Effect of the waterside boundary condition on the seepage and slope stability analyses of the Natomas levees - A case study

Document Type

Conference Presentation


Civil Engineering

Conference Title

Geotechnical Special Publication

Date of Presentation



The Natomas basin is a 21,450 hectare (53,000 acre) area immediately north of downtown Sacramento. The area, which includes an international airport, a civic sports arena, commercial developments, and extensive residential neighborhoods, relies on levees for flood protection along its entire perimeter. The western boundary is a 29.9 km (18.6 mile) long levee that is intended to protect it from flooding by the Sacramento River. Since its construction in the early 1900s, this clay-capped sand levee has experienced a myriad of performance issues stemming from excessive under- and through-seepage. Recently, engineers completed design and construction of remedial measures with the objective of mitigating the historic performance issues. Citing deep and varying bathymetry along the center of the Sacramento River channel, the vertical boundary was modeled as a constant-head boundary. The numerical models and the resulting engineering alternatives and designs were reviewed and approved by the United States Army Corps of Engineers and a blue-ribbon panel of recognized levee experts. This case study will use numerical models established at three different cross-sections to compare the consequences of modeling the vertical boundary along the centerline of the river with a constant-head or a flowline boundary condition. The paper demonstrates that a flowline boundary condition is technically correct and at (and beyond) the toe of the levee results in seepage gradients that are less than or equal to those computed when using a constant head condition. Where the river bottom is lined with low permeability blanket materials, the use of a constant head boundary condition provides seepage gradients and stability factors of safety that are excessively conservative. © 2014 American Society of Civil Engineers.




234 GSP

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