Armfield S12-MKII, Инструкция по эксплуатации

Страница: 49 / 67

45
Exercise E - Effect of base level change
Theory
The third dimension of adjustment in alluvial channel morphology is the long-profile.
The long-profile can adjust through changes in bed elevation, channel gradient, and
overall profile shape. Bed elevation changes occur through aggradation and
degradation, and reflect the channel’s ability to transport sediment. Base level
represents one particularly important control on channel profile.
A reduction in downstream base level leads to channel degradation, initiating incision
as a
nickpoint forms and travels upstream. As the nickpoint migrates upstream, the
sediment generated by it may drive aggradation downstream. In time, secondary
nickpoints may appear downstream of the primary nickpoint and aggraded area, and
the steepened area containing these nickpoints is referred to as the
nickzone . The
sequence of channel adjustments resulting from nickpoint migration and local
aggradation has been termed complex response because a single baselevel lowering
event can generate multiple step changes in bed level. The overall channel
degradation resulting from a fall in base level is a regional phenomenon that involves
the progressive and sustained net lowering of the bed over long distances and
timescales.
An increase in base level leads to aggradation and channel change through the
development of a deltaic type of deposit. Aggradation is a result of the channel
having insufficient energy to transport the sediment supplied from upstream and it
characteristically results in loss of channel conveyance capacity and localised
flooding.
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 2 cm deep.
Set the inlet flow rate to 2.5-3 l/min. Observe the channel during the development of
an alluvial channel environment. Record the sediment yield every 10 minutes.
Record the topography of the bed after 30 minutes (the flow may be turned to a low
level during measurement, so that the channel does not change during this time),
and again after 60 minutes. Make notes on the presence of features such as bars
and terraces, and particularly record the position of the channel thalweg. Measure
the channel length by laying a piece of string along the edge of the channel. The
length of the thalweg can be measured in a similar manner.
To simulate an increase in base level, add one or more additional stop logs to the
deep cut-out. To simulate a drop in base level, remove the stop-log already in
position. Observe the changes carefully, continuing to record the sediment yield rate
and channel planform at regular intervals (15-30 minute intervals for channel
planform, depending on the speed of morphology change). For a drop in base level
there will be an initial rush of sediment, and sediment yield will have to be recorded
every two minutes until the nickpoint has migrated upstream away from the outlet.