Transient Seepage Analysis for Levees and Dams: Numerical and Monitoring Approaches

An investigation into the transient impacts of flood loadings on earthen embankments was conducted. Two embankments were instrumented and monitored over a period of four years. One of these embankments was a levee located along the Mississippi River just north of Cairo, Illinois. The other embankment was part of a catchment basin at the Engineer Research and Development Center located in Vicksburg, MS. Tensiometer and porous block sensors were used to monitor the pore water pressures in the embankments. It was found that when measuring the field soil water retention, tensiometers were more responsive than porous block sensors at low suctions; although, at shallower depths, the tensiometer performance was limited during periods of extended drying. It was shown that during the start of flooding, pore water pressures in the embankment soils were near −10 kPa at depths less than 2 m, which was greater than the normally assumed hydrostatic conditions. An investigation into flood hydrographs collected from across the United States showed that flood durations could be hundreds of days long. These hydrographs were collected over a period of 10 years. It was found that the recorded peak flood stage exceeded the major flood stage 11% of the time. An uncouple transient seepage model of a 2015 Mississippi River flood event that occurred at the Cairo levee showed that an uncoupled model could simulate the field measurements; however, the material properties that resulted in the most accurate simulation differed from those measured in the laboratory. Soil water retention characteristics of the embankment soils were assessed, and it was found that laboratory measured soil water retention curves could be used to bracket field measurements. Slope stability analyses were performed as a proxy to assessing the progression of the wetting front in the levees. Accounting for the increase in shear strength due to the presence of matric suction resulted in minimal impacts to stability factors of safety for levee embankments during flood loadings. The results of this investigation will help to improve the reliability of transient seepage analyses and provides guidance for future embankment monitoring investigations.