Mathijs van Ledden, Maarten Kluijver (Haskoning Inc.), Bob Bass, Nancy Powell (USACE) and Bob Dean

(University of Florida)

HINDCAST OF WAVE OVERTOPPING AND LEVEE EROSION AT GRASS-COVERED LEVEES DURING KATRINA

Hurricane Protection System

Details:Flood protection system with 350 miles of levees, floodwalls and structures

Protecting 24% of the people of Louisiana with 460,000 households and 26,000 businesses (2000) Objective is a 1% protection level in 2011 (i.e. expected exceedance of 50% if you live 75 years in this area!)

Motivation of this work

Current hydraulic design criteria based on current knowledge/guidance: 0.01 cfs per ft with 50% confidence 0.1 cfs per ft with 90% confidence

Impact of this choice has significant costs/schedule impacts

Strong need for more experimental validation, set-up of experiments underway

Also: what are the lessons from Katrina?

Hurricane Katrina

IPET investigated the causes and consequences of Katrina intensively

Katrina was an intense and large storm

Estimated return interval of the storm is around 400-years based on historical data

Levee breaches NO East

Levee breaches St Bernard

Approach

Use the ADCIRC model for the surge levels

Apply SWAN for the nearshore waves

Determine overtopping rates based on empirical relationships

Compare rates with observed damage

ADCIRC – peak of the storm

Surge level modeling at Notre Dame University

Katrina produced 16ft surge in the Lake Borgne area

Source: Notre Dame University

ADCIRC – peak of the storm

Source: Notre Dame University

SWAN grids

600 m / 200 m resolution

Offshore waves from WAM model

No bottom friction

SWAN results

Overtopping - assumptions

Empirical overtopping formulations Van der Meer

Pre-Katrina levee elevations

Grass-covered levees

Constant levee slope (1:5), no wave berm

Perpendicular wave incidence (overtopping rates are conservative)

Overtopping results – St Bernard

MRGO has severe overtopping rates 1 – 10 cfs/ft for hours

Canaervon to Verret levee much lower (< 0.1 cfs/ft)

Overtopping results – NO East

NO East back levee (~ 1 cfs/ft)

Overtopping rates other sections are very low

Summary

Overtopping rates and observations suggest an erosion/breaching threshold between 0.1 – 1cfs/ft Hindcast results do not disagree with the current design criteria but range is large

Suggestions for further work

More detailed nearshore wave measurements (eg. Hurricane Gustav and Ike)

Inclusion of wave direction in wave overtopping analysis

Assessment of uncertainties in overtopping results

Incorporate overtopping duration in criteria

Lab/field experiments

Example of prototype experiments

EU COMCOAST experiment

Conclusions

Comparison between computed overtopping rates and levee observations during Katrina provide reasonable agreement with current hydraulic design criteria

However: discrimination of levee sections/overtopping rates is not sufficiently detailed

More experimental validation is necessary to confirm and refine design criterion

Thank you for your attention