Hydraulics & Waterways
394796 - Assessment of Regime Equations for Predicting General Scour
Tuesday, June 5
4:00 PM - 5:30 PM
Location: Regency Room
There are a number of equations for calculating general scour that can be found in technical papers, design guidelines, and design manuals. Three of these equations are regime equations which were originally published by Gerald Lacey in 1930, Thomas Blench in 1969, and C.R. Neill in 1973, and are therefore referred to as the Lacey Equation, Blench Equation, and Neill Equation. These regime equations are included in the United States Bureau of Reclamation (USBR), Engineering and Research Center, technical guideline titled, “Computing Degradation and Local Scour”, dated January 1984; and in various local agency design manuals such as the Flood Control District of Maricopa County’s (FCDMC) Drainage Design Manual, Hydraulics, dated August 2013. The Lacey and Blench Equations are included in the National Resources Conservation Service (NRCS), National Engineering Handbook (NEH) Part 654 - Technical Supplement 14B, Scour Calculations, dated August 2007.
A review of the equations as found in the USBR, NRCS, and FCDMC publications with the equations presented in the original reference documents by Lacey, Blench, and Neill shows the equations have been modified in the USBR, NRCS, and FCDMC publications from the equations as originally reported by Lacey, Blench, and Neill. As a result, the calculated scour depth using the USBR, NEH or FCDMC procedure is not the same as the scour depth computed when applying the equations as recommended by Lacey, Blench, and Neill. The presentation will compare the various forms of the three regime equations, address the limitations of the equations, identify how the equations as reported in the design guidelines/manuals differ from the equations as originally documented, and provide case studies to illustrate the difference in calculated scour depth by applying different forms of the equations.