=
0.025 SI units
Ku
Ku
=
0.0077 English units
For stratified bed material the depth of scour can be determined by using the clear-water
scour equation sequentially with successive Dm of the bed material layers.
7.6.2 Computer Models for General Scour
The above equations give satisfactory but conservative contraction scour depths. Computer
models, if carefully used by competent hydraulic engineers experienced in their use, will give
a more precise determination of contraction scour. These models can also be used for other
general scour depth determinations. The one-dimensional models presently being used and
maintained are BRI-STARS (Molinas 1990, 2000) and HEC-6 (U.S. Army Corps of Engineers
1993). Also, the one-dimensional water surface profile models HEC-RAS (U.S. Army Corps
of Engineers 2001) and WSPRO (Arneson and Sherman 1998) and two-dimensional models
FESWMS-2D (Froehlich 1996) and RMA-2V (Thomas and McAnally 1985; U.S. Army Corps
of Engineers 2001) can be used to obtain the input variables for the contraction scour
equations.
7.7 LOCAL SCOUR AT PIERS
7.7.1 Introduction
Local scour at piers is a function of bed material size, flow characteristics, fluid properties and
the geometry of the pier. The subject has been studied extensively in the laboratory since the
research of Dr. Laursen in the late 1940s and 1950s (Laursen 1958, 1960, 1963; Laursen and
Toch 1956; Richardson and Lagasse 1999). Richardson (1999) gives a brief listing of scour
investigations in the United States. As a result of the many studies there are many equations.
In general, the equations are for live-bed scour in cohesionless sand bed streams, and they
give widely varying results. Since 1988, through the efforts of the USGS and FHWA, a
considerable number of field measurements of local pier scour depths have been collected
(Landers and Mueller 1999). The data is given by Richardson and Lagasse (1999), page 585.
In this section, we give two equations for determining the ultimate local pier scour and an
equation to determine the topwidth of a local pier scour hole. These equations are as follows:
1. Colorado State University's (CSU) equation. (Richardson et al. 1975)
2. FHWA's HEC-18 equation (Richardson and Davis 2001)
3. Topwidth equation (Richardson and Abed 1999; Richardson and Davis 2001)
This discussion of the equations is only for simpler flow conditions and pier geometry.
Equations and methods to determine local scour depths for piers in more complex flow
conditions (for example tidal flow) and pier geometry (for example a pier on piles with the pile
cap at the water surface) are given in HEC-18 (Richardson and Davis 2001). The HEC-18
equation is a modification of the CSU equation resulting from additional research and field
measurements that have occurred since 1975.
As explained in Section 7.7.2, FHWA's HEC-18 equation is recommended for determining the
ultimate scour depth for both live-bed and clear-water scour. Briaud et al. (1999) present a
method for determining local pier scour depth in cohesive bed material is given for those
special occasions when it is assumed that the ultimate scour depth is not needed. For
7.12
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