Problem

What is the continuity error and what's a good percentage for it?

Solution

The continuity error is the total system computational continuity balance error which accounts for total inflows into the system, outflows, flow losses due to overflows, and volume changes. It's really a measure of how hydraulically stable your results are.

Typically, anything under 5% continuity error is usually acceptable, but it really depends on the agency that is reviewing your model and what type of values that you need your model to adhere to. Obviously, the lower you can get your continuity error in any SewerGEMS model the more stable the model should be, but in order to really know if the results you are getting are good you should compare it to your system results in the field.

Mathematically, the continuity error is a discrepancy in mass balance, expressed in percent (or ratio if you change the units to unitless.)

Continuity Error = |(Ti - To - Tf - Td)/Ti|

Ti = Total Inflow Volume

To = Total Outflow Volume

Tf = Total Overflow volume

Td = Total Volume Change

For example, if the total inflow volume is 1000 gallons, total outflow is 700 gallons, total overflow is 100 gallons and total volume change is 100 gallons, the continuity error is:

|(1000 - 700 - 100 - 100) / 1000|

= 0.1 or 10%

Basically the total inflow is 10% greater than the total outflow+overflow+change. This mass balance error is typically due to instability in the model calculations, which could be due to problems with data input or a hydraulically challenging model. In the latter case, you may need to adjust advanced calculation options to achieve better continuity error.

Note that if the total outflow volume is greater than the total inflow volume, this equation is used instead:

Continuity Error = |(Ti - To - Tf - Td)/To|

You may also want to have a look at the help document titled "Troubleshooting Dynamic Wave Model Calculations" for some of the issues that can cause changes in continuity error and how to adjust the calculation options in an iterative process in order to stabilize your results.

You can also find the continuity error on a per-element basis using the Hydraulic Reviewer tool. This can help pinpoint the location of issues or challenging model arrangements.

See Also

Using the Hydraulic Reviewer

Troubleshooting unstable SewerGEMS and CivilStorm results using the Implicit solver

How to find volume (such as overflow volume) through specific elements

Using the Hydraulic Reviewer to Check Per-Element Volume and Mass Balance