Erodibility of cohesive streambeds in the loess area of the midwestern USA

Excess stress parameters, critical shear stress (tau (c)) and erodibility coefficient (k(d)), for degrading channels in the loess areas of the midwestern USA are presented based on lit situ jet-testing measurements. Critical shear stress and k(d) are used to define the erosion resistance of the streambed. The jet-testing apparatus applies hydraulic stresses to the bed and the resulting scour due to the impinging jet is related to the excess stress parameters. Streams tested were primarily silt-bedded in texture with low densities, which is typical of loess soils. Results indicate that there is a wide variation in the erosion resistance of streambeds, spanning six orders of magnitude for tau (c) and four orders of magnitude for k(d). Erosion resistance was observed to vary within a streambed, from streambed to streambed, and from region to region. An example of the diversity of materials within a river system is the Yalobusha River Basin in Mississippi. The median value of tau (c) for the two primary bed materials, Naheola and Porters Creek Clay Formations, was 1.31 and 256 Pa, respectively. Streambeds composed of the Naheola Formation are readily eroded over the entire range of shear stresses, whereas only the deepest flows generate boundary stresses great enough to erode streambeds composed of the Porters Creek Clay Formation. Therefore, assessing material resistance and location is essential in classifying and modelling streambed erosion processes of these streams.