FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

8 DETERMINING SITE-SPECIFIC LOADS Volume II
8.5.11 Localized Scour
Waves and currents during coastal flood conditions create turbulence around foundation elements,
causing localized scour around those elements. Determining potential scour is critical in designing coastal
foundations to ensure that failure does not occur as a result of the loss in either bearing capacity or anchoring
resistance around the posts, piles, piers, columns, footings, or walls. Localized scour determinations will
require knowledge of the flood conditions, soil characteristics, and foundation type.
At some locations, soil at or below the ground surface can be resistant to localized scour, and the scour
depths calculated as described below would be excessive. When the designer believes the soil at a site may
be scour-resistant, the assistance of a geotechnical engineer should be sought before calculated scour depths
are reduced.
Localized scour around vertical piles. Generally, localized scour calculation methods in coastal areas
are based largely on laboratory tests and empirical evidence gathered after storms.
The evidence suggests that the localized scour depth around a single pile or column or other thin vertical
members is equal to approximately 1.0 to 1.5 times the pile diameter. In this Manual, a ratio of 2.0 is
recommended (see Equation 8.10), consistent with the rule of thumb given in the Coastal Engineering
Manual (USACE 2008). Figure 8-14 illustrates localized scour at a pile, with and without a scour-resistant
terminating stratum.
Figure 8-14.
Scour at single vertical
foundation member, with
and without underlying
scour-resistant stratum
8-34 COASTAL CONSTRUCTION MANUAL