Accounting for bypassed (undetained) flow in PondMaker

  Product(s): Bentley PondPack
  Version(s): V8i, CONNECT Edition
  Area: Modeling


In the Postdevelopment conditions, some of the flow that was discharging to my site outfall bypasses my detention pond. In other words, I have some undetained flow that will end up discharging to the site outfall. How do I account for this in PondMaker to ensure that the pond attenuates my storm such that the flow at the site outfall is reduced to the predevelopment target peak?


By default, PondMaker assumes that the pond outflow hydrograph is what needs to be attenuated down to the predevelopment target peak flow rate. In the case where some of the catchment area bypasses the pond and is "undetained", it needs to be subtracted from the target peak flow used in PondMaker. The pond outflow will add to the undetained flow at the site outfall.

Modified Rational Method

When using the Modified Rational method for your catchments, use these steps:

1) Perform a Q=CIA calculation, using only the area of the watershed that is not being detained in the postdeveloped condition

2) Subtract this peak flow value from your predeveloped peak flow. 

3) Use the resulting value as a user defined target flow in the catchment. In PondMaker, select "user defined" as the "target flow/volume source" in the first step.

Unit Hydrograph Method

When using the unit hydrograph method for your catchments, the peak of the runoff hydrographs tend to be fairly sharp and therefore timing can be an important factor. Meaning, you may not be able to simply subtract the peak flow of the undetained areas from the predevelopment target flow, since the peak of the hydrograph may arrive at the site outfall at a different time than the peak of the pond outflow hydrograph. However, doing this may be a good approximation and starting point. 

1) Compute each postdevelopment scenario and record the calculated peak flow reported at the undetained catchment(s). The pond's "pond type" should be set to "no volume" and the pond outlet link's "has control structure" set to "No". You can find the peak flow in the "Results (Computed)" section of the bypass catchment properties; "Flow (Peak, Computed). You can also see the hydrograph by right clicking on the catchment, choosing Graph and then accepting the default in the graph series options. Do this for each postdevelopment scenario (for example, 2, 10, 100 year)

If you have multiple catchments bypassing the pond, you will need to direct their "outflow node" to a common node element so that you can observe the combined peak flow. For example you could use a junction node element followed by a Conduit element whose "Routing Method" is set to "translation" (to avoid having to specify physical properties, if there is no "real" channel) and then look at the "Flow (Peak, Interpolated Output)" result field in the junction properties.

2)  Compute each predevelopment scenario and record the peak flow at the final site outfall. In the properties of the outfall node, you will see the result field called "Flow (Peak)". Do this for each predevelopment scenario (return event).

3) For each return event, subtract the bypass catchment peak flow (postdevelopment) from the outfall peak flow (predevelopment). This will yield the target peak flow for the pond (such that the combined hydrographs at the site outfall should add up to less than or equal to the predevelopment peak flow at the site outfall.

4) Use the resulting value as User Defined target flow in the catchment. In PondMaker, select "user defined" as the "target flow/volume source" in the first step, then enter the values in the "target peak outflow" field in the worksheet at the bottom. Re-compute PondMaker step 1 using the "Compute step 1" button.

5) After designing the pond and outlet and exporting to the model (step 6), compute the postdevelopment scenarios and check the peak flow at the target element (site outfall). Due to the timing of the hydrograph peaks, it may not be within the tolerance you desire. (compared to the outfall's peak in the predevelopment scenarios). In this case, adjust your pond design until you achieve the desired peak flow at the site outfall. For example you could adjust the target peak flow by the difference seen in your previous attempt. Consider the example below which shows some possible iterations an engineer may go through.

Note: if you are using a conduit element in your model, ensure that you do not see a User Notification stating that the conduit has overtopped - this indicates that the flow through the conduit exceeds the rating table (top elevation of the conduit) If the undetained bypass flow discharges directly into the point of interest (site outfall) without any significant additional attenuation effects, simple connect the catchment directly to the outfall, using the "outflow node" field in the catchment properties. Or, use the "translation" option for the conduit's "Routing Method", to model flow in = flow out.


Take the below Unit Hydrograph-based model as an example. Notice the undetained area on the right side. In PondMaker, the target peak flow is measured at the site outfall and the pond is sized to reduce the pond outflow such that the site outfall peak is below that value. However, flow from the undetained area enters downstream of the pond and is thus cannot be attenuated by the pond, and therefore the pond design must compensate for this. So, even though the PondMaker worksheet shows that the design passes (if the undetained flow is not accounted for - see "trial 1"), this is only because the pond outflow is less than the predevelopment peak flow. When you actually run the model and look at the site outfall, the bypassed flow is added and increases the peak flow at the final site outfall.

Note: each "trial" below represents an example PondMaker design attempt, demonstrating a possible workflow one might use for adjusting pond design in this situation. Step 6 of PondMaker would be completed after each trial in order to export the design to the scenario, compute and observe the flow at the final outfall (since the PondMaker worksheet will only show the results from the pond's perspective). If you are only adjusting data in steps 2 and above in PondMaker, nothing else needs to be done in the model between these trials (you can simply open PondMaker, make the changes, then re-export the design to overwrite the previous designed pond and outlet). If you are changing anything in step 1 (and re-computing step 1), set the pond's "pond type" to "no volume" and the pond outlet link's "has control structure" to "No", before re-opening PondMaker.

Summary (100 year event)

Trial 1 (Initial PondMaker design without accounting for bypass flow): 

Predevelopment target peak as measured from site outfall: 56.40 cfs
Postdevelopment Peak pond inflow: 71.11 cfs
Target peak pond outflow based on design using original target peak: 55.47 cfs (very close to the target of 56.40)
Peak flow at site outfall (sum of the pond outflow plus bypass flow): 63.61 cfs (too high)

Trial 2 (second PondMaker attempt, this time with the undetained peak flow subtracted):

Undetained area peak flow: 12.87 cfs
Target peak minus undetained peak = new adjusted peak: (56.40-12.87) = 43.53
Peak pond outflow based on new design using adjusted target peak: 43.22 (very close to the adjusted pond outflow target)
Peak flow at site outfall: 49.84 cfs (too low compared to site outfall target of 56.40, due to timing of the hydrograph peaks)

Trial 3 (third PondMaker attempt to compensate for timine of hydrograph peaks)

Due to the timing of the peaks of the hydrographs, the design based on the adjusted target peak resulted in a peak flow at the final outfall that was too low. To compensate for this, you might try adding the discrepancy in a third PondMaker attempt:

Discrepency between target at site outfall and computed peak based on adjusted pond outflow peak: (56.40 - 49.84) = 6.56 cfs
New estimated target peak pond outflow to use in next PondMaker trial: 43.22 + 7.23 = 49.78 cfs
Peak pond outflow based on new design using second adjusted target peak: 49.57 cfs
Peak flow at site outfall: 56.94 (good outcome, very close to the target of 56.40)

As you can see, with a couple of iterations in PondMaker, a successful pond design can be accomplished when considering undetained runoff.

Predevelopment Plan:

Postdevelopment Plan:

Trial 1 design (no adjustment to target peak)

Trial 2 design (subtract undetained peak from original target peak)

Trial 3 design (target further adjusted by the discrepency from the previous trial)

Copy of the example model, saved in format:

Pre and Post Pond Design - Undetained bypass

See Also

Using PondMaker

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