NEW WEIRS IN RIVER A SUCCESS; UNDERWATER DIKES SAVE UPKEEP COSTS

Bendway weirs, a new concept in channel maintenance on the Mississippi River between St. Louis and Cairo, is paying off in saving tax dollars and time and money to the barge and towing industry, says a Corps of Engineers official who was involved in the design of the concept.

"The bottom line is this new concept is saving money in dredging costs and making the river safer and more economical for barge traffic," said the official, Rob Davinroy, hydraulics engineer with the St. Louis District Office of the Corps of Engineers.

Bendway weirs are underwater rock dikes similar to wing dikes that extend into the river from the shoreline. Unlike wing dikes, bendway weirs extend from the riverbank under the water, on the bottom of the river.

Wing dikes are used to keep most of the river current in the navigation channel to keep the channel scoured out. However, during low water the Corps has to spend millions of dollars to dredge sand and sediment that builds up along and in the channel, especially in the river bends and at locations where the channel crosses from one side of the river to the other.

Several years ago Davinroy discovered that underwater bendway weirs would accomplish what wing dikes and dredging were already doing, but at a much lower cost.

The first bendway weir prototypes were constructed several years ago in a sharp bend in the vicinity of Price and Daniel's Landing south of Commerce. Since they were built, Davinroy says less dredging has been required in the bend during low water, and towboat captains and pilots report it is much easier and safer to navigate through the bend while coming upstream with a large barge tow.

Last year another series of underwater bendway weirs were constructed by the Corps in the vicinity of Cape Rock at Cape Girardeau. Four other weirs were also constructed on the river bottom between the mouth of Big and Little Flora creeks and the former J.D. Street bulk petroleum terminal north of the Cape Girardeau water intake pumping station at Cape Rock.

After one year of study, Davinroy says the underwater bendway weirs are performing even better than expected.

"To understand how the bendway weirs work, you first have to understand what kind of conditions we were faced with at Cape Rock," said Davinroy. "Cape Rock is located on the outside of a narrow, sharp river bend. When the river flowed around this bend, most of its energy was deflected off Cape Rock. This generated a lot of turbulence in front of Cape Rock that caused the river to dig an enormous hole (as much as 80 feet deep, depending on water levels). The material that was dug up from this hole was then quickly deposited downstream, where it formed a large, shallow sandbar."

Davinroy said the scour hole also caused the main river current to flow toward it, causing the current over the top of the hole to become more turbulent and swift.

"That meant that any barge tow or boat floating downstream toward Cape Rock tended to be drawn toward the hole, increasing the possibility of colliding with the rock ledge at the foot of Cape Rock," he said.

"To avoid a collision, the towboats had to come full ahead on their engines to stay in the narrow channel and avoid hitting the rock ledge. As a result, it would take a longer time, especially during low water, for the towboats and barge tows to pass safely around Cape Rock, while using more fuel. Both of these costs were passed on to the shipper.

"In addition, the sandbar that formed downstream from Cape Rock as a result of the scour hole created a shallow channel that had to be dredged frequently to make sure the channel was deep enough for barge tows and towboats to pass through safely."

Frequent, expensive and time-consuming accidents have occurred near Cape Rock because of the hazardous conditions caused by the swift current and scour hole. Davinroy said Corps and Coast Guard records show that between 1980 and 1992, 15 barge tow accidents occurred on the river near Cape Rock. The accidents involved collisions against the rock or groundings on the sandbar.

Additionally, between 1984 and 1990, approximately 1.5 million cubic yards of material were dredged or removed at the sandbar just downstream from Cape Rock, at a cost of approximately $1.4 million.

"Since the bendway weirs were constructed one year ago, significant changes and improvements have already occurred at Cape Rock," Davinroy said. "The direction of the main current has been pushed away from Cape Rock more toward the middle of the river. The speed of the current against the rock has been slowed considerably.

"Now, it is much easier and faster for towboats and barge tows to navigate around Cape Rock. Delay times and excessive fuel consumption have been minimized.

"The bendway weirs have also caused the current to reduce the width of the large sandbar on the Illinois side of the river, opposite Cape Rock, which has widened the channel at that location."

Davinroy said dredging of the sandbar below Cape Rock was eliminated this past year since most of the material that would have created the sandbar was caught between the bendway weirs where the deep scour hole used to be.

Davinroy said the bendway weirs may also be serving as an environmental benefit. "Engineers and biologists are examining fish populations between the bendway weir structures. The theory is that the structures are acting like underwater reefs, attracting various forms of organisms, which serve as the food chain for fish," Davinroy explained.

"Also, slowing down of the current and widening of the channel could also be improving physical habitat conditions. These combined changes could be attracting larger fish populations to the area around Cape Rock. We'll know more about this as additional data is gathered and analyzed."

Davinroy said the success of the bendway weir projects on the Mississippi has attracted international attention. "We've had design engineers from Canada and the Netherlands ask the Corps for information so they can make use of the bendway weir concept to rivers in their countries," he said.

The Cape Rock bendway weir project used approximately 296,000 tons of rock and cost around $800,000.