Part 9Part 9

LEVEES AND CREVASSES

Model levee designModel levee design

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

Natural crevasses beneath levees 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 contrastsPermeability contrasts caused by clay filled caused by clay filled oxbows create treacherous and contrasting oxbows create treacherous and contrasting foundation conditions beneath levees.foundation conditions beneath levees.

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

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

Raising LeveesRaising Levees

• The rising bed of the Mississippi River has necessitated the heightening of levees, beginning in the 1890s, 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 Levee FailuresFailures

• Levees tend to fail Levees tend to fail during sustained during sustained high flow events high flow events because of because of underseepage underseepage problems, toe problems, toe scour, and scour, and overtoppingovertopping

• 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.