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