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PDHonline Course H145 (4 PDH)
Stable Channel Analysis and Design
2012
Instructor: Joseph V. Bellini, PE, PH, DWRE, CFM
PDH Online | PDH Center5272 Meadow Estates Drive
Fairfax, VA 22030-6658Phone & Fax: 703-988-0088
www.PDHonline.orgwww.PDHcenter.com
An Approved Continuing Education Provider
© Joseph V. Bellini
Overview
Estimating Channel Velocity Method of Maximum Permissible Velocity (soil, riprap, & veg) Method of Maximum Tractive force (soil, riprap, & veg) Geometric Design of Channels Slope Revetment (riprap, gabion, and articulated concrete block) HEC-RAS Stable Channel Design Tool
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© Joseph V. Bellini
References
Sediment Transport Technology, Water and Sediment Dynamics, Simon & Senturk, Open Channel Hydraulics, V.T. Chow, McGraw-Hill Sediment Transport, Theory and Practice, Yang, McGraw-Hill USGS Water-Supply Paper 2339, "Guide for Selecting Manning's Roughness
Coefficients for Natural Channels and Flood Plains," by George J. Arcement, Jr.,and Verne R. Schneider Design of Riprap Revetment, Hydraulic Engineering Circular No. 11 (HEC-11),
Federal Highway Administration (FHWA) Design of Roadside Channels with Flexible Linings, Hydraulic Engineering Circular
No. 15 (HEC-15), Federal Highway Administration (FHWA) Articulated Concrete Block, Revetment Design, Factor of Safety Method, National
Concrete Masonry Association, TEK 11-12 Hydraulic Stability of Articulated Concrete Block Revetment Systems During
Overtopping Flow, 1989, FHWA-RD-89-199 Standard Guide For Analysis And Interpretation Of Test Data For Articulating
Concrete Block (ACB) Revetment Systems In Open Channel Flow, 2008, ASTM7276-08
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ESTIMATING CHANNEL VELOCITY
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Channel Velocity Estimates
Average Channel Velocity (Manning’s Equation)
– v = Average channel velocity (fps)–R = Hydraulic radius = Area/wetted perimeter (feet)–Sf = Friction slope (feet/foot)–Sf = So = Channel bottom slope (feet/foot) for uniform flow–n = Manning’s ‘n’ (roughness) value (see subsequent pages)
2/13/249.1fSR
nv
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Manning’s ‘n’ Values
The Manning roughness coefficient is oftenassumed constant regardless of flow depth.At very shallow depths, the effects of theroughness in the channel bottom are morepronounced producing higher n-values.However, assuming the channel bottom andbanks have similar cover, the n-valuequickly decreases with increased depth tonearly constant until reaching bank full. Inoverbank floodplain areas, n-valuestypically vary significantly with depth.
Manning’s equation is commonly used tocompute the friction slope (Sf) at eachcross-section when developing watersurface profiles under gradually varied flowconditions using the direct-step or standard-step methods.
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© Joseph V. Bellini
Manning’s ‘n’ Values
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Manning’s ‘n’ Values
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Relationship for Manning's roughness coefficient, n, that is a function of the flowdepth and the relative flow depth (da/D50) for gravel/stone lined channels. Thisrelationship is used to compute Manning’s n in FHWA’s HEC-15.
This equation applies where 1.5 ≤ da/D50 ≤ 185.
Where,n= Manning's roughness coefficientda= average flow depth in the channel, ftD50 = median riprap/gravel size, ftα= unit conversion constant, 0.262 (0.319 (SI))
Manning’s ‘n’ Values for Gravel/Stone Lined Channels
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6/1
log23.525.2 Dd
dna
a
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Manning’s ‘n’ Values for Grass Lined Channels
4.04.00
RSCCn nn 528.010.0 hCC sn
where,
Cn= Grass roughness coefficient
Cs= Density-stiffness coefficient
h= Stem height (ft)
α= Unit conversion constant, 0.262 (0.319 (SI))
t0 = Average bottom shear (psf)
γ = Unit Weight of Water, typically 62.4 #/cf
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Depth-Averaged Velocity in Conveyance Tubesfrom HEC-RAS (1D, Steady-Flow)
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1. Select Steady Flow Run
2. Select Flow Dist’n Locations
3. Select # of flow/velocity sub-sections;either globally or by cross-section. Select‘OK’, then select ‘Compute’ from step 2.
© Joseph V. Bellini
Depth-Averaged Velocity in Conveyance Tubesfrom HEC-RAS (1D, Steady-Flow)
120 140 160 180 200 220 240
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343
344
345
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Whitford CC Proposed Plan: Current model 7/21/2010
Station (ft)
Ele
vatio
n (ft
)
Legend
EG 100-yr
WS 100-yr
4 ft/s
5 ft/s
6 ft/s
7 ft/s
8 ft/s
Ground
Ineff
Bank Sta
.039 .042 .036
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After selecting ‘Compute’, plot a cross-section of interest and you will seeconveyance tubes; each showing theaverage velocity per tube.
© Joseph V. Bellini
Vertically-Averaged Velocity from 2D Flow Model
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EVALUATING EROSIVENESS BASED ONMAXIMUM VELOCITY METHOD
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Method of Maximum Velocity – Non-Cohesive Soil
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Method of Maximum Velocity – Cohesive Soil
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Method of Maximum Velocity – Vegetation
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EVALUATING EROSIVENESS BASED ONMAXIMUM TRACTIVE FORCE/SHEAR METHOD
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Method of Maximum Tractive ForceShear Stress in Fluids
Fluids (liquid and gases) moving along solid boundary will incur a shear stress onthat boundary. The ‘no-slip condition’ requires that the velocity of the fluid at theboundary (relative to the boundary) is zero, but at some height from the boundarythe velocity must equal that of the fluid.
For all Newtonian fluids in laminar flow, shear stress is proportional to the strainrate in the fluid where the viscosity is the constant of proportionality. However, fornon-Newtonian, this is no longer the case; for these fluids, the viscosity is notconstant. The shear stress, for a Newtonian fluid, at a surface element parallel to aflat plate, at the point y, is given by:
Where μ = Dynamic viscosity v = Velocity y = Height above the boundary
Shear stress at the boundary is:
yvy
)(
0)0(
yyvy
0
yyv
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© Joseph V. Bellini
Method of Maximum Tractive ForceEstimating Average Bottom Shear
From the Momentum Equation for non-uniform flow:
Solving for the friction force (Ff) andconsidering that the applied shear (tractive)force equals the friction force:
Assuming negligible change in depth andcross sectional area between sections:
Therefore:
Assuming a relatively small friction slope,the average tractive force on wetted area inpsf is:
Assuming a wide channel (B/y > 10), whereR ~ y, the average tractive force on wettedarea in psf is:
fFWPPvvgQw
sin211122
sin21112200 WPPvvgQAFf
21 PP 2211 vv
sinsin00 LAWA sinsin0 R
LWLAp
00 RS
00 yS
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Method of Maximum Tractive Force – Applied Ave. Bottom Shear
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Applied Shear on Curved Channels (from USACE)
5.0max 65.2)(
Brc
o
o
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Applied Shear on Curved Channels (from FHWA’s HEC-15)
dbb K
where,
tb= Side shear stress on the channel(psf)
Kb= Ratio of channel bend to bottomshear stress
td= Shear stress in the approach channel(psf)
Rc= Radius of curvature of the bend tothe channel centerline (ft)
T= Channel top (water surface) width (ft)
05.1
0073.0206.038.2
00.22
b
ccb
b
KTR
TRK
K Rc/T≤2
2<Rc/T<10
Rc/T≥10
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Method of Maximum Tractive Force – Critical (Permissible)Shear for Non-Cohesive Soil (from USGS)
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Method of Maximum Tractive Force – Critical (Permissible)Shear for Cohesive Soil (from USGS)
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Method of Maximum Tractive Force – Vegetation & Riprap (from FHWA’sHEC-15)
500.4 Dp
For riprap:
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Design of Stable ChannelsNorth American Green Software Demo (from HEC-15)
http://www.nagreen.com/27
© Joseph V. Bellini
Design of Stable ChannelsNorth American Green Software Demo
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STABLE CHANNEL GEOMETRY
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Stable Channel Geometry (from USBR)
Side slopes (USBR)
Dimensions oftrapezoidal sections(USBR)
Ad 5.0 zdb
4z
1 d
b
A=Area
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Stable Channel Geometry – Freeboard (from PADEP)
Freeboard (Critical Slope Method – PADEP)Uniform flow at or near critical depth is unstable due to
waves present at the water’s surface.Sufficient freeboard must be provided to prevent waves
from overtopping the channel.Flow is considered “unstable” when So is between 0.7Sc
and 1.3Sc. If unstable flow exists, compute minimumfreeboard as
For a stable channel, the minimum freeboard should be25% of the flow depth.Generally, freeboard should not be less than 0.5 feet
3/4
256.14RDnS m
c
TADm
VDDVF 075.0
3025.0
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Stable Channel Geometry – Ideal Stable Cross Section
From the USBR, ideal section geometry to evenly distribute shear:
x
ddy
maxmax
tancos Tan
dA2max2
213
2
max
sincos1 S
Ed
nv
f
c
Sd
97.0max
Tan
dT 22
max
Equation for bank curves Equations for geometry Equations for velocity
2sin
411
2sinE
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Stable Channel Geometry – Ideal Stable Cross Section
Example:
Given:g=9.81 m/s2; Sf=0.0006; n=0.022; γ=1000 kg/m3; D50=12 mm;Q=10 m3/sec
For a given lining type, it was determined that:
τc = 0.9 kg/m2
Solution:
VQBdA max
213
21fSRn
V
21
35
max49.1 Sd
nQB cent
21
35
max Sd
nQB cent
English Metric
f
c
Sd
97.0max
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© Joseph V. Bellini
Stable Channel Geometry – Stable Channel Slope
2/1
3964
SAD gg
BV
A gg
Where there is insufficient amount of coarse material to develop an armorlayer, degradation will continue until the channel reaches a stable slope.Refer to example on next page.
whereAg = volume of material to be degraded per unit channel width (sf)ΔS = So – SL
SL = Limiting slope (Meyer-Peter-Muller Method)d = Mean flow depth (feet)d50 & d90 = Particle diameter at 50% and 90% passing (mm)
SD
L gg 813
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ddndSL
2/36/19050 )/)()(19.0(
© Joseph V. Bellini
Stable Channel Geometry – Stable Channel Slope
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Stable Channel Geometry – Stable Channel Slope
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SLOPE REVETMENT DESIGN
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© Joseph V. Bellini
Design of Stable ChannelsSlope RevetmentTypes of slope revetment: Riprap Gabion Grouted rock Pre-cast articulated concrete block
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Slope Revetment – Riprap (Safety Factor (SF) Method)
by γ
dSiv
e Fo
rce
Div
ided
M
axim
um T
ract
cossintan
tancos'
SF
2
sin1' τo = Shear along the side
slope (from Lane chart b )
2
sintan
sin2costan
501
21DS ws
o
above) Φ = Angle of repose λ = Angle between
horizontal & velocity vector
© Joseph V. Bellini
tan
50ws horizontal & velocity vector along the slope plane.
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© Joseph V. Bellini
Slope Revetment – Riprap (USBR)
For relatively stable flow (uniform, straight, or mildly curved (curve radius/channelwidth > 30)), impact from wave action and floating debris minimal, and little or nouncertainty in design parameters); stability factor of 1.2.Based on Sg = 2.65; use specific gravity correction factor if other than 2.65.Stability factor is used to reflect the uncertainty in the hydraulic conditions and isdefined as the ratio of the average tractive force/critical shear of riprap. If otherthan 1.2, apply the stability factor correction factor in Table 1 below.
5.0
2
2
1 sinsin1
K
Where,
D50 = Mean riprap particle size
C = Correction factor
Va = Average velocity in channel (fps)
Da = Average flow depth in channel (ft)
θ = Angle of channel bank
φ = Riprap material angle of repose
C = Correction factors
5.11
5.0
3
50001.0KdVCCD
a
asfsg
5.1112.2
g
sg SC
5.1
2.1
SFCsf
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© Joseph V. Bellini
Slope Revetment – Riprap
Design considerations
• Rock gradation
• Filter (granular or fabric)
• Bank slope (maximum recommended is 2:1)
• Bank preparation; cleared and grubbed.
• Thickness
• D100 or 1.5D50, whichever is greater
• > 12”
• Increase thickness by 50% for under-water placement
• Increase thickness by 6” to 12” where riprap may be subject to attackfrom floating debris, ice, or large waves.
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Slope Revetment – Gabions
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Slope Revetment – Gabions
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Slope Revetment – Grouted Rock
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Slope Revetment – Articulated Concrete Block
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© Joseph V. Bellini
Slope RevetmentArticulated Concrete Block (ACB) Safety Factor (SF) Calculation
tdes = to = Ave BottomShear (see Page 18)tc = Critical Shear fromtesting (see note 1)
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Notes:1. Critical shear obtained from tests
conducted per protocol in FHWA-RD-89-199 or ASTM 7276-08.
2. SF is heavily influenced byprotrusion, Δz (assumed to be ‘0’for tapered block).
© Joseph V. Bellini
Articulated Concrete Block (ACB) DesignStability in a Hydraulic Jump
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 10 20 30 40 50
Ener
gy R
atio
Froude Ratio
Test Data Test Data Envelope Curve from Test report
Envelope Curve for 50TBlock
Acceptable
Unacceptable
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© Joseph V. Bellini
STABLE CHANNEL DESIGN TOOL IN HEC-RAS
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© Joseph V. Bellini
Stable Channel Design Tool in HEC-RAS
The Copeland Method (Copeland, 1994) was developed at the USACEWaterways Experiment Station through physical model testing and field studiesof trapezoidal-shaped channels. – Applicable to alluvial channels.
The Regime Method is an empirically based technique originally using actualdata gathered by British engineers for irrigation canals in India. A stream isstable at the design discharge when there is no net annual gain or loss ofsediment through the reach under study. – Applicable for long-term (annual)simulations.
The Tractive Force Method is analytically based. Channel stability is achievedas long as the actual shear stress on a selected particle size of the bedmaterial is less than the critical shear stress (that which just initiates motion ofthe selected bed particle size). – Applicable for use channels lined with gravelor rock.
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© Joseph V. Bellini
Stable Channel Design Tool in HEC-RASCopeland Method Defines stability as sediment inflow = sediment outflow Inflowing sediment must be established; which can be done by providing a
sediment concentration or by allowing RAS to compute the sedimentconcentration by the geometric and sediment properties of an upstreamreach. (Capacity calculation using Brownlies Equation.)
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© Joseph V. Bellini
Stable Channel Design Tool in HEC-RASCopeland Method
MinimumStreamPower –Optimal fordesign
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Stable Channel Design Tool in HEC-RASCopeland Method
MinimumStream Power– Optimal fordesign
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© Joseph V. Bellini
Stable Channel Design Tool in HEC-RASTractive Force Method
Can solve forD75 and any ofthe followingparameters;slope, depth, orwidth.
Input – Discharge, temperature, specific gravity, angle of repose, side slope, andManning’s n.
Pick 2 of D75, depth, width, and slope to solve; RAS solves the other 2.
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