BANK STABILITY RESULTING FROM RAPID FLOOD RECESSION ALONG THE LICKING RIVER, KENTUCKY

River bank instability has been linked with changing land use, deforestation, undercutting by meander migration, and seasonal, rapid fluctuations in river level.  Studies have correlated bank instability with increased pore water pressure resulting from flooding.  During periods of flooding or high water level, soils became saturated, when river level subsequently drops, pore water pressure within the soil may increase dramatically, decreasing the soil’s effective shear strength, causing bank failure.  This is the first study of bank instability involving measurements of matric suction conducted in the Cincinnati area.  The banks of the Licking River exhibit instability features such as tension cracks and rotational landslides, some of which developed as a result of a major flood in 1997. 

 

A set of three tensiometers and one piezometer were installed in five boreholes varying in depth from 4 ft to 12 ft.  Cores recovered from these boreholes show the banks are underlain by clay with low plasticity (CL), and g_wet=120.5 pcf.  Several tests were conducted with undisturbed samples, including a standard consolidation test for saturated soils (which indicates that the soil is overconsolidated).  Unconfined compression tests produced an average value of q_u=1300 psf.  A consolidated-undrained triaxial test with pore pressure measurement was run on saturated samples yielding values of cohesion of 560 psf and a friction angle of 13^o for the clay.  The saturated conditions were analyzed in the triaxial test in order to reproduce the worse case scenario where soil becomes saturated and loses strength in the case of floods, and also loss of confinement pressure when water drops. 

 

After almost three months of suction monitoring, it is possible to see the rapid response of the material to rain events, dry days (increasing evaporation) and saturation after flooding.  The aim of the stability analysis of the banks of this study is to consider strength parameters of the material, the matric suction and the hydrologic conditions of the area (changes in river level) to obtain the proper scenario or the boundary conditions for bank failure.