The mission of the USACE is to improve the hurricane protection system (HPS) around New Orleans to a 100-year level of protection. Part of the long term plans for the hurricane protection system (HPS) around New Orleans is to construct a gate between the Mississippi River Gulf Outlet and the GIWW to close the GIWW from Lake Borgne (2011). In this study an assessment has been carried out to compare the flood risk for several possible interim solutions to protect New Orleans against flooding during hurricanes in the interim period 2008-2011.



The following scenarios have been considered:

Base condition 2007: current elevations of floodwalls/levees;

Raise partial: raise existing levees/ floodwalls along GIWW and IHNC south of I-10 up to 15 ft;

Raise full: raise existing levees/ floodwalls along GIWW and IHNC up to 15 ft;

Interim gate: construct an interim gate at Paris Road Bridge (height 15 ft);

Base condition 2008: current elevations except for Caernarvon to Verret Levee (raised to 17-20ft) and Michoud Slip to Michoud Canal section (raised to 19.5 ft);

Optimal raise 2008: Base condition 2008 with raised levees/ floodwalls along IHNC/ GIWW following 100-year surge levels (GIWW 15-16 ft west of Paris Road Bridge, IHNC south of I-10 at least 15 ft);

Interim gate 2008: Base condition 2008 with an interim gate at Paris Road Bridge (height 15 ft).

Because of the assumptions applied in this study the flood maps and the results from the weakest link analysis cannot be used in an absolute sense. The flood maps will differ from the more detailed IPET flood risk assessment. Furthermore, the results of the weakest link analysis may change if other storms are evaluated. In a relative sense, however, these results can give good insight in the hydraulic effectiveness of the various measures and can therefore be used for decision making.

From this study the following has been concluded:

The flood risk is high in the St Bernard, New Orleans East and Orleans Metro polders. The flooding is mainly caused by a limited number of weak links in the system. Based on the present analysis, these links appear to be the Canaervon to Verret levee (St Bernard), the Michoud Slip to Paris Rd. section (New Orleans East) and the floodwall section south of I-10 (Orleans Metro).

The interim gate has more effect on the overtopping volumes and flood risk reduction than raising the floodwalls to 15ft for 50-year and 100-year event. Furthermore, raising floodwalls or building an interim gate mainly affects the flood risk in the Orleans Metro area, whereas the other polders have less benefit from these interim scenarios.

Raising the floodwalls/levees north of I-10 only slightly contributes to the overall flood risk reduction in the New Orleans East and New Orleans Metro polder.

The “Optimal raise 2008 scenario” (i.e. raising the system along IHNC to the 100-year still water levels) decreases flooding in all three polders and provides similar protection to St. Bernard and New Orleans East polders as the “Interim gate 2008 scenario” at Paris Rd. The Optimal raise 2008 scenario provides slightly less protection to New Orleans Metro polder as the “Interim gate 2008”.

The following aspects are recommended to be studied in further detail:

The current analysis focuses on three different events: 50-year, 100-year and 500-year. The chance of occurrence of a 50-year event an interim period of 4 years is less than 10%. It seems worthwhile to consider also events with a higher probability because of the short duration of this interim period. A 10-year or 25-year event would be appropriate choices since these events have a high probability of occurrence in the interim period (35% and 15%, respectively).

The weakest link analysis appears to be a very useful tool to provide insight in the system behavior. The current analysis in this report only evaluates three different storms. A more thorough analysis of the weakest links is recommended using the 100-year surge levels and waves throughout the entire system (instead of using a specific storm). This additional analysis may change the ranking of the weakest links in this report.

The flood risk assessment shows that there are a small number of weak links in the system. Dependent on the final location the closure in the GIWW/MRGO will partly solve these weak links. Especially for the St Bernard area, this closure should be considered in conjunction with other measures to reduce the flood risk significantly.