How to handle runoff from outside project limits?

[EngineeringBird]

What I'm not sure how to handle is:
1)When the drainage basin starts outside of the project limits, does that still count as site runoff? Do I compute the whole area then knock down the site drainage by the % outside my limits?
and on the flip side of that...
2)When the project limits cuts off a basin, same thing?
3) What if I don't have enough surface model to find a focused discharge point?

I am embarrassed to say I used to know this at one point - but I have totally forgotten.

Attached I have a picture of my site if it helps clarify what I'm talking about.
Thanks for your help!

 

[froude]

Exactly what is your study supposed to determine? It sounds like you are talking about "offsite" flows. Typically, you can't ignore offsite flows if they are entering your facilities. You may not be changing anything offsite, but if the area will still be sending runoff through your channel, storm drain, pond, etc. you have to account for it.

 

[gbam]

froude makes a good point as to what you intend to do with the offsite flows.
The designer needs to determine what is best for the site. For instance if I am accepting the offsite R/O onto my site I will compute pre & post developed conditions at the outfall from my site. The difference is the storage I need to design for. If there is a "solid" concentration point at the upstream site boundary with a measurable discharge, I will convey it through or around the site to the existing outfall. If the offsite contributes sheetflow onto my site I only look at the outfall from the site. I will however, compute the sheetflow onto the site for informational purposes as a development services requirement. Hope that helps.

 

[chicopee]

About berming the troublesome perimeter to redirect the offsite flow.

 

[cvg]

you can't just divert flow around your property! doing that would generally cause harm to another property owner. Good way to make friends with your neighbors. You can channelize flow through your property however, if you return the flow to the original flow path and also if you don't change the nature of it such as by concentrating it or increasing the velocity...

 

[EngineeringBird]

Thanks so much for all the input. My main concern for this project is that per local ordinances, my post-developed runoff must be <= predeveloped+10%. For my detention basin design I am taking into account the entire drainage area. However, I'm still not quite sure how to remove the water that falls offsite to check for compliance.

Is this usually done by %area or %volume? Perhaps this is more of a question for somone who's familiar with local ordinances in WI.

 

[psmart]

You may - or may not - need to "remove the water that falls offsite" when determining the development increase. It will depend on the exact wording of your ordinance. Talk to an engineer who is experienced with the specific ordinance - or ask the agency.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[jim2124]

If you haven't already, you should discuss your project with city/county and regional WI-DNR office. I completed a project in the La Crosse area late last year and found the DNR to be quite helpful. Working with them early in the project clarified my questions and any concerns they had with the project.

When you do speak with local officials and/or WI-DNR, make sure you discuss TSS removal requirements. The DNR's website is a very good resource and thoroughly explains BMPs and how to include them in your site design, but it is still good to know what reviewers will be looking for during the permit process.

 

[froude]

I guess I'm not following completely, so sorry if the following is completely out of left field. It seems you are trying to determine how to compare pre- and post-developed flows, considering that there is offsite drainage? Assuming that is the question...

This will depend on whether or not you route offsite flows through your pond. In either case, you will need to find a common discharge point and route hydrographs in pre- and post- conditions to determine what your project's impact is on volume and peak runoff rate. You may decide that you can make a pond big enough to also detain offsite flows, or you may have to divert offsite flows by some other means through your site and detain enough to show no/allowable increase.

 

[beej67]

Thanks so much for all the input. My main concern for this project is that per local ordinances, my post-developed runoff must be <= predeveloped+10%. For my detention basin design I am taking into account the entire drainage area. However, I'm still not quite sure how to remove the water that falls offsite to check for compliance.

If I follow correctly, your allowable post-development site discharge is 10% less than existing, right? And you're worried that you have to provide that 10% reduction on offsite runon? If so, I've definitely been there and done that. It can get sticky, and can vary depending on your review agency. We used to have to adhere to criteria like that in several metro Atlanta municipalities. I believe Dekalb used to be that way, and I believe they're not that way anymore since they adopted the Blue Book.

What I would typically do, is separate out the offsite portion and the site portion in two different nodes in my existing model, apply a 10% reduction to the existing site runoff (but no reduction to the offsite runon) to determine my "allowable" discharge, and then shoot for that "allowable" in my proposed design. The allowable ends up being somewhere between 1% and 10% less than existing at your study point, based on the pro-rata share of site vs offsite runoff at the study point.

(For what it's worth, it's good practice to always separate site and offsite nodes in your model, so you can ensure you're always comparing apples to apples.)

Then the finesse becomes explaining that approach clearly to your reviewer, which is proportionately more difficult depending on the lack of education or engineering acumen your reviewer has. Heh. Test the waters on that early.

If you've got the Etowah River running through your site, they clearly can't want you to reduce it's flow 10%.



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

http://www.campbellcivil.com

 

[brandonbw]

What did you end up doing? Typically we would take the Pre Q as everything coming onto the site. Otherwise doesn't it seem like you are missing part of the water flow onto your property?

I would find the hydrology engineer at the City or County and ask if there is any special help you can get because of offsite flows onto your property? I have seen some pretty good maps and calcs that will basically make the city agree in your favor as long as you present the facts correctly. This means having a meeting at a table with you and the client at the city.

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

http://bwengr.com

 

[LincolnPE]

Sounds like you are asking for what your overall Storage requirement would be, I think?

Typically you only need to detain for the tributary area within your site boundary.

The peak Q100 for offsite runoff that is coming on to your site would be passed through your detention pond through the spillway. The spillway would be oversized to pass these offsite flows, but you are not required to provide detention volume or attenuate for the offsite flows. Somebody downstream may also be depending on these historic flows/rates from the offsite areas that is now going through your project.

This would go under "not having to improve the whole region with just your project" also.

If the jurisdiction would like you to do the regional detention, they can help kick in for the extra cost for building the bigger pond.