Failure Mechanism of Earthen Levee Strengthened by Vegetated HPTRM System and Design Guideline for Hurricane Overtopping Conditions

Abstract

Prior studies have shown that the protection of landside slope of earthen levees against wave and surge overtopping during large storm surges is important. Overtopping protection system can resist the fast-flowing, overtopping turbulent flow on the landside slope that can damage the protective grass covering and expose the underlying soil to erosion. In this research, the overtopping erosion and failure mechanism of an innovative levee strengthening technique – High Performance Turf Reinforcement Mat (HPTRM) are conducted based on full-scale flume tests conducted during combined turbulent wave and storm overtopping. The HPTRM is a three-dimensional turf reinforcement mat joined at the intersections of randomly oriented nylon filaments with high tenacity polyester geogrid reinforcement at low strains. As the grass roots grow through the open space of HPTRM, roots become entwined within the turf reinforced mat. The interlocking between roots and HPTRM can enhance the resistance against hydraulic and shear forces created by high water flow hydraulic condition. Flow velocity, shear stress and average overtopping velocity on landward-side slope and levee crest are measured and calculated. An examination of soil loss from strengthened levee in the full-scale flume tests showed that overtopping flow velocity more correlated with soil erosion along the levee crest and landward-side slope than the shear stress did. Erosion mechanisms of levee surface were developed to study erosion starting position and the roles of the key hydraulic parameters (flow velocity/shear stress).