Design Guidance for Low-water Crossing in Gravel Rivers
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Transcript of Design Guidance for Low-water Crossing in Gravel Rivers
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Design Guidance for Low-water Crossing in Gravel Rivers
Xing Fang
Lamar University
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Guidance in Estimating Sediment Transport in Gravel River
• Goal: Provide a set of simple tools /procedures for TxDOT engineers to evaluate sediment transport in gravel rivers for various low-water crossing profiles.
• Tasks: Identify and test simple tools and procedures for many low-water crossings in hill-country area.
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Simple Tools/ProceduresPart I – Flow Estimation
• (1). Estimate watershed parameters from 30-meter digital elevation model (low-water crossing as watershed outlet).
• (2). Estimate discharges for different return periods from state-wide regression equations (Asquith and Slade, 1997)
• (3). Check any available discharge measurements from USGS or other agencies.
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Simple Tools/ProceduresPart II – Particle Distribution
• (1). Propose a simple method to determine sediment particle distribution in gravel river nearby low-water crossing, e.g., pebble accounting frame or tape method with gravelometer.
• (2). Test and document methods into simple procedures, which should be easy for TxDOT technicians to apply.
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Determine particle distribution by using pebble counting frame and gravelometer
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Determine particle distribution by tape method and gravelometer (6” interval)
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Simple Tools/ProceduresPart III – Surveying of Channel Geometry
• (1). Propose to survey at least three to five channel cross sections near the low-water crossing: one at the low-water crossing, two at the downstream and two at the upstream of the crossing including estimation of channel slope.
• (2). Provide guidance on cross section survey, especially, recommend appropriate distances between cross sections – important for HEC-RAS.
• (3). Channel geometry will be used for BAGS (Bedload Assessment in Gravel-bedded Streams) and/or HEC-RAS modeling.
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Simple Tools/ProceduresPart IV – Estimation of Bedload Transport
• (1). Propose and test to use BAGS and to estimate bedload transport rate under various flow conditions for natural channel condition and various proposed low-water crossing profiles.
• (2). Identify possibility to verify BAGS results with physical/numerical model results and field observations.
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Simple Tools/ProceduresPart IV – Estimation of Bedload Transport
• (3). Quantify the order of sediment deposition before the crossing for discharges at various return periods – Give possible sediment deposition at that return period.
• (4). Suggest which crossing profile is better – less sediment deposition, easy to pass sediment over the structure, and less possibility to maintenance.
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Guzman
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Simple Tools/ProceduresPart V – HEC-RAS Simulation
• (1). Identify ways to more accurately compute water surface profiles in HEC-RAS for gravel rivers with large gravel transport over low-water crossing, e.g. increase of Manning’s n for channel part, elevate the channel bed with D84 or particle characteristics size (e.g., Limerinos, 1970).
• (2). Use results of physical/numerical or field observation to verify water surface profile computation.
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Two- or Three Hydrodynamic and Sediment Transport Model
• Use NCCH2D/3D - two- or three- dimensional sediment transport model from NCCH (National Center for Computational Hydroscience and Engineering) for hydrodynamic and sediment transport modeling for low-water crossing in gravel rivers.
• 2D model is free from NCCHE, but 3D model is not. It has mesh generator.
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2D/3D Modeling
• Goal: (1) Use 2D model to simulate water surface profiles for gravel river with heavy gravel transport in order to identify ways to more accurately compute water surface profiles by using HEC-RAS.
• Equivalent Manning’s n – link n with characteristic particle size (some literature review)
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2D/3D Modeling
• Goal: (2) Test possibility/ideas of design of structure to fail (sacrificial structures).
• Goal: (3) Test/evaluate possible design a gravel accelerator/guide/funnel which is designed to promote passage of gravel flux over/ through structure (low-water crossing).
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Porous Structure
• Simple model for flow simulation through porous structure.
• Lay a series of small pipes inside porous structure to (a) accelerate flow passage under base flow condition, (b) eliminate any large size of culverts, and (c) make low-water crossing have minimum disturbance to natural channel.