Factors influencing coastal bluff stability

There are numerous, interconnected factors that influence the overall stability of a bluff, bluff erosion, and the formation of landslides. Landslides are one of the biggest hazards associated with coastal bluffs, especially high coastal bluffs made of muddy sediment. Landslides have occurred frequently enough in Maine that geologists have learned from them and identified ways to reduce risk and improve response in an emergency.

The information provided below has been developed from text used for the Maine Geological Survey’s Landslide Hazard Maps, and information from the State of Washington Department of Ecology.

Height
Sediment type
Slope
Slope aspect
Topography

Vegetation
Microclimate and aspect

Waves, tides and sea level
Drainage
Surface water
Ground water
Weathering
Earthquakes
Land use

Height. The taller/thicker the sediment deposit, the more likely its weight will cause movement or slippage. >>top

Sediment type. The finer the sediment, the greater the risk. Muddy clay and silt are the most unstable materials. Mud is structurally weak and prone to slow-motion creep, moderate slumping, or sometimes large landslides. Beneath many Maine bluffs lies a bluish-gray glaciomarine clay known as the Presumpscot Formation. Landslides are most common in this Presumpscot Formation both along the coast and inland; landslides in sand and gravel bluffs are less frequent. Rock or ledge is much more stable and not likely to erode or slide. The elevation of bedrock at the shore and inland beneath a bluff is important in determining landslide risk. >>back to top

Slope. The steeper the slope, the easier it is for gravity to initiate a landslide. The angle of a bluff face varies due to sediment type, rate of erosion at the base of the bluff, and the history of slumps and landslides at the site. Some slopes are uniformly straight while others are terraced or uneven due to prior earth movements.

A concave surface that concentrates water flow and increases pressure on the surrounding sediment is more susceptible to failure than a straight slope or convex slope.

The most accurate way to determine the slope is to use an inclinometer or clinometer. You can make your own using a protractor, string, and a yardstick. When the yardstick is held up and aligned with what appears to be the average slope of the land, the slope angle can be read directly from the protractor. This slope angle can then be converted to the appropriate horizontal/vertical ratio, if needed.

Slope height or the elevation of the land above the shoreline, can be estimated from topographic maps or GPS. The quickest but least accurate way to estimate slope height is to visually estimate the height of some nearby vertical structure on the slope (like a tree or bluff face) and then estimate how many tree heights would equal the overall slope height. Determine slope angle by dividing the measured horizontal distance from the top to the toe of the bluff by the elevation or slope height. >>back to top

Slope aspect. South-facing slopes undergo more extensive freeze/thaw cycles in winter months than slopes that face other directions. Repeated freeze-thaw cycles increase the likelihood of shallow soil slumps. >>back to top

Topography. Swales, gullies, or ditches can direct surface water toward or away from the bluff face and slope. They also affect the recharge of sub-surface water and groundwater. Steep-sided channels concentrate and accelerate runoff, increasing surface erosion. These features often indicate past erosion or landslides. >>back to top

Vegetation. The type, age, health, and abundance of vegetation growing on a bluff can offer valuable clues to determine slope stability. Even the presence of stumps and fallen trees can tell a story to a knowledgeable observer. Vegetative indicators are best interpreted in combination with soil and geological data.

  • Tree trunks that are tilted or twisted in the same direction may indicate soil shift due to previous landslides or gradual surface creep.
  • Curved tree trunks near the roots often indicate land movement down the face of a bluff.
  • Jackstrawed trees that are jumbled in groups on sediment that slid down a slope usually indicates that a groundwater problem or slope instability caused the mass of soil and vegetation to move downslope as a single unit or block. 
  • Distinct lines of trees growing across a slope may indicate one of two different conditions. If the trees are young, fast-growing species such as alder or willow, they may have colonized an exposed area created by a previous landslide. The age of trees growing in this manner can be a clue to when the slide occurred. A distinct line of trees of a similar, water-loving species may indicate an area where water or groundwater seepage is perched above a layer of impervious material underlying a deposit of sandy soil. >>back to top

Microclimate and Aspect.  The weather along Maine’s diversely shaped coastline varies from cove to cove and beach to beach. Microclimates depend on local topography, aspect, and exposure to sunlight. >>back to top

Waves, tides, and sea level. A gradual, but ongoing rise in sea level at a rate of about an inch per decade is causing chronic erosion along the base of many bluffs. As sea level rises, wave action and coastal flooding can reach higher and farther inland and scour more sediment from a bluff. In winter, sea ice erodes tidal flats and the base of bluffs. Tides wash away eroded bluff sediment, which helps wave action move inland. Storm-driven wind, waves, and flooding can cause more extreme erosion at the base of a bluff, increase the bluff slope, and make a landslide more likely. >>back to top

Drainage.  Water can be the most common factor that causes bluff instability, either from groundwater seepage within a bluff, or surface runoff on the bluff itself. Look for drainage issues during or directly after heavy rain, and in spring when water tables tend to be highest as the ground thaws (Ground Water Handbook for the State of Maine). >>back to top

Surface water. Wetlands, ponds, and streams above the bluff can supply water to the bluff face and also to the ground water. The elevation or topography of the land surface determines which way surface water will flow. Water that runs over the face of a bluff can wash sediment to sea, increase the bluff face slope, and weaken the remaining sediment holding up the bluff. Removal of sediment from the bluff face can increase the risk of erosion or a landslide. Direct rainfall to a bluff is sometimes the deciding factor influencing bluff stability. However, wind and frost wedging do act upon some exposed slopes. Many of the other features listed in this section (vegetation, soil type, etc.) are usually related with drainage. >>back to top

Ground water.  Ground water comes from surface sources, such as rain or a stream, uphill in the local watershed. Ground water tends to flow horizontally beneath the surface and may seep out the face of a bluff. Seeps and springs on the bluff face contribute to surface water flow and destabilize the bluff face. In addition, a high water table can saturate and weaken muddy sediment and make the ground more prone to slope failure. >>back to top

Weathering. Weathering in clay and silt can change the strength of bluff sediment and stability of the bluff face. Drying of clay can increase resistance to sliding. The seasonal cycle of freezing and thawing of the bluff face can lead to slumping after a thaw. >>back to top

Earthquakes. Landslides can be triggered by earthquakes. Ground vibration loosens sediment enough to reduce the strength of material supporting a bluff and a landslide results. Most landslides triggered by earthquakes in sediment like that found in Maine have been of Richter magnitude 5 or more. These are relatively rare events, but a few have occurred in Maritime Canada. >>back to top

Land use. Human actions can enhance or reduce the risk of a landslide. Actions that increase surface water flow to a bluff face, such as watering lawns or grading slopes, add to natural processes destabilizing the bluff face. Surface water, collected by roofs, driveways, paths, and lawns flows toward and down the bluff face. Walkways down the face of a bluff can lead to greater erosion from foot traffic or the concentration of surface water flow. Elevated stairs can shade the slope and prevent vegetation from stabilizing the slope. Both surface and ground water above a bluff can be supplied by pipes, culverts, surface drains, and septic systems. Increased water below ground can weaken a bluff and lead to a landslide. Greater seepage of water out of the bluff face can also increase the risk.

Clearing of vegetation from the bluff face can lead to greater bluff erosion and a steeper bluff that is more prone to landslide. Vegetation tends to remove ground water, strengthen soil with roots, and lessen the impact of heavy rain on the bluff face. Removal of vegetation within a shoreland zone to enhance a view may require a permit from the Maine DEP and/or your city or town.

Adding weight to the top of a bluff can increase the risk of a landslide. Saturating the ground with water also adds weight. Even ground vibration, such as well drilling or deep excavation, may locally increase the risk of a landslide.

Shoreline engineering in the form of seawalls, rip-rap, or other solid structures is sometimes used to reduce wave erosion at the toe of a bluff. In some settings, engineering can increase the rate of beach or tidal flat erosion and lower the shore profile over time. This intertidal erosion can undermine engineering and result in less physical support of the base of the bluff by natural sediment. When coastal engineering ends along a shoreline, “end effect” erosion can cause worse erosion on adjacent properties. Engineering alone cannot prevent some large landslides. In general, human activities that increase the amount or rate of natural processes may, in various ways, contribute to landslide risk. >>back to top