Three ways that water has shaped the Scarborough Bluffs

While observing the magnificent Scarborough Bluffs, we are reminded of the incredible force of water. From the visible remnants of glacial lakes at Cathedral Bluffs Park to groundwater seeps all along the Bluffs, evidence of water’s influence is everywhere.

Scarborough Bluffs erosion

Understanding how water has shaped and will continue to shape the landscape, including the subsequent rate of erosion and associated risks, is an important consideration in the Scarborough Waterfront Project (SWP).

The Project Team includes coastal and geotechnical engineering experts who have studied the landscape of the area and the processes that have created the beaches, the Bluffs, and the many habitats within them. Here’s how water has shaped, and continues to shape, the Bluffs and some of the risks.

ONE: Wave action from Lake Ontario

As the waters of Lake Ontario move against the Bluffs, material is washed away from the bottom of the Bluffs (slope toe). This over-steepens the slope toe leading to instability further up the slope.

The Risk

Over-steepened slope faces are unstable in the long term and eventually, the weight of these slope faces overwhelms their ability to remain vertical. Landslides and the loss of land at the top of the Bluffs caused by this erosion can present risks to public use and infrastructure (such as water pipes), resulting in significant management challenges. The Project Team undertook an analysis of the slope failure factors to determine the extent of the hazard area along the Bluffs or which areas are at risk of collapse.

Landslide on the Scarborough Bluffs
Summer 2015 landslide along the eastern section of Grey Abbey.
Interesting historical fact

We’ve mentioned stonehooking before, but how does it impact the Study Area today in terms of wave action?

Stonehooking was a method of removing stone slabs and other rocky material from the shallow areas of the lake for use in construction before we had gravel pits and quarries. Between 1830 and 1930, this activity significantly altered the Scarborough Bluffs.

Prior to stonehooking, wave action from Lake Ontario would be buffered by the rocky material before it reached the toe of the Bluffs. However, once this rocky material was removed, there was nothing in place to help absorb the impact of the waves. This resulted in increased toe erosion and over-steepening which in turn led to increased instability further up the Bluff slope and ultimately loss of tableland (land at the top of the Bluffs).

Historical photo of ship on Lake Ontario, stonehooking

TWO: Water runoff and seepage

Soil erosion can also be caused by water coming off the land. Water runoff (or overland flow) from the tablelands as a result of precipitation, especially storm events, is exacerbated in urban areas where impervious surfaces (roads and buildings) prevent water from infiltrating into the soil. Groundwater seepage out of the slope face plays a smaller role in erosion at the Scarborough Bluffs, but together these water sources, along with the influence of local topography (shape and height of the land) and soil composition (ratio of sands, silts and clays) can play a significant role in surficial soil failures (landslides).

Groundwater seepage on the Scarborough Bluffs
Evidence of groundwater seepage is seen above where the Bluffs appear darker.

The Risk

While bluff slope failure is a natural process that contributes to the character of the site, it becomes a hazard when human use of the shoreline is considered.

Interesting historical fact

The Scarborough Bluffs underwent rapid urbanization in the 1940s, with many areas that were previously farmland becoming urbanized right to the edge of the Bluffs. This resulted in loss of vegetation at the top of Bluffs, which naturally helps mitigate erosion. When vegetation is removed and replaced with impervious surfaces, the impact of surface water runoff is greater.

Rural Scarborough Bluffs in 1947 and 1962

THREE: Freeze-Thaw Cycle

The freeze-thaw cycle is another major factor in soil erosion. During the colder months, water within the soil freezes resulting in expansion, but as the water melts, the soil compresses. This expansion and compression process loosens the soil, which can result in slope failures, especially when spring-time melt water and rain water seep into the ground and elevate the groundwater table.

The Risk

Did you know that water expands about 9% when it freezes?

This process contributes significantly to surficial instabilities across much of the Bluffs. This extra “freeze-thaw” water pressure, in combination with an already oversteepened slope profile, produces failures in the forms of blocks and wedges calving off the slope face during these melt periods. When this type of failure occurs, about 1-3 m of tableland can be lost in a single event.

Pulling it all Together

Sound Project decision-making is not possible without an in-depth understanding of coastal processes, shoreline and bluff erosion, human influences and the other variables that together make up the dynamic system of the Scarborough Bluffs. Careful analysis of physical processes that occur within the Study Area and beyond is an essential component of the SWP Environmental Assessment.

Learn more about how erosion has been influenced by both natural forces and by human activities by viewing the following video: