Mary Blickenderfer, University of Minnesota Extension Service, 888-241-0885

Along undisturbed shorelines, native plants or natural rock guard against erosion by waves and ice. The research sites on County and DNR islands in Rush Lake are eroding primarily due to historic water level changes as well as increased size and frequency of boat wakes. These factors have made it difficult for native plants to reestablish.

Multiple goals were considered in the design of this portion of the Rush Lake project: provide long-term, nomaintenance stabilization of the slope toe, discourage boater foot traffic on the steep slopes above, and create fish and wildlife habitat.

Toe Stabilization – Where Land Meets Water

Six toe protection methods were tested on Rush Lake:

Live fascine

a 1-foot diameter bundle of willow and redosier dogwood branches backed with geotextile and held in place with wooden stakes pounded through the bundles.

Cocoa log

a woven jute sock 1 foot in diameter filled with compressed cocoa fibers. The sock is held in place with nylon rope attached to wood stakes or cable attached to duckbill anchors.

Rock gabion tubes

1.25 diameter tube of 4-8 inch diameter rock surrounded by chain link fencing (secured with tiger ties) and backed with geotextile.

Anchored log rafts

bundles of 3 to 5 logs anchored along the shore with a cable attached to duckbill anchors.

Stump revetment

large stumps placed so that they overlap with roots facing the lake.

Rock rip rap

a layer of 8-12 inch diameter rock placed over geotextile.

The table below summarizes the cost, installation time, maintenance time, and effectiveness of each treatment after one year.

During this initial year, all toe treatments, except for the anchored log rafts and the stump revetments, were effective at stopping erosion. The anchored log rafts were very problematic in that several duckbill anchors did not hold, allowing logs to work loose from the rafts and pose a hazard to boaters. This was corrected by replacing the duckbill anchors with earth anchors, as well as loosely fastening the cables to the logs with fencing staples.

Continued erosion behind the stump revetment was due to waves washing between the widely spaced stumps.

A tighter stump placement may diminish this erosion. In addition, wave action working on the cabled cocoa log cut the log in several pieces. It was replaced with a live fascine.

Our Minnesota winter will continue to test these toe treatments over the next few months. Look for a more detailed report and project updates on University of Minnesota’s Extensions Program web site beginning in January 2006.

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