Breakwaters in the Chesapeake Bay
The use of breakwaters has increased over time in the Chesapeake Bay and it’s no wonder given the promise of both “breaking” the impacts of waves and creating a beach behind them. The reason the beach builds out behind them is because breakwaters block the wave action that causes sand to move parallel to shore called longshore sand transport (see the Blog “Beaches are Rivers of Sand: the Key to Making Groins and Breakwaters Work”). As a result, sand gets deposited behind breakwaters and eventually build out from the beach to the breakwater forming a feature called a “tombolo.”
But, do breakwaters always work to build and preserve nice, wide beaches? Recent research has shown that breakwaters cause nice wide beaches to form behind them immediately after installation, but after about10-15 years the beaches return to their pre-breakwater average widths.
The example in the image above from Silver Downings on the eastern shore of the Bay provides a typical example of how beaches respond to breakwaters in the Bay. At this location, breakwaters were installed between 2008and 2011. The photos and graph show that beaches widened to about 27 meters (that’s88 feet!) on average more than doubling its pre-breakwater width. But, after12 years, the beach returned to the narrowest width the beach experienced before the breakwaters went in. So, the effects of breakwaters appear to be temporary.
It's important to note that beach width is defined by the amount of “clean” sand. While the previously built beaches do narrow behind the breakwaters, they also typically become vegetated by beach-loving grasses. This grass cover removes both clean sand and effective habitat needed for the northeastern beach tiger beetle’s survival.
