THE EROSION PROBLEM
THE IMPORTANCE OF SHOREFORM
The land-sea boundary in characterized by many shapes and configurations. Geologists have
devised elaborate classification systems to describe these various features. For the purposes of
understanding basic shoreline processes and for designing appropriate corrective measures, however, it
will only be necessary to informally classify shorelines as either bluffs, low erodible plains (including
sandy beaches), or wetlands. Many shorelines, of course, contain two or even all three of these basic
features.
Bluff Shorelines
A distinction will be drawn between bluffs and cliffs. Cliffs will be defined as shorelines
composed of relatively sound rock. These rarely undergo severe or sudden erosion problems, but may
experience slow, steady retreat over a long period of years. Such shorelines generally cannot be treated
with low cost solutions because available alternatives are usually less durable than the cliff rock itself.
On the other hand, bluffs are composed of sediments such as clay, sand, gravel, or erodible rock
and erosion problems are often present along these kinds of shorelines. The most prevalent causes of
bluff erosion are toe scour by wave action, surface runoff, and drainage and infiltration problems that
lead to slope stability failures.
An important factor to consider is whether a bluff is high or low. While no precise definition is
possible, many writers have described high bluffs as those being greater than 20 or 30 feet high or, using
a different criterion, a low bluff might be classified as one that can stand alone, while a high bluff must
either be protected, or otherwise treated, to remain standing.
In evaluating conditions at a site, it is necessary to determine which of the above processes is
primarily responsible for the erosion problem. Slope stability problems that are not aggravated by toe
undercutting should be treated using established civil engineering techniques of slope stability analysis
and design. Typical solutions could include vertical or horizontal drains, slope regrading and terracing,
surface drainage controls, elimination of unnecessary surcharges at the top of the slope, and buttressing
the toe.
Wave action at the toe which undermines the bluff, can be treated using a low cost shore
protection device. Important factors in selecting a device will include the relative steepness of the
offshore bottom slope, and whether a sand beach is present at the base of the bluff. These are often
derived from bluff materials that have fallen from above, and they provide a buffer against normal wave
action and may serve as a suitable foundation for various protective devices. During severe wave
activity, however, waves can reach the bluff itself and erode or undercut the toe. Depending on the
strength and characteristics of the bluff materials, this may cause the bluff to fail in a relatively short
time.
The slope of the offshore bottom is also important. If the offshore slopes are steep, deep water is
closer to shore, larger waves can reach the bluff, and maintenance of a protective beach is more difficult.
15
Previous Page
