SWMM5- Average Flow & Time Patterns

Hi Bob (and others)

I went through Example 3 and have difficultly in understanding how Time Pattern data and Average Flow interact. Tried reading the V4 Manual but did not make much headway.

These are the Time Pattern data in Example 3, which have been applied to 30 nodes.

The Time Pattern cannot be representing fractions of the Average Flow, because when you sum them up for the whole day it comes to only 0.63731 (see above). I expected them to sum to 1.

After running Ex 3, the Status Report says the Dry Weather Inflow is 71,980 cu ft. My calculation shows only 68 829 cu ft. (Average Flow does not seem to represent as such per node because then the 24 hr inflow becomes huge).

What can be happening here?

What do Time Pattern and the Average Flow represent in real life?

What am I missing here?


Regards.

Tilake.

Tilake's explanation of dry weather flow time patterns in SWMM 5 is essentially correct. The patterns are simply user-assigned factors that multiply a node's "average" dry weather inflow. They do not necessarily have to sum to 1.0, and the "average" DWF does not have to be the actual average. Maybe we should call it a "nominal" value instead.

The reason that the total DWF inflow reported in the results for Example 3 in the Beta A release did not agree with the value computed by hand had to do with the way that Beta A was updating the simulation clock time for variable time steps, which was used in this example. If you turn off the variable time step feature (by setting the Courant Factor to 0 in the Dynamic Wave Options) the program will report a correct total DWF inflow.

This anomaly with the clock time for variable time step simulations has been corrected in Beta B, which should be posted on the EPA SWMM web site sometime next week.

Lewis Rossman
U.S Environmental Protection Agency
rossman.lewis@epa.gov

Thank you Lewis.

Yes I realise now that the factors need not sum to 1. You may have a good reason for allowing for a "nominal" value rather than the estimated average flow. BTW, I ran Ex 3 with Courant Factor = 0. The D W Inflow is now reported as 108,000 cu ft in stead of 71, 980 as per my hand calculation. Will wait for Beta B.

Regards.

Tilake.
Tilake.Weerasekara@GRIFFITH.NSW.GOV.AU

Hi Lewis

I had run Ex 3 with a different time pattern.

I ran Ex 3 again with the original time pattern and with zero Courant Factor, and got the reported inflow as 68,820 cu ft which tallies with my hand calc.

Sorry for the confusion.

Regards.

Tilake.
Tilake.Weerasekara@GRIFFITH.NSW.GOV.AU