116 *n * 2

(II-6-16)

(b) It seems reasonable to assume a constant *n *for an inlet in sand rather than a uniform *f *value. For

open channels in hard-packed smooth sand, Chow (1959) recommends an *n *value of 0.020 (in English units,

or 0.016 in SI). Since inlets often have rippled or duned bottoms, which generally increase frictional

resistance, a higher *n *value might be expected, for instance between 0.025 and 0.030 (English units or 0.021

and 0.025, SI). Using an estimated value of n = 0.0275 (English units or 0.0225 SI), a friction factor defined

by *f *= 0.088/R1/3 (English units, *R *in feet, or 0.059/R1/3, SI, *R *in meters) is recommended for use in the

equation for *K*.

Jarrett (1975) examined the friction aspect of Keulagan *K *(Equation II-6-4) and defined *F*, inlet impedance,

as

(II-6-17)

4*R*

with *k*en, kex, f, and *L *as defined earlier, and hydraulic radius *R *measured at the minimum cross section of the

inlet. Figure II-6-22 plots inlet impedance against the ratio of inlet length to the 4/3 power of hydraulic radius

for four ratios of minimum cross-section inlet width to hydraulic radius. These curves can be used for

determining *F*.

(4) Inlet length. Inlet length is one of the more difficult parameters to define. The following

standardized method is recommended to determine inlet length. On a detailed bathymetric chart, beginning

in deep water just offshore from the inlet, mark the points of maximum depth moving across the bar, through

the inlet, and into the bay. The bayward end point should be seaward of any major division of the inlet

II-6-22

Hydrodynamics of Tidal Inlets

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