Post on 07-Apr-2020
Designing channels for stream restorationA workshop hosted by Oklahoma State University
June 2, 2016
Arcadia Educational Center
Session 2: Channel design and threshold channels
Doug Shields, Jr. www.friendofrivers.com
Workshop overview
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Doug Shields, Jr. www.friendofrivers.com
• Review of recent research on stream restoration efficacy
• Design for three types of channels• Threshold
• Type I--Bed material immobile at design Q, negligible sediment supply
• Type II—transport capacity exceeds sediment supply but design Q will not erode channel boundary
• Alluvial• Bed material gradation and sediment supply gradation are similar
• Transport capacity = sediment supply
• Available information resources and tools
• Example problems 2
Channel design—two philosophies
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Doug Shields, Jr. www.friendofrivers.com
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Different ways of looking at the same thing
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Doug Shields, Jr. www.friendofrivers.com
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Actually, five design approaches for channels
• Regime method (British engineers, canals, sand beds, Cs < 30 mg/L)
• Hydraulic geometry (Leopold and others)
• Analogy or reference reach (NCD, Rosgen)
• Analytical method (Copeland, SAM, HEC-2)
• Extremal hypothesis (Yang and others)
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Doug Shields, Jr. www.friendofrivers.com
Also see NRCS (2007a)5
Comments on the five approaches (SUNY Buffalo)
• Regime Method: Dependent channel dimensions can be determined from regression relationships with independent variables
• Hydraulic Geometry Method: Dependent channel dimensions can be determined from regression relationships with independent variables
• Analogy Method: Channel dimensions from a reference reach can be transferred to another location
• Analytical Method: Depth and sediment transport can be calculated from physically-based equations
• Extremal Hypotheses: Alluvial channels will adjust channel dimensions so that some parameter is optimized
Doug Shields, Jr. www.friendofrivers.com
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Regime equations (Blench)
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Limitations on regime (Blench)
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Examples of hydraulic geometry
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Hydraulic geometry relation for width based on 32 sand-bed rivers with less or more than 50% tree cover on banks
wBF = aQBFb
a=5.19 (T1)a=3.31 (T2)b=0.5
Another example of hydraulic geometry
Doug Shields, Jr. www.friendofrivers.com
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Channel design guidance recognized by the engineering profession—”analytical method”
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Doug Shields, Jr. www.friendofrivers.com
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Slate et al. (2007)
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“This committee proposes that stream restoration channel design standards be based on measurable criteria supported by current peer-reviewed scientific publications. Design goals should be clearly defined and based on general physical principles, rather than referenced to an empirically defined equilibrium state. More specifically, the questions “What is the supply of water and sediment?” and “What do you want to do with them?” provide a sound framework for restoration design (Wilcock 1997), leading to consideration of physical, ecological and management objectives on a consistent and quantitative basis through space and time. In the absence of more refined materials, the authors support the work previously published by members of the River Restoration Committee (Shields et al. 2003) and suggest using this work as a starting point for channel design standards.”
Doug Shields, Jr. www.friendofrivers.com
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Thoughts on training and credentials
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• Survey of existing training• Practitioner survey
• Lists of disciplines, subjects, skills needed
• Level of mastery required to perform various functions
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Similar content
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An analytical approach to channel design
• This science developed to produce channel geometries that were “stable.”
• “A stable channel is often defined as a channel where the planform, cross section, and longitudinal profile are sustainable over time. A natural channel can migrate en still be considered stable in that its overall shape and cross-sectional area do not change appreciably.”
• Two problems
• Stable channels are rare
• Stable channels are poor habitat
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Analytical method background• Grew out of engineering practice
for design of man made channels—canals, diversions, aqueducts, etc.
• “Stable channel design”
• Channels which were sized (mean W, D, S) to convey the sediment supplied to them but not erode their boundaries
• Largely based on physics-based analysis of forces acting on channel and sediment transport
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But stabilization is not restoration“Strategies for restoration projects often include promoting higher levels of physical dynamism (e.g., flooding, erosion, and deposition) in streams that have been dammed, leveed, or channelized…. Restoring a channel to a state of dynamic equilibrium may not be a socially acceptable outcome if the resulting situation poses threats to riparian resources or infrastructure. …a tension often exists between the dynamism needed for ecological objectives and erosion and flood control interests.”
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Wilcock approach
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Courtesy Peter Wilcock, Utah State Univ
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Wilcock approach
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Courtesy Peter Wilcock, Utah State Univ
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The problem (in general)• Given Q, Qs, ds
• Find W, D, S for a channel reach
• Constraint: the resulting channel will not aggrade or erode its bed or banks
• Three equations
• Flow resistance (Manning)
• Momentum (shear stress)
• Sediment transport
• The last two equations introduce an addition unknown variable, t
• Since there are three equations and four unknowns, the result is a “family” of solutions
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Finding the “givens”• Sv, Wmax, Q, Qs, ds, tc*
• Sv is the valley slope—and thus is the maximum channel slope
• Wmax is the maximum channel top width allowed by site constraints
• Q is the discharge that fills the designed channel• Threshold—what do you want it to be?
• Alluvial--“channel forming discharge”
• Qs is the bed-material load supplied to the design reach from upstream when Q = Qs
• ds, tc*
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Bed sediment size
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• Should be the sediment size in the constructed channel bed• Take borings?• Consider sediment inflows• Natural armoring• Constructed armor or linings
• Not the bank sediment size• Not the current (existing) channel
sediment size• Not the sediment load size• Usually one or more “characteristic”
sizes (d50, d84, d75)• Design is very sensitive to ds
Doug Shields, Jr. www.friendofrivers.com
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Sediment supply• For design, we need Qs,
sediment rate (dimensions of mass/time) when water discharge = Qcf
• To check our design for stability, we need a sediment rating curve Qs = f(Q)
• Rarely measured• Usually must be computed• Computations typically
require special training and skills Sh
ield
s En
gin
eeri
ng
LLC
Doug Shields, Jr. www.friendofrivers.com
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What value to use for tc*• Rep > 500 0.03–0.06
• 4000 < Rep <100000 0.01–0.09 Buffington and Montgomery (1997), coarse-bedded steep streams
• Buffington and Montgomery (1997) attribute the variability to differences in bed stability (or bed mobility) as well as differences in computations and field methods.
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Bunte et al. Water Resources Research 49.11 (2013): 7427-7447.
Doug Shields, Jr. www.friendofrivers.com
Lots of correction factors, etc. to compensate for the differences
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The ultimate wild card
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Analytical design for three types of channels
• Threshold ( t < tc)
• This tc is critical shear stress to mobilize bed sediment particles
• Qs in ~ 0 (Type I)
• Qs in > 0 but ttransport< t < tc (Type II)
• Alluvial ( t >>> tc)
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All three types must obey the same laws of physics.
Doug Shields, Jr. www.friendofrivers.com
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Workshop overview
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• Review of recent research on stream restoration efficacy
• Design for three types of channels• Threshold
• Type I--Bed material immobile at design Q, negligible sediment supply
• Type II—transport capacity exceeds sediment supply but design Q will not erode channel boundary
• Alluvial• Bed material gradation and sediment
supply gradation are similar• Transport capacity = sediment supply
• Available information resources and tools• Example problems
Doug Shields, Jr. www.friendofrivers.com
NRCS (2007b)27
Threshold channels
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• Channel boundary material has no significant movement during design flow
• Bed is composed of o Erosion resistant bedrock or cohesive materialo Coarse material too large to be transported by
flowo Grass, riprap, pavement….
• Examples includeo Bed material formed by glacier outflows or dam breakso Channels downstream from reservoirs with greatly reduced peak dischargeso Channels incised into erosion resistant stratao Channels designed and constructed with immobile boundaries
• Can a sand bed channel be a threshold channel?
Doug Shields, Jr. www.friendofrivers.com
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Examples of threshold channels
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Threshold channel design• Q = WDV (continuity)
• V = (1.486/n)D2/3S1/2 (Manning flow resistance)
• For sediment transport relation, we use a Shields type relation (no relation to me!):
qs = a(t* - tc*)b
(note qs is transport rate of bed material per unit width = Qs/W )
• Since Qs = 0, then t* = tc*, or
0.047 = DS/(1.65ds)
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Threshold channel design• Four equations, five unknowns (W, D, S, V, t)
• Q = WDV
• t = gDS
• V = (1.486/n)D2/3S1/2
• S =[Qn/(1.486WD5/3)]2
• tc* = DS/(1.65ds)
• D = 0.07755ds/S
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For simplicity, assume rectangular channel with Qs = 0, R ~ D and constant Manning n
Doug Shields, Jr. www.friendofrivers.com
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Threshold channel designLet tc*= 0.047 x 0.9
Combine
• S =[Qn/(1.486WD5/3)]2
• D = 0.07755ds/S
To get S = f(W)
• S = 0.0184(Qn) -6/7ds10/7W7/6
“Given”
• Q = 400 cfs
• ds = 45 mm (convert to ft)
• n = 0.035
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0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
0.016
0.018
0.020
0 20 40 60 80
Slo
pe
Channel Width, ft
Slope v Width for Rectangular Channel
Use constraints and design objectives to select a single W or S
Doug Shields, Jr. www.friendofrivers.com
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Threshold channel design
Combine
• S =[Qn/(1.486WD5/3)]2
• D = 0.07755ds/S
To get S = f(W)
• S = 0.0184(Qn) -6/7ds10/7W7/6
• D = 0.07755 ds /S
• If Q = 400 cfs, ds = 45 mm (convert to ft), n = 0.035
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0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
0 20 40 60 80
De
pth
, ft
Channel Width, ft
Depth v Width for Rectangular Channel
Doug Shields, Jr. www.friendofrivers.com
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Threshold design example• Example from NRCS NEH 654.08 pp. 40-41
• Type I threshold design
• Note that this is a trapezoidal channel
• Keys
• Assume n = f(ds) or is constant
• Use constant side slope
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Doug Shields, Jr. www.friendofrivers.com
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Threshold design example 1Given Find
• Channel W, D, S
• Sinuosity
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Threshold design example 1Steps
Determine ds
• Determine prelim W
• Estimate tc
• Estimate flow resistance (Manning n)
• Calculate D and S
• Determine planform
• Assess stability
Methods
• As described above
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Doug Shields, Jr. www.friendofrivers.com
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Threshold design example 1Steps
Determine ds
Determine prelim W
• Estimate tc
• Estimate flow resistance (Manning n)
• Calculate D and S
• Determine planform
• Assess stability
Methods
As described above
Use hydraulic geometry or regime formula (why is this a problem?). W = 2.03Q0.5 = 2.03(400)0.5
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22 to 74 ft
Doug Shields, Jr. www.friendofrivers.com
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Threshold design example 1Steps
Determine ds
Determine prelim W
Estimate tc
• Estimate flow resistance (Manning n)
• Calculate D and S
• Determine planform
• Assess stability
Methods
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As described above Use hydraulic geometry or regime formula Use Shields relation. Consult literature or
use experience with similar streams.
Doug Shields, Jr. www.friendofrivers.com
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Threshold design example 1Steps
Determine ds
Determine prelim W
Estimate tc
Estimate flow resistance (Manning n)
• Calculate D and S
• Determine planform
• Assess stability
Methods
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As described above
Use hydraulic geometry or regime formula
Use Shields relation.
Grain and total resistance using Strickler or Limerinos plus Cowan
Doug Shields, Jr. www.friendofrivers.com
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Threshold design example 1Steps
Determine ds
Determine prelim W
Estimate tc
Estimate flow resistance (Manning n)
Calculate D and S
• Determine planform
• Assess stability
Methods
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As described above
Use hydraulic geometry or regime formula
Use Shields relation.
Grain and total resistance using Strickler or Limerinos plus Cowan
Use iterative process—vary S, compute D, compute Q until desired Q is reached. Table or….
graph of discharge v slope
Doug Shields, Jr. www.friendofrivers.com
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Threshold design example 1Steps
Determine ds
Determine prelim W
Estimate tc
Estimate flow resistance (Manning n)
Calculate D and S
Determine planform
• Assess stability
Methods
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As described above
Use hydraulic geometry or regime formula
Use Shields relation.
Grain and total resistance using Strickler or Limerinos plus Cowan
Use successive approximations or graph of discharge v slope
Compute sinuosity = Valley slope/channel slope
Doug Shields, Jr. www.friendofrivers.com
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Threshold design example 1Steps
Determine ds
Determine prelim W
Estimate tc
Estimate flow resistance (Manning n)
Calculate D and S
Determine planform
Assess stability
Methods
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As described above
Use hydraulic geometry or regime formula
Use Shields relation.
Grain and total resistance using Strickler or Limerinos plus Cowan
Use successive approximations or graph of discharge v slope
Compute sinuosity = Valley slope/channel slope
Consider stability at higher and lower flows and with different values of ds and tc* --Bank protection?
W = 41 ft, D = 1.9 ft, = 0.0047, P = 1.4842
HEC RAS 5.0 Stable Channel Design
• Width is B in HEC RAS literature
• Threshold
• Tractive force
• Regime
• Design is a suitable combination of W, D, S, ds
• Alluvial
• Copeland
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Threshold design with HEC RAS• Referred to as the “tractive force” method
• Reduces the six variable system (W, D, S, Q, Qs, ds) to a five-variable system (W, D, S, Q, ds) since it is assumed that Qs ~ 0.
• The user must supply Q and tc*
• The user must supply two of the four remaining variables and HEC RAS will compute the other two.
• For example, user supplies ds and W, program computes D and S
• Note extreme sensitivity to Shields no. and ds
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Doug Shields, Jr. www.friendofrivers.com
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Compare NEH and HEC RAS results
NEH 654.08
• Limerinos used for Manning n
• W = 41 ft
• D = 1.94 ft
• S = 0.0047
• P = 1.48
HEC RAS 5.0
• Manning n supplied by user or computed with Strickler
• W = 41 ft
• D = 1.88 ft
• S = 0.0048
• P = 1.45
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These dimensions are for planning or preliminary design. Variability and detail must be added, and stability checks across a range of discharges are needed.
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HEC RAS anomalous solutions
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Doug Shields, Jr. www.friendofrivers.com
If you have a very large ds, you can get a very large D…leading to a small B. 46
Threshold design with FHWA• Hydraulic toolbox
• Channel lining analysis
• Threshold type I check
• Allows a range of cross sectional shapes
• Provide ds, Manning n, Q, Width, Slope
• Computes D (flow depth) and assesses stability
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Two stage threshold channel design
• Given
• Slope and valley slope
• Bed D84
• Shields number Qbankfull
• Q5
• Find
• Inset channel width
• Inset channel depth
• Floodplain channel width
• Floodplain channel depth
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Resop, et al. (2014) Journal of Soil and Water Conservation 69.4 (2014): 306-315.
Effect of uncertainty
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Two stage channel design
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Regular deterministic analysis• w1 = 9 m• w2 = 50 m• d1 = 0.5 m• d2 = 0.8 m
Resop, et al. (2014) Journal of Soil and Water Conservation 69.4 (2014): 306-315.
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Monte Carlo varying Qb, D84, n, Shields number
Resop, et al. (2014) Journal of Soil and Water Conservation 69.4 (2014): 306-315.
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To recap…...• Several approaches used for stream restoration channel design
• Analytical approach endorsed by NRCS and recommended by ASCE
• Analytical approach revolves around sediment transport
• Three main types of channel
• Threshold type I—bed does not move, little to no sediment transport
• Threshold type II—bed does not move, significant sediment transport
• Alluvial—bed moves and significant sediment transport Shie
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References• Brookes, A., and Shields Jr, F. D. (1996). River channel restoration: guiding principles for sustainable
projects. John Wiley & Sons, New York.
• Bunte, K., Abt, S. R., Swingle, K. W., Cenderelli, D. A. and Schneider, J. M. (2013). Critical Shields values in coarse‐bedded steep streams. Water Resources Research, 49(11), 7427-7447.
• Buffington, J. M., and Montgomery, D. R. (1997). A systematic analysis of eight decades of incipient motion studies, with special reference to gravel‐bedded rivers. Water Resources Research, 33(8), 1993-2029.
• Copeland, R. R., D. N. McComas, et al., Eds. (2001). Hydraulic Design of Stream Restoration Projects. U.S. Army Corps of Engineers: Engineer Research and Development Center, Vicksburg, MS.
• Federal Interagency Stream Restoration Working Group. (1998). Stream corridor restoration: principles, processes, and practices.
• National Resources Conservation Service (NRCS) (2007a). Basic Principles of Channel Design. Stream Restoration Design, Chapter 7, National Engineering Handbook Part 654.7. USDA NRCS, Washington, D. C.
• National Resources Conservation Service (NRCS) (2007b). Basic Principles of Channel Design. Stream Restoration Design, Chapter 8. National Engineering Handbook Part 654.8. USDA NRCS Washington, D. C.
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References (continued)• Niezgoda, S.L., Wilcock, P.R., Baker, D.W., Price, J.M., Castro, J.M., Curran, J.C., Wynn-Thompson, T.,
Schwartz, J.S. and Shields Jr, F.D. (2014). Defining a stream restoration body of knowledge as a basis for national certification. Journal of Hydraulic Engineering 140(2), pp.123-136.
• Resop, J. P., Hession, W. C. and Wynn-Thompson, T. (2014). Quantifying the parameter uncertainty in the cross-sectional dimensions for a stream restoration design of a gravel-bed stream. Journal of Soil and Water Conservation 69.4 (2014): 306-315.
• Shields, F. D., Copeland, R.R., Klingeman, P.C., Doyle, M.W. and Simon, A. (2003). Design for Stream Restoration. Journal of Hydraulic Engineering 129(8): 575-584.
• Shields, F. D., Jr., Copeland, R. R., Klingeman, P. C., Doyle, M. W. and Simon, A. (2008). Stream restoration. Chapter 9 in Sedimentation Engineering: Processes, Measurements, Modeling and Practice, Manual of Practice 110, American Society of Engineers, Reston, Virginia. 461-499 pp.
• Slate, L. O. Shields, F.D., Schwartz, J.S., Carpenter, D.D. and Freeman, G.E (2007). Engineering Design Standards and Liability for Stream Channel Restoration. Journal of Hydraulic Engineering133(10) ,1099-1102.
• Wilcock, P. R. (1997). Friction between science and practice: the case of river restoration. Eos, Transactions, Am. Geophysical Union 78(41): 454.
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