Download page Limitation: Why Doesn’t the Riprap and Scour Calculator use 2D Results?.

Limitation: Why Doesn’t the Riprap and Scour Calculator use 2D Results?

One of the first questions that arises around these analyses is "why are they limited to one dimensional hydraulics?" HEC-RAS is moving to multi-dimensional hydraulics and many projects are modeled exclusively with 2D. It might seem counter-intuitive to build a 1D model to compute the kind of local velocities required for scour and riprap calculations, when a 2D depth and velocity field would be superior.

While 2D depths and velocities would be more precise, they are not appropriate for the equations. Both the riprap and scour equations are simple, empirical, equations based on cross-section averaged velocity measurements. In most cases, stable rock size and scour depth are not even correlated with the local depth-average velocities, but an idealized, upstream, depth-averaged velocity.

Because these equations are so empirical, they must be used in the manner they were developed. Plugging local, 2D depths and velocities into these equations developed for idealized, upstream, crossing, 1D, velocities violates the assumptions of the equations and is not likely to improve model performance over 1D hydraulics. To use 2D hydraulics with this calculator, a modeler must use a transect to compute a cross-section average hydraulic depth and velocity at an upstream reference cross section, and input it manually into the calculator.

The adjustments that are applied to the average channel velocity in the Maynord equation are intended to represent both horizontal variability (accounted for in 2D models) and vertical variability (not accounted for in depth-averaged 2D models) in the water column that affects localized hydraulic conditions and riprap stability.