Basics: Erosion at the Shoreline

Natural Erosion vs. Accelerated Erosion
Shorelines change naturally over time because the shoreline is constantly being bombarded by waves or ice movement.  In natural conditions this is typically a very slow process over a long period of time.

​However, human activities have changed this balance which accelerates the natural erosion processes - this is called accelerated erosion.

An eroding shoreline can be the result of natural or human elements, can be site-specific or widespread, and may have more than one cause.  In addition the causes of shoreline erosion may differ because of a property's location on the lake, water level changes and season. 

​There are many causes of accelerated erosion but the two most destructive causes are:
​
1)   Native vegetation removal - land and aquatic.
2)  Hardening of the shoreline (example: seawalls).

Lawn --- Lawns Invite Erosion

Changing a lake front from natural vegetation to lawns creates problems!   

Lawns (turfgrass) are not naturally found at lakeshores.  Turfgrass is not suited for being at the lake edge because of it's short and weak roots. These short roots do not have the strength to withstand the energy of waves hitting the shoreline or ice which makes the shoreline area very susceptible to erosion. Native plants that are suited for the lake edge have stronger, deeper and more complex roots systems. 
​(see Plants for Inland Lakes for more information).  

Don't "short" your shoreline - re-think your lawn

Aquatic plants (They aren't just weeds)
Many lakes in Michigan have aquatic plants at the lake edge though not all. The shape and size of the lake help determine where these plants are. The shallower a lake is the more aquatic plants it is likely to have. This is natural! 

Aquatic plants not only provide habitat to support a healthy lake ecosystem but they also help protect the shoreline against erosive forces. They help to reduce wave energy before the waves hit the shoreline.  Once they are all removed the shoreline is exposed to all of the waves' energy.  Share the shore and only remove what is necessary for boating and a small swim area.

​If you want a shore without plants perhaps a different lake that is sandier and deeper will better fit your needs.  

Graphic: Progressive A&E

  There are three different groups of factors that cause erosion: 
​1. Terrestrial (Land based) force       2.  Aquatic (Water based) force      3. Human activities.

​Terrestrial forces

• Slumping – The downward movement of a mass of unconsolidated material moving as a unit. Slumping is commonly caused by groundwater that exerts outward pressure on soil particles, creating a drainage area that can lead to soil movement.
• Soil creep – The gradual downhill movement of soil and loose rock material on a slope due to erosion caused by a combination of aquatic forces and gravity.

• Frost action - Usually happens in poorly drained soils and may result in heaves or depressions in the land.
• Wind erosion – Can affect shorelines with fine and dry soil composition. Wind-breaking structures such as trees, shrubs, sedges and grasses can reduce the effects of wind erosion.

​Aquatic forces

• Splash – Occurs when an area of exposed soil experiences direct precipitation. Heavy storm events can cause a significant amount of soil to be disturbed. Steeper slopes are subject to more intense erosion.

• Sheet erosion – Caused by the flow of storm water over the surface of an area. This movement of water removes “sheets” or thin layers of surface soil. The intensity of this type of erosion varies with the depth and velocity of the water movement, as well as the size and degree of which the land slopes.
• Rilling – The removal of soil in small, well-defined channels as a result of overland water build-up. Rilling is the most common process of rainfall erosion losses. Steep slopes and unconsolidated materials are most susceptible to rilling.
• Gullying – The removal of soil by larger diameter stream channels formed as a result of stormwater runoff. Gullying does not necessarily cause more erosion than rilling, but it can be more challenging to reverse as energy is channeled into specific areas or gullies.
• Wave action – The impact of waves hitting directly on an area of unprotected soil. Wave action may be natural or created by boat wake. Waves vary in velocity and frequency depending on water depth, surface water area and wind. To evaluate the effects of wave action one must consider shoreline slope, vegetation and soil composition.
• Longshore drift – Occurs when waves impact the shore at an angle, displacing unconsolidated shoreline materials and moving them along the shoreline. This natural process is responsible for beach formation and important for beach resupply. The effects can be worsened by structures that deflect wave energy, causing more intense erosion for unprotected areas.
• Ice push – As frozen water from the lake melts in the spring it tends to push or lift objects, especially with rising water levels and high winds. Many times ice is pushed up onto the shoreline in blocks or sheets which pile up and cause erosion.

​Human activities

• Level control structures – May raise lake levels to an unnatural point on the shoreline where soils are less able to withstand wave and ice action.
• Clear-cutting natural vegetation – The removal of vegetation by landowners to maximize the amount of visibility and access to the water. In addition to destroying wildlife habitat, the clear-cutting of natural vegetation also eliminates the erosion-preventing root structures once provided by native plants, trees and shrubs.
• Stormwater runoff – Created by areas with impervious surfaces, such as driveways, parking areas and roofs. Impervious surfaces around lakes collect and deliver precipitation over land instead of allowing it to infiltrate into the soil.

What about Seawalls? 

Property owners look for ways to control shoreline erosion when it becomes a problem.  This typically results in the use of hard structures called seawalls.  A seawall is any hard-surfaced wall installed along the shore to block the waves from reaching the land.  The walls are typically parallel to the shore with a vertical surface facing the water.  They are made out of many materials, including concrete, steel sheet pile, wood and rock filled wood structures.   Property owners are drawn to seawalls because they are perceived to be more stable.  However, seawalls create problems in the lake.  

• Wildlife Barriers: Seawalls are essentially vertical walls that are barriers for wildlife like turtles which cannot climb a vertical wall.  Turtles need to come on land in order to lay their eggs.  If they can't reach the land then they can't lay their eggs. ​

• Scouring of the lake bottom:  Seawalls do not allow for absorption of the energy that waves bring in.   As waves hit the seawall the energy from the waves does not disappear. This energy is directed downwards and sideways.  The energy that is directed down scoops out the lake bottom - this is called scour.  Scouring of the lake bottom increases with lake size and wave height.  Habitat is lost because of this scouring effect and the lake can become deeper in front of a seawall as compared to a nearby property without a seawall.  Scouring can also go deep enough to undercut the bottom of a seawall causing a seawall failure. ​

• Wave Flanking:  When a wave hits a seawall the wave energy is also deflected sideways to neighboring property this is called wave flanking.  This causes erosion on the neighbor's property where there might not have been any erosion.  But now there is a problem.  A domino effect of seawall installations is many times a result of wave flanking problems from a neighboring seawall.  ​

Showing wave flanking from a seawall causing erosion on a neighboring property Photo: Jane Herbert