runoff event in a sand bed material stream. Whereas, it will take many runoff events to get
the maximum depth scour at a pier in a stream with clay bed material. However, as Briaud et
al. (1999) point out, "the maximum depth of scour in sand and in clay appears to be the
same; this is confirmed by the fact that the HEC-18 equation developed from sand
experiments fits this data on clay quite well." This data refers to the Briaud et al. flume
studies of pier scour in clay bed material. Also, in their flume studies of scour at a circular
cylinder in clay bed material, the maximum scour depth occurred behind the pier.
Using an Erosion Function Apparatus (EFA) to measure scour in cohesive soils Briaud et al.
(1999) demonstrated that the scour rate for sand was approximately 1,000 times faster than
clay. Other researchers and field experience have also demonstrated that the scour rate in
clay is many times slower than in sand. In addition, Briaud et al. describe the phenomena,
bonds, and factors that give and affect cohesion in clays. They note "because of the
number and complexity of these bonds, it is very difficult to predict the critical shear stress for
clays empirically on the basis of a few index properties." They propose that the critical shear
stress be measured for the clay bed material at the bridge site directly. The EFA was
developed to measure the erosion rate directly. Briaud et al. propose a method and
equations to use this measurement to predict depth of scour corresponding to the duration of
the flood or the design life of the bridge.
7.7.6 Pier Scour for Other Pier Geometry, Flow Conditions, and Debris
HEC-18 (Richardson and Davis 2001) gives procedures and equations to determine local
pier scour depths for piers with exposed footings and piles groups. multiple columns skewed
to the flow, composite pier configurations (pier on pile cap on piles exposed to the flow),
debris on piers, and piers subjected to pressure flow.
7.7.7 Topwidth of Scour Holes
The topwidth of a scour hole in cohesionless bed material from one side of a pier or footing
can be estimated from the following equation (Richardson and Abed 1999).
W = y s (K + cot θ)
Topwidth of the scour hole from each side of the pier or footing, m (ft)
Scour depth, m (ft)
Bottom width of the scour hole as a fraction of scour depth
Angle of repose of the bed material ranging from about 30 to 44
The angle of response of cohesiveness material in air ranges from about 30 to 44.
Therefore, if the bottom width of the scour hole is equal to the depth of scour ys (K = 1), the
topwidth in cohesionless sand would vary from 2.07 to 2.80 ys. At the other extreme, if K = 0,
the topwidth would vary from 1.07 to 1.8 ys. Thus, the topwidth could range from 1.0 to 2.8 ys
and will depend on the bottom width of the scour hole and composition of the bed material.
In general, the deeper the scour hole, the smaller the bottom width. In water, the angle of
repose of cohesionless material is less than the values given for air; therefore, a topwidth of
2.0 ys is suggested for practical applications (Figure 7.7).