AutoCAD Civil 3D - Stormwater

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Active Member
Posts: 6
Registered: ‎07-16-2012
Message 1 of 8 (253 Views)

How to calculate runoff water that splits to two new subbasins

253 Views, 7 Replies
09-05-2012 08:29 AM

I’m a new user to SSA and am a self-teaching my way through the software.  I’m using 2011, due to printing issues in 2013.  My question relates to determining how runoff gets diverted, or spreads out, after leaving a well defined channel.

 

I have a street intersection scenario.  It’s a “Y” situation in which the main trunk of the “Y” is curbed and contains all the water for the three storms I’m studying.  Runoff flows towards the two streets that branch off.  I need to know what percentage of runoff water splits during the 2-, 10-, and 100-year storms from the main (curbed) roadway to the other two streets - one curbed, one uncurbed. 

 

I assume that velocity, runoff volume, storm intensity, street cross sections, slopes, and other variables play a big role in how the water diverts, but I’m not sure how to set up this scenario in SSA.  Do I somehow use a weir?  Do I use an open channel, or user-defined channel?  I have a subbasin area defined with my concentration point at the end of my main curbed street before the water splits off.  I’m not sure where to go from here when it comes to inserting links, weirs, channels, etc.

 

Any suggestions as to how I can set this up?  I’m not even sure it’s possible to have the software provide this type of calculation.

 

Thanks for any and all help!

 

Regards!

Valued Mentor
fcernst
Posts: 1,014
Registered: ‎01-07-2011
Message 2 of 8 (191 Views)

Re: How to calculate runoff water that splits to two new subbasins

11-27-2012 07:18 PM in reply to: dwillis

You said nothing about pipes playing a role....so I would use HEC-RAS.

Ernst Engineering
Fred Ernst, PE
C3D 2015
W7x64; i7 8gb; Radeon HD 5700
www.ernstengineering.com
Active Member
Posts: 6
Registered: ‎07-16-2012
Message 3 of 8 (182 Views)

Re: How to calculate runoff water that splits to two new subbasins

11-28-2012 06:08 AM in reply to: fcernst

I would agree with using HEC-RAS, but the review agency wants us to use the Rational Method.  There are no pipes in this particular area of concern.  It's strictly sheet flow.

 

Thanks for the reply.

Valued Mentor
fcernst
Posts: 1,014
Registered: ‎01-07-2011
Message 4 of 8 (180 Views)

Re: How to calculate runoff water that splits to two new subbasins

11-28-2012 06:12 AM in reply to: dwillis

You can use the Rational Method.

 

HEC-RAS just wants a flowrate, it doesn't care.

Ernst Engineering
Fred Ernst, PE
C3D 2015
W7x64; i7 8gb; Radeon HD 5700
www.ernstengineering.com
Active Member
Posts: 6
Registered: ‎07-16-2012
Message 5 of 8 (179 Views)

Re: How to calculate runoff water that splits to two new subbasins

11-28-2012 06:13 AM in reply to: dwillis

I found the easiest way to calculate the percentages of water being split between the two roadways was to use Bentely's Flowmaster software.  I created a irregular cross-section of the roadway and used that as my channel.  I then ran the program solving for 'normal depth.'  The program gave me more information that I needed.  From the printout, I was able to calculate the percentages of water for the 2-, 10,- and 100-year storm events.

Valued Mentor
fcernst
Posts: 1,014
Registered: ‎01-07-2011
Message 6 of 8 (171 Views)

Re: How to calculate runoff water that splits to two new subbasins

11-28-2012 06:57 AM in reply to: dwillis
  • The streets probably have different slopes?
  • Are the flows sub-critical?
  • A Flowmaster single open channel section doesn't account for backwater effects.

 

It's an iterative flow distribution process for the modeler where you have to balance energy at sections 2 and 3 below.

 

Hope this helps.

 

 

 

 

Capture.JPG

Ernst Engineering
Fred Ernst, PE
C3D 2015
W7x64; i7 8gb; Radeon HD 5700
www.ernstengineering.com
Active Member
Posts: 6
Registered: ‎07-16-2012
Message 7 of 8 (168 Views)

Re: How to calculate runoff water that splits to two new subbasins

11-28-2012 07:40 AM in reply to: fcernst

Mr. Ernst,

 

Your sketch closely resembles my scenario.  You are correct, the streets have different longitudinal and cross slopes. Flow is subcritical.

 

For my application, I took one cross section where the two roads split.  Per the sketch, this would be along sections 2 and 3.  I paid particular attention to the station and elevation of the point circled in blue, knowing this is where the flow divides.

 

Knowing the initial peak flow (shown in red on the attachment) at section 4, and then looking at Flowmaster’s cross section printout and calculated data, I was able to determine the percentages where flow splits at sections 2 and 3.  From the percentages, I calculated the new peak flows at 2 and 3.

 

Perhaps I made this more cumbersome than it needed to be, but as I said in my initial post, I’m new to hydrology and am fumbling my way through.  Your replies and suggestions are greatly appreciated and I will revisit my numbers and calcs keeping your information in mind.  I did not account for any backwater effects and will research this more to see if it indeed impacts my calcs.

 

Thank you for your replies and your help!

 

Doug

Valued Mentor
fcernst
Posts: 1,014
Registered: ‎01-07-2011
Message 8 of 8 (157 Views)

Re: How to calculate runoff water that splits to two new subbasins

11-28-2012 08:38 AM in reply to: dwillis

"For my application, I took one cross section where the two roads split."

 

Incorrect, in this questionable  abbreviated analysis approach you are taking that negelects backwater effects in a sub-critical flow regime, you would take two sections with their separate longitudinal slopes and balance energy iteratively.

 

 

Ernst Engineering
Fred Ernst, PE
C3D 2015
W7x64; i7 8gb; Radeon HD 5700
www.ernstengineering.com

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