Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Impervious areas - connected or not connected?

Impervious areas - connected or not connected?

Impervious areas - connected or not connected?

Say I have a pre-construction site which has a total area of 3.8 acres. In this 3.8 acres lies two homes (about 1400 sf - one floor), two separate garages (not connected to the homes and about 450 sf each) and a small shed. The rest of the acreage is grass, in good condition. When the runoff hits the roofs of the buildings, it'll sheet flow until it runs off the roof, where it'll land in the grass, then flow for several hundred feet before discharging onto roadway (over the curb). When I compute the weighted CN value, would these impervious areas be considered connected or un-connected? If un-connected, how to figure the percentage of this impervious area over the total impervious area?

The whole area (all 3.8 acres) is in a Hydrologic Soil Group B Soil.

The flow path from the most distant point to the discharge point of the area is approx. 600'. This path does not appear to include the roof runoff from the buildings.

If this is considered un-connected, wouldn't the percentage of impervious areas be 100%? Or am I missing something?


RE: Impervious areas - connected or not connected?

Only impervious surface that is hydraulically connected directly should be considered a CN of 98 (assuming this is your impervious #). All other non-directly connected impervious areas should be considered a CN of whatever surface they join.

If you think about it a different way, a roof that drains onto the grass and sheets is going to develop a different amount of runoff than a roof that drains to the road/driveway. Instead of behaving like impervious surface, the water acts like a heavy rainfall landing on the grass, and the grass CN properties will apply.

RE: Impervious areas - connected or not connected?

Thanks, Kevin. Your answer makes sense. I think I'm thinking too hard about this; I tend to do this! ponderhammer

RE: Impervious areas - connected or not connected?

According to TR-55 documentation, a CN adjustment is ONLY made if the impervious is less than 30% of the total area. The document states that for greater impervious area, "the absorptive capacity of the remaining pervious areas will not significantly affect runoff."

In the example given, if one correctly applies the rules with the areas given, it is doubtful that there will be any CN adjustment.

RE: Impervious areas - connected or not connected?


I would recommend taking a 1,000-foot view of the drainage before considering curve numbers. For instance, what is the slope of the impervious surface draining to pervious areas? Does it fall to the ground, losing its energy and subjecting it to any level of infiltration? Does it flow across a very large area of pervious with lots of time to absorb into the soil, or does it travel a short distance? 30% can represent any size of land, so I would recommend using "engineering judgement" when determining if the surface should be classified as CN 98 or lower.

RE: Impervious areas - connected or not connected?

One always needs to employ "engineering judgement". In my experience, If one were questioned on this judgement, regulators will typically rely on regulatory documentation. Since we are talking about NRCS methods, I cited an NRCS documentation regarding unconnected impervious. "All other non-directly connected impervious areas should be considered a CN of whatever surface they join." does not apply if one follows the methods described in the documentation.

RE: Impervious areas - connected or not connected?

Absolutely correct! All TR-55 related documentation applies here, and regulators will for sure check against it if that ever came into question. If you're submitting to an authority that will look at it through that lense for review, it is important to cross your t's and dot your i's using their assumptions for calculating runoff.

RE: Impervious areas - connected or not connected?

Thanks Bill for a great question, and thanks to Terry and Kevin for the great follow up! I've been at stormwater for a while and haven't quite grasped this subtlety 'till now. Having the documentary backup, along with engineering judgement and consideration of regulatory requirements is key :)

RE: Impervious areas - connected or not connected?

I had to read each of the answers three or four times to understand the intent! The total of the impervious areas (roofs and driveways) came out to be 6.6% of the total drainage area, or 0.25 acres. So....since that is less than 30%, an adjustment would have to be made.

By the way, the very little that runs off the driveways would drain up to 200' over grass before outfalling to the road, while the runoff from the roofs would drain an average of 225' over grass (or pervious) surface before outfalling to the road. I would think that part of the roof / driveway runoff would have a likely chance of permeating into the ground, on its way to the road.

Thanks Terry and Kevin for great, thought-provoking answers!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


eBook - Integrating the Engineering Ecosystem
Aras Innovator provides multiple options for integrating data between systems, depending on the scenario. Utilizing the right approach to meet specific business requirements is vital. These needs range from authoring tools, federating data from various and dissimilar databases, and triggering processes and workflows. Download Now
Research Report - Simulation-Driven Design for SOLIDWORKS Users
In this engineering.com research report, we discuss the rising role of simulation and the paradigm shift commonly called the democratization of simulation. In particular, we focus on how SOLIDWORKS users can take advantage of simulation-driven design through two analysis tools: SOLIDWORKS Simulation and 3DEXPERIENCE WORKS. Download Now
White Paper - Industry 4.0 and the Future of Engineering Education
With industries becoming more automated, more tech-driven and more complex, engineers need to keep their skills and knowledge up to date in order to stay on top of this wave—and to be prepared for the Industry 4.0 future. The University of Cincinnati offers two online Master of Engineering degree programs designed specifically for practicing engineers. Download Now
eBook - The Design Gridlock Manifesto
In this eBook, you’ll learn 6 ways old CAD technology slows your company down and hear how design teams have put those problems to rest. “The Design Gridlock Manifesto” shares first-hand modern CAD experiences from 15 companies around the world. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close