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 ft 2 ) 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

Step 4 – Soil Properties • Input stream bed substrate grain size (45.0 mm) • 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 of 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? 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 Download: www.engr.colostate.edu/~bbledsoe/streamtools/