## 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 __yc__and
__yn__,
flow can be either subcritical or supercritical

· Type
3 curve: Depth is less than both __yc__and
__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.