Download page Step 2: Identify Design and Reference Cross Sections.

Step 2: Identify Design and Reference Cross Sections

The Scour calculator uses the same "Upstream Reference Cross Section" concept as the riprap calculator. But unlike the riprap calculator, it also uses hydraulic parameters at the design cross section. The figure below illustrates reference and design cross sections on a 1D HEC-RAS model. In general,

The general scour equations use the design cross section hydraulics and

The bend scour equations use the upstream reference cross sections.

General scour equations tend to be used for pipe crossings or other analyses on river runs or crossings where flow hydraulics are approximately one-dimensional. Therefore they use the local hydraulics. However, hydrodynamics at the outside of a bend are fundamentally multi-dimensional. Therefore, like the riprap computations, the bend scour regressions use hydraulics from a run or crossing cross section upstream of the more complicated bend geometry. Bend scour equations correlate complex multi-dimensional processes with one-dimensional hydraulic properties by tying the equation to an upstream "refence" or "approach" cross section. The reference or approach cross section should be a relatively symmetrical upstream section where one-dimensional assumptions are appropriate. This is usually the closest cross section in the straight crossing or run, upstream of the bend that also has similar hydraulic properties as the bend (i.e. bankfull depths/widths, channel slope, overbank depths/widths, roughness, etc.).