Integrating Field Monitoring and Numerical Modeling to Evaluate Performance of a Levee under Climatic and Tidal Variations

Abstract

Several short-duration and extreme hydraulic loadings impose time-dependent variably saturated seepage conditions on earthen slopes and embankments. Difficulty assigning appropriate input parameters and lack of full-scale validation are among the main factors introducing uncertainty and lack of confidence when performing numerical transient seepage analysis. This case study demonstrates how to effectively use field-monitoring data to improve the numerical analysis of a levee under climatic and tidal variations. The case study includes a silty sand setback levee located near Seattle. An array of tensiometers and soil moisture sensors installed within the levee’s embankment and foundation, along with a nearly real-time data acquisition system, were used to collect and process the in situ data for a period of about 15 months. Climatic and weather data, including precipitation, temperature, humidity, and wind speed, were collected from a weather station at the site, and tidal water fluctuations were monitored using a water level sensor. The field-measured matric suction and water content were used along with a suction stress–based representation of effective stress to compute suction stress and effective stress profiles versus time. A finite-element model of transient seepage under saturated-unsaturated conditions was developed. The numerical model was calibrated and then validated using the measured pore-water pressures and the piezometric surface. The application of the numerical model was illustrated by modeling the seepage and stability of the levee during a 100-year flood event. The results highlight the need to consider climatic variables and soil–atmosphere interaction when performing transient seepage analysis.