Levee

Position: Floodplain

 

Form

Raised elongate asymmetrical ridge that borders the channel (i.e. along the proximal floodplain). The channel margin is steeper than the floodplain margin. Levees scale in proportion to the adjacent channel. Levee crests may stand several metres above the floodplain surface or be relatively shallow, laterally extensive features. Composed almost entirely of suspended load sediments (dominantly silt, often sandy).

 

Process description

Levee form is influenced by, and in turn influences, the channel-floodplain linkage of biophysical processes, influencing the lateral transfer of water, sediment, organic matter, etc. Levees are produced primarily from overbank suspended-load deposition at high flood stage. During overbank events, flow energy dissipates when flows spread out over the floodplain. Under these conditions, the flow has insufficient energy to carry its load. The marked reduction in velocity results in deposition of coarse sediment on proximal floodplains. Interbedded flood-cycle deposits, termed flood couplets, reflect rising- and falling-stage sedimentation. Finer materials are carried into the distal parts of the floodplain. Highly developed levees along extensive fine-grained floodplains infer a laterally fixed channel zone and well defined segregation of water and sediment transfer between the channel and floodbasin. As the levee grows, the deposition rate of coarser sediment near the crest is reduced, leading to a generally fining upward sequence of deposits within the levee profile.