SWMM5 Major Storm System

[nrosin]

I have been worked with swmm5 on a couple of projects now and have been given a drainage plan to do for a subdivision with several ponds. The minor system has been modeled and sized with relative ease based on the 1 in 5 year event. Once the event is switched to the 1 in 100 year many of the nodes surcharge as expected.

The problem is that when I place the parallel conduits for the street flow, in the same manner as the applications guide shows, the EGL drops for some reason and no flow is routed through the street conduits?? At several of the nodes there is considerable flooding then once the additional pipes are put in it is like the excess flow just disappears as if the minor storm system can handle the entire flow with minor surcharging??

Any ideas or suggestions would be greatly appreciated as I am not sure what to do.
Thanks.

 

[Ryb01]

Try checking the following pertaining to the way you've coded the overland flow component, from Chapter 7 of the SWMM Applications Manual;

1)Max Depth
The maximum depth of the manhole junction is the distance from its invert to the ground surface elevation where street flooding would begin to occur. If this maximum depth is left set at zero, SWMM automatically uses the distance from the junction’s invert to the top of the highest connecting link, which would be the top of the gutter/street channel in this particular example.

2)Surcharge depth
The surcharge depth of a junction is the additional depth of water beyond the maximum depth that is allowed before the junction floods.

3) Conduit Offsets
The inlet offset for a conduit is the distance that its inlet end lies above the invert of the junction that it connects to. A similar definition applies to the offset for the outlet end of a conduit. In the parallel pipe and gutter system, the elevations of the gutters are set above the pipes using their inlet and outlet offsets

4)Are you using a Dynamic Wave Modelling approach? This is the best approach for modelling a dual drainage system.

The overland/roadway conduit needs to be coded using the above paramters. Figure 7-2, in the SWMM Applications Manual gives a good illustration of how the overland flow component needs to be coded.

Hope this helps.

 

[nrosin]

The parameters I am using are as follows
Max Depth - 1.2m
Surcharge depth - 0 as no surcharging will occure above the street
Conduit Offsets for street - 1.2m
And I am using dynamic wave.

Still, prior to adding the second conduit the max depth observed is 1.2m plus flooding, then with no other change except the second conduit the max depth is suddenly less than 0.9m??

 

[Ryb01]

Would you be able to send the .inp file to have a further look?

 

[nrosin]

Here it is...

 

[Ryb01]

If you look at Fig 7-2 in the SWMM application manual it shows the top of the overland flow component to be at the maximum depth of a junction. In your case the inverts of the overland flow are located at the maximum depth. Meaning that the flow can't see the road surfaces which you are trying to model, thus resulting in the surcharge condtions.

If the maximum depth of the flow node needs to 1.2m, then say your maximum road surface flow depth is 0.15m, your inlet offset would then need to be 1.05m and not 1.2m.

Looked at section J38-Out1, revised the inlet/outlet offsets as per above and the hydraulic grade line appears to be reasonable.

Hope this helps.

 

[nrosin]

I agree that the offset of the street conduit should be 1.05 or the max depth of the junction 1.35, however, try deleting one of the overland flow routes, the HGL will come up for some reason and cause flooding at the node even though there was no flow over the street.

 

[Ryb01]

By switching the inlet/outlet offsets to 1.05 as previously discussed in section J38-Out1, the overland flows are as follows;

Conduit OL-1 Q=0.06cms
OL-2 Q=0.014cms
OL-3 Q=0.233cms

In all cases there is an overland flow in this section...deleting the overland flow conduit will indeed cause the HGL go up and ulimately surcharge the surface.

 

[nrosin]

try OL-133 or anything around there.
The three you mentioned appear to be the only ones that are working correctly.

 

[Ryb01]

I see what you're saying. There seems to be only an issue at the upstream end of the system at J45.The peak flow generated from Catchment 31 is 0.197cms. The capacity of conduit P54 is approx 0.195cms, therefore producing a surcharge condtion within the storm sewer at this location. Based on the 2.4m of head that the model is calculating it would appear as though SWMM has switched to using an orifice equation, to calculate the HGL at the top of the system. This head seems to dissapear with modelling of the road surface.

I have not run into this issue before in previous modelling, it may be beneficial to post your question on the SWMM users forum, see the following website;

http://www.chiwater.com/Communities/Listservers/swmm-users.asp

Hope this helps.