Think Small: Avoid the Impacts of Large Boulder Riprap on Your Shoreline

THINK SMALL

Avoid the Impacts of LARGE BOULDER Riprap on Your Shoreline

By Jennifer DeMoss, Tip of the Mitt Watershed Council Communications Director

Shoreline erosion: it’s a nightmare for lakefront property owners. You invest a tremendous amount of time, money, and effort into creating the perfect home or vacation getaway, only to lose parts of your land to the waves. The encroaching water can cause some to resort to drastic shoreline modifications, such as piling large rocks or boulders along the shore to “hold” the soil in place. Using any kind of rock to stabilize a shoreline is called riprap, and the bigger the rock, the better it is at protecting the shoreline…right? But, is bigger always better when it comes to riprap?

First, Tip of the Mitt Watershed Council is here to tell you, those large boulders hurt more than they help as there are ecological consequences to using oversized rocks at the shoreline. When waves hit large boulders, wave energy is directed downwards and sideways. This causes erosion to the lake bottom behind the boulders, called scouring, and causes erosion to neighboring properties. This wave deflection not only magnifies erosive processes, it also eliminates vital habitats for wildlife who live near the lakeshore. Bigger is not better in the case of using rocks to stabilize the shoreline, and there’s an art to the process.

If you’re not using huge boulders, what’s the alternative? Are all rocks off-limits for erosion control? Of course not. Because riprap is often associated with large boulders, we use the term RIPRAP “revetment” when discussing the use of rocks in bioengineering projects. Revetments are sloping structures on shorelines designed to absorb wave energy and bioengineering is a set of techniques that primarily use vegetation to control erosion. Plants, as well as appropriately-sized stones, in gently-sloping shoreline revetments are what we recommend for preventing erosion.

Slope and stone size and shape are important considerations. A properly designed revetment will have a gentle slope of 3H (horizontal):1V (vertical) or flatter - meaning the slope is three times longer than it is high. This gentle slope can be dressed with appropriately-sized, round fieldstones. Why round? Revetments act as wedges that assist ice up and over the shoreline rather than directly into it. Also, flat rocks are easily destabilized by waves. On most high-energy lakes in Northern Michigan like Walloon, a 9 to 12-inch diameter fieldstone is typically the maximum size necessary to anchor the revetment in place and a 4-inch diameter fieldstone is, generally speaking, the minimum size. This does not factor in smaller drain stone and pea gravel used for the filter layer underneath the revetment.

Hand placing revetment fieldstones can help stabilize the revetment and minimize spaces between the individual stones. In revetments with a lot of empty space in the structure, waves can pass through and continue to erode the shoreline—another reason to avoid placing huge boulders with gaping voids between each rock on the shoreline. Waves will continue to eat through the structures wherever they’re allowed, which is why thoughtful planning and construction are needed to truly protect one’s valuable investment.

What else can stabilize a shoreline like nothing else? You guessed it: plants. At the Watershed Council, we’re big fans of using native plants as an attractive, cost-effective method to grab hold of your beach sand and keep it there. As Connor Dennis discusses in his article, if you’re seeking to prevent erosion and preserve your property’s beauty and value, you probably have what you need right at your shoreline. A buffer zone of deep-rooting shoreline vegetation offers many benefits: filtering stormwater runoff, preventing pollution, adding beauty to a lawn, cutting down on time and money spent on lawn care, and discouraging geese and their droppings.

One thing to note is that many shoreline modifications require a permit through the Michigan Department of Environment, Great Lakes, and Energy (EGLE). You’ll want to check out mishorelinepartnership.org/shoreline-permitting-information for more info before any project.

Overall you want to protect your investment in a shoreline property, enjoy all Walloon Lake has to offer, and have a beautiful view, and we want the same for you! Consider using native plants, installing a revetment over large boulders or seawalls, and utilizing bioengineering to support a healthy shoreline.