6 Impact of Dredging Planform
At present, the upstream and downstream ends of the dredged region of the
Port of Anchorage have vertical sidewalls cut out of hard, nonerodible material.
The dredge transitions are oriented at a 30 deg angle to the shoreline. The abrupt
vertical transition may cause quiet flow regions which allow suspended sediment
to deposit in the harbor. The objective of this task was to test the hypothesis that
sloping dredge planform transitions on both ends of the harbor would decrease
areas of flow reduction and increase harbor flushing.
Comparisons were made between existing dredging practice and the
proposed dredging planform featuring sloping transitions. Both alternatives were
fabricated and placed on the precision flow table for testing. Flow fields near the
transition were quantified using the laser Doppler velocimeter, and dye injection
was used as a qualitative tool to evaluate the two dredge planforms.
A rectangular flow channel of width 380 mm (14.9 in.) was created on the
flow table using Plexiglas blocks as illustrated in Figure 50. The water depth
was set to 44 mm (1.7 in.), and a flow rate of 1.7 L/sec 3(.5 pints/sec) was used
for the experiments. Average flow velocity over the channel cross section was
approximately 10 cm/sec (3.9 in./sec). Relevant scaling factors (prototype-to-
model ratios) for the dredge transition models are listed in Table 9 along with
approximate model equivalences. Model geometric distortion was Ω = NX /NZ =
5 for this simplified model of the Port of Anchorage dredging transitions.
Two dredging transition models were fabricated out of Plexiglas by the
ERDC Model Shop. The first model, shown on the left side of Figure 51,
featured a vertical transition along the dredge cut that is sloped at 30 deg to the
shoreline. This represented the present dredging practice. The second model,
shown on the right side of Figure 51, is similar except for the sloping transition
from the deeper dredged area up to the existing bottom elevation.
In the first tests each transition was oriented as shown in Figure 50 with the
deeper portion upstream of the shallower portion. This represented flow leaving
the harbor over the transition. Next, the transitions were reversed on the flow
table to represent tidal flow from the shallower area entering the harbor over the
transition. Velocities on the horizontal portion adjacent to the transition were
Chapter 6 Impact of Dredging Planform