FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

Volume II DETERMINING SITE-SPECIFIC LOADS 8
EQUATION 8.10. LOCALIZED SCOUR AROUND A SINGLE VERTICAL PILE
Observations after some hurricanes have shown cases in which localized scour around foundations far
exceeded twice the diameter of any individual foundation pile. This was probably a result of flow and
waves interacting with the group of foundation piles. In some cases, scour depressions were observed or
reported to be 5 to 10 feet deep (see Figure 8-15). This phenomenon has been observed at foundations with
or without slabs on grade but appears to be aggravated by the presence of the slabs.
COASTAL CONSTRUCTION MANUAL
(Eq. 8.10)
where:
Smax = maximum localized scour depth (ft)
a = diameter of a round foundation element or the maximum diagonal cross-section
dimension for a rectangular element
Figure 8-15.
Deep scour around
foundation piles,
Hurricane Ike (Bolivar
Peninsula, TX, 2008)
Some research on the interaction of waves and currents on pile groups suggests that the interaction is
highly complex and depends on flow characteristics (depth, velocity, and direction), wave conditions (wave
height, period, and direction), structural characteristics (pile diameter and spacing) and soil characteristics
(Sumer et al. 2001). Conceptually, the resulting scour at a pile group can be represented as shown in
Figure 8-16. In this Manual, the total scour depth under a pile group is estimated to be 3 times the single
pile scour depth, plus an allowance for the presence of a slab or grade beam, as shown in Equation 8.11.
The factor of 3 is consistent with data reported in the literature and post-hurricane observations.
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