Surface Protection
Moderate
Surface Protection
Very High BEHI
Bank Materials
Very Low Risk
High Risk
Extreme Risk
Near Bank Stress
Low
Near Bank Stress
Moderate
Near Bank Stress
High
Near Bank Stress
Very High