Why is the flow at the outfall less than I expect it to be?  

If the flow at the outfall is less than you expected, based on quantities in Flow Files, this could be caused by many possible reasons.

Flows were not selected for the analysis run

Flow may not be injected into the model system if all the Flow Files were not included when you set up the analysis run.  When you create an analysis run, in the Analyze dialog box Flows tab, make sure each Flow File you want to include is specifically selected (it will be highlighted).

Selected Flows are not getting into your collection system

Flow may not be getting injected into the model system if all your service areas are not connected to an entity on the collection system layer.  Check to make sure you have values in the SY_ID field on the service area or drainage area layer.  If you make corrections to the connects, you will need to re-generate your Flow Files.  If you have added pipes to the collection system, you will need to update the Service Connects, and then re-generate your Flow Files.

Also, remember that with Stormwater flows, depending on the storm’s starting time, a portion of the rainfall may occur outside the time period of the analysis, and so never enters the collection system.

Flow is staying in the system

Remember that the collection system starts out “dry” at the beginning of an analysis period.  If your collection system includes pumps with wet wells, and/or reservoirs or detention facilities, these may fill during your analysis.  So a significant amount of flow may never make it to the outfall.  Check to see what total storage capacity you have set at reservoirs.

Flow is exiting the system through Overflows

Flow may be leaving the model system through pumps, diversions, and manholes.  Check to see if you have entered data in the SY_IDOVER field which determines where overflow is directed.  If all entities have 0 in that field, and you have any surcharging, then flow is leaving the system to the ground.

Flow is exiting the system through Exfiltration

Flow may be leaving the system through exfiltration from channels or pipes.  Check the parameters you have used in your Design paragraphs.

If you still have questions on the quantity of flows in your modeled system, contact technical support.  You can request a project review by an experienced modeler to spend time looking over how you have set up your model
and comparing that to how your real system works.

Make sure your “Safety Factors” are not set to zero.  Safety factors are normally set to “1” unless you are in design mode, or otherwise want to increase your calibrated flow by some percentage.  The safety factor parameter for each entity is shown in the Hydra® interface in the following fields:  HG_SF_SAN, HG_SF_INF, HG_SF_RAP, HG_SF_STO, or appear in the Hydra Command File as: SAF  1  1  1  1

Check your Percent Active Values.  One of the factors applied to population-based sanitary flows is a Percent Active field.  Normally this is intended to account for a percentage of population on septic systems, however it has been used for other purposes.  If you have values other than 100 in these fields, it will reduce the amount of sanitary flow used by the model.

NOTE:  If you use spreadsheet programs to edit your sanitary flow layer DBFs, be careful the spreadsheet program does not change the 100% value to “1”!

Check units.  Did you input flow data using the wrong unit?  Did you use the wrong unit for pipe diameter?