Restoring Natural Habitats

ECO-SPAN®: Restoring Stream Ecosystems

A recent article by Emily Clegg in the National Precast Concrete Association (NPCA) publication Precast Solutions highlighted a some of the many benefits that precast concrete arches such as ECO-SPAN® offer for stream ecosystems. While the focus of the article was the Great Lakes Basin, the same principles apply to almost any natural stream.

A question we often hear is “Why is such a large span required?” The article explains some of the reasons why engineers may replace a pipe or series of pipes with a wider clear-span structure. The bottom line is with today’s permitting and environmental concerns, hydraulic performance is just one of many issues that must be addressed.

One such issue is “Natural Connectivity.” This refers to the ability of a stream corridor to allow for the transportation of nutrients, natural water flows, and migratory aquatic organisms such as fish, insects, mussels, amphibians, reptiles and mammals. Lack of connectivity can result in population fragmentation or isolation, limiting habitat for affected species.

The article highlights ways that bridges and culverts may become obstacles to connectivity:

Perched Culverts: If a structure with an enclosed bottom causes a significant increase in velocity, scour at the downstream end eventually results in a waterfall and splash pool.  At the upstream end, lower velocities, channel widening and sedimentation often occur.  These conditions are a common obstacle to connectivity.

Undersized structures: Scour countermeasures may eliminate the perched culvert condition, but undersized structures also block passage in other ways.  Increased velocity and debris blockage are common in structures that do not span the base flow channel of a stream. Other possible blockages are sedimentation and too little flow at low flow times.

A 2012 ECO-SPAN® installation at US 31 near Kokomo, Indiana addressed the connectivity issue by not only spanning the base flow channel width but adding an animal crossing area adjacent to the stream. In addition to the benefits to the stream’s ecosystem described above, the crossing should improve roadway safety by allowing deer and other animals to pass under the road instead of across it.

The complete Precast Solutions article may be downloaded here. Select the Summer, 2013 edition of Precast Solutions Magazine and go to page 16.