Classification of Water Surface Profiles


The water surface profile within a culvert can be classified two different ways:

1.    Hydraulic Slope, which is based on the slope of the culvert bottom and

2.    Hydraulic Curve, which is based on the relationship of the water depth relative to critical depth and normal depth. 

FishXing uses these classifications to determine the appropriate inlet and outlet boundary conditions.

Hydraulic Slope Classifications

The Hydraulic Slope of a culvert at a specific flow classifies the hydraulic regime and defines the type of solution generated from the Gradually Varied Flow calculations. 

Hydraulic Slope is determined from:

1.       The culvert bottom slope (So) and

2.       The relationship between critical depth (yc) and normal depth (yn) at a specific flow.  


There are five slope classifications:

         Adverse (A) if So < 0  (slope is positive in the downstream direction)

         Horizontal (H) if So = 0

   Critical (C) if Yo = yc

         Mild (M) if So> 0 and yn > yc

         Steep (S) if So > 0 and yn < yc


For culverts with slopes > 0, the slope classifications change as flows change.  It is not uncommon for a culvert to switch from being Mild Slope to Steep Slope as flows increase.

Hydraulic Curve Classifications

Hydraulic Curve classifications are used to describe the shape of the water surface profile at a specific flow. The curves are based on the Hydraulic Slope (A, H, C, M, or S) and the relative position of the actual flow depth to normal and critical depth as designated by the numbers 1, 2, and 3.

         Type 1 curve: Depth is greater than yc and yn, flow is subcritical

         Type 2 curve: Depth is between ycand yn, flow can be either subcritical or supercritical

    Type 3 curve: Depth is less than both ycand yn, flow is supercritical.            


Since normal depth (yn) is undefined for Horizontal and Adverse slopes, they only experience Type 2 and Type 3 curves.

Note: Since FishXing does not model slope breaks within a culvert, therefore type 3 curves do not occur within the model.