Methods

A predictive method will be developed from NBSS measurements and vegetation/channel properties from the flume data. The predictive method will be verified with field data from the prototype stream.

Hydraulic radiusSlope

Bank slopeBlockage factor

Vegetation densityFrontal Density

Relative submergence

NBSS

Near Bank Shear Stress Along Vegetated StreambanksL. Clark and T. Wynn

1. Field method to quantify NBSS on vegetated streambanks

2. Evaluation of velocity profiles along hydraulically rough streambanks

3. Insight into observed differences in stream width as a function of riparian vegetation type

4. Evaluation of spatial variation in NBSS in vegetated streams

5. Identification of relevant measurable vegetation properties which describe the influence of vegetation on NBSS estimates

6. Prediction of NBSS based on easily measurable vegetation properties

Expected Outcomes

Subaerial Processes

Fluvial Entrainment

Bank Failure

Freeze-thaw and wet-dry cycling

weaken soil

Soil entrained during high flows

Mass failure from slope instability

Streambank retreat can contribute up to 85% of total sediment yield2 and occurs due to a combination of three processes:

1. Determine a method for measuring NBSS in the field along hydraulically rough streambanks

2. Evaluate effects of streambank vegetation on NBSS

3. Develop predictive methods for estimating NBSS based on measurable vegetation properties

Goals and ObjectivesThis study will examine the theory and instruments used to measure vegetation influences on NBSS, following three specific objectives: References

1. ARS. 2003. Helping states slow sediment movement: a high-tech approach to clean water act sediment requirements. Agric. Res. Magazine 51(12): 12-14.

2. Wallbrink, P.J., A.S. Murray, and J.M. Olley. 1998. Determining sources and transit times of suspended sediment in the Murrumbidgee River, New South Wales, Australia, using

fallout 137Cs and 210Pb. Water Resources Res. 34(4): 879-887.Trimble, S.W. 1997. Contribution of stream channel erosion to sediment yield from an urbanizing

Watershed. Science 278: 1442-1444. Prosser, I.P., A. O. Hughes, and I. D. Rutherfurd. 2000. Bank erosion of an incised upland channel

by subaerial processes: Tasmania, Australia. Earth Surface Processes Landforms 25(10):1085- 1101.

3. Hanson, G. J., and A. Simon. 2001. Erodibility of cohesive streambeds in the loess area of the midwestern USA. Hydrological Processes 15: 23-38.

Damages associated with sediment cost $16 billion annually1.

Introduction

HerbaceousMixed (shrubs)

The upper reaches contain three distinct vegetation types:

Forested

Tom’s Creek in Blacksburg, VA was selected as the prototype steam.

Bed Surface Pebble Count, Tom's Creek-Forested Reach

silt/clay sand gravel cobble boulder

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.01 0.1 1 10 100 1000 10000

particle size (mm)

perc

ent

finer

than

0

5

10

15

20

25

num

ber o

f partic

les

cumulative % # of particles

Initial field measurements were collected to characterize the three stream reaches.

VegetationGrain

RoughnessTopography

A flume model will be created using the field measurements.

This study focuses on fluvial erosion predicted by the excess shear stress equation3, specifically on estimating near bank shear stress (NBSS) along vegetated banks.

where = erosion rate a = exponent (assumed 1) a = applied shear stress (NBSS)kd = soil erodibilityc = critical shear stress

= kd(a-c)a

Measurements will be made along a grid in the test section of the flume (n=20), and a method for measuring NBSS along vegetated streambanks will be determined.

Cross section View

Upper

Middle

Lower

Toe

Side View (of test section; assume 30o bank slope)

Bed

0.5 m

0.125 m

Bed

Bank

Cross sections1 2 3 4

Biological Systems Engineering Department, Virginia Tech, Blacksburg, VA

One model streambank of the prototype stream will be constructed for each experimental field reach.

90o 30o

D0.25 m

0.4 m

2*D0.07 m

Vegetation (wooden dowels, metal mesh and/or synthetic grass)

0.43 m

1 m x 6 m x 40 cm Test section, 1.5 m

Scaling will follow a Froude scale modeling (FSM) technique to construct model streambanks in the flume.

Research Flume

Near bank shear stress will be calculate by four methods, using three instruments to measure velocity. For this study, measurements made by a hot-film anemometer (CTA) will be considered representative of NBSS.

3D ADV

Mini-propeller

Pitot Tube

CTA

Average boundary shear stress

Reynolds Stresses

yu*/U=f (u*

=-u’v’

)

o

=C2w’2

Law of the Wall

Turbulent Kinetic Energy (TKE)

NBSS Theory Velocity Instruments

Valid field method for measuring NBSS

along vegetated streambanks

Develop scaled model and conduct flume study Instrument

TheoryField measurements of prototype

stream

Develop NBSS

prediction equation

Determine NBSS field

measurement method

Field testing

= f?

+ =?

+ =?