Presentation on theme: "U.S. Department of the Interior U.S. Geological Survey Numerical Analysis of the Performance of River Spanning Rock Structures: Evaluating Effects of Structure."— Presentation transcript:
U.S. Department of the Interior U.S. Geological Survey Numerical Analysis of the Performance of River Spanning Rock Structures: Evaluating Effects of Structure Geometry on Local Hydraulics http://www.usbr.gov/pmts/sediment/kb/SpanStructs River Spanning Rock Structures Research Chris Holmquist-Johnson
Background Rock weirs capabilities include: fish passage, bank protection, channel profile stability, and improved aquatic habitat. Rock weirs are ideal structures for conditions of…steep slopes, diversions, scalability, habitat, and aesthetics. Failures or loss of function can result in: disruption of service, expensive repairs, maintenance cost, failure to meet biological and geomorphic requirements, and loss of prestige.
Background Qualitative field evaluations and Initial laboratory results found widespread performance issues. We think this is a flaw in the designs, not in the concept. The Rock Weir Design Guidelines Project was developed to investigate failure mechanisms and provide methods for designing sustainable structures.
Research Components The Research Project takes a multifaceted approach incorporating mutually supporting field, laboratory, and numerical studies. Field LaboratoryComputer Provides a controlled setting Includes scour processes Physically replicates hydraulics Inexpensively simulates large numbers of cases Increases range of applicability Provides a design tool Evaluates current methods Hypothesizes successful techniques Identifies failure mechanisms Creates a practical link
Need for Physical Modeling Field investigation identified growth of a scour hole causing geotechnical movement as the primary failure mechanism for rock weirs Understanding the scour processes and quantities provides vital information on failure modes and potential counter measures required to design sustainable structures Understanding failure modes in the field is COMPLICATED Analyzing scour hole development is best answered in a controlled environment such as the laboratory
Need for Physical Modeling Assumed Physical Model (Q bankfull, Small cobble ) Difference in anticipated scour hole definition and physical model results Rosgen (2001) - Max scour U-weir generated two defined scour hole locations
Physical Modeling Objectives Rating curves for flood elevations Scour prediction method Potential counter measures and retrofits Low flow depths and velocities for fish passage criteria
The field evaluation included 21 sites consisting of 127 structures and included various structure designs by Reclamation, Natural Resources Conservation Service (NRCS), Dave Rosgen, and Oregon Department of Fish and Wildlife (ODFW). The observed range of conditions included: Evaluations of Field Performance Approximate channel widths from 15 feet to 250 feet Approximate channel slopes from 0.5% to 2% 75 U-or V-weirs, 28 J-hooks, 12 A-weirs, 4 W-weirs, and 2 rock ramps
Evaluation of Field Performance Field measurements provide a practical link to guide and validate laboratory and numerical investigations. Field evaluations identified several failure mechanisms. Scour and Slumping Flanking Loss of Pool Depth Incipient Motion Identifying failure pathways allows for design of countermeasures. A-weir with throat buried by sediment Displacement of Header rocks along arm Redirection of flow at bankfull Q
Velocity Vectors Parallel to Bed Velocity Vectors Returning Parallel to Bed Plunging Flow Hydraulic Jump Rapid Vertical Contraction and Expansion over the Weir Crest Importance of Numerical Modeling Field and laboratory provide data over a limited range of conditions. Narrower ranges of conditions limit the applicability of results. Numerical modeling provides opportunity to test numerous design parameters over large ranges for less money and in a shorter amount of time to extend the applicability.
Numerical Model Design Parameters Bed Material and Slope Coarse Gravel to Large Cobble (23mm to 181mm) Slope.001,.005, and.01 Channel Geometry Regime Equations (Parker et al. 2007) S = bed slope; Q bf = bankfull discharge (m 3 /s); D s50 = median particle diameter, (m); W bf = bankfull width (m); H bf = bankfull depth (m).
Numerical Model Design Parameters Structure Geometry Drop height 0.4, 0.8, 1.2 ft Throat Width .25W,.33W, and.5W Arm Length (arm slope & departure angle) .5 and 2 times minimum deviation (4.5% and 25 degrees) 2-7 Percent
Numerical Design Parameters
Numerical Model 3D model U 2 RANS- (Yong Lai, Reclamation) Unsteady and Unstructured Reynolds Averaged Navier- Stokes solver Preprocessor, CFD solver Mesh Generation and Post Processing Auto mesh generator, SMS, Tec Plot
Automatic Mesh Generator Flow-WiseBank-WiseStructure-WiseLaterally UpstreamLeftBedThroat DownstreamRightHeaderToe FooterTop bank PoolOverbank
Automatic Mesh Generator
Numerical Model Validation Laboratory comparison U-Weir Bank Full 2-7 Percent 0.8 ft 1/3 W 20-30 Design Concept ConstructionBefore Test
Numerical Model Validation Bed shear stress distribution at Q bkfull Laboratory comparison U-Weir Bed topography from LIDAR scan Surface velocity distribution with stream lines
Numerical Model Validation Laboratory comparison U-Weir water surface profile comparison
Numerical Model Validation Laboratory comparison U-Weir Velocity Comparison
Numerical Model Validation Field U-weir Velocity at Q bkfull Field comparison
Numerical Model Results Results from the numerical model show a significant change in velocity and shear stress distributions when the geometry of the structure is altered Currently working on a statistical method for describing the variation in velocity and bed shear stress to structure configuration
Numerical Model Results Assist In: Optimizing structure geometry for given restoration needs (Diversion, Fish passage, bank protection, pool development, etc.), Analysis of flow hydraulics resulting from differing structure configurations, Predicting DS scour hole development, and Providing techniques for analyzing, designing, and constructing sustainable rock weir structures.
Research Summary The research produces tools and guidelines for structure design or retrofits based upon predictable engineering and hydraulic performance criteria. Understanding the processes governing success and failure allow designers to construct robust and sustainable structures requiring fewer repairs, less disruption of service, and lower maintenance costs. Guidelines simplify and reduce future design efforts while increasing the likelihood of successful structures. We hope that solidly developed criteria will simplify the regulatory permitting processes.
In the End Reliable structure designs reduce failures, Fewer retrofits saves money, Saving money while meeting regulatory requirements governing habitat and fish passage makes everyone happy Reduced failures results in fewer retrofits,
Colorado State University Dr. Chris Thornton and Dr. Chester Watson Tony Meneghetti, Mike Skurlock Reclamation Kent Collins, Kendra Russell, Elaina Holburn, David Mooney Science & Technology program Pacific Northwest Area office Special Thanks To: