Morganza to the Gulf Hurricane Protection Feasibility Report

March 2002

Hydraulic Analysis

Interior Drainage Analysis

• UNET (gradually varied unsteady flow 1D model that conserves energy & volume)

• Geometry - mixed, some new & some from earlier studies back to 1987, datum to NGVD

• Datum errors as much as 1 foot in both topography and measured water levels.

• Calibration - Hurricane Andrew (1992) and 1991 rainfall event

ADCIRC Modeling

• Grid developed from surveys, nautical charts & quadrangle maps

• Verification – Carmen(1974), Juan (1985) & Andrew (1992)

• 17 experienced storms from 1893 to 1992 used to generate data set

• Storms experienced prior to ~1945 produced unacceptable results. Sparse storm data during this earlier period did not permit accurate computation of surge elevations

Frequency Analysis

• EST (Emperical sampling technique) – not used to analyze ADCIRC results due to the lack of data during the 100 year storm analysis period, but used for UNET results with some smoothing

• Plotting position formulation – used to generate exterior frequencies at gage locations and interpolated between.

• HEC-FDA used to compute confidence bands based on estimated record length

Table 2.5.1 Exterior Stage Frequencies                                                                                                                                                                                                                                             

Levee Design

• Windspeeds from GM – Larose GDM, 1972 (77mph = 100-yr frequency)

• ACES used to determine Hs, T & Levee Height

• Runup from Hs & T added to SWL determined levee height

Table 2.6.5. 100 Year Frequency Hurricane, Wave Characteristics And Design Wave Runup And Design Elevations Of Protective Structures, Highway 57 Alignment – Base Conditions                                                                             

Morganza to the Gulf Hurricane Protection Restudy

2008

Interior Drainage

• HEC-RAS • Geometry – from ADCIRC grid• Grid composed of ~ 10-year old Lidar• Original datum adjusted to

NAVD88(2004.65) • May 2008 - Corrections grid datum using

latest survey data• Calibration - Hurricane Rita (2005) and

a recent rainfall event

‘New’ ADCIRC Modeling

• Preliminary stages generated in LACPR study not used due to errors in grid datum and recent availability of new survey data

• Restudy will use a sub-set, 54 hypothetical storms, specifically for Morganza region

• Verification – Rita(2005)

• Model - base & future, w & w/o levee alignments

Frequency Analysis

• JPM-OS (Joint probability-optimum sampling technique)

• Products will be 50-, 100-, 500- & 1000-yr frequency stages

• & Confidence bands

Levee Design

• Waves – STWAVE (using ADCIRC grid) Hs & T plus confidence bands generated for required frequencies

• Standard deviations (std) for SWL, Hs, & T• Runup & overtopping at 50% confidence

from PCOverslag determines levee slope.• Design Height determined at overtopping

limit of 0.1 cfs/linear foot at 90% confidence for base & future conditions

Structure Design

• Waves – STWAVE (using ADCIRC grid) Hs & T plus confidence bands generated for required frequencies

• Standard deviations (std) for SWL, Hs, & T

• Design Heights determined for vertical walls by overtopping limit of 0.1 cfs/linear foot at 90% confidence for future conditions