(14) A new principle on morphodynamic stability and its application to shallow sea reclamation project
Port and Airport Research Institute, Japan
Sandbars play an important role in beach stability since they reduce the energy of waves by breaking them, thereby preventing severe erosion. The persistent nature of intertidal sandbars was the subject of much speculation concerning the hydrodynamic mechanisms involved, but its origin remained enigmatic. We recently found on the basis of the geophysical evidence and theoretical modeling and analysis that the interplay between the effects of the dynamics of suction, i.e. negative pore water pressure relative to atmospheric air pressure, and sediment transport and morphology plays a crucial role in such intertidal sandbar morphodynamics. Here, we applied this new principle of the morphodynamic stability to a shallow sea reclamation project and further clarified this mechanism in light of artificially created sandbars in Tokyo Bay, Japan. We performed integrated observations and analyses of the morphological and geoenvironmental changes as well as the benthos diversity and geo-stratigraphy variations, combined with a series of laboratory soil tests. The results reveal the salient new geophysics concerning erosion and deposition in intertidal zones. Notably, the combined results demonstrate that in the morphodynamics processes where the sediments moved from the unsaturated zone to the saturated zone, the feedback between the suction-dynamics induced cyclic contraction, strength gain and sediment transport became pronounced. Namely, the sediment relative density at the bar crest increased to as high as 80% and this caused the surface shear strength to develop beyond three-fold magnitudes there. As a consequence, the morphological changes due to repeated erosion and deposition became markedly suppressed, yielding the distinctly stable sandbars under severe wave and current conditions which would otherwise lead to unstable bar behavior without the effects of the suction dynamics. This is also verified by the numerical analysis accounting for the feedback between the sediment transport and the suction-dynamics effects. The stable geomorphology also gave rise to a significant enhancement in the biodiversity in the field. On the basis of these results, we propose an optimal design of such dynamically stable sandbars which could effectively contribute to disaster mitigation as well as diverse ecological activity in intertidal zones.