Coastal Processes

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Coastal Processes

All the information found on the Coastal Processes web pages are excerpts from the publication entitled "Living on the Coast". For more detailed information concerning coastal processes, please visit this link to download the PDF version of: "Living on the Coast".

There are numerous processes that shape and mold the Great Lakes. Historical processes such as glaciations and erosion helped create the lakes. Further erosion from wind, waves and lake level fluctuations has formed the basins as they are today. It is important to understand the physics behind the formation of the lakes to truly appreciate these national treasures.

Glacial Geology

Glaciers covered most of the Great Lakes basin about 14,000 years ago (Figure 1). They began receding approximately 10,000 years ago (Figure 2). There were numerous minor advances of the ice edge. During each of these advances clay, silt, sand and rocks were left behind as layers of till which are now exposed in eroding bluffs in many places.

Figure 1 - 14,000 Years Ago

Figure 2 - 9,000 Years Ago

Between these till layers are layers or lenses of sand and gravel (Figure 3). Many of these layers were deposited as beaches and stream deltas at the margins of glacier and lake. Water drains through these porous layers, creating bluff instability problems.

The varieties of soil types are particularly noticeable in high coastal bluffs. Some soils like clay (Figure 4) can stand as very steep slopes when dry, but then fail as large landslides when wet or severely undercut.

Figure 3 - Layers of Till

Figure 4 - Clay Bluff

Sand holds a more gentle slope and rarely fails catastrophically (Figure 5). In some places the shoreline consists of rock, with little or no sediment cover. This is especially true in the northern Great Lakes area where the glacier was mostly erosive, and the rock was resistant enough to withstand glacial erosion (Figure 6).

Figure 5 - Sandy Shoreline

Figure 6 - Rocky Shoreline

The present shoreline position is not the shoreline position of the past. In bluff areas the shoreline may have retreated several miles since the last glacier melted away. Even bedrock shorelines have been eroded by waves, though to a lesser extent.

Coastal property owners who plan to own the property for a long time are advised to be prepared for future lake levels beyond the ranges indicated in the historical records (see Sources of Information for the location of these ranges). The Great Lakes have been in their present connected arrangement for the past 3,000-4,000 years. The water level fluctuations over this time were the result of natural climate variability occurring from season-to-season, year-to-year, and from decade-to-decade. The record of measured lake levels (more than 140 years) is inadequate to confidently predict lake levels that will occur in the next 20, 50 or 100 years.

Wind and Waves

Storms and Storm Surges

The storm systems that cross the Great Lakes Basin are spawned primarily from the collision of two air masses. A frontal boundary along the path of the high altitude jet stream separates relatively cool, dry air originating in Canada from warm moist air originating over the Gulf of Mexico (during the winter season) and over the Eastern Pacific during the other three seasons. Along this frontal boundary, low and high-pressure systems are steered by the jet stream flow from west to east.

As storm winds blow across many miles of open water on the Great Lakes , they drag some water towards the downwind side of the lakes. This causes a temporary rise in water level along the downwind shore and a lowering of water on the upwind shore.

The temporary rise in water level is called a storm surge, storm set-up, or storm-induced rise (Figure 7). The drop in water level is a set-down. Set downs are sometimes a problem for vessel navigation and for recreational boats tied to piers. Storm surges occur on all of the Great Lakes shorelines. Similar, but shorter, periodic oscillations of lake levels are called seiches. Seiches are caused by rapid changes in air pressure or rapid shifts in wind direction as weather systems pass over the lakes.

Figure 7 - Storm Surge

Wave Development

As the wind blows across the surfaces of the lakes, energy is transferred from the wind to the sea surface. Most of this energy generates currents. The rest builds waves (Figure 8). Although the wind energy that goes into wave creation is a small percentage of the total energy transferred from wind to water, it is an enormous quantity of energy.

Wave conditions vary greatly over time at deep-water locations on the Great Lakes . One minute, the lake is "flat calm". Within the hour there may be waves two feet high. Seven hours later, the waves may be seventeen feet high.

Figure 8 - Wave Development

Shoreline and Lakebed Erosion

Shoreline Erosion

Great Lakes shorelines are retreating. They retreat at various rates. There are few exceptions. These shores consist of cohesive materials (clay and bedrock) that have binding forces, or non-cohesive materials (sand and gravel) that have weak or no binding forces. These shores are composed of rock, fine soil ground up by the glaciers or sediment laid down during much higher lake levels, sand and gravel. Rock is the least erodible and sand the most erodible of these materials. Distribution of these soil types varies along each lakeshore and from lake to lake.

Banks and bluffs are eroded by wave attack, sudden slumping and sliding of massive blocks, and by modest but steady surface erosion. As they erode, the shore recedes. Figures 9 and 10 are good examples of bluff erosion on the Great Lakes.

Figure 9 - Bluff Erosion, Sheboygan Cnty, WI

Figure 10 - Bluff Erosion, Berrien Cnty, MI

Sandy beach ridges, banks and beaches are the exception to the rule of retreat. They advance and retreat as water levels rise and fall, storms come and go and sand supplies shrink or expand. Figures 11 and 12 are examples of a sandy beach and fluctuating water levels.

Figure 11 - Beach at Low Water, St. Joseph, MI

Figure 12 - Beach at Higher Water, St. Joseph, MI

Erosion in rock shores typically involves rock falls where the toe of the bluff has been gradually undercut by wave action.

Erosion rates vary over time and space. These variations occur in response to many factors. Among them are:

soil slope and composition

erodibility of material

lake level

nearshore lakebed shoals and slopes

storm wave energy and duration

precipitation

ground water and soil conditions

ice cover

shoreline orientation

beach composition, width and slope

shore protection structures.

Erosion is a natural process. Erosion in one location supplies sand and gravel to build or maintain beaches in other locations. Erosion provides fine sediments that move long distances to settle on the deep lakebed beyond the reach of waves.

Lakebed Erosion

Erosion of the lakebed (or downcutting) is a common feature along cohesive shorelines of the Great Lakes (Figure 13) as well as shorelines developed in relatively weak bedrock such as shale and some sandstone.

A key feature of these shorelines is that when erosion of the nearshore lakebed takes place, it is irreversible - it cannot be restored as is the case with sandy shores. The fine sediments and soil are lost to circulate in the lake and settle out in deep water basins.

Sand or gravel may form a narrow beach or a thin layer over the erodible lakebed. Grains of sand and pieces of gravel moved by nearly constant wave motion are abrasives wearing away the lakebed (Figure 14). A thin cover of sand and gravel increases the rate at which erosion takes place through abrasion and the impact of the sediment particles.

Figure 13 - Downcutting of Cohesive Nearshore

Figure 14 - Sand Starved Nearshore

If enough sand and gravel accumulates to form stable deposits, it can protect the underlying lakebed from erosion (Figure 15). In one situation, lakebed erosion decreased where there were sand thicknesses greater than 15 centimeters. Erosion during storms will occur even when the sand is quite thick because of the migration of sandbars and the troughs between them. Studies indicate that because of this migration, probably more than 50 centimeters of sand is needed to protect the lakebed from erosion.

With lakebed erosion, any structure built to protect the toe of the bluff is subject to increasing wave energy and to undermining of the foundations as the water depth in front of the structure increases (Figure 16). In areas where bedrock occurs in shallow water, or an accumulation of cobbles and boulders forms a protective lag deposit over the cohesive lakebed, a nearly horizontal platform will develop and this will ultimately reduce the rate of recession of the bluff toe at this location.

Figure 15 - Sand Rich Nearshore

Figure 16 - Undermining of Shore Protection

Measurements have shown rates of vertical erosion in the range of one to 15 centimeters per year. More typical erosion rates are three to five centimeters per year. Lakebed erosion rates tend to be highest close to shore where the waves break and where there is lots of turbulence due to wave breaking. Erosion rates tend to decrease further from shore to just a few millimeters per year in water depths greater than a few meters.

The underwater erosion of the lakebed controls the rate at which erosion and recession of adjacent cohesive bluff and bank shorelines takes place. Recession of the bluff or bank takes place as a result of wave erosion of the toe. If this occurred without lakebed erosion then a shallow platform would be left as the bluffs receded and waves would then dissipate all their energy on this platform, thus eventually reducing the ability of the waves to erode the bluff toe. However, as lakebed erosion occurs it continues to allow large waves to reach the toe of the bluff and so lakebed erosion and bluff recession proceed together

The rate of vertical erosion at a point on the profile can be predicted from the profile slope - the steeper the slope the greater the erosion rate, and this accounts for the concave shape of most cohesive profiles with steep slopes close to shore where erosion rates are highest, with the slope decreasing offshore into deeper water as erosion rates decrease.

While erosion of banks and bluffs may decrease during periods of low lake level and increase during high lake levels, the opposite is true of nearshore lakebed erosion. During periods of low lake levels, the nearshore lakebed at a given location is subject to higher water velocities from wave motion and the zone of wave breaking where erosion is highest occurs further offshore than during high lake level periods. When high water levels return, the water depth close to shore is greater than it was during the previous high water period; increasing wave impacts on the shore.

Longshore Transport

Sediment transport is the method by which dynamic coastline features, such as beaches, spits, dunes and offshore bars, are built and maintained.

Littoral transport is nearshore sediment transport driven by waves and currents. As shown in Figure 17, this transport occurs both parallel to the shoreline (alongshore or longshore) and perpendicular to the shoreline (cross shore or on-off shore).

Figure - 17 Long Shore Transport

Storm waves carve beaches, ridges and banks, transporting large volumes of sand to nearshore bars. Where the rate of offshore sand transport exceeds the rate of supply from updrift sources, the beach erodes. During calmer periods, waves transport sand from offshore bars and deposit it on the beach face. Through these cycles, there is a movement of sand and gravel along shore in response to the shifting directions of waves. In many places this is a net movement in one direction. The transport direction depends on such factors as wave climate, bathymetry, shoreline orientation, and the presence of natural or artificial features that deflect waves and currents. Cross-shore transport is affected by changes in lake levels.

The "littoral zone," where littoral transport occurs, extends roughly from where the waves begin to break offshore to the limit of wave uprush on the beach (Figure 18). Wave conditions and current speed determine the size of material that can be transported. The rate of transport within the littoral zone is relatively small along erosion-resistant rocky shorelines but may reach several hundred thousand cubic yards (a hundred thousand cubic meters) per year along some sandy coastlines.

Figure - 18 Cross Shore Transport

Water on the Land

Water arrives on the land as either surface water runoff or as groundwater. Some of this water originates on the coastal property. Other surface water and groundwater is flowing through on its journey to the lake from inland sources.

Surface water runoff may come from rain water, snow melt, groundwater seeps or springs, and lawn or garden sprinkling systems.

There are a number of indictors of surface water problems. Many exposed soil surfaces on bank and bluff slopes have miniature troughs or larger gullies. Some slopes have exposed lengths of drain pipe or exposed foundations of stairways or other structures. Areas of decayed vegetation in low areas indicate possible prolonged periods of standing water that may have infiltrated into the groundwater, rather than evaporated. Exposed soil surfaces on the land indicate possible easy infiltration into the groundwater.

Groundwater infiltrates into the soils of coastal properties and moves to the slope face from surface water sources, off-site groundwater sources, septic systems or dry wells. Active bluff slumping (Figure 19) is a visual indication that on-going erosion is occuring due to groundwater. The hidden activity of groundwater can be more dangerous than the visible effects of surface water runoff because groundwater can trigger large, deep landslides that sometimes have catastrophic consequences (Figures 20 and 21). The presence of water in soil pores and soil fractures beneath a slope weakens the soil by adding weight and by reducing the frictional resistance among soil particles that are in contact with one another.

Figure 19 - Bluff Slumping

Figure 20 - Catastrophic Bluff Failure

Figure 21 - Catastrophic Bluff Failure

All coastal properties have groundwater flow beneath them; the ground adjacent to and lower than the lake surface elevation will generally be saturated. The surface of this zone of saturation (called the water table) is at lake level at the shoreline and rises gradually in the inland direction. For any banks consisting entirely of sand and/or gravel, this will be the only groundwater flow system present. Infiltrating water moves directly into the lake-level groundwater flow system and causes little weakening of the soil.

Many coastal bluffs contain soil layers (clays and tills) that retard water flow into the water table near lake level. Coastal landslide problems develop primarily where there are zones of water saturation above the lower, main water table; these are called perched groundwater. At such sites, groundwater collects in the sand and gravel layers because underlying soil layers that are resistant to flow slow downward movement of the water. The water flow in these sand and gravel layers is usually toward the slope face, where the water emerges in the form of seeps or springs.

Groundwater's influence on slope stability is controlled by several factors, including the quantity and distribution of groundwater beneath coastal property. The amount and rate of water infiltration is also important. The greatest infiltration comes from prolonged, slow application of water at infiltration locations. The soil moisture content and the soil structure's ability to pass water through the soil are also important.

Ice on the Shore

The type and amount of ice that forms along the shores varies from location to location and from day to day. A frozen beach is the first ice feature to form. Waves drive slush ice to shore to form an icefoot. On beaches exposed to waves, a nearshore ice complex forms (Figure 22), extending lakeward from the icefoot and containing relatively smooth sheets of ice. Ice ridges form where waves break, such as over nearshore sandbars, and may provide a lakeward boundary for this ice mass.

Figure 22 - Nearshore Ice Complex

Waves breaking against grounded ice ridges scour the lakebed, and the lakebed is gouged by contact with the keels of ice ridges moved by the wind. Slush ice and anchor ice that releases from the bottom incorporate sediment. Drifting ice transports significant quantities of sediment along and away from the shore.

Coastal property can be significantly damaged by ice shove. Ice shove (Figure 23) is caused when wind and wave energy is transmitted to an existing ice sheet and pushed onshore. Structures not designed to withstand ice could be extensivelys damaged.

Figure 23 - Ice Shove

Water Levels

The Great Lakes have had their present connections for the past 3,000-4,000 years (Figure 24). Water level fluctuations over this time were due to natural climate variability, except for some effects from diversions and dredging of connecting channels since the 1850s. There has been a lot of experience in dealing with high levels over the last half of the last century but relatively little experience with low lakes levels.

Figure 24 - 4000 Years Ago

The very short recorded history of Great Lakes water levels is inadequate to forecast lake levels that will occur in the next 20, 50 or 100 years. Future climatic conditions may be quite different.

The lake level adjustment process does not work well when natural climate changes are rapid and/or extreme. At such times, human actions to alter lake levels are often too little and too late-producing water level changes of a few inches when changes of several feet are needed. Only part of the effects of these human actions can be realized in the three years (or more) during which some of the lakes may have risen or fallen three to five feet (1-1.5 meters). Efforts to adjust Lake Ontario 's water levels have had considerably better results than efforts with the other lakes.

Coastal property owners who plan to own the property for a long time are advised to anticipate future lake levels beyond the ranges indicated in the historical records.