greg jennings, phd, pe professor, biological & agricultural engineering north carolina state...
TRANSCRIPT
Greg Jennings, PhD, PEProfessor, Biological & Agricultural EngineeringNorth Carolina State [email protected]
BAE 579: Stream RestorationLesson 3: Stream Stability Assessment
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
50%
10%
15%55%
Courtesy NEMO, Univ. of CT
Impervious Surfaces Across the Landscape
Roads
Parking
Buildings
Sidewalks
Driveways
Center for Watershed Protection
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
VI
IV
V
III
I
Original Nickpoint
III
IIIIV
I
I
IIII
III
I
II
I
I
G4c Alabama
Stream Channel Succession (WARSSS)
Stream Channel Succession (WARSSS)
http://www.epa.gov/WARSSS/sedsource/successn.htm
Entrenchment Ratio
Lateral Stability
Meander Width Ratio Bank Erosion (BEHI)
Meander Width Ratio
Wbkf Wblt
MWR = Wblt / Wbkf
Wbkf = Bankfull Width of Riffle Cross-Section
Wblt = Belt Width
Bank ErosionMonitoring
Harris Reach, South Fork Mitchell River X-Sec 7+65
0
2
4
6
8
10
0 4 8 12 16 20 24 28 32 36
Distance from Toe Pin (ft)
Ht
ab
ov
e T
oe
Pin
(ft
)
1/8/97
1/8/97
6/18/97
8/15/97
7/9/98
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
Extreme RiskHigh Risk
Very Low Risk
Near Bank Stress
Low
Near Bank Stress
Moderate
Near Bank Stress
High
Near Bank Stress
Very High
