Coastal Construction Manual - National Ready Mixed Concrete ...

3 IDENTIFYING HAZARDS
Erosion rates have been calculated by many States and communities to establish regulatory construction
setback lines. These rates are typically calculated from measurements made with aerial photographs,
historical charts, or beach profiles. However, a number of potential errors are associated with measurements
and calculations using each of the data sources, particularly the older data. Some studies have estimated
that errors in computed erosion rates can range up to 1 foot or more per year. Therefore, even if published
erosion rates are less than 1 foot per year this Manual recommends siting coastal residential structures
based on the larger of the published erosion rate, or 1 foot per year, unless there is compelling evidence
to support a smaller erosion rate. Basing design on erosion rates of less than 1 foot per year can lead to
significant underestimation of the future shoreline and inadequate setback to protect the building from longterm
erosion.
3.5.2 Causes of Erosion
Erosion can be caused by a variety of natural or manmade actions, including:
Storms and coastal flood events, usually rapid and dramatic (also called storm-induced erosion)
Natural changes associated with tidal inlets, river outlets, and entrances to bays (e.g., interruption of
littoral transport by jetties and channels, migration or fluctuation of channels and shoals, formation of
new inlets)
Construction of manmade structures and human activities (e.g., certain shore protection structures;
damming of rivers; dredging or mining sand from beaches and dunes; and alteration of vegetation,
surface drainage, or groundwater at coastal bluffs)
Long-term erosion that occurs over a period of decades,
due to the cumulative effects of many factors, including
changes in water level, sediment supply, and those factors
mentioned above
Local scour around structural elements, including piles
and foundation elements
CROSS REFERENCE
Chapters 12 and 13 provide
information about designing
and constructing sound pile and
column foundations.
Erosion can affect all coastal landforms except highly resistant geologic formations. Low-lying beaches and
dunes are vulnerable to erosion, as are most coastal bluffs, banks, and cliffs. Improperly sited buildings—
even those situated atop coastal bluffs and outside the floodplain—and buildings with inadequate foundation
support are especially vulnerable to the effects of erosion.
3.5.2.1 Erosion During Storms
Erosion during storms can be dramatic and damaging. Although storm-induced erosion is usually short-lived
(usually occurring over a few hours in the case of hurricanes and typhoons, or over a few tidal cycles or days
in the case of nor’easters and other coastal storms), the resulting erosion can be equivalent to decades of longterm
erosion. During severe storms or coastal flood events, large dunes may be eroded 25 to 75 feet or more
(see Figure 3-31) and small dunes may be completely destroyed.
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