Outfalls and Area Grates

[tmccoy3]

It's my understanding that with Hyrdodynamic Routing, an outfall can only have one inlet link. I have three flows that combine at the outfall (one surface flow, two flows from detention pond). Can I just route these through a junction box for the sake of the model without producing inaccurate results?

How do you recommend modeling an area grate that, say, sits in the middle of a parking lot instead of against a curb?

[Chris Maeder]

With regard to your outfall question, the routing limitation is actually from SWMM 5.0. To overcome this issue, place a junction node where you were wanting to place your outfall node. Then, connect the junction node to a "dummy" outfall node, a short distance downstream with a pipe of sufficient diameter not to cause any backwater effects.

With regard to your storm drain inlet question, simply connect the contributing drainage area to the storm drain inlet at the center of the parking lot. If you have additional upstream drainage runoff that is being routed to this storm drain inlet, then develop a open channel link that is sufficiently wide to represent the routed flow path from the upstream area to the storm drain inlet. I would recommend a rectangular open channel of between 10 to 20 feet wide (unless you have an actual drainage channel cross section geometry to use). Then, run the model and check the depth of the flow in the routed cross section. If the flow is too deep, then you may need to make the channel wider. Similarly, if the flow is too shallow, you may decide to make the channel narrower. An appropriate depth of flow would be an engineering judgement by you, based on what you think would be reasonable for the design storm being analyzed.

[jhmiller]

Regarding the inlet question, I don't believe that it was fully answered if I understand correctly. I have the similar situation often where the grate is in a sump situation away from the curb. I have ended up setting the inlet up as a Neenah Foundary grate (look at their website to find a grate similar to yours rather than toggle through them all on Stormnet.)

This is sufficient for myself as my runoff amounts are not so large that the grate opening is a limiting factor, or there is no concern that if Stormnet conducts the model assuming that water can't enter the structure from one side due to curb. I just set the Roadway and Gutter Specifications to minimal amounts. Unless, Chris can confirm that when a Neenah grate is used, the model automatically doesn't assume that there is a curb?

[Chris Maeder]

From what you are stating, StormNET should handle it fine. An inlet can sit by itself, and then you can have a link defining a method in which surface runoff can enter the link. You should be able to them quickly determine if the inlet is a limiting factor.