Levee Erosion by Overtopping in New Orleans during the Katrina Hurricane

Abstract

Erodibility of a soil is defined here as the relationship between the erosion rate of a soil dz / dt and the velocity v of the water flowing over it, or the relationship between the erosion rate of a soil dz / dt and the shear stress developed by the water at the water-soil interface. This is called the erosion function. The test used to measure the erosion function of the levee soils is the erosion function apparatus test. The test consists of eroding a soil sample by pushing it out of a thin wall steel tube and recording the erosion rate for a given velocity of the water flowing over it. Several velocities are used and the erosion function is defined. A new erosion category chart is proposed to reduce the erodibility of a soil or rock to a single category number. Twenty three samples were retrieved from 11 locations at the surface of the levees around New Orleans. Thirteen were samples from Shelby tubes while ten were bag samples. The results obtained show a large variation of erosion resistance among the soils tested. Some of the levees associated with the location of the samples resisted the overtopping erosion very well; others eroded completely. On the basis of the erosion test results and of the observed behavior of the levees during the hurricane, a chart is presented which can be used to select soils for overtopping resistance. Numerical simulations were performed using the program CHEN 3D to obtain the distribution of velocity vectors in the overtopping flow and of shear stresses at the interface between the water and the levee surface. The comparison of the numerical simulation results and of the erosion function gives added credibility to the proposed levee overtopping erosion chart.

Overtopping breaching of river levees constructed with cohesive sediments

“…4), which shows the water surface and flow shear stress distribution in a certain moment after overtopping of a levee calculated by Briaud et al (2008). The large negative shear stress at the toe is caused by flow separation due to sharp corners between the levee and bed surface (Briaud et al, 2008).…”

“…If the flow velocity increased, the overflow departed from the headcut surface, forming an Briaud et al (2008) impinging jet. For multi-step headcuts, when the flow velocity was not large enough, multiple small impinging jets may appear due to the steps (shown in Figs.…”

Analyses, Simulations, and Physical Modeling Validation of Levee and Embankment Erosion

“…Based on numerous experiments using various soil samples from New Orleans area, Briaud et al (Briaud & Chen 2006, Briaud et al 2008 determined the relationship between the hydraulic shear stress applied by the water flowing over the soil and the corresponding erosion rate experienced by the soil, namely erodibility. Although no explicit erosion formula is provided in these publications, we use the presented data to estimate this relationship for different soils.…”

Levee reliability analyses for various flood return periods – a case study in southern Taiwan

“…The built-up water pressure against the flood wall pushed over the flood wall and thus caused inundation around the navigation canal Duncan et al, 2008). Besides, overflowing for the levee can also erode the backfill at the protected side (also known as a landside; "protected side" is used throughout the text hereafter) of the levee, leading to a loss of support from the backfill material at the protected side of the levee (Briaud et al, 2008). Wave overtopping was considered as a dynamic failure mechanism and therefore it is not the main focus in this study.…”