PROTECTIVE WATERFRONT STRUCTURES
Seawall. A seawall is a soil retaining or armoring structure whose
purpose is to defend a shoreline against wave attack. It differs from a
breakwater in its capacity as a soil retention structure. Seawalls are forms
of shore protection and are not intended for use as berthing facilities
(refer to NAVFAC DM-26.2, Coastal Protection).
Bulkhead. A bulkhead is a soil retaining wall structure comprised
of vertically-spanning sheet piles or other flexural members. Bulkheads
derive their stability through mobilization of passive earth pressures between
the mudline and embedded tip, and, in most cases, from a lateral restraint
system installed between Mean Low Water (MLW) and top of the wall top.
Bulkheads are installed to establish and maintain elevated grades along
shorelines in relatively sheltered areas not subject to appreciable wave
attack, and are commonly used as berthing facilities.
Quaywall. A quaywall is a gravity wall structure having the dual
function of providing shore protection against light to moderate wave attack
and a berthing face for ships. Its function is similar to a bulkhead but
should be chosen when overall height requirements or wave environment severity
exceed the practical capabilities of typical bulkhead constructions.
Quaywalls differ from bulkheads and wall-type seawalls in that they do not
necessarily retain a soil backfill.
Selection of Type of Facility. The boundaries of functional
application between basic structure types are not well defined. Within the
general definitions of each, there exists a wide variety of construction types
and techniques, resulting in considerable overlap in wave resistance capacity
and applicability. In general, selection among the basic forms for a
particular application depends upon the severity of the wave environment, the
physical requirements of the berth of the ship, if applicable, and the
relative construction costs associated with each in the geographic area under
Bulkheads vs. Seawalls. The terms "bulkhead" and "seawall" are
frequently and inappropriately interchanged, and bulkheads are sometimes
installed to serve as seawalls. The forces imposed by breaking or broken
waves can be considerable and can prove to be irresistible to the flexible,
multi-jointed sheet element construction inherent to bulkheads.
Seawalls are required along exposed shorelines subject to attack by
wave spectra defined by significant wave heights of 5 ft (1.52 m) or more.
For shorelines exposed to lesser degrees of wave action, the decision between
seawall and bulkhead construction will be based upon economics and land-use
considerations. Where maximization of waterfront real estate is required,
the vertical-wall construction of bulkheads is preferable. The finished grade
elevation of the retained backfill can be established at or above the extreme
high water line, and maintained immediately up to the shoreline. Where
development of the waterfront is not anticipated and the lateral extent
required by revetment or rubble-mound types of seawalls is acceptable, these
typed should be considered.