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HSDRRS/MRL Considerations

HSDRRS/MRL Considerations. Briefing to Coastal Protection and Restoration Authority Board. Thomas A. Holden Jr., P.E. Deputy for Programs and Project Management New Orleans District U.S. Army Corps of Engineers 27 March 2013. HSDRRS MRL Orientation. Updates.

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HSDRRS/MRL Considerations

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  1. HSDRRS/MRL Considerations Briefing to Coastal Protection and Restoration Authority Board Thomas A. Holden Jr., P.E. Deputy for Programs and Project Management New Orleans District U.S. Army Corps of Engineers 27 March 2013

  2. HSDRRS MRL Orientation

  3. Updates • In March of 2012 briefed CPRAB and identified need to follow up on three items: • 1. Overtopping rates above the crossover point • 2. Armoring MRL above the crossover point • 3. LPV 149 Tie-In to MRL

  4. Item 1: Overtopping for 1% Event • Along the west bank of the MRL above RM 85.5 (west bank crossover point) and the east bank above 81.5 (Caernarvon tie-in) there are isolated locations that do not meet the MR&T authorized grade or the1% HSDRRS design elevation • USACE concluded that these locations are acceptable for a positive finding for the Levee System Evaluation Report (LSER) based on: • Average overtopping rates along an entire reach • Performance capabilities of established turf • Capabilities of interior drainage

  5. Locations That Do Not Meet 1% HSDRRS Elevation RM 85.5 West bank crossover point RM 77.5 East bank crossover point Slide 5

  6. Item 2: Armoring Needs • Armoring is not a requirement for a positive LSER • No authority to armor above the crossover point on the MRL • At the locations that do not meet the 1% design elevation, analysis shows that the overtopping rates for the 500 year event exceed the 0.5 cfs/ft threshold for armoring • Although overtopping rates exceed the armoring threshold, studies at Colorado State show well established turf performs well up to 4 cfs/ft

  7. 500 yr Overtopping Rates Based on Existing Elevations Based on 0.2% 50%CI Overtopping MR&T; assumes RM 70 to RM 85.5 constructed to 1% design elevation with EAMs Overtopping rates for 500yr event in cfs/ft

  8. Item 3: LPV 149 Tie-In to MRL • Higher elevation transition tie-in from HSDRRS floodwall to MRL is necessary (from El +21.5 to +24.0 feet) • T-wall and earthen levee tie-in options were investigated – earthen tie-in is recommended plan • Work will extend 100 linear feet up river and 30 linear feet down river along the MRL from the T-Wall • 65% Design in Progress • Expect to advertise: August 2013 LPV 149 Typical Section HSDRRS and MRL Earthen Levee Tie-in

  9. Back Up

  10. HSDRRS-MRL • Mississippi River Levees provide risk reduction • Riverine Flood Event • Based on 1973 Project Flood Flowline (refined) plus freeboard • Freeboard can vary for east and west bank • Flood Control Act of 1928 (Authorized MR&T) • 1% Hurricane Event • Based on storm surge, coincident river flow, and associated waves • Goal is to provide risk reduction for 1% chance exceedance • Hurricane and river flow are independent events, but both must be considered in analysis • Emergency Supplemental Appropriations Act (4th Supp. 2006) – (Authorized HSDRRS) Slide 10

  11. Orientation - Definitions • Crossover Point - The point at which the MR&T authorized design grade is equal to the 1% HSDRRS design grade • For 2011 this is RM 85.5 on the West Bank and RM 77.3 on the East Bank • The 1% HSDRRS elevations govern downstream and the MR&T elevations govern upstream of this point • HSDRRS-MRL Co-Located - The area along the MRL where HSDRRS and MR&T coexist and HSDRRS design elevations govern • Engineered Alternative Measures - Alternative measures have been constructed to raise the levee height to the 1% elevation while still staying within the existing footprint. EAMs meet LSER requirements • Resilient Features - Construction work which will ensure the WBV-MRLs meet all applicable design and O&M requirements Slide 11

  12. Timeline • Completed analysis of overtopping rates • No change to work required on MRL for 2011 • Commitment to complete further analysis on overtopping rates for east and west bank above the crossover points • Provide SLFPA-E letter addressing several questions and concerns (April 2012)

  13. Eastbank Crossover Point

  14. Westbank Crossover Point

  15. MRL Future Needs Miles of work required as crossover point moves upriver in future years 91 95.5 89 85.5 79.5 77.5 81.5 70 Slide 15

  16. ISAAC EOC Briefing Mississippi River Update 12.4 ft 12.4ft Carrollton West Point a la Hache 11.3ft Still Rising…. IHNC Lock Baton Rouge

  17. Mississippi River Surge Levels from Venice to Red River Red River Landing Baton Rouge RM 302 St. Francisville Bonne Carre Carrolton Donaldsonville Alliance Point a la Hache West Bay RM 8

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