EM 1110-2-1100 (Part II)
30 Apr 02
EXAMPLE PROBLEM II-6-5
FIND:
Using the information provided in Example Problem 1, find the potential stability of the proposed channel
cross section. Remember the channel has vertical sheet-pile walls, so its cross section can only change in the
vertical.
SOLUTION:
By varying the cross-sectional
area of the channel AC , assuming
that the channel width B remains
constant and varying the channel
depth d and recalculating the tidal
prism as described above, the effect
of channel area on the bay tidal prism
can be evaluated and compared with
the appropriate equation from
Table II-6-3 (dual jettied inlets, AC =
7.489 10-4 p0.86). This is done
graphically in Figure II-6-46, which
shows a plot of P versus AC from the
hydraulic response calculations and
from the stability equation. The
intersection of the two curves on the
right side is the stable solution. It
yields a channel cross-sectional area
of 1,440 m2 or a depth of 8 m. This
shows that the 180-m by 3.7-m
design channel would be unstable,
with a strong tendency to erode.
Where the hydraulic response
curve lies above the stability curve
(as in the example) the tidal prism is
too large for the inlet channel area
and erosion will likely occur until a
stable channel develops. If the
hydraulic response curve crosses the
stability curve twice, the lower point
is an unstable equilibrium point from
which the channel can either close or
scour to the upper stability point. If
the hydraulic response curve is
substantially below the stability curve
Figure II-6-46. Example Problem II-6-4. Plot of P versus AC from the
at all points, a stable inlet channel is
hydraulic response calculations and from the stability equation
unlikely to develop and the channel
should eventually close.
The stable inlet cross-sectional area depends on other factors (e.g., wave climate, monthly tidal range
variations, surface runoff) besides the spring or diurnal tidal prism. As a result, the tidal prism-inlet area equations
given in Table II-6-3 only serve as an indication of the approximate stable cross-sectional area. The analysis
performed in the example demonstrates that the design channel is very likely to erode to a greater depth; however,
that depth, which will fluctuate with time, can vary substantially from the indicated depth of 8 m.
Hydrodynamics of Tidal Inlets
II-6-55
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