More than a hundred million tons of dirty sediment mixed with tree limbs and junk float behind the Conowingo Dam, and Hurricane Sandy, a giant faucet nicknamed “Frankenstorm,” could send it pouring into the Chesapeake Bay.

This is just one of the potentially devastating environmental scenarios biologists are fearing as Sandy churns up the Atlantic coast. In the Hampton Roads area that includes Norfolk — where the earth is sinking and the sea level is rising, a potential storm-water flooding nightmare — officials are hoping that meteorologists are correct in their prediction that the storm will pass them by.

The hurricane is on track to smack the United States somewhere between the Delmarva Peninsula to the south and Cape Cod to the north, more than likely across Delaware and Pennsylvania. There lies the Susquehanna River, which cuts through the Keystone State and rushes pass the Conowingo, where it pours nearly 20 million gallons of water per minute into the Chesapeake Bay.

More water means more grainy sediment, which wreaks havoc on the bay by burying life-sustaining grasses, the nurseries of many species of baby fish. It can smother slow-moving bivalves, such as oysters, and the living things that attach to their shells, such as mussels.

A bit of good news this year is that the little fish have grown up and moved on and the summer grasses are gone, according to the Maryland Department of Natural Resources. Last year, Tropical Storm Lee did exactly what biologists fear will Sandy will do.

Lee loitered over the Mid-Atlantic, dumping enormous amounts of rain. The Susquehanna picked up volume and speed, bench-pressed four million tons of the sediment behind the dam and sent it cascading over in four days.

That was a million tons more sediment than what usually flows over in an entire year. Last year, the Susquehanna sent more phosphorus and sediment into the bay than in any year since monitoring started in 1978, according to a U.S. Geological Survey report.

Days after Lee, satellite images captured the normally blue bay colored like coffee with cream.

Plenty of sediment is left for Sandy: An estimated 160 million tons of sediment has piled up at the dam in the past 80 years. Some sediment, larger particles, contributes to the bay’s life cycle. But most of it, small granular particles, is a burden.

A U.S. Geological Survey study showed that the sediment behind the Conowingo Dam has become a main source of pollution that plagues the bay — nitrogen from fertilizers, phosphorus from manure and sediment from erosion, along with dust and construction site debris. The pollution threatens to undermine a multimillion-dollar bay cleanup.

“We’re likely to see additional scouring from behind the dam pushed down into the bay,” said Mark Bryer, director of the Nature Conservancy’s Chesapeake Bay Initiative. “That sediment moves downstream.”

Bryer said Sandy underscores the urgency of removing the sediment from behind the dam, a costly task that Maryland and the Army Corps of Engineers are studying. In the past few years, monster storms including Lee, Hurricane Irene and now Sandy have lined up to take a shot at the bay.

“The storm is extraordinarily large, a huge swath,” Bryer said. “There’s just going to be a lot of rain coming . . . that’s going to go down on the Potomac and Susquehanna, with a lot of pollution in it, and hit the bay. What I’m seeing is a lot more coming, based on what seen over the years.”

Bruce D. Michael, director of resource assessment for Maryland’s Department of Natural Resources, said his agency is focused on what happens next. “This will be a huge event for the Chesapeake Bay in terms of storm surge and potential property damage,” he said.

It’s not just the storm and sediment that’s worrisome, Michael said. It’s the carry-over effect.

The more sediment, the more chance there is for a dissolved oxygen event, Michael said, using science-speak for a dead zone — when pollution causes algae to grow and die in a rapid cycle, creating a muck that robs the water of the oxygen marine life needs to live. Fewer plants mean fewer fish, which waterfowl need to survive.

People with waterfront property should be wary of Sandy, Michael said. “We certainly are concerned about erosion, loss of property and flooding around the bay,” he said.

The moon is full, the tide is high and places such as Chincoteague, Va., and Norfolk, including the greater Tidewater region, are on their toes. Officials in those places gathered for emergency operations meetings Friday, and preparations continued through the weekend.

Norfolk continues to suffer shell shock from the nor’easter that dropped a huge volume of flood-causing rain in 2009.

“The experience folks have had if the tide’s up and rain on top of that is that there’s nowhere for the water to go in the drainage system,” which is old and outdated, said John Carlock, deputy executive director of the Hampton Roads Planning District Commission.

Norfolk is one of the nation’s oldest cities, with communities built on marshy ground where they probably shouldn’t be. They flood even at high tide — never mind a nor’easter or hurricane.

“Some of those areas are extremely vulnerable to flooding,” said Benjamin McFarlane, a regional planner for the commission. “That’s why we have to pay attention. Some of our areas are so close to the water threshold that it doesn’t take much.”