Causes of Bluff Erosion

The southeastern shoreline of Lake Huron, home to communities like Bayfield, and Goderich, features 10-20 meter high clay-till bluffs. Deposited during the last ice age, these bluffs contain pebbles from as far afield as northern Ontario, transported south by the ice that formed them. These west-facing shore cliffs provide spectacular views over Lake Huron, with sunsets here being ranked amongst the top 10 in the world according to National Geographic.

2019 and 2020 saw high water levels on Lake Huron, the highest since 1986. With this high water came renewed concerns surrounding bluff erosion, which is a recurring problem in this area. Bluff retreat is a significant liability for property owners and municipalities, and recent research suggests that it may be contributing to harmful algal blooms (HABs) in the Great Lakes.

While water levels in Lake Huron have since receded to their historical average, this episode of bluff retreat - the first to occur since the advent of modern GIS, drones, and airborne LiDAR - provides a unique opportunity to better understand Lake Huron's bluffs. By examining bluff erosion using these technological advances, we can be better prepared for the next rise in Lake Huron's water levels.

Shoreline slope is an indicator of the relative strength of terrestrial and marine (or in this case lake-related) geomorphic processes. Gradually sloping shorelines indicate that erosion from waves is weak compared to terrestrial processes that flatten out the landscape. In contrast, steeper slopes, like the ones along Lake Huron, are indicative of sustained wave-based erosion of the shoreline over time. The fact that steep bluffs exist throughout this stretch of shoreline - even in areas that have not seen erosion during the historical period - shows that erosion here is a natural process that has likely been occurring since Lake Huron settled into its modern form around 4000 years ago.

From just north of Grand Bend all the way up to Goderich and beyond, Lake Huron's bluffs are composed of a silty-clay till unit called the St. Joseph till. When it was deposited, this till unit was compacted by the weight of the ice sheet above it, making it remarkably dense and thus somewhat resistant to erosion. While the St. Joseph till is rigid, its high clay content makes it especially vulnerable to moisture, so shallow slips and flows are quite common when the bluff becomes oversaturated.

The part of the bluff that interacts with waves during storms and high water levels is known as the toe. Erosion of the toe of the bluff steepens the rest of the bluff, making it unstable. The remarkable density and rigidity of the St. Joseph till allows it to hold steep to even vertical slopes for short periods of time. However, after between 2 years and a decade (depending on vegetation characteristics) weathering and moisture weaken the material, causing the slopes to become more shallow (~30 degrees) during periods when the bluff is not being eroded by the lake. It is in this manner that toe erosion leads to bluff retreat over time.

Several atmospheric, hydrologic, hydrogeologic, geotechnical, and anthropogenic factors interact on the Lake Huron shoreline to determine when, where, and to what degree bluff retreat occurs over time. Many of these factors are relatively constant in their impact throughout the shoreline between Grand Bend and Goderich, with such factors including:

• Lake Level Changes (controls the timing of bluff retreat, but NOT the location)
• Orientation of the shoreline (shoreline faces roughly west throughout the study area)
• Longshore Drift Direction (sand moves north to south on this stretch of shoreline)
• Precipitation

And factors that resist erosion, including:

• Bluff Material
• Ice Cover (protects the toe of the bluff from waves in the winter months)

Since these factors are the same throughout the entire shoreline, and bluff retreat occurs only along certain portions of the shoreline and in varying amounts, none of these factors can explain where bluff retreat occurs along the lake. However, there are other variables that do vary from site to site that explain why the bluff retreats in some places but not others. These factors include:

• Beach width, which is controlled by
  • Sediment supply
  • Perpendicular shore structures, such as piers, jetties, and groynes
• Bluff toe defenses (e.g. gabian baskets, steel walls, rock walls)
• Residential and agricultural drainage onto the bluff face
• On-bluff vegetation

Perpendicular shoreline structures such as piers, jetties, and groynes trap sand that is moving south along the shoreline due to the process of longshore drift. Since this sand becomes trapped on the updrift (in this case, north) side of the structure, the downdrift (south) side of these structures becomes sediment-starved, so the beaches here become quite thin. Beaches are the bluff toe's first line of defense, dissipating wave energy before it reaches the bluff. Thus, during periods of rising lake levels, areas south of these structures with thinner beaches are more vulnerable to toe erosion than areas to the structure's north.

The clay soil found along the Lake Huron shoreline necessitates drainage for agriculture, so tile drains have been installed on many of the farms that adjoin the bluff. These tile drains often empty directly onto the bluff surface, oversaturating and weakening the St. Joseph till. Where tile drains empty directly onto the bluff, the bluff material is weakened against toe erosion and toe erosion propagates more quickly up the surface of the bluff than in drier areas. Further, large gullies may form in the bluff over time where runoff is exceptionally high.

In a similar manner to tile drains, residential drainage, such as that from storm drains, can also weaken the bluff. It is thus best practice to install tubing or piping so that water does not drain directly onto the bluff surface. It is also beneficial to install porous toe defenses such as rock walls in place of impermeable ones like steel walls. This is because over time, steel walls can trap water behind them, saturating the bluff.

While lake levels are constant throughout Lake Huron and thus do not dictate where bluff erosion occurs, the timing of lake level rises is the primary factor determining when these erosive periods occur.

Lake Huron has experienced several water level "highs" over the past century, with notable ones occurring in 1973, 1986, and 2019/20. However, erosion does not only occur during periods of high lake levels. Significant erosion occurred in the late 1960s, when lake levels rose rapidly from historical lows to historical averages. Thus, rapidly rising lake levels, rather than high ones, control the timing of toe erosion. This can be explained by the fact that beaches, which protect the toe of the bluff, always re-form at the lake level of the day, but it can take several years for this to occur.

It is unclear whether the most recent episode of coastal erosion on Lake Huron was made worse by climate change. Further, there still remains a lot of uncertainty regarding how climate change affects lake level oscillations. However, one clear impact that climate change has had on the Great Lakes is a precipitous decline in winter ice cover, which has been occurring since the 1970s. This has important implications for bluff erosion in southeastern Lake Huron because the bluff toe in this area has historically been protected by large ice banks throughout the winter. Since the windiest months of the year in this area are November, December, and January, this decline in ice cover could significantly increate the amount of erosion that will occur during the next lake level high.