It appears that 14 miles of 120-year-old sand-and-grass dike near Portage, Wis., has survived the flood-swollen Wisconsin River’s recent rise to a record-level 20.6-ft, or more than 3 ft above flood level.
With the Wisconsin River&rsquos water level now dropping at about an inch an hour, the series of dikes, built by farmers in the 1890s and now maintained by the Wisconsin Dept. of Natural Resources, appears to be essentially intact, according to the DNR.
“The river has dropped to 18.7 ft this morning, and is going down at about an inch per hour,” DNR spokesman Greg Matthews said Sept. 28.
“We inspected all 14 miles of dike this morning. There is a little erosion and the dikes are still saturated, but it looks like they’ll continue to hold,” he added.
The DNR’s emergency response team expects to leave the town in south central Wisconsin, about 40 miles north of Madison, where it has been since the previous weekend.
Matthews said the DNR’s regular team in the area will continue to monitor the dikes for another week or so until the Wisconsin River returns to normal level.
Heavy rain far upriver last week swelled the river to a record height of 20.6 ft at Portage on Sept. 26.
The DNR dispatched an emergency team to the site to help monitor the dikes and help make any repairs needed to help them withstand the high water. The emergency team and regular DNR workers in the area totaled about 40 people.
The antiquated dikes were built of sand dredged from the river and topped with sod and grass.
They vary in size, but are generally up to about 12 ft tall, 6 ft to 12 ft wide at the top, and 100 ft or wider at the bottom.
Their side slopes range from about 1 ft of rise in 6 ft of run to 1 ft of rise in 3 ft of run, according to one expert.
The sandy dikes eventually developed 14 “boils,” spots where water began flowing through them from the river side to the land side.
“These dikes were built by farmers more than a century ago to prevent frequent small floods that destroyed crops. They weren’t meant to protect against a flood of this magnitude. They get saturated and leak like sieves,” says engineer Meg Galloway, chief of the DNR’s dam and floodplain section.
Galloway says that some clear-water leakage is acceptable, but when the leaking fluid starts to contain fine sand and has the consistency of light mud, there’s danger that the dike is eroding from the inside, and something has to be done.
At the 14 boil sites, the DNR teams built semicircular “silos” of sandbags about 4-ft high to provide some backpressure to slow the flow. The remedy appears to have worked.
Low-lying areas near the river that weren’t protected by the dikes flooded, particularly the Blackhawk subdivision, where some 300 people and an estimated 100 homes were affected�mostly because the road leading into the area flooded.
Patrick Beghin, Columbia County’s director of emergency management, said this morning that it is still too early to estimate how many buildings were actually damaged by flooding because the area is still not easily accessible and because there are some seasonal homes that may have been unoccupied when the flood hit.
“Many of the homes in the Blackhawk subdivision were built high, so they were not actually flooded, but the road to the subdivision flooded, cutting the subdivision off,” he says.
Beghin estimates that the flooding will have receded enough by sometime Sept. 29 for many people to get to their homes, get belongings, and assess property damage.