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Bed Mobility

Bed Mobility. David Thompson Will Asquith, Meghan Roussel, Frank Heitmuller Ted Cleveland Xing Fang. Progress to Date. Literature Review Field Studies Laboratory Work Modeling. Three Hypotheses.

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Bed Mobility

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  1. Bed Mobility David Thompson Will Asquith, Meghan Roussel, Frank Heitmuller Ted Cleveland Xing Fang

  2. Progress to Date • Literature Review • Field Studies • Laboratory Work • Modeling

  3. Three Hypotheses • Context and Natural History: The entire gravel bed moves down-gradient (hydraulic gradient) • Raising the grade of a structure has no affect on gravel passage, that is low-water crossing built at grade is better than elevated low-water crossing • A porous structure can mitigate failures.

  4. Hypothesis: Context and Natural History • Three components to this hypothesis: • Remote sensing -- image, mapping analysis, and historical aerial photograph interpretation will yield significant understanding of mechanics of motion (Bed motion is episodic.) • Field investigations/geophysics (The entire bed moves.) • Flood frequency (Recent years have been unusual from a hydrologic perspective.)

  5. Remote Sensing Analysis • Determine the extent upstream/downstream from existing low-water crossings • Determine if migration is homogeneous or "pulsating/episodic.” • Estimate $50K

  6. Field Investigations/Geophysics • Determine depth of "mobile" gravels; mobile layer has fewer (define fewer) fines relative to immobile layer • Determine if gravel bed descends in a series of steps or a smooth gradient • Electromagnetic Survey - $120K

  7. Flood Frequency • Is current "period" unusual from a hydrologic perspective? • Flood-frequency analysis: Estimate $50K • Indirect flood measurements: Estimate $60K • Is there a simple way to predict (in a probabilistic sense) a potential problem: • Are there any characteristic predictors that can be measured in the field (i.e. slope, sinuosity, size distribution, etc.)? Field work: $150K • Transport rate distribution function • BAGS predictor estimates: $150K

  8. Hypothesis: Crossing At Grade • If a structure traps debris then it will affect passage rates of materials which in turn unfavorably affects the local hydraulics • Field work: $50K • Video monitors: $36K (3 sites) • RADAR monitoring: $550K (water surface velocity, 4-years duration, 3 sites)

  9. Hypothesis: Porous Crossings • There exists evidence that downstream hydrostatic failure of aprons occurs during flooding events, therefore the hydrostatic pressures in the structure could be important. • This hypothesis is testable in a laboratory (idealized geometry). • Results: optimal porosity; at grade or above grade; kind of anchoring required. Gabion-based crossing. Transition from flow through to flow over; critical depth at transition to submerged flow.A product is a conclusion on porosity. • Costs: • 150K -- laboratory/DNS • 75K -- instrument 3 sites

  10. Conclusions • It may be better to place crossings near “natural grade” • Porous crossings may be better than solid crossings • We may be in a period of unusual flooding • The entire gravel bed may move as a unit, or it may move episodically • The mechanics of bed motion are unknown • Development of effective designs is dependent on knowledge of the above conditions

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