Watershed district proposes erosion solution in Scandia

The view from the bottom of the ravine, looking up. (Photos submitted)

The view from the bottom of the ravine, looking up. (Photos submitted)

$40,000 to $50,000 to stabilize ravine

 

The sediment delta at the bottom of the ravine.

The sediment delta at the bottom of the ravine.

Mary Bailey
Community Editor

A drainage pipe that keeps the road dry is gouging a ravine and filling in a wetland between 197th Street and the St. Croix River in Scandia.

At a workshop with directors of the Carnelian-Marine-St. Croix Watershed District on Saturday, Oct. 5, residents of the neighborhood learned about the problem and possible fixes.

The erosion began in 2007 or 2008 when the city of Scandia paved the road, which heads east from St. Croix Trail (TH-95) and then turns south, going down a steep hill to the 13 properties along the river. Four are year-round residences and the rest seasonal.

Where the road turns, the city installed a catch basin. A pipe under the road carries rain water from this depression to the top of the hill. From there it flows freely down the hill, on its way to the river.

Large rain events since the road was paved have scoured out a deep ravine about 70 or 80 feet long and deposited about 18 inches of sediment in a black ash seepage swamp at the bottom. The ravine cut is more than 10 feet deep in places.

A black ash seepage swamp is considered a unique ecosystem of vegetation, including black ash trees, which prosper in wet, peaty areas.

Todd Shoemaker, engineer at Wenck, presented potential solutions.

At the top of the hill, a rain garden would be planted in the catch basin. Water would infiltrate in about 48 hours; most of the time the basin would be dry.

To stabilize the ravine, Shoemaker recommended removing some trees. With tree canopies gone, more sunlight would reach the ground and more plants could provide soil-holding roots. Jim Shaver, administrator of the watershed district, has marked trees for possible removal and is waiting for the National Park Service to respond.

The trees would be cut at ground level, so their roots stay in the soil, and the trunks would be dropped parallel to the slope to act as another form of erosion control. The remaining large trees would be spaced 35 to 70 feet apart.

Any hollow or diseased trees would be removed first, and smaller-diameter trees would be selected over larger, more mature trees. Species would also be taken into account, with box elder more likely to be cut than oak.

A pipe would be laid in the bottom of the ravine, with a flattened end to slow the water as it emerges at the bottom. The pipe would be covered with fill and topped with rocks to hold the soil in place, and a woodland seed mixture scattered.

A second option is to fill in the existing ravine but place the pipe in another location. Using a nearby city-owned utility easement would allow for easier maintenance in the future. The ravine crosses two private properties. Shaver recommended this option.

Costs were estimated at $6,360 for the rain garden, plus either $37,728 to stabilize the ravine and place the pipe on private property or $42,648 to use city-owned land.

The project could occur next fall, a good time of year to avoid inconveniencing seasonal residents. That would also provide an opportunity to observe the flow of water next spring.

The swamp at the bottom “is no longer a black ash seepage swamp,” Shaver said, with black ash trees dying and reed canary grass spreading. Restoring the swamp may be done in another phase of the project, he said.

Besides providing information, another reason for the meeting with property owners was to secure their cooperation. “The watershed district does not force projects on property owners,” Shaver said. “We look for cooperation and consensus.”

Road maintenance workers for the City of Scandia would like to widen the steep north-south part of the road, which is currently one lane and has forest growing up to the edge. Shaver said any plans to wide the road are unrelated to this project.

Watershed districts were created in 1971 by the federal Clean Water Act, Shaver said, and since then have expanded their focus, initially on flooding, to include solving water quality problems.

The seven-person board, appointed by county commissioners, has the authority to levy taxes and sell bonds.

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