Figure 6. Comparison of grain-size distribution for sandy native ebb shoal sediments and the finer dredge material sediments

ERDC TN-DOER-N6

March 2000

Current Velocity and Direction

Measurements. A 1,500-kHz

SonTek Acoustic Doppler Profiler

(ADP) was installed in the study

area prior to placement of the

dredged material to obtain current

velocity and direction. The ADP

was mounted in a stable housing and

placed on the seabed. A water level

recorder in a subsurface case re-

corded the absolute pressure of the

water column above the instrument

from which waves and water levels

were derived. The pressure was

corrected for barometric pressure

via another pressure gauge located

to record atmospheric pressure Figure 6. Comparison of grain-size distribution for sandy native

ebb shoal sediments and the finer dredge material

changes. The water level gauge

sediiments

sampled at 1 Hz for 1,024 sec every

hour. The barometer sampled pressures over 1 min and formed an average, which was then used

to correct the water level gauge pressures. Each recorder stored the information on a data logger

located in the waterproof subsurface unit. The velocity and water level data were retrieved every

4 to 6 weeks.

Water level and wave data were collected almost continuously for the period from the end of October

1998 through July 1999. Tidal currents were recorded during that period as well, but problems with

sediment accumulation on the gauge limited the amount of useful data. The data are summarized

in the following paragraphs.

The Wave Information Study (WIS) (Hubertz and Brooks 1989) hindcast summary data for Station

27 in the Gulf of Mexico, which is 24-32 km (15-20 miles) south of the study area, are presented

in Table 5 (after Hubertz and Brooks 1989). The table provides the monthly maximum significant

wave height hindcast for each year between 1956 and 1975 and the means of significant wave height

for each month (averaged over the 20-year hindcast record). The significant wave height maximums

and means measured by the wave gauge and analyzed for this period are also provided. The

measured data tend to be lower than the hindcast data. One likely reason for this is that the WIS

station is at 24-32 km (15-20 miles) south of the study site. Waves from the north then would be

larger at Station 27 than at the study site because of the longer fetch to the WIS station. Waves

from the north occur less frequently, as the dominant wave direction is from the southeast. Another

likely reason is simply that the weather during this period was mild. The average peak periods of

the measured waves were between 5 and 6 sec, which is consistent with the hindcast data.

Tidal and meteorologically induced currents were measured during the study period. Only the

months of December, January, and March provided useful data. Table 6 provides the summary data

regarding the currents for those months. The currents contain a north-south diurnal oscillation as

water moves in and out of Mobile Bay.