By Patrick Jackson
It may be several months until all the water is removed from New Orleans.
Sandbags weighing more than 900kg may also be dropped
Before the vital permanent pumping stations can all be reactivated, the city swamped by Hurricane Katrina has to be drained - a delicate task given the number of bodies in the floodwater.
The floodwalls along the canals must be repaired and levees (embankments) specially opened for gravity drainage must be dug over.
As of 7 September, about 60% of the city remained "under water" and the city's pumping capacity was only about 10% of the norm, the US Army Corps of Engineers (USACE) said.
Pumps now in operation in the downtown area of the city are discharging floodwater at around 64,000 litres (2,250 cubic feet) per second while, according to a Louisiana State University estimate, there were about 95 billion litres (3.6bn cubic feet) of water there as of 2 September.
The weather will largely dictate the speed of work and the hurricane season is predicted to run into October.
Engineers working at one breach site, the 17th Street Canal, have managed to seal the broken floodwall with rock and sandbags and have begun removing water with pumps brought in specially.
However, four other breaches remain at two other canals, one of them twice the size as 17th Street.
Repairs in stages
Whether patching up a breached floodwall to keep water out or opening a levee to let water escape, the USACE engineers have been working flat out under the clatter of helicopters and the splash of the giant sandbags they deliver.
They are working in stages to seal off the canals hit by storm surges out in Lake Pontchartrain, USACE spokesman John Hall told the BBC News website from operational headquarters in Vicksburg, Mississippi.
Four levees are also being deliberately breached with earthmoving machines to allow floodwater in the St Bernard and Plaquemines Parishes (districts) to drain out into the Gulf of Mexico.
According to the USACE, as of 6 September, the situation on the three canals was as follows:
- the 90-metre (300-foot) breach at the 17th Street Canal is now plugged by a rock dyke and supplemental pumps are in operation; the lake mouth of the canal has been dammed with steel sheet pile
contractors have begun work on damming the London Avenue Canal at a bridge near its lake mouth with sandbags; the canal has two breaches, 180km (600 ft) in the west bank and 60m (200 ft) in the east bank, to be plugged
THE FLOODWALL BREACHES
One at 17th Street Canal: 90m (300 ft)
Two at London Avenue Canal: 180m (600 ft) and 60m (200 ft)
Two at Industrial Canal: widths unclear
a contractor is sending lorries and ramps to the Industrial Canal (also known as the Inner Harbor Navigation Canal) which has two breaches; work should be easier here as it is a navigation canal and barges can be used.
The work has involved dropping 300lb (136kg) sandbags by helicopter into the canals and building a rock road to allow heavy machines to approach the 17th Street site from dry land.
"People here are extra short of time and working round the clock - we have a day shift and a night shift," Mr Hall said.
Bodies in the water
After the water level falls and electricity is restored, the engineers will be helping to restore the 22 Drainage Pumping Stations and scores of other pumps which serve a city which has expanded below sea level over the decades.
GETTING THE WATER OUT
Temporary repairs to floodwall breaches
Drainage through temporary pumps and holes specially made in levees
City pumps reactivated after electricity returns
Engineers say they taking special care to watch for human remains being swept into the grates of pumping stations, noting that there is a "tremendous amount of debris" accumulating there.
"We're going to be as diligent as we possibly can in working with the local and state governments and other agencies to treat these remains with as much dignity and respect as possible," said the USACE Washington spokesman Tom Waters.
Scientists at the UK's Benfield Hazard Research Centre predicted earlier this year that the hurricane season between July and October would be "very active".
The tactic of deliberately breaching levees, therefore, involves some risk.
"We want to make sure that we don't catch ourselves with levees open and another storm front moving on us," USACE commander Lt Gen Carl Strock told reporters in Washington.
USACE officers have been predicting that the city may not be properly pumped out until mid-November.