Effects of delayed drainage on subsidence modeling and parameter estimation
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The use of delayed drainage in land subsidence modeling greatly complicates model calibration, particularly when the thickness of the fine-grained interbeds varies throughout the modeled region. This thesis documents two separate projects (chapters) related to the use of delayed drainage in groundwater flow and subsidence modeling with parameter estimation. The overall goal of these projects was to better understand how delayed drainage affects accurate parameter estimation and how it is currently affecting the subsidence processes occurring in Las Vegas Valley. Chapter 1 describes an investigation on the value of subsidence data for groundwater model calibration considering delayed drainage. The calibration results of 13 hydraulic parameters of a synthetic conceptual model evaluated for 24 test cases indicate that (1) the inverse of the square of the observation values is a reasonable method to weight the observations, (2) spatially abundant subsidence data typically produce superior parameter estimates even with observation error under constant and cyclical pumping, (3) when subsidence data are limited and combined with drawdown data, outstanding results are obtained for constant pumping conditions. However, for cyclical pumping with observation errors, the best parameter estimates are achieved when multiple years of seasonal subsidence data are provided. The results provide useful suggestions for real-world calibration problems. Chapter 2 outlines the development of an updated flow and subsidence model for Las Vegas Valley covering the entire period of development of the basin. The new model includes a subsidence package that takes into account delayed drainage of fine-grained interbeds. Previous models used subsidence packages that assumed instantaneous equilibration of heads across all hydrogeologic units. The new model resulted in an agreement with measured water-level and improved the simulation of land subsidence. The analysis shows that the typical residual subsidence in Las Vegas Valley can be accurately simulated by incorporating delayed drainage in a long-term model. The study also indicates the need for more sophisticated modeling practices that use delayed drainage with parameter estimation processes to accurately calibrate flow and subsidence models.
- Masters Theses