| Product(s): | Bentley SewerGEMS, SewerCAD, StormCAD, CivilStorm | ||
| Version(s): | 08.11.xx.xx | ||
| Environment: | N\A | ||
| Area: | Modeling |
Problem
How do I calculate design depth of flow based on average and full flow?
Background
Some countries like Israel and India follow their own way of calculating design depth of flow for pipes, based on average and peak/full flow. This technote explains how to achieve that calculation method using Bentley OpenFlows products.
Solution
As per the guidelines we know the sewers shall not run full as otherwise the pressure will rise above or fall below the atmospheric pressure and condition of open channel flow will cease to exist. Also from consideration of ventilation sewers should not be designed to run full. In case of circular sewers, the Manning’s formula reveals that:
The velocity at 0.8depth of flow is 1.14 times the velocity at full depth of flow.
The discharge at 0.8 depth of flow is 0.98 times the discharge at full depth of flow. Accordingly, the maximum depth of flow in design shall be limited to 0.8 (80%) of the diameter at ultimate peak flow.
Here d is defined as actual depth of flow considering average flow in the pipe and D is full depth of flow considering peak flow.
And, q is average flow and Q full flow.
To achieve this, we need to set up the formula based user data extension for the manning’s formula for calculating,
- Velocity at full depth
- v/V
- q/Q
Once you have configured the formula based user data extensions for these values, you can have the design depth of flow values based on these.