mathijs van ledden, maarten kluijver (haskoning inc.), bob bass, nancy powell (usace) and bob dean...
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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
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
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
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