Issues on Flow Peaks / Volume Relating to Pumps and Reservoirs
This version of the Hydra® Engine gives you the option of either maintaining the peaks or the volumes of hydrographs. This option is set in the Analyze dialog box, General Tab. If you have pumps or reservoirs in your collection system, you need to understand how Hydra combines flow components of differing time steps.
In the real world, hydrographs are analog in nature, or put another way, they are smooth curves representing the change in flow throughout the day. In the digital world however, hydrographs are represented by a series of steps. In Hydra, these steps range from 6 to 60 minutes in length.
Generally, using steps to represent an analog curve present few problems with one exception – when 2 hydrographs are added together, it is possible to “loose” or at least mush down the peaks. For example if a hydrograph consists of only one 15 minute step of 10 CFS, and we desire to delay the flow 7.5 minutes, then the results will be that ½ the flow will be shifted to one 15 minute step and ½ to the next, and as a result we have 2 steps, each with 5 CFS peak flow. In such a case the volume is correct, but the peak has been “mushed” down by the process of shifting the time step.
The above example is an extreme case, and because most hydrographs consist of many steps and the changes between steps are not great, the suppression of the peak is minor, but it is wise to be alert for this situation.
What must be understood is that this is not an error, but simply the nature of a digital representation of an analog function. It is less pronounced as the step length is decreased, but it is always there when using normal adding algorithms.
There is an alternate approach however, and the HydraEngine gives you this option call “Preserve Peaks”. If you make this selection, we insert logic that scans the resulting hydrograph and simply reset the peaks to the maximum value during the step. In the above case, this would result in both steps being 10 CFS. This is now correct from a peak standpoint, but the volume of the resulting hydrograph is incorrect as it would set the flow for both 15 minute steps to 10 CFS and that doubles the total volume during that period.
You must make the decision which approach you desire. The default is set to maintain peaks (SET ADD PEAK), but if you desire to maintain volumes, you need to use the “SET ADD VOLUME” command at the start of your command file. The SET ADD VOLUME assures that the volumes are correct but perhaps the peaks are muted. The SET ADD PEAK, assures that the peak flows are maintained, but the volumes are increased.
What is Your Purpose of Analysis?
Generally if you are trying to determine the required sizes of pipes and channels, the SET ADD PEAK is the best option, but if you need to determine the sizes of detention facilities, then SET ADD VOLUME is the natural option. In many situations, it is a good ideal to make a run one way and then the other, which generally covers all bases.
A special note on pumps.
If you select fixed speed pumps that have sufficient capacity to drain the wet well in just a few minutes or a fraction of the step length, then the same problem arises. For example suppose the pump capacity is set at 10 CFS but the flow into the pump station never exceeds 0.5 CFS. In this case the wet well will slowly fill, perhaps taking as long as 60 minutes to reach the on switch for the pump and then totally empties the wet will in 1 minute. If you are set to run in 15 minute time steps and your used SET ADD VOLUME, then the discharge from the pump will display as 10/15 or 0.67 CFS, because that is the average discharge over the 15 minutes. If you use SET ADD PEAK, then the discharge will be 10 CFS, so watch for this situation carefully.
Also, if the pump cycles “on” for less than 1 minute, there is an inherent problem in converting to 6-minute or larger time steps. In this case, Hydra uses “preserve peak” even if step is less than 1 minute.
All this being said, in most cases these situation do not arise, and SET ADD VOLUME does the job very well, but it is suggested that you make a run using SET ADD PEAK just to make sure the peaks are reviewed.
Technical Discussion. Hydra actually does all its analysis on 1 minute time steps. The problem here is balancing storage, speed and need. Most input data is only known in 15 minute time steps (at the very best), and to change Hydra to 1 minute time steps just because it would seem better, would not really accomplish much over using SET ADD VOLUME and checking with SET ADD PEAK while increasing storage by 15 times and slowing the program down significantly. A 1,000 pipe system (really quite small) can already take 2.5MB of storage and so we might be talking about nearly 40MB of storage for every 1,000 pipes in your system and because there is about 1 pipe for every 10 individuals in a typical city, for a city of 50,000 we might be looking at almost 200MB of storage for every run.
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