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Storage Unit, No Volume Change

For my job during the summer, I've been assigned to model a storm water pond using SWMM 5.1. My goal is to estimate the distribution of the ponds total storm water inflow at each of the pond's inlets (The pond I'm working on has 4). I intend to build a fairly high resolution model, but at the moment it is fairly simplified. I have a basic storage unit with 4 attached inlets and 4 sub-catchments which drain into the pond. 
When I run the simulation I obtain a continuity error of over 18% for routing and when I look at the simulation results, specifically the depth of the pond, I see no change, even after running a 100 year storm event. This makes no sense to me as I've set the evaporation and seepage values in the storage unit to 0 AND when I look at the total inflow graph for the storage unit, I can see the expected inflow into the pond. So the pond receives an inflow, but I see no noticeable change in depth even when zooming in on the depth graph.
I figured that this may point to a mistake on the storage curve, but I checked and the curves values make sense to me:
Depth   Area      (in feet and feet squared)
0     0
1.64042    178.2503679
3.28084    3140.048148
4.265092    10689.96304
5.249344    40818.47314
6.233596    99769.07745
7.545932    195384.254
8.530184    236999.2565
9.514436    270525.9334
10.826772    306948.7481
11.811024    332108.9598
12.795276    356250.4749
14.107612    389311.6126
15.091864    414820.6826
16.076116    541104.7175
I've also ran the simulation for kinematic and dynamic routing (Which I don't know anything about really) and I see completely different results in my simulation. For kinematic routing, I see no depth change. For dynamic routing, I get drastic continuity error (Like in the magnitude of around one billion %) which points to numerical instability. I've tried adjusting my time steps and I cannot seem to make the model converge though.

I've been troubleshooting this thing for over a week and I'm super lost. Would anybody be able to assist me with this issue?
I should also add a couple things:
- I have set the initial depth of the pond to approximately 7.23 feet
- I have not connected the pond to the outlet node yet as I want to ensure I understand the storage unit's response to my design storms
- I should clarify, the 100 and 10 year storms do not correspond to this specific pond. What I mean by that is that they don't raise the water level of this specific pond up to the 100 or 10 year water levels. They are just design storms that the city I live in uses for analysis on storm water facilities


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  • Hello Brent,

    Have your tried to adjust the number of iterations and head convergence tolerance for your model?


  • Hello,

    I am finishing up my summer job using SWMM 5.1. I am observing a fair amount of oscillation in my model results. I have 4 inlet pipes in my model which connect to a storage unit. In two of the inlet pipes, I observe the attached hydrograph. There are very unusual oscillations that can be identified on these graphs. I am wondering if this points to numerical error? What are the possible ways that I can reduce this error? Can this be attributed to physical issues in the model (small manhole depths for example) as well? Thank you,

    Note: I would post my current model of the pond I am modelling, but cannot due to confidentiality issues
    Untitled.pngThe oscillations can be seen on the blue line from 1140-1290 and a smaller more spastic section around 1060

    Again, any help is GREATLY appreciated


  • Hello Brent,

    I made a few changes to your model:

    1. You did not have a maximum depth at the nodes, so I made then all 10 feet

    2. you had a few 0.1 feet long links which was the main source of your problem - i used the option to lengthen the links automatically

    3. I used a 1o second time step to speed up the model,

    4. The storage node is only sink in the model, it would be better to have an outfall for this storage for your long runs

    5. I am using dynamic wave

    I hope this helps you.


    • My goodness!!! Thank you so much! I was pulling my hair out trying to make it work! Ill buy ya a beer if I ever meet you!

      • Hey, could you help me with a few more things?

        We are collecting rain data/ time series and I am not sure what form I need to be giving the rainfall in. Should it be in [units]/ hour? Now second, if I wish to do a model in metric, should my units be in mm/ hour? Or m/ hour? I feel like mm/h makes much more sense, but I want to make sure. Thanks in advance


        • The units can be inches, inches /hour, millimeters or millimeters per hour.  The types are basically intensity or volume.   Yes, mm/hour or mm over the precipitation interval makes sense.

          • So reporting a timeseries on 10 minute intervals would need to have rainfall intensitys for 10min? Or mm/10min?

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