Sand Bar Flushing Test through Gates #19~24 March
20~21
Sand packs over west side
gates (#19~24) flushing were tested. Deposited
sediment depth was measured and removing time was acquired.
Gate #19 was opened for 30
minutes, and then gate #20 was opened for 1 hour only. Sand pack depth upstream of gate #19 was
measured with calibrated paddle (figure 1).
Gate #21 and #22 were opened
for 2 hours and 3 hours, respectively. Sand pack depth measured after gates were raised.
Finally, gate #23 and 24
were opened in same time for 1 hour because of bank erosion.
Measured sand pack depth (ft)
depending on opening time is summarized in table below. These sand pack depths are roughly measured average
values.
First time step (11:30,
20-Mar) sand pack depths measured before gate opening. The blue colored characters represent beginning
time of sand pack erosion.
Sand pack erosion was begin together
with previous gate opening and continued to next gate opening. For example, sand pack over gate #20 was
eroded when gate #19 was
opened at 11:30, and the
erosion was continued up to gate #21 opening.
Gate #23 and #24 had to be
opened only 1 hour to keep west bank erosion and high amount of sand pack is still
existing. At least 3 hours of opening
needs to flush sand pack
and 5 more hours are expected
for heavy load such as gate #23 and #24.
|
Opening Time |
#19 at 12:00 |
#20 at 13:00 |
#21 at 15:00 |
#22 at 10:00 |
#23 at
11:00 |
#24 at
11:00 |
|
11:30, 20-Mar |
3.5 |
3.5 |
4.0 |
4.5 |
4.5 |
4.5 |
|
12:00 |
3.0 |
3.0 |
4.0 |
4.5 |
4.5 |
4.5 |
|
13:00 |
2.0 |
1.5 |
3.5 |
4.0 |
4.5 |
4.5 |
|
15:00 |
0.5 |
0.0 |
1.0 |
1.0 |
4.5 |
4.5 |
|
10:00, 21-Mar |
0.5 |
0.0 |
0.0 |
0.5 |
4.0 |
4.5 |
|
11:00 |
0.5 |
0.0 |
0.0 |
0.0 |
3.0 |
3.5 |
Sand pack seems moved
downstream because of the gate operation. Based on previous gate operations, high amount of sediment transport
from upstream was trapped and
deposited on the upstream
pool during the piano-like operation (task #3). Only one gate was opened at a time and water depth was really high
(about 4.3 ft) during the task #3
as shown in figure 2. After completion of the task #3, all gates were
fully lowered to survey channel bed change, and at this time, sediment deposited
upstream pool was moved
down over the lowered gates
as figure 3.
Figure 2. Sediment
transportation during Task #3 and #4 Figure 3. Sediment moved down when all gates
were fully lowered
When gates are raised for
task #4, sand packs placed on the gate panel were exposed as figure 4. At task #4, east side gates were opened to
keep water depth,
and deposited sediment was
washed away. Downstream sand pack, shown
in figure 4 (b), would be easily eroded, but upstream sand pack would be tough
to be removed.
Sand pack and flushing procedure
was analyzed based on the field observations and measurements.
To figure out sand pack volume,
moving direction, and speed, more study, (maybe numerical modeling) is required
for detailed analysis.
(a) (b)
Figure 4