By Lynn Geiger
7 Lakes Alliance
Riprap, those large stones you see blanketing lakeshores, has been the go-to method for shoreline stabilization for decades. But is it really helping? That depends on what you are trying to protect.
Armoring the shoreline with riprap or a seawall will protect the land directly behind it from wave energy. But that energy has to go somewhere. In some cases, it will be reflected and scour out the shallows in front of the armor (there goes your nice beach) or will increase erosion on the side where the armor stops (then your neighbor needs armoring!).
Riprap can also be used to stabilize the land around a culvert to keep roads from being undercut by stormwater. If the only goal is keeping a road’s gravel out of the lake, riprap is a decent solution.
But the land is only half of the equation; we must also keep in mind the water.
The biggest threat to lake water quality is stormwater runoff. When it rains, stormwater will pick up nutrients and pollutants as it runs across driveways, roads and lawns. If it picks up too much speed, it erodes whatever it runs across and takes the sediment and dirt with it.
When stormwater runoff reaches the shore, riprap does nothing to stop polluted water from flowing over the hard surface and straight into the lake.
Those rocks baking in the sun can also transfer heat to stormwater, which may have been flowing across hot roofs and pavement. Warming up lake water may seem great for swimmers, but adding heat to the ecosystem will increase biologic activity and decomposition, which can deplete oxygen levels and kill fish.
Installing a natural, vegetated buffer will slow runoff and give stormwater a chance to percolate into the ground before it reaches the lake. As the water moves through the soil, the excess nutrients and pollution it picked up will be filtered out, and the water will have a chance to cool before it enters the lake. The roots in vegetative buffers hold soil in place, and once the plants are well-established, they will effectively stabilize the bank.
In a lake, waves can’t get as large as they do in the sea, and additional armor is rarely necessary. There is a time and a place for riprap, but typicallyreturning the system to a natural state works better for protecting lake water quality.
Lynn Geiger is 7 Lakes Alliance’s erosion control policy manager.