discharging to and modeling an undersized downstream system

Question

discharging to and modeling an undersized downstream system

sam74 (Civil/Environmental)

(OP)

15 Jan 13 18:40

We have an undeveloped site that currently discharges to an undersized downstream storm water system. That system is a relatively new 12" culvert in a street that discharges into an undersized detention pond. The existing inflow to the system is the street it is in and a 2 acre motel and parking lot. Calculations show that this system currently should flood during the 2yr storm event.

This is our only discharge point when we develop this site. I don't think Hydrocad appears to be modeling the discharge from our site due to backwater effects in either predev or postdev situations although I don't have much to base this on. Normally (in the past) I would be modeling a fix to this situation instead of adding to the problem.

We don't have many options other than to try and release postdev flows at predev rates but how do you know it is modeling this accurately. I do have the Dyn-Stor-Ind reach routing and pond routing methods specified.

Any thoughts?

psmart (Civil) · Answer 1 · 2013-01-15 19:36:17

Are there any warnings messages? If so, these need to be resolved before you can draw any meaningful conclusions from the model.

For further details on modeling storm sewers please see www.hydrocad.ne/sewers.htm Since you system is over capacity, pay particular attention to the modeling of overflow outlets and overflow storage.

Peter Smart
HydroCAD Software
www.hydrocad.net

sam74 (Civil/Environmental) · Answer 2 · 2013-01-15 20:15:34

Oh, there are all kinds of warnings. There is basically a series of curb inlets and manholes along the 12" storm system from our site to the detention pond. They are all overflowing with a head of 10 feet out of the ground during the 25yr event which is our design storm. I assumed the manhole covers should be getting blown off during most any rainfall event.

My experience with HydroCAD is not that great. Should I or can I model the overflow as flowing into a reach (the street).

Thanks for the assitance.

psmart (Civil) · Answer 3 · 2013-01-15 20:50:54

You must fix the warning messages - that's why they're there!

A head of 10 feet above grade suggests that you haven't included an overflow device, usually a weir or orifice. Without the overflow devices you're basically modeling a sealed pipe network, and the reported head is the pressure required to force the entire flow though the pipe.

For an accurate model you MUST include an overflow on each CB. This will reduce the head to a reasonable overflow depth. The overflow can then be routed to the downstream node where the flow will rejoin the network. For details please see www.hydrocad.net/sewers.htm (there was a typo in my pervious link)

Stepping back, exactly what are you trying to determine with your model? Do you need to model the storm sewer system? Or is it sufficient to restrict the model to your site and maintain your undeveloped flows in the developed scenario?

Peter Smart
HydroCAD Software
www.hydrocad.net

sam74 (Civil/Environmental) · Answer 4 · 2013-01-15 21:11:14

Thanks for the tip. I will explore CB overflows tomorrow.

Technically yes we need to maintain undeveloped flows in developed areas but I believe all flows will be affected by a tailwater produced by undersized piping and detention downstream.

I originally modeled just our site until we got some downstream offsite information that led me to beleive that the downstream system contained undersized piping and detention that will have an affect on our flow offsite due to backwater effects. We must provide detention storage but tailwater depths seem to be a factor on flow offsite.

Thanks for the advice and any other ideas/information you can provide.

sam74 (Civil/Environmental) · Answer 5 · 2013-01-17 15:59:18

If all my storm manholes and catch basins are going to be overflowing out of the top into the street there are some warnings that will not go away isn't there?

psmart (Civil) · Answer 6 · 2013-01-17 16:58:20

A warning message generally indicates a problem with the simulation, and that the model should be adjusted in order to get meaningful results. Even if your system is "failing" or "undersized", you need to address the warnings.

An overflowing CB will not necessarily generate a warning. HydroCAD is "OK" with an overflowing system. But you need to model it properly in order to get an accurate simulation of the overflow.

Peter Smart
HydroCAD Software
www.hydrocad.net

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discharging to and modeling an undersized downstream system