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