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

• 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.

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.)

Remote Sensing Analysis

• Determine the extent upstream/downstream from existing low-water crossings

• Determine if migration is homogeneous or "pulsating/episodic.”

• Estimate $50K

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

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

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)

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

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