Contrasting Erosion & Recession
The terms erosion and recession are often used interchangeably. However, recession is the landward movement of a feature, such as an elevation contour or the bluff or dune crest, while erosion is the wearing away of land. Recession is expressed as a distance or a change in distance, while erosion is expressed as a volume or a change in volume. Recession can be thought of as a consequence of erosion.
Bluff Erosion & Recession
To that extent it is reflective of bluff erosion. In addition, bluff recession might be reflective of coastal erosion processes as a whole, if averaged over a sufficiently long period. Bluff recession is the most visible aspect of coastal erosion and receives the most attention.
However, using only bluff recession as an indicator of erosion rates or erosion trends might be misleading because of the length of time, or lag, that usually occurs between erosion and bluff recession. Coastal erosion occurs over the area roughly from the top of the bluff out into the near shore region to about the 30 feet of water depth. As a result, erosion processes (particularly those that occur to the near shore lake bottom) often do not become apparent as bluff erosion or bluff recession until days, weeks, months, or even years have passed.
In addition, erosion, particularly bluff erosion and recession, do not occur at a constant rate. Over relatively short time periods of days, weeks, and months, the rate of erosion and recession might vary greatly. It is very common for a reach of coastline to have no bluff recession for months or years and then experience severe bluff recession over a period of days or weeks. This bluff recession might occur during a period of little or no storm activity.
Cause & Effect Relationship
It is not possible to precisely determine the relationship between any of the erosion forces, or a single factor influencing erosion, and the bluff recession rate, particularly over short time periods. It is generally possible only to correlate a cause and effect relationship among all the erosion forces and factors influencing erosion, taken as a whole, to the bluff recession rate averaged over a period of many years or decades. The rates of recession thus derived are referred to as long-term, average, bluff recession rates.