USDI Bureau of Land Management Roseburg, Oregon District Larry Standley Hydrologist Cory Sipher Fisheries Biologist Rick Shockey District Engineer 1
Introduction Background why model buoyancy? The buoyant force The model Field methods Slide Creek LW Project Background Observation of other LW projects. Needed a practical method to keep logs in place Maintain desired effects longer Cost effective Reduce risk to downstream structures Recent literature buoyancy is dominant force (2000, Braudrick & Grant). 2
The Buoyant Force Archimedes Principle: (3 rd century B.C.) The buoyant force on a body immersed in a fluid is equal to the weight of the fluid displaced by that object. Water weighs about 62.4 lbs/ft3 Log Drives 3
LW Buoyancy Model Simple model for field application Approximates true buoyant forces. Other stream forces are ignored (drag, friction, turbulence, debris flows). Don t spend more on analysis than the cost of being wrong. (Wise Person) Submerged Log Log Weight water surface LOG Buoyant Force Log = 40 ft x 2 ft Density = 35 lbs/ft 3 Volume = 126 ft 3 Weight = 4,398 lbs Buoyant force = - 7,841 lbs Resultant force = - 3,443 lbs 4
Off-bank Log Design Water Surface Log Weight LOG Hinge Point Buoyant Force Free Body Diagram for Off-bank Log Wl (+) rl Hinge Point LOG B (-) rb Resultant torque = Wl rl + B rb 5
Embedded Log Design Water Surface Log Weight Soil Weight LOG Buoyant Force Hinge Point Pin Log Log 2 Log 1 Design Water Surface Buoyant Force Log 2 lays on top of Log 1 6
Design Water Surface Elevation Determine project life expectancy (25 yrs.) Determine risk acceptable probability and consequences of failure. Calculate return period flood (50 yrs). Design water surface elevation Percent probability of occurrence of one or more events equal to or greater than the T year event in N years. No. of Years (N) Return Period (T) Years 5 10 20 25 50 75 100 200 5 67 41 23 18 10 6 5 2 10 89 65 40 34 18 13 10 5 20 99 88 64 56 33 24 18 10 25 * 93 72 64 40 29 22 12 50 * * 92 87 64 49 39 22 75 * * 98 95 78 63 53 31 100 * * 99 98 87 74 63 39 *>99.5 Hydrologic risk and return period selection for water related projects, BLM Technical Note #337 7
Field Methods Lay out design water surface elevation Establish Bankfull elevation Establish 50 year flood elevation Estimate log size and place end stakes Run spreadsheet hand held computer Log floats? Alter log size, log placement, or site Iterate until model shows logs won t float at design water surface Key Points Importance of buoyancy to LW movement Buoyant forces dominate drag forces If it doesn t float, it s less likely to move Design water surface elevation Spreadsheet Model to assess buoyancy Free download on Oregon BLM website 8
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Slide Creek - existing conditions Coho salmon and steelhead trout Lacking large woody debris Incised bedrock channel Riparian condition Large conifers present Accessible 10
Goals of the project Increase gravel retention Increase pool complexity Increase juvenile survival and productivity Project life of 25 years 11
Methods Selected trees from riparian corridor Calculated log lengths and buoyancy Felled and moved with minimal impact Excavation in incised reaches Limited disturbance/rehabilitation 12
Preliminary results Bankfull event in December 2003 2 to 3 year event in December 2004 All structure locations were stable Minor erosion of the banks near embedded structures Butt ends of all logs did not move Pin logs generally did not move 13
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Preliminary results n butt moved tip moved Off bank logs 41 0 2 Pin logs 12 0 1 Embedded logs 8 NA 0 15
Lessons learned Buoyancy method was effective at securing logs Doesn t account for debris flows or drag forces Can be used with other methods Acknowledgements Matt Fairchild, Fisheries Biologist/Project Lead Larry Standley, Hydrologist/Contracting Officer Mike Vallance, Contract Administration Tony and Brandon Lee, Lee Enterprises/ contractors 16
Spreadsheet model to estimate buoyancy www.or.blm.gov/gis/resources/other.asp 17