Mechanisms for Damage and Failure from the Great East Japan Tsunami - Tohoku, Japan Earthquake and Tsunami of 2011 - page 56

Figure 47. Natori Beach shoreline dike
— Collapsed sections of seaward facing
concrete paver units due to
combination of hydrostatic and
hydrodynamic loads on seaward side
and overtopping scour of landward side

Figure 48. Natori Beach shoreline dike —
Complete collapse of landward facing
concrete paver units due to combination
of hydrostatic and hydrodynamic loads
on seaward side and overtopping scour of
landward side

Figure 50. Otsuchi — Concrete panels
removed due to flow behind wall; caused
scour of earthen fill and structural failure
of the wall

Figure 49. Utatsu — Effects of flow
obstruction (Greater area facing flow is
subject to higher hydrostatic and
hydrodynamic forces.)

3.7 Supercritical Flows

Due to extremely long wave periods, tsunami waves typically appear as fast-moving
turbulent bores when propagating towards the shore. Due to the immense volume of
displaced water movement and the high energy involved, supercritical flows typically
occur during the tsunami runup at flow obstructions such as walls and dikes, and
particularly during the tsunami backrush (draw-down). This high-discharge,
supercritical runup and backrush flow provides a mechanism for massive beach
erosion and erosion around the foundations of coastal structures (Figures 51 - 54).