Backwater Analysis vs Capacity Analysis flow profile methods

  Applies To 
  Product(s):
SewerGEMS, CivilStorm, StormCAD, SewerCAD
  Version(s): V8i, CONNECT Edition
  Area:  Calculations
  Original Author: Mark Pachlhofer, Bentley Technical Support Group

Problem

What is the difference between the Backwater Analysis and Capacity Analysis flow profile methods? 

Solution

The default option for the Flow Profile Method calculation option is 'Backwater Analysis'. It is used most of the time when computing a system because it uses the gradual varied flow algorithms, which are more rigorous and generate solutions that more closely reflect reality. This analysis starts at the outfall and solves the network equation upstream. The depth of flow can vary over the course of the length of pipe, based on the flow regime (how steep the pipe is, etc.). Hydraulic jumps can also occur.

With the 'Capacity Analysis' option, the program assumes the pipes are flowing steady (uniform flow). It still uses a backwater approach, with the profile type for a pipe being primarily dependent on the pipe's full flow capacity and downstream hydraulic grade. It’s a more simplified approach and is only advantageous over the gradually varied flow analysis in terms of processing time or compatibility with other software. If you are dealing with a large network and you wish to arrive quickly at reasonable approximation, or if you are trying to get results comparable to other software that use a uniform flow approach, then the capacity analysis should be selected. Otherwise, the backwater analysis is the best option.

When using the Capacity Analysis, there are two basic approximate profile cases: the Full Capacity Profile and the Excess Capacity Profile.

Full Capacity Profiles 

Full capacity profiles occur when the pipe's actual discharge is greater than or equal to the pipe's full flow capacity (the theoretical capacity based on normal depth being equal to the top of the pipe). In these cases, the downstream depth is taken as the greater of the actual downstream hydraulic grade or the free discharge tailwater elevation. The free discharge tailwater depth is commonly approximated as halfway between the crown of the pipe and the pipe's critical depth (in accordance with the U.S. Federal Highway Administration's HDS-5). Starting from the tailwater elevation, the pipe's full flow friction slope is used to determine the hydraulic grade at the upstream end of the profile. 

Excess Capacity Profiles 

Excess capacity profiles occur when the full flow capacity of the pipe (again, based on Normal depth equal to the top of the pipe) is greater than the actual flow in the pipe. For these profiles, there are three basic tailwater conditions:

  • Case 1 - Hydraulic grade downstream less than or equal to normal depth.
  • Case 2 - Hydraulic grade downstream greater than normal depth, and less than or equal to pipe crown.
  • Case 3 - Hydraulic grade downstream greater than or equal to pipe crown.

Excess Capacity Profile, Case 1 (Hydraulic Grade <= Normal Depth):

If the downstream depth in the pipe is at or below the pipe's normal depth, normal depth is assumed for the pipe's entire length.

Excess Capacity Profile, Case 2 (Normal Depth < Hydraulic Grade <= Pipe Crown)

When the hydraulic grade is above the pipe's normal depth but below the top of the pipe, a friction slope of zero is assumed until it either intersects the pipe's normal depth or reaches the end of the pipe.

Excess Capacity Profile, Case 3 (Hydraulic Grade >= Pipe Crown)

If the hydraulic grade is above the pipe crown, the hydraulic grade continues upstream following the pipe's full flow friction slope. This slope will continue until it either intersects the pipe crown or reaches the end of the pipe.

Note: If the full friction slope intersects the crown of the pipe, the profile will continue with a Case 2 profile analysis.


Composite Excess Capacity Profiles 

An excess capacity profile may actually be a composite of two more simple profiles. Consider the case below, where the tailwater is above the crown of the pipe. In this case, the profile begins as a Case 3 profile. Where the full flow friction slope intersects the crown of the pipe, the profile changes to a Case 2 profile, following a flat slope until it reaches normal depth. Where normal depth is intersected, a Case 1 profile begins, extending all the way to the upstream end of the pipe.



Example Profile

Below is an example profile from one of the included example models, comparing Backwater Analysis vs. Capacity Analysis

Backwater Analysis:

Capacity Analysis:

See Also

Why does the profile appear to be at odds with the results for capacity?

Why is the HGL reset at the rim elevation of flooded structures?

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