Temporary Cofferdams Tame Water for Work

“For 20 or 30 years, algae was growing in the springs, and there was decreased output because of the water table being lower for awhile in Florida. So there was less flow, and it wasn’t getting flushed out.” Underwater Engineering Services was involved in a restoration project.

An interesting facet of the project is that hand-dredging was required to clean out the springs because of the wealth of cultural artifacts found in the river. Although much trash was dug out, there were also some artifacts dating back thousands of years.

Among the more interesting items found were Native American tools and paddles, 16th century Spanish pottery, and arrowheads. In total, some 3,800 cubic yards of sediment and more than 10,000 artifacts have been pulled from the river, according to Cofer.

“There are a lot of beer bottles, a lot of whiskey bottles, a variety of things. We had an archaeological company working with us to classify all of this stuff. We have found anchors, ancient spear points. It’s been catalogued, at great expense.”

Cofer explains that the project involved construction of a temporary cofferdam berm, on top of which sat 10 geotubes. A sand and water mixture was pumped from the springs into these bags.

“The water passes into the bag, which works like a big filter, collecting the sediment that was dredged from the headspring,” he says. “Clear water then filters out, leaving the sediment in the bags. We hauled them off after the bags were filled with the dredged material.”

The project took place from March to August 2013, and the job specifications from the Southwest Florida Water Management District included guidelines that the nearby boat ramp remain open during the entire procedure. As a result, notes Cofer, crews also installed approximately 1,000 linear feet of turbidity curtains. This allowed work to proceed without undue silt and sediment movement from nearby boat wakes.

The hand dredging not only cleared the area of tons of sediment, but also deepened the project area by approximately 2–4 feet.

From a Hole in the Ground to Something Grand
On the banks of the North Saskatchewan River in Edmonton, Canada, a private club was looking to build a new boat ramp.

Myles Zuk, owner of Horizon Cleaning, explains that the group had previously simply carved out an area in the ground, but it wasn’t very effective for them.

“To build what they wanted, we needed to go about 20 yards or so into the river itself, so we created a U-shaped dam around the area and dewatered the work space. We used a temporary cofferdam, a bladder dam, from Dam-It Dams. With this system, you essentially fill the dam with water, and this creates the barrier. You then dewater the side you need to work in.

“Once we dewatered the area, we had to excavate about a yard down. In that void, we put 80-centimeter riprap. So we excavated almost a full meter down into the riverbed and surrounded our platform with heavy, heavy riprap. This holds the ramp pieces in place.

“The riprap held it in, but there was also a gravel surface that we created, and that was topped with precast, ribbed concrete planks. Each rib is about an inch and a half wide. This helped with grip, similar to what might be placed on a slippery roadway.”

The bladder dams were 4 feet high, and extended for a length of 250 feet. When filled with water, the bottom of the dam swelled to 10–12 feet wide.

“It will expand about two and a half times its height,” says Zuk, “so if you have a four-foot dam, then at its base, it’s 10 to 12 feet across. It’s like a big sausage. As it fills up, it goes high and wide.

“They’re looking for stability with this design, so it’s not just the height; you’re looking for something that’s not going to move.

He notes that he chose Dam-It Dams’ product because “what they wanted here was a fully synthetic product. It had to be non-earthen, to reduce sediment. The fact that it was to be a bladder dam was specified.

“One of the things about this system is that the river bed itself dictates how well you do. What I mean by that is that our conditions were very porous, so it wasn’t a muddy, silty bottom; it was a very coarse, gravel bottom. As a result, what we experienced was some penetration from underneath the dam. The fellows at Dam-It gave us some good ideas about how to mitigate that, and we were successful.”

In addition to the bladder dams protecting the river, there was also silt fence placed along disturbed areas of the riverbanks.

Zuk adds that another consideration was the aquatic life in the area. “There are more than 20 species of fish in the river, so there was concern about disturbing their spawning beds. The timing of the project involved two considerations to minimize impact to the fish. One was determining which fish were doing their thing at what time, and the other was to schedule the project while the river’s water level was at a low point, so it was arranged for October.”

The bulk of the work was completed within a couple months, before the end of 2014. The only work left was the landscaping, which had to wait for the then-present frost to thaw.

How does Zuk describe the transformation of this boat launch from its original carved-out area of ground?

“Now, it’s almost Olympic-style. If anything, it’s overdone!”