Short course for UNIVERSIDAD NACIONAL DE INGENIERIA January 26-29, 2016
Planning and Design for Rehabilitation of Rivers Using Large Wood Metodología para Reforestar Ríos Degradados por Actividades Humanas usando Técnicas de Bioingeniería
5.0 Selection of design condition
Course overview Day I (Jan 26)—Foundational topics
• Three design approaches • Key issues for large wood design
Day 2 (Jan 27)—Designing large wood structures • Case study I—Little Topashaw Creek, Mississippi • Design life for wood structures/selection of design event or condition
Day 3 (Jan 28)—Risk, uncertainty and construction • Sensitivity and Monte Carlo analyses
• Constructability assessment • Case study II—Trinity River, California • Monitoring
Day 4 (Jan 29)--Field trip
Shields Engineering LLC
• Introductions • Review of information resources (design handbooks and spreadsheets) for large wood • Is wood appropriate for your site?—criteria for screening (Planning)
• Types of wood structures • Findings of recent research on drag and lift coefficients
• “Road testing” selected design spreadsheets
2
How long will wood last? If continuously wet….almost forever. BUT, if NOT then……
Carll. 2009. FPL-GTR-179. USDA US Forest Service, Forest Products Laboratory.
How long will wood last?
Crossman, M. and Simm, J. 2004. Manual on the use of timber in coastal and river engineering. Thomas Telford Publishing. London, England.
How long will wood last? • Red River Waterway • Southern yellow pine, Douglas fir, spruce—untreated • 5 to 10 years “rots out”
Decay rates vary by species
Failure definition • Structure transported away • Structure causes undesirable erosion or deposition • Structure fails to provide habitat benefit
LW additions should not be designed to be permanent or fixed. They interact with organic, inorganic and biotic components of stream. Messy is not failure.
What is reasonable design life based on wood in your region? • In my opinion, it is hard to argue for design life > 10-15 years in any region if wood is wet/dry • Assuming design life is 10 years, what is design Q? NOAA event probability calculator
What is the design condition? • During the expected life of the structure, when will it experience maximum destabilizing forces? • Peak discharge for design life • Bankfull discharge—highest velocity • Prolonged high flow • Just barely overtopped after long dry period • Maximum wave drag • Maximum buoyant force Doug Shields, Jr.
www.friendofrivers.com
Drag forces a max when log just barely overtopped
Smooth Log (PVC)
Moderately Rough Log (Hackberry)
1.4
Velocity, m/s and Depth, m
Temporal variation of forces on cylindrical logs during hydrograph Flow depth, m Velocity, m/s
1.2
1.0 0.8 0.6 0.4 0.2 0.0
Vertical force (lift + buoyancy), N
Force, N
600
300
0
Force, N
600
Horizontal force (drag), N
300
0 00:00
05:00
10:00
Time from start of run, mm:ss
15:00
00:00
10:00
20:00
Time from start of run, mm:ss
30:00
Based on project site and design condition, select values for….. • Design life for wood (very regional, but ~ 15 yr max) • Wood density (0.4 to 0.5 x rw) • Drag coefficient (up to 6, but 1.0 typical) • Lift coefficient (-4 to 31, but 0 typical) • Soil strength (nil to 2000 psf?) • Safety factor (Do you feel lucky?-1.2 to 5.0)
Photo from Tracy Drury
Doug Shields, Jr.
www.friendofrivers.com