For structures backed by fill material having a permeability
between 10-3 and one ft/min:
1) If the wall is of low permeability (sheet piling and few or
no weep holes), assume tidal lag equal to one-half the mean tidal range.
2) If the wall is permeable (lagging, with spaces between the
boards, gravel chimneys connected to weep holes or similar devices [see
Figure 6]), treat the water load (saturation level above low water level) as a
surcharge. The curves of Figure 6 assume semi-diurnal tides. For diurnal
tides, the effects of tidal lag will be less. See Figure 7 for a sample
computation illustrating the use of the curves of Figure 6.
d) These allowances for tidal lag do not consider the effects of
inundation of the backfill due to overtopping. If overtopping is anticipated
and positive provisions have not been made to drain the overflow water away
from the wall and to prevent its penetration into the backfill design for an
assumed ground water level at the top of the wall. Allow 50 percent
overstress (or reduce overall load factor to 1.2) for all loading combinations
which include this assumption.
e) Add the tidal lag effect with low water condition on the
Allowable Stresses. For Service Classification B, use allowable
stresses given in MIL-HDBK-1002/2, Loads.
Scour. Except in deep water, scour at the toe of a seawall during
severe storms is inevitable. Provide a toe wall or toe armor to prevent
undermining. The following may be used for guidance in detailing toe
Width of Toe Armor.
(See Figure 3, type H.)
Depth to Toe Wall. Assume scour depth below the natural seabed
equal to the maximum unbroken wave height consistent with the depth of water
at face of wall. For attack by broken waves, and for conditions of
progressive shoreline erosion in general, refer to method in Section 5.28 of
the Shore Protection Manual, U.S. Army Coastal Engineering Research Center.
Design to ensure the stability of the wall in the scoured condition
and provide excess toe armor or toe stone in rubble mounds of revetments to
allow for the inevitable settlement and displacement during heavy wave attack.
Overtopping. Overtopping of a seawall in a severe storm is
difficult to prevent. If substantive erosion of the upland due to the fall
and wash of the water cannot be tolerated, the upland adjacent to, and a
distance back of, the wall must be armored against erosion. Pavement close to
the wall is required to resist the fall of the water thrown into the air by
the impact of waves on the wall. If the upland slopes inshore, provisions
must be made to collect and transport the overtopping water and to prevent
erosion due to the run-off of said water. Plantings in lieu of pavement
seldom work well. Use devices described in para. 3.2.4 to reduce overtopping.