Greg Jennings, PhD, PEProfessor, Biological & Agricultural EngineeringNorth Carolina State Universityjennings@ncsu.edu

BAE 579: Stream RestorationLesson 3: Stream Stability Assessment

Stream Stability

1. What is Stability?2. Causes of Instability3. 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

IIIIII

IV

I

I

IIII

I III

II

I

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

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 a

bove

Toe

Pin

(ft)

1/8/971/8/976/18/978/15/977/9/98

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