Building Artificial Islands

Best Management Practices for Soft Engineering of Shorelines
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...On the Upper Mississippi River, managers, agencies, and industry have worked together to restore habitat diversity by building islands, often from dredge spoils.

Construction of an artificial island begins by building a sand base . Sand is supplied by dredging nearby main-channel or backwater sites.

This technique often provides a useful method for disposing of dredge material.

Hydraulic dredging is typically used because it is generally cheaper than mechanical dredging for large amounts of sand.

Final shaping and contouring of the sand is accomplished with the use of bulldozers. For safe operation of heavy equipment, the top of the sand base should be at least one foot above water level.

The desired shoreline slope of 20:1 is very difficult to build. Thus, a sacrificial berm, designed to erode naturally to a 20:1 slope, is placed along the shoreline.

On shorelines that may be subject to severe erosion, protective features are often added. This can be limestone rip-rap applied around the upstream head and tips of islands, but where possible, limestone groins (about 30 feet long) are used instead. Building groins are cheaper than armoring the entire shoreline with rip-rap and the beach areas between groins provide better access to the island for shorebirds, reptiles, and other animals.

After the sand base is complete, a cap of 1-4 feet of fine soil is applied. The height of the island is typically the elevation of a 10-year flood event at that site. Higher mounds can be added to support plants that require dryer habitats.

Finally, the island is planted with willows along the sandy shoreline and a mix of grasses, trees, and legumes in the fine soil. Using legumes in the mixture helps to maintain adequate nitrogen in the soil.