Methods A predictive method will be developed from NBSS measurements and vegetation/channel...
1
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 radius Slope Bank slope Blockage factor Vegetation density Frontal Density Relative submergence NBSS Near Bank Shear Stress Along Vegetated Streambanks L. 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 Subaeria l Processe s 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 yield 2 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 Objectives This 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 annually 1 . Introduction Herbaceou s Mixed (shrubs) The upper reaches contain three distinct vegetation types: Foreste d Tom’s Creek in Blacksburg, VA was selected as the prototype steam. Bed Surface Pebble C ount, Tom 's C reek-Forested R each 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 (m m) p erce nt fine r t 0 5 10 15 20 25 num b er of pa rti cum ulative % # ofparticles Initial field measurements were collected to characterize the three stream reaches. Vegetation Grain Roughness Topography A flume model will be created using the field measurements. This study focuses on fluvial erosion predicted by the excess shear stress equation 3 , specifically on estimating near bank shear stress (NBSS) along vegetated banks. where = erosion rate a = exponent (assumed 1) a = applied shear stress (NBSS) k d = soil erodibility c = critical shear stress = k d ( 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 Midd le Lowe r Toe Side View (of test section; assume 30 o bank slope) Bed 0.5 m 0.125 m Bed Bank Cross sections 1 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. 90 o 30 o D 0.25 m 0.4 m 2*D 0.07 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 =C 2 w’ 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 Instrumen t Theory Field measurement s of prototype stream Develop NBSS predictio n equation Determine NBSS field measuremen t method Field testing = f ? + = ? + = ?
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Transcript of Methods A predictive method will be developed from NBSS measurements and vegetation/channel...
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?
+ =?
+ =?
