There is uncertainty in the analysis, in addition to model assumptions. As
discussed by Kraus and Rosati (1999), uncertainty can be estimated by expanding all
quantities in a Taylor series as, for example, as Q → Q δ , where the average
Q
rate on the left is interpreted as the actual rate and the right side is interpreted as a
best estimate plus an uncertainty δ . Expanding the right side of Eq. 14 and
Q
keeping lowest-order terms, one obtains for an expression for the maximum
uncertainty
F
I
δ
δ δ δS δ
Q
W
D
x
t
G
J
=
+
+
+
(15)
H
K
Q
W
D
xS
t
which can be estimated from Eq. (15) as
F
I
δ
G
J
Q
2 0.5 10 0.1
H
K
≈ 29%
=
+
+
+
(16)
Q
20 3.5 500 4.1
If, instead, we can assume the individual uncertainties are independent and
randomly distributed, then a root-mean square (rms) approach would give a more
c
h
realistic value of the uncertainty as δ Q rms = 18% . The estimate of transport
Q
rates made by Kieslich and Brunt (1989) contains similar uncertainties, so both
estimates, 9,200 and 8,500 m3/year, are equivalent.
PHYSICAL MODEL STUDY
Spit evolution was examined in the Idealized Inlet Model installed at the U.S.
Army Engineer Waterways Experiment Station (Seabergh 1999). This physical
model supports basic and applied research through the Corps of Engineers' Coastal
Inlets Research Program. The concrete basin (46 m wide, 99 m long, 0.6 m deep)
contains an ocean and bay separated by a barrier spit bisected by an inlet. A
movable wave generator located on the ocean side creates waves of fixed direction
and variable height and period. Tidal variation can be specified, and storage tanks
allow simulation of large back bays. Flows can be created and directed with piping
and without changing the water level.
The present study was devised to examine dependencies of geometric parameters
appearing in the analytical model and to observe integrated spit processes. For the
study, 0.13-mm uniform quartz sand was placed along the updrift barrier islands as a
10-cm-thick veneer, and truncated approximately 2.1 m updrift of the entrance.
Waves were applied at 20-deg angle (water depth at generator, 18 cm) to the shore to
generate a longshore current and sand transport directed toward the inlet. With
initiation of wave action, a spit emerged from the terminated movable bed and grew
toward the inlet. The tests were performed in the presence of a curved ebb-tidal
shoal, which tended to focus waves toward the inlet. Twenty-two cases were run
with different wave height and period, fixed water level or with tide, and flow
through the inlet as no flow, tidal flow, or a steady flood current. Wave height
varied between 2.4 and 3.4 cm, and wave period between 1 and 2.2 sec.
Kraus
13
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