×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

# Contact US

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!

*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

# Modeling Disconnected Impervious

 Forum Search FAQs Links MVPs

## Modeling Disconnected Impervious

(OP)
TR-55 states that the composite CN value for disconnected impervious greater than 30% should be computed from Figure 2-3.

In the attached screen capture there is a 20ac site with 10ac unconnected pavement and 10ac pervious. HydroCAD is taking the weighted average to compute a composite CN of 69 instead of using Figure 2-3.

Is there a step I am missing?

### RE: Modeling Disconnected Impervious

Per TR-55 and NEH, this adjustment only occurs when the total impervious portion of the subcatchment is less than 30%. Your example is 50% impervious, so there is no adjustment.

Try 4 acres unconnected with 10 acres of grass. Please note that the data entry screen will continue to show the standard average value, but the subcat summary and all the reports will show the adjusted value.

Peter Smart
HydroCAD Software
www.hydrocad.net

### RE: Modeling Disconnected Impervious

If you look carefully at figure 2-3 it's gust giving you a straight algebraic weighting without any special adjustment for UI. That's exactly what HydroCAD is doing.

Peter Smart
HydroCAD Software
www.hydrocad.net

### RE: Modeling Disconnected Impervious

(OP)
Ok, I see it now. Thanks

#### 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

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:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!

Already a member? Close this window and log in.