DETERMINING Q-pre

[notsocivil]

Hello all,

I am in the process of doing storm calcs for a relatively small site that is mostly developed (85-90% impervious). The site is a redevelop and I am trying to determine the best way to determine my pre-develoepd runoff rates. I am using the rational method. The redevelopment consists of removing and rerouting the existing system around my site. My question is: what is the best(and most economical) method to determine my pre developed rate if I do not have sufficient information about the existing system? The only information I have is what was picked up by the survey on my site and the county doesnt have extensive records available in that area. I have pipe sizes coming in to the most remote inlet but that is about it. Would it be reasonable to assume full flow for the existing pipe and just model it from there?

Many thanks!

 

[gbam]

Have you reviewed the drainage report for the existing development? It should be in there.

 

[Hoagie]

i've been in your situation. i found it best to involve the approving authority in such decision making. i recommend that you put a worksheet together and make an appointment to show him the situation. he will be able to answer you about the assumptions/criteria stuff...and this way you will know what he expects. later, you can develop an explaination (to your client)about the need for additional fee or extension in schedule.

 

[sammyk]

Assuming full-flow would not be good, there are too many scenarios that could invalidate your bypass drainage calcs.
For example, what if the upstream development used overtopping of a roadway as part of the natural flow path. Your pipe flow would be totally insufficient.

Look at USGS contour data, county topo to get a feel for whats coming to your site. GBAM and HOAGIE's comments are very important for your diligence.

Just remember that if you mess up this calculation you can flood out your development with the possibility of doing some serious downstream damage (from an undersized drainageway).

Also, try to use TR-55 to quantify flows for the upstream watershed.

 

[Crystalct1]

Maybe I'm missing something, but in my experience, you need to do these calculations. This is why there are so many "unexplained" flooding areas.

You need to find the time of concentration for the existing site. You need to come up with a composite C value and find the peak runoff from the site as it is. Then you need to do the same with the new development. If you have pipe data, you can assume full flow in the pipes in order to determine Tc. You can use aerial photos or field visits to find C values. You may have to break the site up into subareas if the runoff exits the site at more than one place. Note that if the drainage paths/quantities are being altered, you'll need to do something to manage the peak flow to the area that has a lower peak in the existing condition. Some authorities will let you increase the predeveloped peak by 10% without futher stormwater management.

This is very important. Engineers tend to blow this part of the analysis off or "fudge it" to the numbers they need. Don't. Imagine a new development upstream of your home that all of a sudden leaves water in your backyard from a storm that previously left your yard unflooded.