LA LARGE WOOD E WOOD STRUCTURE RE STA TABILITY Y AN ANAL - - PowerPoint PPT Presentation

LA LARGE WOOD E WOOD STRUCTURE RE STA TABILITY Y AN ANAL ALYSIS T TOOL

Michael Rafferty, P.E. AMEC Environment & Infrastructure Thesis Study

Introduction

• Large wood design requires numerous complex, iterative,

and time-consuming calculations

• High quality tools have been developed for large

Engineered Log Jams (ELJ’s)

• Restoration community would benefit from a publically

available design tool for small- to medium-sized large wood structures that incorporates the latest design guidelines and science

Model Development

• 1. Based on nationally accepted design standard

(NRCS NEH 654 Tech Supplement 14J, 2007)

• 2. Applicable to a wide range of structure types and

anchoring techniques

• 3. Clear and user-friendly data input
• 4. Automated to the extent possible
• 5. Ability to handle interactions between logs in

small to medium-sized multiple log structures

• 6. Flexible to account for characteristics of different

stream systems and geographic regions

Large Wood Structure Types

Single Log Structures: Rootwads, Log Vanes, Log Weirs, Tree Revetments, In-situ Logs Multiple Log Structures (~2 to 10 logs): Flow Deflection Jams, Full-Spanning Jams, Transport Jams, and In-situ Jams

Image Source: NRCS TS14J

Force Balance Method

• Experimental approach pioneered in the late 1990’s
• Basis of NRCS 2007 method (Technical Supplement 14J)

Force Balance Method

VERTICAL FORCES (floating)

Driving: Buoyancy and Lift Forces Resisting: Weight of Log and Soil, and Anchors

HORIZONTAL FORCES (sliding)

Driving: Drag Forces Resisting: Friction, Passive Soil Pressure, Anchors

MOMENT FORCES (rotating)

Moment arms of Driving and Resisting Forces

LOG INTERACTION FORCES (various)

Resulting Forces from Adjacent Logs

Sample Application - Rootwad

Data Input

• Step-by-step organization of worksheets
• Final tab in spreadsheet has a complete list of

notation, symbols, abbreviations, and units

Step 1 – Edit Cover Sheet

• Enter Project

Name, Revision Date, and Designer Name

• Optional: Insert

project-specific photograph

Step 2 – Assign Factors of Safety

• Verify/edit design

Factors of Safety

• “Stability” and

“Risk” are commensurate with project setting

Step 3 – Hydraulic Characteristics

• Assess flow conditions: Typically use a hydraulic model
• Input maximum flow depth (6 ft), average velocity (5.65

ft/s), and wetted area (1,178 ft2) for the design discharge (1,000 cfs)

• Given the bankfull width and radius of curvature, the

tool estimates the velocity at the outer meander bend (8.06 ft/s) using an USDOT formula (HEC No. 23, 2009)

• User can input data for up to 20 site locations

• Input stream bed substrate grain size (45.0 mm)

Step 4 – Soil Properties

• Input bank soil type (gravel/sand) based on field observations
• Tool uses a lookup table or formula to determine friction

angles and unit weights (dry and buoyant)

• Custom soil properties can be entered into the lookup table

Step 5 – Select Wood Species

• Use map to select

geographic region

• Select up to 10

species from a dropdown list of common riparian tree species

• Tool inserts the unit

weights of the wood (Source: USFS NRS-38)

Step 6 – Define Site Cross Section

• Use dropdown lists to select the Site ID, structure

type, position in the channel

• Input channel cross section geometry

Step 7 – Input Structure Geometry

• Select the wood species (Douglas-fir)

and add a rootwad to the log

• Input the log length (24 ft), diameter

(2 ft), orientation to flow (315° ), tilt angle (-2.0° ), and location of the log at a key point (in this case, the bottom of the root collar)

Structure Geometry Layout

• Model automatically calculates various design

parameters using geometry equations, the slice method, and the partial area method

Step 8 – Vertical Force Analysis

• Model calculates the vertical force balance
• Factor of Safety is 1.05, which is below the target

value of 1.50. An additional 2,753 lbf of resisting force is required to meet the design criteria.

Add Anchor Forces

• Designer may choose to add additional soil ballast,

boulder ballast, or mechanical anchors

• Manufacturer load ratings are included in lookup

tables for several types of mechanical anchors (Manta Ray, Stingray, Duckbill, and Platipus)

• For this design, boulder ballast is added to the log

Updated Structure Geometry Layout

• Verify anchor position, which in this case, is two

boulders (~3 ft dia) sitting on top of the log

• Tool calculates a force balance for each boulder

Check Vertical Force Analysis

• New factor of Safety is 1.57, which is above the

target value of 1.50

• Vertically stable

Step 9 – Horizontal Force Analysis

• Model also calculates the horizontal force balance
• Drag forces are calculated using Gippel formula (1992),

which includes adjustments for wave drag and the blockage ratio of the channel cross section

• Factor of Safety is 5.72, which is above the target value
• f 2.00
• Horizontally stable

Step 10 – Moment Force Analysis

• Model also calculates the moment force balance
• Centroids are found for each force
• Point of rotation is determined (stem tip for this

sample design)

• Factor of Safety is 2.03, which is above the target

value of 2.00

• Structure is stable

Design of Multi-Log Structures

• First analyze force balance for “non-key members”
• Then design “key members” and input resultant

forces, relative position, and connection type of adjacent “non-key members”

• Tool determines which forces are transferable to

the next layer of log

• If necessary, apply anchoring techniques to “key

members” until design criteria is met

Additional Design Considerations

Designer must also consider:

• Ecological design criteria
• Decay rates of wood
• Loading from trapped debris
• Scour protection for bank soils
• Bed scour depths
• Suitability of soils for anchors
• Regulatory codes (e.g., no-rise)
• Risks to recreational users
• Construction considerations (e.g., wood availability

and cost, site access, protection of existing trees)

Limits of Applicability

Tool may not be applicable or require manual adjustments for the following design scenarios:

• Larger engineering log jams (10+/- logs) with

complex structure geometry

• Logs with branches or partial rootwads
• High energy streams
• Sites with highly erodible banks or stream bed
• Large rivers with complex geometry

Distribution of Design Tool

• Large wood design tool and companion paper

(thesis) are available for download at: www.engr.colostate.edu/~bbledsoe/streamtools/ Designers shall verify the calculated values and take full responsibility for the final wood structure design and performance. User feedback is encouraged.

Questions?

Download: www.engr.colostate.edu/~bbledsoe/streamtools/

Contact Information: Michael Rafferty, P.E.

• Sr. Habitat/Civil Engineer

michael.rafferty@amec.com Special Thanks to: Rob Sampson, NRCS CSU advising committee:

• Dr. Bledsoe
• Dr. Wohl
• Dr. Nelson