Example 3: Nonquartz Sediment

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CETN IV-18

March 1999

Using the plot in Figure 2, an equilibrium scour depth (relative to tide elevation of maximum

discharge) of he = 6 m on the vertical axis corresponds to a discharge per unit width of qe .

9.7 m2/s for sand with median diameter of 0.6 mm = 0.0006 m. Adding the freshwater discharge

of 3 m2/s gives a new discharge of 12.7 m2/s. From the same plot, this increased discharge

corresponds to a new equilibrium scour depth of 7.7 m, or a 1.7-m depth increase.

This estimate is likely conservative because the combined freshwater and ebb-tidal flow are

maximum for a relatively short time during each tidal cycle. Whether or not several days will be

sufficient time to reach a new scour equilibrium is unknown. Also note that any effect of flow

stratification because of the influx of less dense fresh water is not considered.

Example 3: Nonquartz Sediment. The curves in Figures 2 and 3 pertain to quartz sand

with specific gravity of 2.65. Estimates for inlets having nonquartz sediments must use

Equation 7 or 8.

For example, consider another fictitious tidal inlet at a location where the sediment is primarily

broken shell material having specific gravity of Ss = s/w = 2.4 and de = 0.5 mm = 0.0005 m.

What will be the equilibrium depth corresponding to a discharge per unit width of

8 m2/s?

Substituting numerical values for the variables in Equation 7 yields

0.234 (8 m 2 / s )

0.234 (6.35)

8/9

he =

m = 5.8 m

[(9.807 m / s ) (2.4 - 1)]

(3.204 ) (0.0794 )

(0.005 m )1 / 3

4/9

An equivalent estimate for quartz sand produced an equilibrium scour depth about 0.4 m less

than found for the shell sediment. This difference is probably less than the error that might be

expected for this primitive estimation technique, so use of Figures 2 and 3 for nonquartz

sediments should not introduce too much error except for exotic sediments that are either

extremely heavy or nearly neutrally buoyant.

ADDITIONAL INFORMATION. Questions about this CETN can be addressed to Dr. Steven A.

Hughes (Voice: 601-634-2026, FAX: 601-634-3433, e-mail: ). For

information about the Coastal Inlets Research Program, please contact the Program Manager,

Mr. E. Clark McNair (Voice: 601-634-2070, e-mail: ). Dr. Nicholas

Kraus provided a beneficial review of this CETN. This technical note should be cited as follows:

Hughes, S. A. (1999). "Equilibrium scour depth at tidal inlets," Coastal

Engineering Technical Note CETN IV-18, U.S. Army Engineer Research and