Description:
Current velocities were measured on a number of transects across the Columbia River in Washington State, to understand how velocity varies within a channel. The velocity at any point in a river is controlled by a number of factors, including the river's slope or gradient, roughness of the channel bed, turbulence of the flow, depth of the river, etc. Typically, water moves faster away from the bed of the river, where obstacles create drag and turbulence. The highest velocity overall is usually in the deepest part of the channel, just below the surface.
Data on the variation of velocity with time at 3 different depths were obtained along a transect below Priest Rapids Dam on the Columbia River in Washington State. Velocities were measured every minute for 66 minutes. Even though all other variables are fixed, velocity still varies with time as the current surges and wanes. Knowing the mean velocity in a channel is extremely important when calculating river discharge, the volume of water flowing past in a given amount of time. The data emphasize the importance of thorough measurement.
The histograms and accompanying statistical parameters show some interesting patterns. Mean velocity decreases with depth, as expected. Roughness of the bed has a strong influence on velocity, due to both drag and turbulence. The standard deviation increases with increasing depth, perhaps reflecting this increasing turbulence.
