Post on 09-Dec-2015
description
Streambank Protection Planning and Design
Approaches
Instructor:
David T. Williams, Ph.D., P.E.David T. Williams and Associates, Engineers
david@dtwassoc.com
Overview
Project Planning
Project Design
Revetment Design Materials
Design Criteria and Procedures
Design Software
Planning Steps Determine cause of streambank failure
Decide if bank is worth protecting
Inventory available resources
Select a bank protection measure
Develop a project plan
Obtain permits
Construct the project
Inspect and maintain the project
Project Considerations Planning-Stakeholders / Funding / Regulations
Conduct Geomorphic Assessment
Perform Hydrologic Design
Calculate Hydraulic Design Parameters
Select Bank Protection Approach
Perform Design Calculations
Prepare Plans and Specifications
Obtain Permits
Construction, Inspection, and Maintenance
General Design Considerations
Great variation in site conditions
Numerous materials and design details
Handbook type analyses lead to poor solutions
“Cookbook” approach impractical
Good design practice involves judgement and experience
Checklist of Major Design Factors Geomorphology
Hydrology and Hydraulics
Geotechnical
Environmental
Toe Protection
Surface Drainage
Safety Factor
Manufacturer’s Recommendations
Design Geomorphology
“Location of Work”
Helps answer three basic questions:
Where do we begin work? Where do we end the work? What alignment do we follow from beginning
to end?
Beginning and Ending Points
Length of Protection
Time sequence analysis of channel migration (Planform or “horizontal” stability)
• Channel surveys (hydrographic or topographic)• Aerial photography
Plan form interpretation Talk to locals Numerical or physical modeling
Beginning and Ending Points Spot Protection
Downstream is more critical than upstream. Extend protection downstream at least 1.5
(prefer 2) times approach channel width.
Transition to existing bank Upstream: shallow “key-in” may be sufficient. Downstream: deeper “key” is recommended.
Braided Streams Stabilize a considerable distance upstream
and downstream from the active erosion.
Channel Alignment Considerations
Use existing alignment where possible. Principle: Existing alignment has resulted from an integration of all
pertinent variables. Relocation is undesirable (environmental, engineering and cost
factors)
Relocation may be appropriate for: Irregular alignment or smoothing sharp bends
Check envelope of stable values for: Sinuosity Pool and bar spacing Ratio of bend radius to channel width
Design Discharge
Design discharge is not necessarily an “extreme” event.
Is it the flow that stresses the protection most severely?
Is it the flow that governs the geomorphology of a stream? Theoretical channel-forming discharge or
dominant discharge or bank full discharge.
Design Hydraulics
How deep? How high? How strong?
Hydraulic design factors:
Design discharge Variations in discharge and stage Tractive force Secondary currents Prediction of toe scour Top elevation of protection
Top Elevation of Protection
Extend protection up to design flowline + freeboard (most conservative).
Consider placing the top elevation at a more frequently occurring flowline based on:
Stage duration Erodibility of upper bank material Type of protection Bank slope Consequences of failure
Geotechnical Design Considerations Presence of groundwater can have a significant impact
on the stability of a bank.
Two methods of groundwater control: Surface drainage to prevent surface water infiltration. Subsurface drainage to remove water from the soil.
• Horizontal drains (slotted pipe)• Vertical drains (for areas with high ground water tables)• Drainage trench
Slope stabilization Flattening Benching
Prediction of Toe Scour
Matter of judgement and experience
Scour prediction tools
Analytical Methods Empirical Methods Numerical Modeling
Will talk about it in later lectures
Revetment Design Materials Numerous product options
Consult manufacturer’s design manuals
Most common revetments:
Stone or Rock Riprap Gabions Cellular Blocks Articulated Concrete Blocks (ACB) Soil Bioengineering or Vegetated Revetments
Selection Criteria Selection of material is very project specific
Hydraulic conditions
Scour and sediment transport conditions
Slope stability
Visual impact
Environmental compatibility
Selection Criteria Material installation and maintenance costs
Effectiveness in reducing bank erosion
Availability
Vegetation compatibility
Feasibility of installation
Durability and longevity
Time to establish effectiveness
Riprap Design Methods U.S. Army Corps of Engineers (EM 1110-2-1601)
Federal Highway Administration, Design of Riprap Revetment (HEC-11)
U.S. Bureau of Reclamation (EM-25)
California Department of Transportation Bank and Shore Protection Manual.
U.S. Geological Survey (Blodgett & McConaughy, 1986)
USACOE Riprap Design Criteria
Based on experimental data
Coefficients account for safety factor, side-slope, stability, velocity distribution in bends.
Generally not for highly turbulent areas (e.g., stilling basins)
Channel slopes of 2 percent or less
Representative stone size is D30
Gabions Gabions are stone filled wire mesh baskets tied
together to form a flexible, continuous protection.
Gabion baskets are made of heavily galvanized or PVC-coated steel wire. Baskets are manufactured in three different shapes: box, mattress, and sack.
Capable of withstanding high velocity flows.
Follow design procedures by manufacturer or use gabion design software.
Gabion Terms
Gabions – Woven (double twisted) Wire
Reno Mattress
Welded Wire Gabions
Gabion Design Procedure
Determine the hydraulic shear stress acting on the lining.
Determine rock size and critical shear stress.
Determine lining thickness.
Determine filter requirement (fabric or gravel filter)
Determine toe scour depth and specify toe protection method (e.g., toe scour apron)
Major Gabion Manufacturers
Maccaferri Gabions, Inc. www.maccaferri-usa.com West Coast Division Office
Maccaferri, Inc., 3650 Seaport Blvd. West Sacramento, CA 95691, Tel: (916) 371-5805
Terra Aqua Gabions gabions@terraaqua.com P. O. Box 7546, Reno, NV 89510, Tel: (775) 828-1390
Other Revetment Systems
Cellular Confinement Systems – 3 dimensional cells confine and reinforce fill material
Articulating Block (AB) Fabric form - a series of compartments linked by an interwoven perimeter with grout ducts that interconnect the compartments. High strength cables are installed between and through the compartments and grout ducts. Grout is injected.
Articulating Concrete Blocks (ACBs) - A matrix of individual concrete blocks placed together by interlocking or cables
Cellular Confinement Systems
Cellular Confinement Systems
Articulating Block (AB) MatFabric Form Concrete (Fabriform)
Articulating Concrete Blocks (ACBs)
ACBs consist of concrete blocks that may be interlocked or cabled to form a flexible revetment.
No standard design procedure.
Consult manufacturers for design procedures and software.
Trilock ACB
Trilock ACB
Armorflex ACB
Failure of ACB Mat Systems
Definition of Failure: Loss of intimate contact between the revetment and the subgrade.
Progressive failure mechanisms:
Flow beneath the armor layer causing increased uplift pressure and separation of blocks from the subgrade.
Loss of subgrade soil through piping or washout. Rapid saturation and liquefaction of the subgrade soils
in fine sands and silts. Loss of block or blocks from the revement matrix.
Failure of ACB Mat Systems
Failure of ACB Mat Systems
ACB Mat Design Procedure
Factor of Safety (FOS) method.
Compute hydraulic conditions.
Assume a block class.
Calculate FOS.
If FOS is greater than or equal to 1.5 (okay).
If FOS is less than 1.5 (Not okay). Select a larger block class and compute FOS again.
Major Manufacturers of Cellular Systems and ACB Mats
Armortec, 3260 Pointe Parkway, Suite 200, Norcross, GA 30092, Website:www.armortec.com Western Region, 17113 Minnetonka Blvd., Suite 223
Minnetonka, MN 55375, Phone: (952) 476-5979
Presto Products Company - Geosystems 670 N. Perkins St., P.O. Box 2399, Appleton, WI 54912-2399, Website: www.prestogeo.com, Tel: 1-800-548-3424 or 920-738-1118
International Erosion Control Systems L.L.C., Cable Concrete, P.O. Box 119, 30630 Forest Boulevard, Stacy, MN 55079, Website: www.IECS.com, Tel: 1-800- 433 9143 or (612) 462 4466.
PETRATEC, Inc., 4444 West 78th Street, Minneapolis, MN 55435, Tel: 1-800-787-2359 or (612) 897 1617
Filter Design Why is a filter required?
To prevent migration of fine soil through revetment (retention). If filter openings are too large, excessive piping of subgrade soil will cause failure.
To relieve hydrostatic pressure within soils under the revetment (permeability). If filter openings are too small, hydrostatic pressure will build up behind the filter and cause a slip-plane to form and result in a massive slide failure.
To distribute the weight of the armor and provide uniform settlement.
Filter Design Types of filters
Granular filters Geotextile filter fabric
• (quick installation, less expensive, difficult to install underwater)
Granular Filter ratio requirement (applies between soil and filter and also filter and revetment cover): D15 (coarser layer)/D85 (finer layer) ≤ 5 D15 (coarser layer)/D15 (finer layer) > 5 but < 40
Multiple layers may be used to satisfy the filter ratio.
Granular filter blanket thickness ranges from 6 to 15 inches.
Geotextile Filter Design Criteria
Apparent Opening Size or AOS (soil retention)
For soil with ≤ 50% particles by weight passing U.S. #200 sieve (0.074 mm), use filter with AOS less than 0.6 mm.
For soil with > 50% particles by weight passing U.S. # 200 sieve, use filter with AOS less than 0.3 mm.
Permeability (K) Kfabric > Ksoil
Selected Design Software RIPRAP Design System Program, WEST
Consultants, Inc., 11848 Bernardo Plaza Court., #140-B, San Diego, CA 92128, Tel: (858) 487 9378
AFLEXOCF Computer Program and Armorflex Design Manual for ACB Revetments, by Armortec, Tel: (952) 476 5979
GABION DESIGN SYSTEM User’s Manual, Terra Aqua Gabions, P.O. Box 18158, Reno, NV 89511, Tel: (775) 828 1390
GEOWEB Cellular Confinement System Program by Presto Products Company
Selected Design Software GEOFLTR, Geotextile Filter Design Program by TC
Mirafi, 365 South Holland Drive, Pendergrass, GA 30567, Tel: (706) 693 2226
EROSIONWORKS Computer Program by American Excelsior Company, 850 Avenue H East, Arlington, TX 76005, Tel: (817) 640 1555
ECMDS Erosion Control Blanket Design Computer Program by North American Green Inc., 14649 Hwy 41 North, Evansville, IN 47711, Tel: 1-800 772-2040
EROSIONDRAW and BIODRAW by John McCullah, Salix Applied Earthcare, 491 South Street, Redding, CA 96001, Tel: 1-800 403 0474