EM 1110-2-1100 (Part V)
31 Jul 2003
V-3-2. Coastal Armoring Structures
(1) Seawalls and dikes. The primary purpose of a seawall (and dike) is to prevent inland flooding from
major storm events accompanied by large, powerful waves. The key functional element in design is the crest
elevation to minimize the overtopping from storm surge and wave runup. A seawall is typically a massive,
concrete structure with its weight providing stability against sliding forces and overturning moments. Dikes
are typically earth structures (dams) that keep elevated water levels from flooding interior lowlands.
Various types of seawalls and dikes are depicted in Figure V-3-4. When vertical, they are labeled
nonenergy absorbing, whereas if with a sloping surface or rubble mound, they absorb some energy
(Pilarczyk 1990). The front face may also be curved or stepped to deflect wave runup. Typical
damage modes for seawalls include: toe scour leading to undermining; overtopping and flanking;
rotational slide along a slip-surface below and shoreward of the seawall; and corrosion of any steel
reinforcement. Vrijling (1990) discusses 14 damage/failure mechanisms for dikes including "stability
of the protective revetment." Part VI presents details for functional design.
Construction of the massive, concrete seawall to protect Galveston, Texas, against overflows from
the sea began in 1902, in the aftermath of the major hurricane of September 1900. Over 6,000
(16 percent) of the citizens lost their lives. An original construction photo (top) and chronology of
seawall and embankment (dike) cross-section development (bottom) are shown in Figure V-3-5 (from
Davis 1961). Major features are the wood piles, a sheet-pile cut-off wall, riprap toe protection, and
the curved face to deflect wave runup. Modification and extension occurred in 1909, 1915, 1926,
and the last extension to the west completed in 1963. In the almost 100 years of existence, many
lives and millions of dollars of property damage have been saved by this project (Davis 1961).
The City of Virginia Beach has opted for a low-crest elevation, sheet-pile, concrete cap seawall that
also serves as a new boardwalk. Figure V-3-6 displays an artist's perspective (top) with cross section
and an aerial photo (bottom) of a recently (1998) completed section. Construction of the newly
designed, interior drainage system with pumping stations for an ocean outfall and a widened sandy
beach will complete the project in 2002.
Part VI-2 also discusses many other typical cross sections and layouts of seawalls and sea dikes.
(2) Bulkheads. These are vertical retaining walls to hold or prevent soil from sliding seaward. Their
main purpose is to reduce land erosion and loss to the sea, not to mitigate coastal flooding and wave damage.
For eroding bluffs and cliffs, they increase stability by protecting the toe from undercutting. Bulkheads are
either cantilevered or anchored sheet piles or gravity structures such as rock-filled timber cribs and gabions.
Cantilever bulkheads derive their support from ground penetration; therefore, the effective embedment length
must be sufficient to prevent overturning. Toe scour results in a loss of embedment length and could threaten
the stability of such structures. Anchored bulkheads are similar to cantilevered bulkheads except they gain
additional support from anchors embedded on the landward side or from structural piles placed at a batter on
the seaward side. For anchored bulkheads, corrosion protection at the connectors is particularly important
to prevent failures. Gravity structures eliminate the expense of pile driving and can often be used where
subsurface conditions support their weight or bedrock is too close to the surface to allow pile driving. They
require strong foundation soils to adequately support their weight, and they normally do not sufficiently
penetrate the soil to develop reliable passive resisting forces on the offshore side. Therefore, they depend
primarily on shearing resistance along the base of the structure to support the applied loads. Gravity
bulkheads also cannot prevent rotational slides in materials where the failure surface passes beneath the
structure. Typical bulkheads are shown in Figure V-3-7.
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