Armfield S12-MKII, Инструкция по эксплуатации

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47
Exercise F - Scour in open channel flow
Theory
Local conditions can affect channel flow. These can be natural conditions, but man-
made obstructions and artificial structures are becoming increasingly common.
Changes in land use can also affect channel conditions by altering runoff,
transpiration and evaporation rates. Local conditions do not necessarily have local
effects, as the river environment is a complex system with many interrelated parts.
One of the most common type of artificial structure is a solid obstruction placed in the
channel flow, such as a bridge pier or groyne. General and local scour are two similar
morphological responses to such flow obstruction. General scour is a site-specific
phenomenon involving bed lowering in a reach over a long period, or only during high
flow events. It is usually caused by an accelerating flow velocity through a narrowed
channel width.
Local scour is a local phenomenon where a section of the bed is lowered in response
to increased velocity and turbulence adjacent to a solid obstruction. Channel
response can be seen in both upstream and downstream directions. On the
upstream side a horseshoe vortex develops and results in the formation of a scour
trench. On the downstream side a line of vortices called a
vortex street is produced.
Procedure
Connect the flexible piping from the river inlet tank to the quick release connector on
the 5 l/min flow meter. Fit one of the two 50mm stop logs into the deep cut-out at the
foot of the tank.
Set the slope of the sand tank to approximately 2%. Smooth and lightly tamp down
the sand level with the top of the sand tank. Use the scoop provided to cut an initial
straight channel into the sediment bed from the river inlet tank to the deep cut-out.
The channel should be approximately 4cm wide and 2cm deep.
Switch on the pump and set an inlet flow rate of 2 l/min. Allow the channel to develop
for 30 minutes. Sediment yield and channel discharge may be recorded if desired,
but this is not essential.
Accessories may be placed in the channel in order to simulate bridge piers, groynes,
vanes and so on. These items should be partially sunk into the streambed to prevent
movement during the run. A variety of shapes have been provided so that the effects
of streamlined, flat-ended and cylindrical shapes may be investigated. Other items
may be used to simulate additional flow obstructions, such as a pebble to represent a
boulder. Any such items must be heavy enough to remain motionless in the flow.
Place the chosen object into the main channel approximately one third of the way
down the sand tank from the river inlet. Any changes in channel shape and the
development of any scour holes and sediment depositions should be observed and
recorded. Flow turbulence and eddies may be located by observing the reflection of
light off the water surface.
Increase the valley slope in increments of 0.3-0.5%, observing the development if the
scour hole and the channel along the length of the sand tank. The channel should be
left for at least 15 mins after each change in slope. Scour hole geometry, channel
width and depth, discharge, sediment yield and valley slope may be recorded if
desired.