THE EFFECT OF LATERAL DIFFERENCES IN STREAM VELOCITY ON HYPORHEIC INTERCHANGE

The Effect of Lateral Differences in Stream Velocity on Hyporheic Interchange

Generally, hydrogeology deals with negligible kinetic head, but stream-subsurface interactions may require that it be considered. Bernoulli’s equation describes fluid potential, the dominant force driving flow on both sides of and across the stream bed:

P₁ + 1/2 rv₁² + rgh₁=P₂ + 1/2 rv₂² + rgh₂

For two flow lines of equal elevation (h₁=h₂) and with v₁ approaching 0 where water is standing along the point bar, the equation reduces to:

P₂=P₁ - 1/2 rv₂²

This implies that stream bed pressure will be less than water column static weight by an amount equal to the kinetic energy of the water in the column. Because water does not flow at equal velocities at all points, the bed pressure varies across a transect.

This study measures the effects of these differences in velocity on the hydraulic head (fluid potential divided by acceleration of gravity) beneath the stream bed using 3 sets of 3 nested piezometers along a transect across a sharp meander, and one set of 3 nested piezometers on each bank. Preliminary data show that water pressure changes unevenly across a meander transect as stage and velocity increase. During baseflow, an upward subsurface gradient masks stream velocity induced differences. For two unattended storm events, auto-logged data and subsequent observations suggest that debris trapped against the piezometers along the cut bank and in the thalweg diverted flow to the point bar. Subsurface pressure in the piezometers along the point bar increased less than the pressure in the piezometers downstream from the obstructions. Future storm events will be monitored and attended to clear the obstructions and to measure the flow velocity at each of the nests to correlate pressure with velocity. Although data does not yet support the predictions, it does indicate that kinetic head is significant in the calculation of stream fluid potential.