HY8:LOWFLOW SS Gradations
Low Flow Hydraulics, Gradation Data (Optional)
The gradations are used to determine the Manning's n value for the channel and culvert as well as the stability of the embedment within the culvert. This step is NOT required for the Low Flow Hydraulics method. It is provided to allow the user to use a computed Manning's n value and to provide embedment check computations. If the culvert has a metal, plastic, or concrete floor, the user should not enter gradation data. If there is gradation data already entered, the user should clear it out. The gradations should only be used on embedded culverts or open-bottom culverts.
Manning's n Methods
From HEC-26, Appendix C:
An appropriate equation selection must consider the basis on which the equation was developed and how it might apply within a closed conduit. The Bathurst, Jarrett, and Mussetter equations tend to better represent n values on steeper channels or channels with larger roughness elements. Limerinos and Blodgett attempt to encompass a wider range of conditions. The Bathurst equation depends on channel top width for calculation of Manning’s n (See Kilgore and Cotton (2005) for details). However, in a closed conduit, top width does not monotonically increase with depth as it does in a natural channel. Therefore, the Bathurst equation would be problematic to apply within a culvert.