please see attached model, i have a drainage area that generates 1.74 cfs, the inlet reports a total inflow of 1.74 cfs as well, but the outfall report about 19 cfs, because the catchbasin flow is calculated by the model as a 19 cfs number. can someone please explain how that is possible? does not make sense to me how the catchbasin flow is much higher than the total inflowhe same catchbasin, when there are no other incoming pipes to it. there are also no external flows defined.please rename attachment to remove ".txt".
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Put a 100 s.f. ponded area over the catch basin to see if it works.
a coworker of mine researched and found a link from the forum prescribing the same solution:
thanks for responding with the "fix". a follow up question would be, is 100 sq.ft ponded area the magic number for every simulation with this kind of setup (structures with SCS)? and if it is, I would like to know the theory behind it. thanks!
I am not exactly sure what is the actual field scenario you are trying to model however, a catch basin connected by a direct link to an outfall would actually mean, the inflow at the catcbasin is discharge directly to the outfall without any routing involved. You may want to first stabilize your model before analyzing the results. Please see the instability in your model: http://screencast.com/t/T8YNFnZOud Those spikes in the time series is what is reporting incorrect values.
Adding a ponding area of 100 sq.ft seems to stabilize the model however it may not work in every situation.
If the field condition is such that the subbasin drains to a catchbasin and then is routed through a conveyance to an outfall, you may as well define the conveyance as a pipe or a channel.
Next, you may want to use Hydrodyanmic routing to obtain the most realistic result. Note that with this routing method, you may need to use smaller routing and reporting time steps. ( I have changed your model to Hydrodynamic routing + changed your direct link to a pipe + reduce your routing time step to 5 seconds and report it every 5 min) Please see the new result: http://screencast.com/t/wyNak9qCV
Hope this helps.
I came accross this post and the one referenced in the reply (http://forums.autodesk.com/t5/AutoCAD-Civil-3D-Sto
Ponded area is just a constrained cylindrical cross sectional area. Not sure about SSA of course... but SWMM will fill this conceptual cylinder with surcharged nodal flooding volume to whatever height is dictated by the system response.
I see. So we just need to use the ponded area and the depth it calculates to get the volume of surcharge then just "spread" that volume out over the actual surface of the design. That makes sense but yet seems a little repetative when it comes to having nearly 40 inlets on the site that I have to do that with. I would think that with the integration it has with Civil 3D and the capabilities of Civil 3D that it would calculate that for you.
Thanks for your help and explanation, I am new to this software and was just a little confused. It would be nice is there was more explanation in the "help" documentation as to this and a few other things.
Rich Bauer, MS, PE, SIT, M.ASCE
Engineering Department Manager
RB & Associates Consulting, Inc.
Web site www.rb-associates.net
Now that you seem to have your mind around around that concept. It is not really a cylinder per se, just an amount of constrained area. It's just easier to picture at first as a conceptual cylinder over your junction.
The important thing is this ponded area volume will be held up above your junction, until the hydraulic gradient of the overall system response allows this volume to be introduced back down into the system. Therefore, it is not lost from the system.
Be aware, caution, this ponded volume is also subsequently impinging its piezometric head relationship on your system as a function of its constrained height...
This is how EPA SWMM works, not sure about the SSA implementation.
Yes I understand its a "conceptual" cylinder. The head must make a significant difference. I tried the Auto Dealership sample that came with SSA 2012 and changed one of the junctions to an inlet and using a ponded area of 1000 sf, the ponded depth is showing to be 0.23 ft which in theory would be 230 cf. However, when I change the ponded area to 100 sf, the depth is now only 0.26 fr which is 26 cf. That is a huge difference and this would skew the number however you feel like it. The issue I am having is that one fo the requirements for the development we are doing is that there is only 0.5 ft of ponding over the inlet in the 10 yr event. I really hate to say it, but unless I get this figured out, I will have to use a completely different program for analysis, which is really sad.
I am looking into the difference between the different methods and the effect on the results. I am using the rational method right now but do have other options that I can choose if I want. I will let you know my results.
Unfortunately for now, it just gets more sad and weepy for us engineers regarding the current implementation of SSA for the very typical project scenario that you are working on:
Now, just imagine a better world for a minute, that is if SSA was included in Model space, much as the DTM interactive drainage tools were once included in LDD (see attached file). All the nice grading effort around your inlets could automatically be used from your Proposed surface.
The C3D team has done an absolutely tremendous job with Corridors, SAC, Code Set Styles and Feature Lines for us. I really throughly enjoy using these tools. Now since that implementation is a wonderful success, it is time to get drainage back into Model space, for efficient interactive analysis.
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