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I have a design to work on that is a lot line site (building covers the site) and the design intent is to create storage for the 100 year event. chances are the site limitation preclude me from store the 24 hr storm and the client is requesting the design utilize an infiltration rate to ground water.
I am taking the approach of a detention pond but do not know how I can incorporate the infiltration rate to ground water. I started thinking outlet control structures to match the infiltration rate but I am unsure. is there a better way to model this
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I suppose that would be an outlet using a rating curve based on depth. I've never modeled infiltration personally so I'm not sure what parameters would be involved. Is it a function of head or time or both?
Hi Joe,
For Storage Nodes there are exfiltration options (e.g. constant flow, Horton, etc) that can be turned on from Project Options so that they are then available in the Storage Node dialog box. Is that not what you are looking for?
Charlie Ogden, PE
Product Manager
Hi Charlie
I'll start with I'm a little over my head.
I'm waiting for geotech to give me the infiltration rate. Meanwhile I'm not sure what to enter in the dialog
the inverts I understand: bottom of structure and max height of water, right
The flow properties and storage type section... not sure what to do
Exfiltration... I'm guessing all elevations and the flow rate would be that from geotech, yes?
Is the storage volume understood from the inverts and area requested? I realize I will be trial and error the size of the storage node.
Thanks.
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Aha! There they are! I knew they were there somewhere! I was looking in the options for the storage nodes for that, not the project options.
Once you set that in the project options, the storage node exfiltration options become available (and will change depending on the exfiltration type you speficied).
Your geotech will likely give you an infiltration rate in in/hr. You could use that or convert it to a fix flow rate in CFS. The options you choose depends a little on the type of storage device, local regulations, and designer preference.
Constant Flow - means you enter a fixed flow rate in CFS.
You'd probably only do that if the only pervious material of the storage was a fixed area at the bottom (or if someone tells you to only use the bottom area).
Constant Rate - would be in in/hr which get multiplied by an infiltrating area to determine the flow - so it varies at each time step if the area varies. Help describes the different area options. Projected area may be appropriate for your storage chambers if you cannot use the side walls for infiltration area... Or wetted area if you can.
The Horton options could be used if you had a decay factor (and someone told you to use a method where infiltration decreases over time).
For the options inside the storage node you'd likely only use "above elevation" if you have a liner or other impervious bottom... otherwise you'd likely just use "at all elevations".
Charlie Ogden, PE
Product Manager
Hello Charlie
Thanks for the info its is a big help. Until I hear from the geotech I figured I would fudge in a few number to give it a try.
My storage node is going to be a rectangular box with an open bottom and I am in sandy soil. I filled out the dialog as shown, but the analysis doesn't really tell me anything (everything is N/A) and the maximum depth looks wrong. I was expecting 4 feet.
Any Idea where I may have gone wrong? The warning in help for maximum elevation confused me. I probably need a piezometric tube but dont see where to add one. More than likely my flow will flood the vault.
Thanks for your help
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I think I figured out the no results. I added a Direct link to an outlet whose invert is the bottom of my structure. Sound right?
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Yes, that sounds right. I think SWMM needs a link in the model to compute hydraulics. A direct link is probably fine in this case, especially if you are proving that there is no flow through it - i.e. your storage is not overflowing. If you do have a flow and you do have an overflow structure that can outlet to something... you could also create a direct link to a junction with combination of orifices and weirs coming out of that to represent the outlet geometry.
Charlie Ogden, PE
Product Manager
Hello Charlie
Your input was a great help and once I started working through h the dialog boxes the model process became very clear.
I am still going to rely on the geotech for the perc test but I though I bring this to your attention since you are on the inside. The chart provided by SSA shows infiltration rate that do not correspond to the USDA charts. I'm concerned if the SSA charts are wrong. I may be reading the USDA chart wrong? For Sandy Soils there seems to be a discrepancy by a factor of 10
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