A lot of beaches are eroding, basically washing away, because they aren’t getting enough natural sediment to replenish themselves. One reason for this is that hard structures have been built along beaches (like jetties, ports, harbors, and factories) that block sand from being naturally pushed along by waves; sand gets stuck behind these structures and doesn’t replenish the downdrift beaches. One option to combat this problem of beach erosion is called renourishment. When a beach is renourished, sand from somewhere else is taken and added to the beach, making the beach bigger and building habitat and recreational area. Dunes are also often constructed to help protect the land from energetic waves.
The new sand must be the same as the old natural sand (color, size, texture, etc.) so that the natural habitats and beach shape can remain in equilibrium. So, what do scientists and engineers do if they don’t know what the natural sediment was like because the environment has already been altered by human construction? Scientists from Indiana University and the National Park Service set out to answer this question about beaches along Indiana Dunes National Lakeshore.
Indiana Dunes National Lakeshore is on the southern end of Lake Michigan. Naturally, it receives sand that is pushed by waves from northern beaches towards the south. But, as you can see in this picture, Lake Michigan’s coastline has become speckled with hard structures like factories that block this natural flow. Sand can no longer migrate down the entire coastline and is trapped in isolated “drift cells” between hard structures. These isolated drift cells have altered the natural sediment found along individual beaches and led to unnatural shifts in beach erosion that destabilize coastal communities, industry, and the natural resources of the National Lakeshore.
Before beach renourishment can be a viable option to restore these beaches, scientists had to find a way to define what the new sand should look like. Since they couldn’t determine what historical conditions were like, they had to be more innovative. They chose to study the present sediment at nine beaches, which ranged from currently eroding (bad) to currently stable or growing (good). They found that beaches that were eroding had different sizes of sediment than those that were growing and were therefore able to determine what type of sediment was best suited to stabilize each beach. They also found out what kind of sediment was best for local marine species. For example, at Central Beach, pebbles that are long and flat are the best choice to restore the natural habitat.
With pioneering studies like this one, scientists and engineers can find out the best way to restore beaches and habitats that have been dissected into drift cells, even if no one knows what their natural state used to look like.
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Photo taken by Tom Gill (Licensed under Creative Commons: CC BY-NC-ND 2.0): https://www.flickr.com/photos/lapstrake/10843064813/in/photolist-8s6Ced-e6QkDB-HvqtNh-m4K6he-hQ5yzt-m7nwoG-7WX8fx-rSAAh9-99JQuv-9Sa8AK-5YSabd-nqRdZw-pQso5P-5YS9Py-aitExd-b5rTiK-6EfNx2-bCCJca-6EjEh1-6EfooK-iGbPP7-atHFK2-5YMXCB-6EfU3a-b6kNKn-7j6Lnh-