DERIVATION OF THE INDEX GOVERNING THE SEEPAGE FLOW AND DYNAMIC SIMILARITY CONDITION OF LEVEE FAILURES DUE TO SEEPAGE FLOW

It is necessary to establish an estimation method of levee weak points due to seepage flow during floods for making appropriate levee reinforcements and risk management measures in a river basin preparing for large-scale floods. The internal flow with large phreatic surface gradient in the body of the levee tends to concentrate around the landside toe of the levee during a flood event, which may trigger the levee failure. This study shows that the idea of the levee failure is effective for primary selection of dangerous levee failure positions to the seepage flow. First, the only dynamic index governing the seepage flow in soils, SFn (Seepage Flow number) is derived by nondimensionalizing Richards' governing equation for a general seepage flow. Next, by applying SFn to the seepage flow in the levee, levee vulnerability index t* is derived in the expression of a dynamic dimensionless quantity. Also, the vulnerability index of the levee on the permeable foundation is derived by using unsteady quasi two-dimensional seepage flow analysis method. Finally, the vulnerability index t* is calculated for the actual field levee and model levee experiment in which each failure mode such as leakage, sliding or collapse and breach occurred. As a result, it is confirmed that t* shows approximately common values in the field levee and model levee taking the same failure mode. Therefore, it is demonstrated that t* gives a dynamic similarity condition on the levee failure phenomena due to the seepage flow and the judge of the degree of levee failures.