Problem Description

How are the profile descriptions derived in SewerCAD's GVF-Convex solver or StormCAD's GVF-Rational solver?

Resolution

The profile description in the conduit is a calculated result field:

It is a combination of the slope classification and the zone classification. For free surface flow, depth rarely remains the same throughout the length of a channel or pipe. Starting from a boundary control depth, the depth changes gradually, increasing or decreasing until normal depth is achieved (if the conduit is sufficiently long). The determination of a boundary control depth depends on both the tailwater condition and the hydraulic characteristics of the conduit. The areas of classification for gradually varied flow analysis are:

Slope Classification
Zone Classification
Profile Classification

These are described in detail below.

Slope Classification

The constructed slope of a conduit is a very important factor in determining the type of gradually varied flow profile that exists. Slopes fall into one of five types, all of which are handled by the program:

• Adverse Slope
• Horizontal Slope
• Hydraulically Mild Slope
• Critical Slope
• Hydraulically Steep Slope

Any pipe can qualify as only one of these slope types for a given discharge. For differing flows, though, a pipe may change between qualifying as a mild, critical, and steep slope. These slopes do not relate to just the constructed slope, but to the constructed slope relative to the critical slope for the given discharge.

Adverse Slope
Adverse slope occurs when the upstream invert elevation of a pipe is actually below the downstream invert elevation. Normal depth is undefined for adverse slopes, since no amount of positive flow would result in a rising friction slope. Most flow conditions for adverse sloping pipes are subcritical.

Pipes are typically not designed to be adverse, so most situations with adverse slopes are due to construction errors or other unusual circumstances. Adverse pipes may cause some concern beyond the hydraulic capacity of the system, because stagnant water, excessive clogging, and other non-desirable conditions may result.

Horizontal Slope
As the name suggests, a horizontal slope results when a pipe's upstream and downstream invert elevations are the same. Normal depth for a horizontal pipe is theoretically infinite, although critical depth may still be computed. Like adverse slopes, most flow conditions for horizontal pipes are subcritical.

Hydraulically Mild Slope
A hydraulically mild slope is a condition where the constructed slope is less than the critical slope. For this condition, the section's normal depth is above critical depth, and the flow regime is usually subcritical.

Critical Slope
A pipe or channel may have exactly the same slope as the critical slope for the discharge it carries. This is a very uncommon occurrence, but it is possible and the program does calculate it appropriately. Critical depth is an inherently unstable surface, so flow is most likely to be subcritical for these slopes.

Hydraulically Steep Slope
A hydraulically steep slope is a condition where the constructed slope is greater than the critical slope. For this condition, the section's normal depth is below critical depth, and the flow regime is usually supercritical. However, high tailwater conditions may cause flow to be subcritical.

Zone Classification

There are three zones that are typically used to classify gradually varied flow:

Zone 1 is where actual flow depth is above both normal depth and critical depth.
Zone 2 is where actual flow depth is between normal depth and critical depth.
Zone 3 is where actual flow depth is below both normal depth and critical depth.

Profile Classification

The gradually varied flow profile classification--or Profile Description, as it is called in the conduit properties--is simply a combination of the slope classification and the zone classification. For example, a pipe with a hydraulically mild slope and flow in zone 1 would be considered a Mild-1 profile (M1 for short). The program will analyze most profile types, but will not analyze certain flow profile types that occur rarely in conventional sewer system such as H3, M3, and S3.

The screenshot below shows the general behavior for the different profile descriptions. The actual results may be less exaggerated than shown in the screenshot.

See Also

• Additional information on gradual varied flow can be found in Stormwater Conveyance Modeling and Design
• Hydraulic grade discontinuity at steep pipes or across nodes (frontwater analysis)