A combination of time-series sate
llite imagery, he
licopter-borne video footage and fie
ld observation is used to identify the impact of a major tsunami on a
low-
lying coasta
l zone
located in eastern Japan. A comparison is made between the coast protected by
armoured ¡®engineered¡¯ sea wa
lls and the coast without. Changes are mapped from before and after imagery, and sedimentary processes identified from the video footage. The resu
lts are va
lidated by fie
ld observations. The impact a
long a ¡®natura
l¡¯ coast, with minima
l defences, is erosion focussed on the back beach. A
long coasts with hard engineered protection constructed to defend against erosion, the presence of three to six metre high concrete-faced embankments resu
lts in severe erosion on their
landward faces. The erosion is due to the tsunami wave acce
lerating through a hydrau
lic jump as it passes over the embankment, resu
lting in the formation of a ditch into which the foundations co
llapse. Engineered coasta
l defences are thus found to be sma
ll defence against high
ly energetic tsunami waves that overtop them.
There is little erosion (or sedimentation) of the whole beach, and where active, it mainly forms V-shaped channels. These channels are probably initiated during tsunami inflow and then further developed during tsunami backflow. Tsunami backflow on such a low lying area takes place energetically as sheet flow immediately after tsunami flooding has ceased. Subsequently, when the water level landward of the coastal dune ridges falls below their elevation, flow becomes confined to rivers and breaches in the coast formed during tsunami inflow. Enigmatic, short lived, ¡®strand lines¡¯ are attributed to the slow fall of sea level after such a major tsunami. Immediately after the tsunami coastal reconstruction begins, sourced from the sediment recently flushed into the sea by tsunami backflow.