22. In riverine situations, piles are typically driven to bedrock to a 

 layer which is not expected to move, whereas in coastal or offshore 

 situations, construction conditions and distance to bedrock often preclude 

 being able to base the piling on firm foundations. Because of this, scour 

 around piles in this environment is more critical and should be given greater 

 consideration in design of the structure. Scour holes associated with 

 non-oscillatory flow (such as is found in rivers) differ from those found near 

 pier pilings where waves and associated currents supply the energy for 

 sediment transport. In riverine situations, the transport is in the direction 

 of the current. In a wave/current climate, however, the transport, while 

 generally in the direction of the angle of approach of the waves, may also be 

 affected by longshore currents, reflected waves, etc. Laboratory and field 

 studies have been conducted to determine the maximum depths of scour for 

 various situations, and most show that the maximum depth is related to several 

 variables including sediment mean diameter, wave height and period, 

 still-water depth, sediment density, angle of repose of sediment, dimensions 

 of the structure, elapsed time, and the free stream velocity. 



Scour Problems at Vertical Seawalls 



23. Perhaps the most common of all coastal protection structures is the 

 vertical seawall. Under certain wave and/or current conditions, the base 

 which supports vertical seawalls can be eroded and partial or total failure 

 can occur. To properly design such structures, it is important to be able to 

 estimate the potential depth of scour at the toe. The problems associated 

 with a vertical structure in the presence of a wave climate are amplified by 

 the reflected wave energy that can accompany such a structure. The net result 

 of wave reflection usually is to increase the depth to which the wave can 

 influence the bottom. In most cases where scour at vertical seawalls has 

 caused failure, sand or sediment was eroded beyond or near the bottom of the 

 structure (Figure 8). Following this, the incoming waves exert pressure on 

 the upper part of the structure and failure occurs when the sediment at the 

 toe of the wall is scoured to the point where its resisting ability is 

 overcome by the wave forces and any back pressure exerted on the wall. For 

 additional discussion on the problem of scour at vertical seawalls or other 

 vertical wall structures, consult Fowler (1992), Kraus (1988), Athow and 

 Pankow (1986), Powell (1987), and Herbich et al. (1984). 



24. Another case where scour at vertical walls is a problem occurs as a 

 result of tidal or river-related flows. In this case, there may be some wave 

 action on the walls (typically from boat or ship traffic) but the predominant 

 scouring force is the current at the base of the structure. Here, sediment is 

 moved from the base and not replaced. When this occurs over an extended 



25 



