Hydraulics in the Field
Location Map
The following field observations were taken at the Crimson Hiking Trail Head, just above Third Dam of the Logan River. The goal was to observe multiple types of river hydraulics and understand how they are influenced by the river regime.
Field Map Sketch + Cross Section
Denoted is a cross section of the Logan River, depicting the thalwag. The Planform drawing of the same location (A-A1) shows rippled and smooth surface flow types. The rippled is caused by the thalwag while the smooth surface causes sediment deposition. The flow is diverged around the exposed bar. During a high flow period, the bar would be submerged.
Channel Annotations
Above depicts convergence of flow around a long island. The velocities of the combining flows are different, resulting in a mixing zone.
The divergent flow is a result of the exposed bar. The flow converges once again down stream, resulting in a small eddy.
This section of the Logan River is immediately down stream from the above annotations.
The uniform flow results after the above convergence. A small stick jam results in a small wake before the flow separates.
The flow separates further around the exposed in-channel bars. Reattachment occurs and results in several eddies.
The thalwag is located on the right bank of the river, creating a flow seam along an eddy.
Past this point, flow is uniform once again.
The flow types present just past the bridge are rippled and smooth. As indicated in the Field Map Sketch section, the difference in surface flow has to do with the depth difference across the channel.
The thalwag is on the left side of the channel, where the velocity is higher. This results in rougher flow.
On the right side, next to the exposed in-channel bar, is slower flow resulting in sediment deposition.
The surface flow change occurs because there is a flow convergence around the exposed bar.
Discharge Calculation
To determine discharge we selected a very uniform section at Second Dam on the Logan River. Average depth was estimated to be 1 foot and width 60 feet, allowing us to calculated area as 60 feet. To determine velocity a stick was dropped from a bridge with a width of 6 feet. A stop watch was used to time how quickly the stick moved underneath the bridge, a total of 4.98 seconds. Velocity was calculated to be 1.2 feet per second. Area was multiplied by velocity, giving us 72 cubic feet per second (cfs).
Hydraulic Jump
The hydraulic jump occurs across the width of the river.
Hydraulic Jump.movJoe explains to us how subcritical flow creates waves upstream, but once in supercritical flow waves are no longer propagated upstream.