When storm surge threatens to exceed the maximum operating water level of a canal, the Corps closes the gates and operates the pumps. Interior pumps operated by the Sewerage and Water Board pump rainwater out of the sub-basin and into the outfall canals. The interim pumps then pump rainwater out of the outfall canals, around the gates and into Lake Pontchartrain. The closed gates reduce the risk of storm surge entering the canals and threatening the city. When the surge recedes to a safe level, the gates are reopened and normal drainage resumes. The Corps received authorization to build the interim closure structures under emergency flood control legislation passed in September 2005, specifically Public Law 109-61 and Public Law 109-62. It cost about $400 million to build the interim closure structures. The Corps did not have the authority, however, to build permanent canal closures and pumps under this legislation. It was not until emergency appropriations in the years after Hurricane Katrina that the Corps received authorization to build the permanent gates and pumps. This authority superseded any previous legislation prior to Hurricane Katrina that prohibited the Corps from building closure structures at the mouths of the outfall canals. The appropriations for this project total about $804 million. In the years since Hurricane Katrina, the Corps of Engineers has been constructing and bolstering of levees, floodwalls, barriers, gates and other structures as part of the Greater New Orleans Hurricane and Storm Damage Risk Reduction System (HSDRRS), which stretches across five parishes and includes much of the original Lake Pontchartrain and Vicinity Basin, as well as the West Bank and Vicinity Basin on the West Bank of the Mississippi River. When complete, the HSDRRS will reduce the risk from a storm that has a one percent chance of occurring in any given year, or a 100-year storm. The interim closure structures already provide a temporary 100-year-level of risk reduction, and the permanent canal closures and pumps will continue to provide that same level of risk reduction. The contract for the permanent canal closures and pumps will be awarded in 2011 and construction will be complete in the fall of 2014. Although interim closure structures (and eventually the permanent canal closures and pumps) prevent storm surge from entering the canals and provide the 100-year-level of risk reduction, several portions of the outfall canal floodwalls are being remediated, or strengthened, to meet the more stringent post-Katrina design requirements. When remediation is complete, all canals will be able to operate under a maximum operating water level of +8.0 NAVD 88.