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