Effects of water inifltration on soil erodibility

A new flood risk approach for design and testing of primary and secondary flood defence structures in the Netherlands relies on the probability of loss of the water retaining capacity. Determining this probability for levees requires a better understanding of consequences of extreme conditions leading to failure, starting with a study of soil. In breaching of levees high flow velocities occur. Conventional erosion models for non-cohesive sediments cannot handle this adequately and therefore overestimate erosion rates. Due to these high flow velocities and corresponding high shear stresses soils do no longer erode due to the pick-up of individual particles but fail as entire layers. Therefore in this report first a new process based erosion equation for dilatant material under the influence of high flow velocities is under development based on findings from the scientific field of Dredging Engineering. The soil can therefore be described as a saturated porous medium with a specified angle of internal friction and cohesion. Further the soil is assumed to be dilative, which means that over the depth of failure, soil particles need to dilate in order to be sheared off. The equation is then applied to a steady state situation at which no further erosions takes place in the case of jet flow applied to soil. Research over the last decade identified three key factors affecting erodibility, namely material texture, compaction moisture content and compaction energy. The importance of these factors is well known by now, however they are not integrated into erosion models so far. This study aims to provide thoughts in the erodibility of sand under the influence of water infiltration by means of experiments performed at the laboratory of fluid mechanics at the Delft University of Technology. Infiltration is generated by introducing a head difference between the top and the bottom of the sand layer. The soil is thereby kept fully saturated. A water tank set-up with a double bottom was created to be able to apply a water pressure at the bottom of a well-defined sand layer. With a tube connected to the double bottom of the tank the water pressure and the associated infiltration rate was regulated. The water head on top of the sand layer was thereby kept constant. Determining the erodibility of the soil was done with a submerged jet erosion test (JET). Therefore an impinging jet with known constant velocity was directed perpendicular to the sand surface and created a scour hole. The scour depth development was recorded over time and later processed into data sheets. This was repeated for several different infiltration settings. From the erosion rates first effects of water infiltration were detected. Based on two different analytical methods, the Blaisdell Solution and the Scour Depth Method the critical shear stress τc and the erodibility coefficient kd were determined. Results of the two methods differ significantly by order of magnitude which is why in the analysis for this research both solutions were used and compared with each other. With the scour depth recordings erosion rates were determined for the different water infiltration rates, showing than with increasing infiltration velocity the maximum scour depth decreased. Evaluation of the critical shear stress and the erodibility coefficient showed an inverse relationship between each other where τc increased and kd decreased with increasing infiltration rates. An explicit mathematical relationship between the critical shear stress and the water infiltration velocity could not be determined with sufficient certainty based on the limited data set. The validity of the two most common analytical methods to determine erodibility have been questioned since both provide non zero critical shear stress values whereas the stability of the scour hole could be explained from a continuum geotechnical model under the normal stresses and shear stresses applied on the stable wall of the scour hole.Civil Engineering and GeosciencesHydraulic Engineerin