Design Considerations for Confining and Guiding Levees on Alluvial Fans

The dynamic nature of alluvial fans in arid and semi-arid environments offers numerous floodplain management challenges primarily due to unpredictability of flowpaths and delivery of significant sediment. Construction of levees on alluvial fans is one of the primary structural control measures utilized in historically successful "whole fan" floodplain management that address flood protection requirements. Levee design and hydraulic evaluation for alluvial fans are much different than similar applications for standard riverine locations. Corresponding requirements by FEMA in determining flood hazard definition for alluvial fan locations also complicate the issues. The primary design elements associated with levees which are unique for alluvial fans include (1) geometry and alignment, (2) embankment cross section and (3) armoring requirements. Geomorphic and flooding characteristics on an alluvial fan involve various sources of uncertainties that require the application of statistical procedures to evaluate flooding at a given point on the alluvial fan. Armoring of the levee face is an essential requirement to ensure successful operation and specific analysis must be applied to evaluate the vertical limits of the armoring, specifically toe-down depths, since scouring of the slope protection is one of the most common failure modes. The hydraulic evaluation must include the analyzes of potential alternative flowpaths, flow-impingement on the levee, degradation, deposition, sediment ramping, and maximum flooding depths from a variable and random flowpaths. This paper presents the previously described design considerations for the construction of guiding and confining levees on alluvial fans with detailed discussions on each topic.