Erosion and Other Damage in Ravines
Ravines formed by geologic weathering serve as natural drainage channels. Rainfall drains into the ravines and forms a stream at the bottom which empties into Lake Michigan. This stream of water can cut a channel at the bottom of the ravine, but, as long as the velocity of flow remains low, the cutting of this channel is very slow. Due to urbanization, however, ravines serve today as conduits for much larger volumes of water, and the larger volumes travel at much higher velocities.

Any construction of impervious surface--buildings, patios, driveways--covers the natural surface of soil which could otherwise absorb large quantities of water. But when impervious surfaces replace the natural soil, the water can only drain off into the ravines, increasing not only the total volume of water but also increasing the velocity of the water down the sides of the ravines and into the channels at the bottom.

This higher rate of flow does much more damage than a slow rate. (Erosion is proportional to the square of the velocity.) It does damage to the slopes and damage to the bottom of the ravine which is eroded more rapidly into deeper and deeper channels. When the channels get deep enough, the soil just above is undercut and weakened. This weakened soil often slumps, carrying debris into the channel, together with trees and shrubs that were growing there. If the channel becomes blocked, the diverted water will create a new channel and succeeding rainfall may create damage to new areas.

Automatic lawn sprinklers, especially near the edge of the ravine, often contribute to erosion if they are set to go on at regular designated times. If the ground is already saturated by a heavy rain, additional water coming from an automatic sprinkling system only aggravates over-saturation of the ground and causes the spillage of water, called "sheeting," over the edge and into the ravine.

In the many areas, rainwater collected from streets into storm sewers discharges into the ravines. The amount of flow from a major storm can be tremendous, pouring large volumes into the ravines at high velocity. These major storms are main contributors to ravine erosion.

Vegetation on the slopes of ravines is very important, for the root systems tend to hold the soil together. Destroying the vegetation will rapidly increase erosion on the slopes. Vegetation can be destroyed or injured by constant trampling, by heavy machinery, by dumping material such as lawn wastes on top of it, by heavy flow of water such as from a pipe opening or break at some point on the slope, by overhangs that create a shadow and prevent light from reaching the plants, and by misguided attempts to stabilize the slope by inappropriate methods.

Destroying the vegetation will rapidly increase erosion on the slopes.

Another source of instability arises from the pressure created by construction too near the top edge of the ravine. As mentioned, there are sand layers scattered in the soil which form weak spots. When there is too much pressure on the surface, the clay may slide away and the soil under the pressure area can slump into the ravine, carrying the structure with it. Also, heavy machinery used too close to the edge may put excessive pressure on the soil which may then fail.

A further hazard which contributes to ravine instability is excessive moisture on the ground. Water which cannot drain away saturates the soil, making it more prone to movement and slippage.

Eventually, when the slope angle of the adjacent ravine is too steep, the saturated soil may give way and slump into the ravine. This exposes bare soil where the slump had occurred, and this bare soil is very vulnerable to further erosion by water.

Finally, excessive runoff of water down the slope (sheeting) often occurs after very heavy rain and is very detrimental to the soil on the surface of the slope.

There are methods to minimize the damage from water and from loss of vegetation.