Itinerary and Structures Observed - Tohoku, Japan Earthquake and Tsunami of 2011 - page 36

2.3 Coastal Structures
During its survey, the COPRI team observed examples of five major categories of
coastal protection structures.

 Coastal dikes — These structures, typically constructed parallel to the

shoreline, guard coastal areas from tsunami, storm surges, typhoons, and high
waves. Dikes built along large river basins also serve as flood control
measures during tsunamis. The coastal and river dikes that the team observed
sloped on both the seaward and landward faces, and consisted of a packed
earthen core covered with precast concrete slabs, pavers, or stone. The dikes
range in size from approximately 5 m wide to 20 m wide at the base. Their
crests generally consist of either asphalt topping or a concrete deck.

 Tsunami seawalls or walls (barriers) — Tsunami seawalls, aligned along the

shoreline, serve to prevent tsunamis from overtopping local coastal structures
and flooding uplands behind the shoreline. Tsunami seawalls are comprised of
concrete units that are approximately 10 meters high. In contrast, tsunami
walls (barriers) are located onshore and usually separate the inner port/harbor
facilities from the town structures further inland. Also made of concrete, these
walls are 5 m to 10 m high. Steel gates, closed in the event of a tsunami,
provide vehicular access between the inner harbors and towns; stairways over
the walls provide pedestrian access.

 Water gates — Tsunamis can often travel far up a river valley, causing

massive destruction extending for kilometers inland. Water gates are large
flood control structures that span a river, close to the river mouth. They
resemble dams or sluices, with multiple lift gates. Open during normal
conditions, water gates allow natural tidal exchange and flow of river water to
sea. Shut during a tsunami, water gates prevent the tsunami wave from
propagating up the river system.

 Breakwaters — Breakwaters are structures built offshore of a port or coastline

to shelter the landward area from high waves and storms. Japanese
breakwaters typically consist of massive precast concrete caissons on top of a
rubble mound base. Precast concrete units such as tetrapods, dolos, or other
engineered concrete units usually armor the seaward side of the breakwater.
Offshore breakwaters constructed across natural harbor entrances protect
shoreward port areas from a tsunami. At various smaller fishing villages along
the coast without natural protection, breakwaters form small artificial harbors.

 Greenbelts — Greenbelts (vegetative barriers) along the Japanese coast

provide a second or third line of defense behind wide beaches, dikes and
seawalls to dissipate wave energy, educe runup and protect upland areas from
high velocity flows. Rows of tall trees form the main part of the greenbelts
along with smaller shrubs and park areas.