Part 9 LEVEES AND CREVASSES Model levee design

Model levee design • The theory of levees proposed to confine the river’s mass

Natural crevasses beneath levees Crevasses are sand filled distributary channels that form at high

Seepage crevasse exposed at the east levee of the IHNC breach in New Orleans

• Permeability contrasts caused by clay filled oxbows create treacherous and contrasting foundation

• The worst combination of foundation conditions is the ‘gore point’ formed between

• Clay filled oxbows consolidate under the load imposed by the earthen levees,

Raising Levees • The rising bed of the Mississippi River has necessitated the heightening

Levee Failures • Levees tend to fail during sustained high flow events because of

• Most levees are compromised by underseepage. When a levee breaches, the sudden

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Part 9 LEVEES AND CREVASSES

Model levee design • The theory of levees proposed to confine the river’s mass in its main flow channel, encouraging scour during high flow.

Natural crevasses beneath levees Crevasses are sand filled distributary channels that form at high flow, and lie beneath earthen levees like ticking time bombs, waiting to explode.

Seepage crevasse exposed at the east levee of the IHNC breach in New Orleans after Hurricanes Katrina and Rita

• Permeability contrasts caused by clay filled oxbows create treacherous and contrasting foundation conditions beneath levees.

• The worst combination of foundation conditions is the ‘gore point’ formed between two infilled oxbows, as shown here.

• Clay filled oxbows consolidate under the load imposed by the earthen levees, causing these levees to settle and sink. Differential settlement is a major obstacle in maintaining levees.

Raising Levees • The rising bed of the Mississippi River has necessitated the heightening of levees, beginning in the 1890 s, as shown above. • Lower image shows dynamite charge being detonated on levee south of New Orleans in Plaquemines Parish during the Great 1927 flood.

Levee Failures • Levees tend to fail during sustained high flow events because of underseepage problems, toe scour, and overtopping • This shows the 1999 Celotex levee failure in Louisiana, along the Mississippi River

• Most levees are compromised by underseepage. When a levee breaches, the sudden change in base level promotes rapid bed erosion, as shown here in the 1993 Missouri River flood.