Jefferson County - East-West Gateway Coordinating Council

A Regional Overview All-Hazard Mitigation Plan 37
FIGURE FIGURE J22A J22A J22A LEVEE LEVEE DESIGN
DESIGN
Figure Figure 4: 4: Cross Cross-Sectional Cross Sectional View of of an Earthen Levee
Generally, levees are specifically designed and constructed to withstand a certain flood
frequency. A ten to a fifty-year levee is usually considered to be an agricultural levee
designed to protect floodplain in agricultural areas from floods that may occur once every
ten or fifty years. These areas will experience flooding during major flood events (e.g.,
100-year flood events). Urban levees protect floodplains from 100-year floods or higher.
Other levee structures in the system include tie back or lateral levees, which extend from
the main stem levee to bluff lines (high ground) and are part of the line of protection
against backflow during periods of high water (FIGURE J30C). Other key components of a
levee system include pumping stations, gravity drains or outlets, street closure gates and
relief wells. Gravity drains or outlets are openings built through the base of the levees and
are designed to drain and convey floodwaters. These gravity structures permit the
outflow of stormwater that comes off the bluffs to the east when the river stage is low.
Gravity drains are equipped with closure gates to prevent the river flows from entering the
protected area during time of high river stages. Pump stations may be also operated to
drain the protected area from interior flooding (FIGURE J30C).
Figure Figure 5: 5: Schematic Schematic of of of Standard Standard Levee Levee Levee System System Source: FEMA
The four general ways a levee may fail include: overtopping; piping; saturation; and
underseepage (FIGURE J22B). During an extreme flood situation, floodwaters may actually
exceed the designed water level of the levee and overtop it. For example, the Metro East
levees have a design water level of 52.0 feet. In 1993 floodwaters reached a level of 49.6
feet, which approached but did not reach overtop conditions. Piping, or internal levee
erosion, occurs when floodwaters enter the levee through animal burrows and/or plant and
tree root channels. Saturation failure is attributable to a levee that is saturated with
floodwaters for an extended period of time. Floodwater permeates and weakens the core
of the levee, making it unstable. Underseepage refers to floodwater that travels under the