Environmental Engineering Reference
In-Depth Information
barrier beach or offshore bar is built parallel to the shore (Figure 7.45), creating a lagoon
between the bar and the mainland. Strong currents extend the bar along the shoreline that
conflicts with river flow, causing the channel to migrate around a spit as illustrated in
Figure 7.24.
Wave and wind forces, particularly during storms, push the bar inland.
Since the bars and spits are deposited by current and wave action, they are composed
of well-sorted materials, usually in a medium-compact to compact state, free of fine-
grained particles. During periods of high current and wave activity, gravel beds are
deposited and become interbedded with the sands.
Tidal Marsh
The tidal lagoon fills with sediments from stream activity. When the water becomes suffi-
ciently shallow to provide protection against wind and currents, biotic activity begins, and
a tidal marsh with organic soils is formed. If the sea level is rising, as is the case along the
east coast of the United States, these organic deposits thicken and enlarge to fill depres-
sions in the lagoon bottom. Along the Atlantic coastline, these organic deposits can range
from 6 to 60 ft in thickness and cover large areas.
Dunes
Wind action moves the finer sands from the beach area to form dunes, which eventually
migrate to cover the organic soils.
Engineering Characteristics
A geologic section through the barrier beach at Jones Beach, Long Island, New York, is
given in
Figure 7.46.
The logs of the test boring from which the section was made show
the materials of an older barrier bar to be gravelly and very compact on the shoreward
side. The very high SPT values are caused by the gravel particles. These older deposits
have interbedded landward with thin strata of soft organic silt, and strata of interbedded
sand and organic silt. Overlying the older beach soils are the more recent beach and dune
soils; the latter migrate inland to cover the tidal marsh soils. Fine-grained soils in tidal
lagoons are usually extremely soft, since the salinity of lagoon waters causes the clayey
particles to flocculate as they settle out.
Logs of test borings from three other locations of barrier beaches and tidal marshes are
given in
Figure 7.47.
It is apparent that soil conditions attendant upon marine depositional
Sand dunes
20
10
N
N
Tidal marsh
13
Sand dunes and
beach deposits
23
N
N
6
Ocean
14
62
0
10
20
30
40
2
10
27
1
10
11
25
15
34
137
25
83
51
Older barrier beach -
sands and gravelly sands
132
41
1
6
152
132
230
19
29
2
26
22
28
37
Peat and/or soft organic silt
Interbedded sands and organic silt
500
1000
Distance (ft)
1500
2000
FIGURE 7.46
Geologic section illustrating migration of dune and beach deposits over tidal marsh organic materials (Jones
Beach, Long Island, New York). (Courtesy of Joseph S. Ward & Associates.)
