Applying this scaling criterion to the Ventura Harbor physical model with Nz =
NL = 25, Nde = 1.46, and Ng = Nγi = 1 gives a velocity scale of
Vproto
= (1)1/2(1.46)3/8(25)1/8 = 1.72
NV =
Vmodel
In other words, to reproduce the scour hole formed in the prototype under an estimated
mean velocity of 1.1 m/s, the model would need a mean velocity of
model = Vproto = 1.1 m/s = 0.64 m/s
V
1.72
1.72
Note that strict Froude scaling of the velocity would have yielded a model velocity
of 0.22 m/s, which is beneath the velocity required for incipient motion of the model
sediment. Therefore, the model scaling given by Eqn. 4 distorts the Froude velocity
to achieve similarity of the maximum scour depth. We note that the discussion of
scaling originally given in Hughes and Schwichtenberg (1998) is incorrect.
Model Calibration
There was enough uncertainty surrounding the proposed movable-bed scaling criterion
and estimate of total discharge that occurred during the scour hole formation, that
was necessary to "calibrate" the movable-bed model by attempting to reproduce the
original Ventura scour hole in the physical model. Calibration of the physical model
was an essential component of this study if any credence were to be given to the model
scour results.
Calibration was performed by incrementally increasing the total discharge through
the gap and comparing the equilibrium scour depth to the actual target depth. This
procedure follows the philosophy of distorting the velocity scale to achieve similarity.
Eventually, the equilibrium scour depth and general planform of the scour in the model
approximately matched the prototype at which time the bathymetry was recorded,
contoured and plotted. Two calibration tests were performed without the rock sill
in place across the gap. The first test served as a trial run, and the second test
was considered the calibration run. The second calibration test lasted 14 hours, and
produced a scour pattern quite similar to the prototype scour. Figure 4 compares
model and prototype scour hole contours. The plots are reproduced at approximately
the same scale, and contour labels refer to prototype depth in meters. Additional
details are given in Hughes and Schwichtenberg (1998).
Scour After Rock Sill Construction
The rock sill spanning the gap between the north jetty spur and detached breakwater
at Ventura was constructed to scale in the physical model with the sill crest elevation
corresponding to a prototype elevation of -4.5 m MLLW. This test configuration rep-
resented the existing condition at Ventura, and the test was conducted to determine
the likelihood of scour developing downstream of the rock sill under conditions similar
to what caused the original scour hole. Model water depth was once again 18 cm
(water surface corresponding to MLLW in prototype), and currents were generated
using the same flow rate as was used in the second calibration test. Scour equilibrium
was reached after 12 hours, and the bed was surveyed and photographed.
9
Hughes/Schwichtenberg
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