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Helpful Member!  scott910 (Civil/Environmental) (OP)
9 Feb 12 16:11
I have been tasked with determining pre- and post-development runoff for a project that is 4.5 acres.  However, I am having trouble determining the time of concentration (Tc) because the property slopes side to side.  In other words, the topography of the land sheet flows onto the adjacent property, rather than to the roadside ditch where my discharge point will be.

If the pre-development discharge point, which would be the adjacent landowner currently, is different from the post-development discharge point, how do I calculate and compare the pre- and post-development runoff for detention purposes?

beej67 (Civil/Environmental)
9 Feb 12 17:15
Set up your hydrologic model to study all the way from your site to where the existing site discharge and proposed site discharge come together.  I presume from your description that's in the ditch further down, after your neighbor's property has also drained into it.

In your existing model, you have one node for your site, with a Tc determined by the sheet flow onto your neighbor, then you've got the neighbor's property, which will have it's own basin and Tc, and you have some sort of conveyance conduit in your model to show the flow from your site crossing his site.  Then those come together, possibly get conveyed further down via the ditch, to your study point.  Each watershed has a Tc that carries it to the downhill most spot of the watershed, and each conduit has some way built into it to lag the flow.  These vary by model.

In your proposed model, you have your site, maybe a detention pond or similar, then the outlet culvert to the ditch.  It meets back up with the runoff from your neighbor's property before your study point, and you compare study points existing to proposed to make sure you're following whatever regulations apply.

Make sense?  The key to comparing apples to apples is to pick a study point far enough downstream that it includes all your site in both the existing and proposed conditions, as well as how the water gets there, and any other offsite basins that also contribute to that point.   

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

scott910 (Civil/Environmental) (OP)
9 Feb 12 17:32
Thanks for the response.  I understand your suggestions, but are you suggesting that I would have to survey my neighbors property also to find these points?  I don't know how the owner will feel about that.  Guess that is engineering...?
psmart (Civil)
9 Feb 12 17:40
You might also want to talk to the reviewing agency to see how they expect you to compare pre/post in this situation.  Although I tend to agree with beej67's comments about comparing at a fixed point of confluence, the regulations may require you to compare flow at the boundary line.  On the other hand, other regulations require comparison at a downstream point, in order to assess how your proposed changes will interact with the adjacent sites.

Peter Smart
HydroCAD Software

beej67 (Civil/Environmental)
9 Feb 12 17:41
I often make assumptions off of municipal GIS (if available) or USGS quad maps (if GIS isn't available) for off-site basins when I do a hydro study.  In Georgia we have to not only model all the way to where our flows come together, but 10 times farther, to check the capacities of downstream structures.  Clearly nobody gets 500 acres of survey for a 50 acre project.


Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

beej67 (Civil/Environmental)
9 Feb 12 18:17
Peter's comments are important.  Regulations vary widely.  Sometimes your discharge is limited to a fixed number per acre, ignoring the existing condition entirely.  Sometimes your only obligation is to check downstream capacity.  Sometimes they want different storms treated differently.  Sometimes they want you to show a percentage reduction over the existing condition, or to assume that the existing condition has different land cover (e.g. trees) than it may already have (e.g. developed).  It all depends.

Weirdest reg I ever saw was one around here that required you to attenuate your 100 year post developed flows to the 50 year predeveloped rate, the 50 year post to the 25 year pre, etc, all the way down until your 2 year flows were set at the predeveloped 1 year rate.  No real explanation how they came up with that, either.


Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

cvg (Civil/Environmental)
9 Feb 12 18:25
some agencies may have a little heartburn if you are changing your site outfall location. Even if you reduce the total peak discharge leaving your site, you might be subjecting downstream property owners to differing conditions than pre-development. Worth checking before you get too far along in the process
brandonbw (Civil/Environmental)
16 Feb 12 2:44
Have you figured out what the agency is looking for?

For some cities we work with they want us to eliminate a certain discharge point because of flooding an alley or small street.  So they want to see that the drainage pattern completely changes.

Or sometimes they want to make sure that we are containing a certain volume of water, infiltrate the first flush, before discharging because of flooding problems.

We are having an issue with this on a 2.3 Acre Commercial project right now.

Can you post a simple sketch of the pre and post points along with the patterns?

B+W Engineering and Design
Los Angeles Civil Engineer and Structural Engineer

scott910 (Civil/Environmental) (OP)
17 Feb 12 12:47
From what I have gathered so far, the agencies are not necessarily getting any heartburn with changing the outfall points.  As of now, their main concern is ensuring that pre- and post-development discharge is equal no matter where the discharge point occurs.

The city is requiring me to use a Chambers County Triangular Hydrograph method for determination of the detention requirement.  TxDOT is requiring their standard SCS method.

My thought is to run both to figure which one would be most stringent, and use that one for the whole design.

I have never posted an attachment, but will do that this afternoon.

psmart (Civil)
17 Feb 12 12:59
Even if the agency is OK with changing the "outfall points", you still have to determine the "study point(s)" to be used when comparing the existing and proposed conditions.  To avoid the risk of having to re-do your study, it would be preferable to have an understanding with the agency about the study points before you start work.

Peter Smart
HydroCAD Software

scott910 (Civil/Environmental) (OP)
17 Feb 12 13:08
I agree, Peter.  I have sent an email to both agencies to start the conversation.

i really appreciate all of the input.
beej67 (Civil/Environmental)
19 Feb 12 23:01
Something else to keep in mind ..

Agency approval does not ensure that you're following the law, and does not ensure you're not "creating a nuisance."  You can get sued for the latter even if you follow every reg you're provided.  For that reason I would heavily advise against increasing the flow at any given outfall point without studying how that increased flow makes it down to the limits of your study.  Taking an extreme example, reducing your sites outflow from 20 cfs to 10 cfs, but directing that 10 cfs through a neighbor's kitchen, will get you sued.


Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

brandonbw (Civil/Environmental)
24 Feb 12 5:06
beej67:  That's funny yet a very good example.  I felt like I have fought very hard against some City Engineers in recent years to not flow through a neighbor's kitchen.

My example a few posts above, the City Engineer wanted us to break a few structural codes with what he really wanted.

B+W Engineering and Design
Los Angeles Civil Engineer and Structural Engineer

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