Max Cv

The Bagnold term in the O'Brien quadratic (the same term that includes the representative particle size) estimates losses from particle collisions. The term approximates the relative frequency of these collisions, in part, from the density of particles in the fluid relative to the maximum packing density.
Max Cv option becomes available when the O'Brien Equation is selected. In this case the Cv is greater than the default max (61.5%). Therefore, the Max Cv was increased to a max debris flow concentration from a literature review. If the solids were not in motion and/or dry, they would still have a volumetric concentration less than 1 (<100%). The solids are a porous media, so even at rest they have porosity. The maximum volumetric concentration of the solids is the inverse of the porosity of the solids in their highest density packing configuration. Porosity (void volume divided by total volume) of natural materials roughly vary between 0.35 and 0.45. Larger particles tend to have larger porosities but well graded (poorly sorted Engineers and geologists (unhelpfully) use opposite conventions to describe particle size distributions. Uniform distributions with very little particle-size diversity are poorly graded (in geotechnical terminology) and well sorted (in geologic terminology). Debris flows tend to be extreme examples of the opposite phenomenon, porous media that include a wide range of particle sizes. Soil and sediment that include significant components of a wide variety of grain classes are well graded or poorly sorted.) tend to have smaller porosities as the finer particles fill the interstitial space between the larger particles.
The O'Brien equation uses the maximum volumetric concentration from the Bagnold equation – which is the theoretical packing maximum for uniform spheres: 0.615 or 61.5% which corresponds to 39.5% porosity. Debris flows can have higher volumetric concentrations than this default while in motion. Users must be careful that the volumetric concentration is never higher than the maximum packing concentration. Therefore, if modelers select either of the methods (O'Brien or Maron and Pierce) that use Max C_v, the Max C_v field will become active and editable. It will populate with the Bagnold default (61.5% - like C_v, this variable is always in percent) and will use this value or a user specified value for both methods if both are selected.