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CONCEPTS – Conservational Channel Evolution and Pollutant Transport System. Eddy J. Langendoen Robert E. Thomas USDA Agricultural Research Service National Sedimentation Laboratory Oxford, Mississippi. Instructors. Eddy Langendoen
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CONCEPTS – Conservational Channel Evolution and Pollutant Transport System Eddy J. Langendoen Robert E. Thomas USDA Agricultural Research Service National Sedimentation Laboratory Oxford, Mississippi
Instructors • Eddy Langendoen • Research Hydraulic Engineer with USDA-ARS National Sedimentation Laboratory • Developer of CONCEPTS • Develop technologies that integrate fluvial hydraulics, sedimentation engineering, and computer modeling techniques to examine the effectiveness of stabilization and restoration practices • Background in computational hydraulics • M.S. in Civil Engineering and Ph.D. from Delft University of Technology, The Netherlands
Instructors (cont.) • Rob Thomas • Research Associate with USDA-ARS National Sedimentation Laboratory • Investigating lateral and vertical adjustment of channels to disturbances, including monitoring and modeling of the upstream migration of knickpoints. • Application of CONCEPTS to Goodwin Creek and the Yalobusha River, Mississippi. • B.S. in Geography from University of Nottingham, England
Course Outline • Introduction • Why CONCEPTS? • An application of CONCEPTS • CONCEPTS overview • Hydraulics • Sediment transport and bed adjustment • Streambank erosion mechanics
Course Outline (cont.) • Input data • Required data • How do we get it? • Output data • Output types • Output options • Execution of CONCEPTS using sample applications • Preparing input files • Running CONCEPTS • Viewing/interpreting output files
Why Do We Use/Develop CONCEPTS? • Most of our work is in the Mid-Continental USA • Highly erodible soils • Channelized stream systems • Unstable stream systems • How can we reduce impact or stabilize these stream systems
Characteristics of Unstable Streams • Migrating knickpoints • Mass wasting of streambanks • Recruitment of large woody debris • Impairment • Stream • Habitat • Infrastructure
Stream Rehabilitation/Restoration • Stabilize stream • ‘Hard’ structures (concrete, rock) • ‘Soft’ bioengineered or vegetative solutions • Combination • (Re)construct channel • Does our design work? • Need to evaluate design • CONCEPTS
Sample Application of CONCEPTS – Yalobusha River, Mississippi
Yalobusha River – Background • European agricultural development early 19th century • Channel capacity restricted by silt and debris accumulation • Excavation of 12-mile ditch through the river’s valley early 20th century • Sand, silt, and debris closed lower part of the channel • Clearing in the 1940s • New channelization in the 1960s • Extending ditch downstream • Realigned and dredged the mouths of all major tributaries • Construction of erosion control structures
Yalobusha River – Background (cont.) • Channel response: • Headward migrating knickpoints in all tribs • Extensive streambank erosion • Recruitment of large woody debris • Deposition of sands and debris at the terminus of channelization • Flooding problems at Calhoun City • Mitigation: plug removal
Channel Evolution • Channels respond to: • Hydrologic changes • Changes in sediment loadings • Man-made changes • How do channels adjust? • Vertically: scour – fill, degradation – aggradation • Horizontally: widening & migration • Models must capture these processes • Hydraulics • Sediment transport and bed adjustment • Streambank mechanics
Output: Changes in channel geometry Time series of hydraulic variables and sediment yield Input: Channel geometry Composition of bed and bank materials Erosion resistance and shear strength of bed and bank materials Rates of flow and sediments entering the channel Bendway weir Bed evolution and sediment transport Streambank erosion Flow hydraulics CONCEPTS – CONservational Channel Evolution and Pollutant Transport System CONCEPTS simulates long-term response of channels to loadings of water and sediments, and to instream structures
Hydraulics • Unsteady • One-dimensional • Long term simulations • Cross-stream variations are neglected • Gradually-varying • St. Venant equations • Special techniques to handle ‘flashy’ runoff events • Solution method • Generalized Preissmann scheme • Gaussian elimination
Hydraulics – Boundary Conditions • Boundary conditions • External boundaries • Upstream and downstream ends of stream corridor • Internal boundaries • Locations where the flow is rapidly varied • Hydraulic structures • Knickpoints • Rapids
Hydraulics – External Boundary Conditions • Upstream boundary • Discharge hydrograph • Downstream boundary • Rating curve • User specified • CONCEPTS’ generated loop-rating curve h= flow depth (m) K = conveyance (m3/s) Q = flow discharge (m3/s) Sf = friction slope (m/m) t = time (s) a= coefficient of rating curve b= exponent of rating curve
Hydraulics – Hydraulic Structures • Hydraulic structures in CONCEPTS • Box and pipe culverts • Bridge crossings • Grade control structures • Generic structures • We need two equations • Continuity equation • Dynamic equation Relate discharge with stages upstream and downstream of the structure
Sediment Transport • Sediment transport rates are a function of: • Flow hydraulics • Bed composition • Upstream sediment supply • Sediment deposited on or scoured from the bed changes bed composition, flow hydraulics, and fractional transport rates • Sediment discharge is therefore calculated using a transport (advection) equation with source terms representing loadings (entrainment and lateral contributions) and losses (deposition)
Sediment Transport (cont.) • Sediment mixtures • 13 predefined size classes • Sediment transport capacity is predicted by SEDTRA • CONCEPTS tracks the composition of both surface and substrate • Distinguish different erosion characteristics of cohesive and cohesionless bed material • Cohesionless: local erosion or deposition is proportional to the difference between sediment transport rate and capacity • Cohesive: excess shear stress approach
Sediment Transport – Approximation • Fractional step method. The transport equation is split into an equivalent system of: • Advection equation • Sediment mass is transported downstream without the effects of sources and losses. • Solved by the method of characteristics • Rate-of-Change equation • Sediment mass is adjusted by taking into account erosion and deposition • Analytical solution assuming sediment transport capacity is constant during a time step
Sediment Transport – Bed Adjustment deposition partly wetted bed erosion partly wetted bed erosion fully wetted bed deposition fully wetted bed
Sediment Transport – Boundary Conditions • Upstream boundary • Sediment load as a fraction of local sediment transport capacity • Time series of sediment load (kg/s) • Downstream boundary • Not required by the solution method, however only processes occurring in the reach then determine the evolution of the outlet. Change in bed elevation can be adjusted by multiplying it with a coefficient varying between 0 and 1.
Sediment Transport – Internal Boundary Conditions • Sediment transport rate at a hydraulic structure is a function of the supply of sediment and the invert of the structure • If invert is above channel bed, particles transported as bed load are deposited • If invert is below or at the channel bed all sediment passes the structure
Streambank Erosion • Channel-width adjustment occurs in a wide variety of geomorphic contexts • Equilibrium approaches are unlikely to accurately predict width adjustment over time • Fundamental processes responsible for bank retreat • Hydraulic erosion • Mass bank failure
Streambank Erosion – Hydraulic Erosion • Transport, deposition, and erosion of cohesive sediments are extremely complex • Erosion rate is given by an excess shear stress relation • Critical shear stress: • Arulanandan et al. (1980) if sodium adsorption ratio, dielectric dispersion, and pore fluid salt concentration are known • In situ measurements (Hanson and Simon 2001) • Historical data on the retreat of the base of the bank combined with flow data • The effects of weathering processes and vegetation can be included by adjusting critical shear stress
Streambank Erosion – Hydraulic Erosion (cont.) • Average shear stress ti on each soil layer • Average erosion distance DEi
Streambank Erosion – Streambank Stability • Weight is the primary force tending to move the failure block • Mobilized shear at the slip surface is the primary resisting force • Shear strength is affected by • Pore-water pressures • Vegetation • Mechanical reinforcement • Reduction of soil moisture through canopy interception and evapotranspiration • Increase soil moisture through stem flow
Streambank Erosion – Planar Failure Analysis • Stability is analyzed using limit equilibrium methods => FOS • Methods are based upon static equilibriums of forces and/or moments • Method of slices Fw= confining force (N/m) N= normal force (N/m) S= mobilized shear force (N/m) W= weight of composite soil (N/m) b= angle of slip surface
Streambank Erosion – Inclination of Failure Surface • The inclination of the failure plane is that for which the factor of safety is a minimum • CONCEPTS uses a search method to find smallest FOS • Factor of safety is evaluated at Ne number of points along the bank profile
Test Case 1 • Prismatic channel • 25 km long • 0.002 m/m bed slope • Manning n is 0.04 • 51 Cross sections composed of a rectangular main-channel right-overbank sections at 500 m intervals • Single runoff event
Run Control File • The Run Control file sets: • case name, • project title, • lateral inflow due to tributaries, • downstream rating curve, • the proportion of sediment transport capacity for all the sediment size classes being transported into the upstream end of the reach, • the silt fraction and degree of downstream bed control,
Run Control File (Cont.) • The Run Control file sets: • selection of the bank failure options, such as whether positive pore-water pressures, confining pressures, and matric suction are accounted for, and the complexity of the analysis- the number of time steps at which factor of safety is computed plus the number of potential failure planes in the bank, • selection of the flow resistance formulation, • water temperature, • activation of sediment transport mechanisms and streambank mechanics, • sets the start and end time of the simulation, the initial time step length • the makeup of the modeling reach (the number of links, cross-sections and their names), • output options.
Output Data. • CONCEPTS creates three types of output: • output at a certain cross-section and for a certain runoff event • time-series output at a chosen cross-section • output for a certain runoff event along a section of the modeling reach • Names of the output files are in the form: run identifier + ‘ ’ + number + ‘.TXT’. e.g., if run identifier is SIMULATION, then output file names are ‘SIMULATION 050.TXT’, ‘SIMULATION 051.TXT’, etc..
Output at a Certain Location and for a Certain Runoff Event • To request data for a chosen cross-section and runoff event, user has to: • enter number of locations at which output is requested. • for each location, user must enter: • type of data required, • location of required cross-section within modeling reach, • number of runoff events for which output is requested, • dates of those events, • repeated for however many output cross-sections are required.
Time-Series Output at a Certain Location • CONCEPTS checks if time falls between start and end time of all requested time series. When model time is within time series boundaries, the requested parameters are printed. • To request output at a certain cross-section over a period of time, the user has to: • enter number of locations at which output is requested. • for each location, user must enter: • type of data required, • the location of the required cross-section within the modeling reach, • number of time series for which output is requested, • the start and end dates of the time series, • repeated for however many output cross-sections are required.
