(Above) Basic levee terminology
Overtopping. Water in the river raises higher than the crown of the levee. The water flows over the crown and down the land side of the levee, eroding the levee from the back side.
The above figure shows the levee erosion sequence due to high water. Surface erosion can occur from high water on the water side of the levee. When the water overtops the levee, erosion can begin on the land side of the levee.
Surface erosion. During high water when the river water is against the levee, waves generated by wind will erode the water-side of the levee.
(Above) Eroded California levee. Photo from CA Department of Water Resources photo library.
Shear failure (slope instability) of the levee. Manifested as large slabs of the levee sides sliding down the side of the levee during or immediately after high-water. Many days of high water will saturate the surface layers of the levee (by phreatic water movement) making them heavier than the underlying layers. The heavier surface layers lose cohesion with the lower layers, resulting in the surface layer sliding down the slope of the levee. In older levees constructed of sand the entire levee can become saturated during a flood resulting in total collapse of the levee.
(Above) An example of shear failure of a Washington levee. Photo from skagitriverhistory.com
Piping and underseepage. During a flood the water column exerts a weight upon the levee sides and adjacent floodplain that pushes water into any holes or cracks in the levee structure. If the water reaches the landside of the levee it will flow from the levee and carry levee material with it, resulting in the levee eroding from the inside out. A special type of piping that is common in the Central Valley of California is underseepage. Flood water flows through permeable soil material (e.g., old sandy channel) that underlies the levee. The water arises to the surface near the landside base of the levee in what is termed a ”boil”. Again, the water is flowing, so it is carrying levee foundation material out from under the levee.
As the above figure shows, during high flows, water can seep through pipes from the water side to the land side of the levee and release through boils on the land side surface.
(Above) Boils in orchards on the land side of a Feather River levee after a January 1997 flood, from here.