Question

Annual mean wave height and period at NDBC Buoy 44009 off Delaware Bay are 1.1 m and 5.5 s, respectively. The buoy is moored in 28 m of water.

a. Are these deep-water waves or shallow water waves at the buoy?

b. What is the maximum orbital velocity at the bottom?

c. Is the computed umax large enough to exceed the movement threshold for 1 mm sand?

d. What values of wave height would be required to move that sand for T = 5.5 s, and for T = 8 s?

e. What do these results tell you about sediment transport under waves?

Answer 1

1. to find whether it is a deep water wave or shallow water wave, we need to find the wavelength first.

Wavelength and time period relation is given as,

= 1.56 (5.5)² m

= 47.19 m

deep water if d> wavelength /2

shallow water if d< wavelength /2

since 28>23.595 m

the wave is deep water.

2. Orbital velocity is given by,

here H is the amplitude of wave, T is the time period, k(wavenumber) = reciprocal of wavelength and d is the depth

= 3.14(1.1)/5.5(sinh(0.593))

= 1