SHELL BEACH, La. -- Under a murky, low-hanging sky, the Lacey Marie pitches to and fro on choppy Lake Borgne, using sonar to map a muddy bottom littered with rusted-out cars, boats, oil platforms, washing machines, and other junk swept into the water by Hurricane Katrina 18 months ago.
The oyster boat's mission is twofold: to identify navigational hazards just below the surface, and to produce a detailed, up-to-date map of the lake floor that could be useful in predicting what another hurricane could do to New Orleans.
Lake Borgne is actually a bay along the Gulf of Mexico. Katrina drove its water into Lake Pontchartrain and, from there, straight through New Orleans's levees.
Along Lake Borgne, fishermen complain bitterly about torn nets and other equipment damaged by junk 10 to 12 feet below the surface. In the fishing village of Shell Beach, they are still scratching their heads about what happened to a seafood refrigerator truck that is probably in Lake Borgne. Fishermen have run across a sunken offshore oil platform, and lots of neighbors' boats and cars are missing.
Aboard the Lacey Marie, two hydrographers hired by the National Oceanic and Atmospheric Administration have the tedious job of combing the lake foot by foot, one long pass at a time, with side-scan sonar. On this morning, a lap of Lake Borgne showed little.
"Nothing to speak of. A lot of trawl scars and things like that to give us confidence checks that we're seeing the bottom," Jason Infantino, one of the hydrographers, said of the day's work.
The trawl scars -- marks left by the automatic dredges oystermen use -- are the work of people like Frank Campo, the Lacey Marie's owner, who was at the steering wheel. His family has fished this lake, abundant with oysters, shrimp, and crabs, for 200 years. NOAA hired him to work with the hydrographers.
He looked bored as kept his bloodshot eyes trained on a monitor displaying the boat's position in relation to the grid it was running along. "Keep that boat between the red lines -- that's my job," he said, guffawing at how mundane the task was.
In the lexicon of hydrographers, this repetitive work is called "mowing the field": You go back and forth across the water, bouncing pulses off the bottom to get a reading of what's there.
By the end of this survey, Campo and the Lacey Marie will log more than 10,000 miles back and forth across Lake Borgne -- the distance to France and back. Hydrographers have also surveyed coastal waters in Alabama, Mississippi, and western Louisiana where Hurricane Rita hit a month after Katrina, and found huge fuel tanks, cars, air conditioners, and steel pipes.
When a navigational hazard is spotted, the crew notes its location and passes the information to state officials. A contractor is then dispatched to remove the junk.
Pinpointing debris is not the only aim. An accurate picture of Lake Borgne is an important step in understanding the flood danger to New Orleans. NOAA officials say existing charts of the Louisiana coast are woefully inadequate because of the changes wrought by erosion, hurricanes, and rising sea levels.
To forecast storm surge, meteorologists need to know Lake Borgne's depth and contours, because it is a 280-square-mile saltwater welcome mat to New Orleans.
Katrina passed over Lake Borgne the morning of Aug. 29, 2005, the storm's surge reaching heights of more than 20 feet. The water pushed west into canals and 630-square-mile Lake Pontchartrain, breaking levees, and flooding 80 percent of New Orleans.