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Basin with two outlets

Basin with two outlets

Basin with two outlets

Hey all.

I have 25 Ac. site with a pond at the bottom. The site is located within a rate restriction zone where the post development flow rates have to be 50% or less than the predevelopment rates. The pond is too small to provide any meaningful stormwater management so I need a stormwater basin to control peak flow rates. The problem is that there are wetlands around the pond and the local regulators want some portion stormwater to go into the pond to maintain the wetlands. In addition, we need to do some kind of routing through the pond to ensure the pond doesn't overtop.

The thought is to have the regular basin with a normal primary outlet structure and a secondary 6" pipe running from the basin to the pond. The small discharge pipe would provide some inflow into the pond for every storm event and the primary outlet would provide most of the rate control. The problem is that I can't figure out how to model the basin & the pond. I would need a separate outflow hydrograph for both the primary outlet and the 6" pipe. The outflow from the 6" pipe would need to be routed though the pond.

Also, I already thought of having a small dedicated area of the site to feed the pond but due to site constraints, its difficult to do.

Anyone have any ideas?

Thanks in advance


RE: Basin with two outlets

I interpret these as being cascading; storm water basin will drain to the pond through a 6" pipe and then where does the main storm water basin outlet? The two outlets from the storm water basin can be modeled independently then combined into one rating curve to be routed through the storm water basin. A second diversion rating curve could be used to divert flow to the different outfalls. Then the hydrograph from the 6" pipe could be routed through the pond plus any additional R/O reaching the pond. Hope my interpretation is correct and that it helps.

RE: Basin with two outlets

What you describe does not sound much different from simply having two ponds on a site that are interconnected and then there is a single point of discharge, presumably from the second larger pond. In your case, is the site's existing point of discharge the existing pond? It sounds like it must be if the pond is the low area of the site.

If the pipe is large enough to ensure the small existing pond equalizes with the proposed pond, is that sufficient to satisfy the regulator's requirements to maintain the wetlands? If so, then you effectively can model it as one pond with stage-storage of the two ponds combined. A 6" pipe sounds incredibly small though unless the pond is the size of a swimming pool. If you're using routing software, the model will help you size the pipe appropriately as you can review the time-stage of each and increase the pipe size iteratively until they are relatively equal.

If you must maintain the existing pond at a higher elevation than the proposed pond, then I don't see how you can do that without routing the site's rainfall into the existing pond first and only allowing overflow into the proposed pond at a certain stage. The best you'll be able to do is to maintain the existing pond at a stage equal to the proposed pond which is functionally the same as a single pond.

RE: Basin with two outlets

I am assuming
-There some kind of existing sheet flow going and some flow makes it to the pond and some don't; otherwise, if you are matching 50% of the existing, there is no way the pond gets overtop.
-You have the ability to take the flow around or away from the pond once you intercepted so as not to overtop the pond.

The issue is that the pond is at the bottom and it can Not overtop.
1.- Ergo it can NOT be interconnected because the pond HGL has to be lower than the flow storage HGL, which elevation is higher than the Pond, you will have to break some physics law to do so.
2.-The outlet going onto the pond cab NOT overtop the basin, which means you have to control the outlet at different storms events; i.e. what works for the 2 year event may overflow the pond at the 100-year event.

IMHO, you need to models

I would use (HEC-hms or SWMM or HEC-1 with diversion cards for every outlet)
Model 1,
I would run the Existing pond model at 3 stormevents, 2- 10 and 100-year to obtain a Design Volume for each event (2Volume, 10Voluem, 100Volume) and the Design Flows (50%2Q, 50%10Q, 100%2Q)

Model 2
The Proposed Watershed (W1) will drain onto a Flow Storage Basin(B1) which will have 3 outlet systems each consisting on 1 low levels (or pipe) and 1 high level outlets (or weirs)
System 1
Low level drain onto pound
Will match 2Volume
Will match 50%2Q

System 2
Low level drain onto pound
Will match 10Volume
Will match 50%100Q

System 3
Low level drain onto pound
Will match 100Volume
Will match 50%100Q

As a result, the lower level pipes will always drain onto the pond and the weirs will always drain away from the pond.

Hope this helps!

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