Member
Posts: 3
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Message 1 of 14 (913 Views)

total inflow and vs. catchbasin flow

913 Views, 13 Replies
02-08-2012 12:54 PM

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".

Put a 100 s.f. ponded area over the catch basin to see if it works.

Member
Posts: 4
Registered: ‎10-03-2011
Message 2 of 14 (896 Views)

Re: total inflow and vs. catchbasin flow

02-09-2012 10:07 AM in reply to: AC_1980

Put a 100 s.f. ponded area over the catch basin to see if it works.

Walter M. Lewinski, P.E.
Solution Specialist - Civil
MICRODESK, INC.
(800) 336-3375
Member
Posts: 3
Registered: ‎02-02-2012
Message 3 of 14 (887 Views)

Re: total inflow and vs. catchbasin flow

02-09-2012 04:32 PM in reply to: Wlewinski

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!

Product Support
Posts: 100
Registered: ‎05-27-2011
Message 4 of 14 (857 Views)

Re: total inflow and vs. catchbasin flow

02-13-2012 04:06 PM in reply to: AC_1980

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.

Matt Haokip
Contributor
Posts: 11
Registered: ‎11-28-2012
Message 5 of 14 (688 Views)

Re: total inflow and vs. catchbasin flow

01-24-2013 03:15 PM in reply to: Matt_Haokip

I came accross this post and the one referenced in the reply (http://forums.autodesk.com/t5/AutoCAD-Civil-3D-Stormwater/Storm-amp-Sanitary-Sewer-Analysis-Question... and am confused on what the "ponded area" actually is. Now I understand what it is in theory and reality, but how woud we know this from the beginning? Is it not based on the depth of the "surcharge"? If you have 2% in every direction and 0.5 ft of ponding, that is 625 sq ft of ponding area but if you have 0.3 ft of ponding, that is only 225 sq ft. So why would we put in only 100 sq ft? Thanks for your help.

Rich

Rich Bauer, MS, PE, SIT, M.ASCE
Engineering Department Manager
RB & Associates Consulting, Inc.
www.rb-associates.net
Valued Mentor
Posts: 1,104
Registered: ‎01-07-2011
Message 6 of 14 (684 Views)

Re: total inflow and vs. catchbasin flow

01-24-2013 03:44 PM in reply to: rdbauer50

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.

Fred Ernst, PE
C3D 2015
Ernst Engineering
www.ernstengineering.com
Contributor
Posts: 11
Registered: ‎11-28-2012
Message 7 of 14 (672 Views)

Re: total inflow and vs. catchbasin flow

01-25-2013 07:09 AM in reply to: fcernst

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

Rich Bauer, MS, PE, SIT, M.ASCE
Engineering Department Manager
RB & Associates Consulting, Inc.
www.rb-associates.net
Valued Mentor
Posts: 1,104
Registered: ‎01-07-2011
Message 8 of 14 (665 Views)

Re: total inflow and vs. catchbasin flow

01-25-2013 07:41 AM in reply to: rdbauer50

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.

Fred Ernst, PE
C3D 2015
Ernst Engineering
www.ernstengineering.com
Contributor
Posts: 11
Registered: ‎11-28-2012
Message 9 of 14 (658 Views)

Re: total inflow and vs. catchbasin flow

01-25-2013 08:18 AM in reply to: fcernst

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.

Rich Bauer, MS, PE, SIT, M.ASCE
Engineering Department Manager
RB & Associates Consulting, Inc.
www.rb-associates.net
Valued Mentor
Posts: 1,104
Registered: ‎01-07-2011
Message 10 of 14 (652 Views)

Re: total inflow and vs. catchbasin flow

01-25-2013 09:22 AM in reply to: rdbauer50

Hi There,

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:

1. The SSA inlet objects do not currently handle surcharged flooding, or "upwelling" flows as shown in the manual. Search for my test in this forum.
2. To get the correct head relationships for your 40 sag/sump inlets you will need to create little stage/storage curves representing the grading around all of those inlets. This due to the current arms length Import/Export drainage paradigm implementation that the C3D team has chosen to go with so far.

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.

Fred Ernst, PE
C3D 2015
Ernst Engineering
www.ernstengineering.com
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