Monitoring and modeling of peat decomposition in Sacramento Delta levees

The integrity and reliability of levees are essential components of homeland safety. The failure of such systems due to a natural or manmade hazard can have monumental repercussions, sometimes with dramatic consequences on human life, property and the country's economy. This paper presents some results of integrated monitoring and modeling to assess the performance-based response of a levee section. The modeled levee is part of the Whale's Mouth section on Sherman Island where satellite images and in-ground GPS sensors are used for displacement measurements. The Whale's Mouth levee is modeled using the large deformation option of the finite element program PLAXIS 2D. The model is used to establish a deterministic performance response under maximum water level loading and to investigate the effect of peat decomposition on the deformation response of the levee section. The remote sensing and in situ data are used to calibrate the numerical model. The results are compared to the pre-defined limit state and illustrate how the peat layer decomposition affects the modeled levee section performance. The concept of performance limit states of these critical structures provides a means to quantitatively assess the functionality of an earth structure under severe storm loading events. The probability of exceeding a prescribed limit state is defined based on the strain or gradient levels in potential emerging failure zones. The variation in strength properties and hydraulic conductivity of the levee embankment, as well as the rate of rising water level and duration of flooding, may lead to the progression of the structure state from a low probability of exceeding adequate functionality to the probability of exceeding a high limit state, i.e., imminent failure. The displacement data collected during these loading and unloading events is used to establish the levee condition assessment on the basis of the performance limit states.