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 *A*C , 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, *A*C =

7.489 10-4 p0.86). This is done

graphically in Figure II-6-46, which

shows a plot of *P *versus *A*C 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

at all points, a stable inlet channel is

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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