Analysis of Cost-Effective Rehabilitation: Principles and ......Can provide analysis of...
Transcript of Analysis of Cost-Effective Rehabilitation: Principles and ......Can provide analysis of...
Analysis of Cost-Effective Rehabilitation: Principles and Tools for Reducing
Uncertainty in Design
Natasha Bankhead and Andrew Simon Cardno ENTRIX, Oxford, MS
Often the Questions are Basic Ones
• Is the bank/bed stable? • How much will it erode over “x” number of years or for a given range of flows? • Can erosion be reduced/stopped using various techniques? • Can we allow for “natural” processes without threats to assets, infrastructure and human safety? • Should we do nothing, use rock, engineered wood structures, flow deflectors, vegetation? • What is the most effective? • Is it affordable?
But My Stream is Different… • The physics of erosion are the same wherever you
are…no matter what hydro-physiographic province, stream type or river style you are in…channel response is a matter of quantifying available force, and resistance of the channel boundary
• Channel adjustment is driven by an imbalance between the driving and resisting forces
• Differences in rates and magnitudes of adjustment, sediment transport rates and ultimate channel forms are a matter of defining those forces…deterministically or empirically
The Processes: Force and Resistance • Hydrologic: The principle driver (expressed as
discharge over time);
• Hydraulic: How discharge is translated to a quantifiable force acting on the channel boundary (expressed as shear stress vs. critical shear stress);
• Geotechnical: Downslope gravitational force (resisted by cohesive and frictional strengths)
The Analytic Approach Static and Dynamic Numerical Modeling
• Collect field data to define the variables that control the processes (force and resistance);
• Use that data with physically-based analytic solutions/models to test plans and designs;
• Use the best available numerical models for prediction
Hydraulic Shear Force and Resistance τo = γw R S τo = mean boundary shear stress γw = unit weight of water R = hydraulic radius = A / 2y + w S = channel gradient
τ* = τo / [ (γs – γw) d] τ* = dimensionless shear stress γs = unit weight of sediment d = characteristic particle diameter
Re = ( U* d) / υ Re = boundary Reynolds number U* = shear velocity = (g R S)0.5
υ = kinematic viscosity (function of water temperature)
= measured in the field
y and x axes of Shields diagram
Collect the Data Hydraulic Resistance (bed and banks): Non-cohesive: bulk particle size or particle count for τc Cohesive: submerged jet-test device
Equivalent to critical shear stress where rate of scour
becomes 0.0 mm/min
Geotechnical: Force vs. Resistance Resisting Forces
Driving Forces If Fs is greater than 1, bank is stable. If Fs is less than 1 bank will fail. (We usually add a safety margin: Fs>1.3 is stable.)
Resisting Forces Driving Forces (gravity)
soil shear strength bank angle matric suction weight of soil mass root reinforcement weight of water in bank confining force bank height
Factor of Safety (Fs) =
Components of Shear Strength (ie. for saturated conditions)
Where: τf = shear strength (kPa); c’ = effective cohesion (kPa); σ = normal load (kPa); µw = pore water pressure (kPa); and φ’ = effective friction angle (degrees)
τf = c’ + (σ-µw) tan φ’
= measured in the field
Collect the Data Geotechnical Resistance (banks):
• Coarse-grained, non-cohesive: particle count for φ’; c’ = 0.0 • Cohesive: borehole shear tester (BST) or laboratory analysis
for c’ and φ’
Default values by soil type are available but increase uncertainty
Differentiate Between Hydraulic and Geotechnical Protection
• Hydraulic protection reduces the available boundary hydraulic shear stress, and increases the shear resistance to particle detachment
Hydraulic Protection
Geotechnical Protection
• Geotechnical protection increases soil shear strength and decreases driving forces
Vegetation as a River Engineer • Above and below-ground biomass • Process Domains • Role of vegetation can be quantified
Process Domain
Geotechnical Hydrologic Hydraulic
Above Ground
Surcharge Interception Evapo- transpiration
Roughness Applied shear stress
Below Ground
Root reinforcement
Infiltration Matric suction
Critical shear stress
Benefits of Process-Based Approach
• Can test design scenarios for performance under a range of flow conditions before they are built; • Can modify designs and re-test; • Avoid under-design and reduce risk of project failure; • Avoid over-design and cost over-runs; • Can provide analysis of effectiveness of different designs; • Can provide analysis of cost-effectiveness in real terms of cost per amount of reduction (ie. $/m3 or $/m)
Analysis as part of the design process has been shown to be cost effective by reducing risk and uncertainty (Niezgoda and Johnson, 2007)
Technology is available and cost effective!!
Bank-Stability Model Version 5.4 • 2-D wedge- and cantilever-failures
• Tension cracks • Search routine for failures • Hydraulic toe erosion • Increased shear in meanders • Accounts for grain roughness • Complex bank geometries • Positive and negative pore-water
pressures • Confining pressure from flow • Layers of different strength • Vegetation effects: RipRoot • Inputs: γs, c’, f’, φb , h, uw, k, τc
Confining pressure
Tensiometers (pore pressure)
shear surface
12/29/97 01/05/98 01/12/98 01/19/98 01/26/98 02/02/98
FACTOR OF SAFETY
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
RIVER STAGE, IN METERS ABOVE
SEA LEVEL
80
81
82
83
84
85
BFactor of safety
Effect of confining pressure
Bank failures
Stage WAT
ER L
EVEL
, M
For Bank Erosion:
Toe Model OutputVerify the bank material and bank and bank-toe protection information entered in the "Bank Material" and "Bank Vegetation and Protection"worksheets. Once you are satisfied that you have completed all necessary inputs, hit the "Run Toe-Erosion Model" button (Center Rightof this page).
Bank Material Bank Toe MaterialLayer 1 Layer 2 Layer 3 Layer 4 Layer 5
Moderate cohesive Moderate cohesive Moderate cohesive Moderate cohesive Moderate cohesive Moderate cohesive Material
5.00 5.00 5.00 5.00 5.00 5.00 Critical shear stress(Pa)
0.045 0.045 0.045 0.045 0.045 0.045 Erodibility Coefficient(cm3/Ns)
Account for:Stream Curvature
Effective stressacting on each grain
Average applied boundary shear stress 51.060 PaMaximum Lateral Retreat 24.966 cm
Eroded Area - Bank 0.232 m2
Eroded Area - Bank Toe 0.423 m2
Eroded Area - Bed 0.000 m2
Eroded Area - Total 0.655 m2
-1.00
0.00
1.00
2.00
3.00
4.00
5.00
6.00
-1.00 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00STATION (M)
ELE
VA
TIO
N (M
)
Base of layer 1
Base of layer 2
Base of layer 3
Base of layer 4
Base of layer 5
Eroded Profile
Initial Profile
Water Surface
Run Toe-Erosion Model
Export New (Eroded) Profile into Model
Example: The Role of Toe Protection
Slope = 0.0035 m/m
Depth = 2.5 m
Toe material: silt
Eroded: 0.66 m2
Slope = 0.0035 m/m
Depth = 2.5 m
Toe material: wood
Eroded: 0.28 m2
Toe Model OutputVerify the bank material and bank and bank-toe protection information entered in the "Bank Material" and "Bank Vegetation and Protection"worksheets. Once you are satisfied that you have completed all necessary inputs, hit the "Run Toe-Erosion Model" button (Center Rightof this page).
Bank Material Bank Toe MaterialLayer 1 Layer 2 Layer 3 Layer 4 Layer 5
Moderate cohesive Moderate cohesive Moderate cohesive Moderate cohesive Moderate cohesive Rip Rap (D50 0.256 m) Material
5.00 5.00 5.00 5.00 5.00 204.00 Critical shear stress(Pa)
0.045 0.045 0.045 0.045 0.045 0.007 Erodibility Coefficient(cm3/Ns)
Account for:Stream Curvature
Effective stressacting on each grain
Average applied boundary shear stress 51.060 PaMaximum Lateral Retreat 24.159 cm
Eroded Area - Bank 0.232 m2
Eroded Area - Bank Toe 0.044 m2
Eroded Area - Bed 0.000 m2
Eroded Area - Total 0.276 m2
-1.00
0.00
1.00
2.00
3.00
4.00
5.00
6.00
-1.00 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00STATION (M)
ELE
VA
TIO
N (M
)
Base of layer 1
Base of layer 2
Base of layer 3
Base of layer 4
Base of layer 5
Eroded Profile
Initial Profile
Water Surface
Run Toe-Erosion Model
Export New (Eroded) Profile into Model
Channel-Modeling Capabilities
Process BSTEM HEC-RAS
SRH-2D
HEC+ BSTEM
SRH-2D+BSTEM
Shear in meanders
Bank-toe erosion
Mass-failures
Bed erosion
Sediment transport
Vegetation effects
‘Hard’ engineering
Channel evolution
Rapid Assessments
Alpha version of HEC-RAS/BSTEM is now being tested by CoE, Cardno
ENTRIX and USDA-ARS
0
5000
10000
15000
20000
25000
30000
12/6/1999 4/19/2001 9/1/2002 1/14/2004 5/28/2005 10/10/2006 2/22/2008
DIS
CH
AR
GE
, IN
CU
BIC
F
EE
T P
ER
SE
CO
ND
DATE
June 1, 2000 - June 30, 2008
Sandy River, OR
Flow series for calibration
Objectives: 1. Predict migration of meander bend
and timing of threats to park infrastructure;
2. Quantify reduction in retreat rates under various mitigation measures
In cooperation with Brian Vaughn, Portland METRO
50-Year Simulation Results
y
Infrastructure Type (points) Year Impacted
2011 2022 2037 2047 2062
Play Structures (1 Point)
Play Structure #1 X
Play Structure #2
Group Picnic and Campgrounds (2 Points)
Group Picnic A X
Group Picnic B
Group Picnic C X
Group Picnic D X
Group Picnic E
Group Camp #1
Group Camp #2 X
Restrooms and Showers (3 points)
2 Door Restroom #1
X
2 Door Restroom #2 X
4 Door Restroom #3 X
4 Door Restroom #4
2 Door Restroom #5 X
4-Door Showers #1
4-Door Showers #2 X
Roads (4 points)
Main Road X X X X X
Boat Ramp X X X X X
Essential Infrastructure (5 points)
Sewer System X
Total Score
Cumulative Relocation Cost Score 11 20 16 19 14
Timing of Associated Threats to
Park Infrastructure
Modeled Conditions High GW case Fs (GW 9.31 m above
bed)
Bank top retreat
(m)
Geotechnical erosion
(m3)
Existing (with stream curvature) 0.99 10.7 97.6
Grade to 1:1 (with stream curvature) 1.53 0 0
RipRap to 15% bank height (with stream curvature) 1.07 0 0
Grade + RipRap (with stream curvature) 2.18 0 0
Flow deflection (stream curvature turned off) 1.49 0 0
Flow deflection + riprap (stream curvature off) 1.50 0 0
Flow deflection + grade (stream curvature off) 1.67 0 0
Vegetation (with stream curvature) 1.00 10.7 97.6
Vegetation + grade (with stream curvature) 1.98 0 0
Vegetation + riprap (with stream curvature) 1.23 0 0
Vegetation + flow deflection (stream curvature off) 1.49 0 0
Testing Alternative Treatments
A range of alternative treatments would be successful here. Ultimate decision is a function of cost, permitting and policy
Bayou Pierre, MS Encroachment on
Gas Pipeline
Simulated Retreat: 30 Years (Calibrated with 2-D Hydraulic Modeling)
0
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-30 -25 -20 -15 -10 -5 0 5 10 15
EL
EVA
TIO
N A
BO
VE
AR
BIT
RA
RY
DA
TUM
, IN
M
ET
ER
S
DISTANCE FROM BANK EDGE, IN METERS
Original Geometry5 years10 years15 years20 years25 years30 years
0
1
2
3
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7
-250 -200 -150 -100 -50 0
EL
EVA
TIO
N A
BO
VE
AR
BIT
RA
RY
DA
TUM
, IN
M
ET
ER
S
DISTANCE FROM BANK EDGE, IN METERS
Original Geometry5 years10 years15 years20 years25 years30 years
Spoke 2-4
Spoke 2-6
Predicting Bend Migration
E = Existing pipeline; A, A/B, B and D represent alternative routes
Burnett River, Queensland, AUS
January 2013 flood of record: RI at least 140 years
Objectives: 1. Protect local assets
from bank erosion; 2. Determine cost-
effective treatments 3. Reduce fine loads to
the Great Barrier Reef
Bank erosion: 48 Mt over 3.8 years!!
Calibration: 2010-2013
Once calibrated, 10-year simulations for “no action” and alternative strategies were conducted
Erosion under Alternative Treatments
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5
10
15
20
25
-80 -30 20 70 120
ELEV
ATIO
N, i
n m
eter
s
STATION, in meters
2013 START PROFILEEND NO ACTIONEND with vegetated bankEND with rock toeEND with rock toe and bank vegetationEND rock toe and bankEND rock toe and bank, and vegetationEND vegetation and bendway weirEND vegetation, bendway weir, rock toe and bank
0
5
10
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-80 -30 20 70 120
ELEV
ATIO
N, i
n m
eter
s
STATION, in meters
END NO ACTIONSTART 32 degree bankSTART 2:1 bankEND 2:1 bank with vegetationEND 32 degree bank with vegetation
10-year flow series
0.001
0.010
0.100
1.000
10.000
100.000
1000.000
10000.000
100000.000
DIS
CHA
RGE,
IN
CU
BICM
ETER
S PE
R SE
CON
D
BMRG-02
“No action”
Includes 2011 and 2013 floods
Estimate Costs (similar relations for wood structures; heavy machinery;
plantings, etc.)
0
2
4
6
8
10
12
14
0 20 40 60 80 100 120 140
VERT
ICAL
HEI
GHT
OF
ROCK
, IN
M
ETER
S
TONNES OF ROCK REQUIRED
Horizontal placementVertical placement
$$ = 259.93 h 1.9528
r² = 0.9985
$$ = 187.91 h 1.4165
r² = 0.999
100
1000
10000
100000
0 5 10 15
COST
PER
MET
ER O
F RO
CK
PLAC
EMEN
T*
VERTICAL HEIGHT (h) OF ROCK, IN METERS
Bendway weirs
Rock toe protection
From D. Derrick, written comm.,
(2013)
Assuming $81.50/t (AUS)
Scenario Protection Unit Cost
Cost effectiveness Unit Cost Total Cost
Height Length By volume By retreat (m) (m) ($/m) ($/m3) ($/m) ($)
No Action - - $0 $0 $0 $0 With vegetation 13.2 530 $ 66 $81 $1,230 $35,000 Rock at toe 4.1 530 $ 1,360 $988 $15,200 $722,000 Rock toe with vegetation 4.1 530 $ 1,430 $1,020 $15,600 $757,000 Rock toe and bank face 11.1 530 $ 5,680 $3,890 $61,600 $3,012,000 Rock toe and lower bank with vegetation 6.6 530 $ 2,790 $1,930 $30,200 $1,480,000 2:1 battering with vegetation 13.2 530 $ 186 $259 $2,440 $98,600 32 degree bank with vegetation 13.2 530 $ 186 $308 $2,710 $98,600 Vegetation with bendway weirs 13.2 530 $ 1,560 $1,080 $2,700 $824,000 Vegetation with ELJs 13.2 530 $ 260 $181 $30,300 $138,000 Vegetation with bendway weir and rock toe 13.2 530 $ 2,920 $1,990 $30,300 $1,550,000 Vegetation with ELJs and rock toe 4.1 530 $ 1,620 $1,110 $16,800 $860,000
Cost Effectiveness (to hold the line or not)
Cost Effectiveness
Summary and Conclusions •Gravity and the physics of erosion and sediment transport are a constant, allowing us to quantify force and resistance mechanisms.
•We can collect all of the data we need at a site/reach to quantify driving and resisting forces operating on the channel boundary
•Using principles of hydrology, hydraulics and geotechnics we can use our field data to inform numerical tools and models.
• Analyses and models can then be used to quantify performance and effectiveness of a range of alternative strategies
• By applying a cost basis to each strategy, we can provide a range of cost-effective options to be considered.