pre and post development runoff calculations 1

[azukarita]

Hello

I am trying to do a stormwater management / drainage report per city of Paterson, NJ requirements that includes the following:

- Pre and post development runoff calculations for runoff for 2 and 25 yr storms.
- Design stormwater management facilities zero net increase in runoff rate for the 2 and 25 year, one hour storms.
- Design stormwater management facilities (detention ponds, control structures, seepage pits, etc) per Residential SIte Improvement Standards NJAC 5:21.

Please help! i'm right out of school and is the first time for me doing this in my new job. Any contribution?

Thank you!

 

[azukarita]

Also the project is about 7500 sq feet

 

[beej67]

Did you have a hydrology class in college? This task is not one you can do on your own without the appropriate training or a lot of personal research.



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

http://www.campbellcivil.com

 

[SMIAH]

http://www.waterbucket.ca/gi/sites/wbcgi/documents/media/316.pdf

 

[brandonbw]

Ask someone where you work. No one would have learned how to write a specific report for a particular city in school. Or get a previous report someone did at your work and plug and chug the words.

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

http://bwengr.com

|

http://bwstructuralengineer.com

|

http://bwcivilengineer.com

 

[cvg]

not sure voodoo hydrology is the best learning tool for a beginner, just sayin...

 

[SMIAH]

Maybe he'll see that it's a tricky thing.

 

[francesca]

Does a 7500 sq ft lot require a permit? I would start with the Patterson, NJ city storm water ordinance... unless they're big enough to have a manual. I don't imagine you'd need to do much more than capture downspouts and maybe outlet to a rain garden on a lot that size.

 

[azukarita]

Francesca,

According to the town ordinance is a Non-major development. They have a checklist with the items we must submit.

As per ordinance, they ask to follow RSIS NJAC 5:21, but they do not specified what method to use to calculate runoff. NJ Stormwater Best Management Practices Manual gives a better guidance mentioning NRCS, Rational Method, and Modified Rational Method.

I chose to use the NRCS Methodology and I am using the WinTR-55 sofware to run the calculations.

I designated only one sub-area with the total square footage of the project (not sure if is a correct decision). In the part of the software provided to calculate Tc, I am trying to determine the right parameters (flow type, length, slope, Manning's n).

Since the site is bare soil, <50% grass, some gravel, developed urban area, I chose sheet flow, not sure how to calculate the length, and surface manning "smooth surface, 0.011"

Can you give me any tip on how to do this correctly?

 

[cvg]

sheet flow on dirt Manning is much higher than 0.011. Frankly, for a site this small I would have used rational method (modifed will give you a "hydrograph"), and you would have been done by now.

Tc minimum is usually 10 minutes or if you want to be conservative, use 5 minutes

 

[azukarita]

cvg: What criteria do you use to choose Tc=10 mins or Tc=5 mins?

 

[cvg]

it is often specified in the drainage standards for most agencies that minimum Tc is 10 min

 

[gbam]

There are many different methods to compute Tc. It is oimportant that you use the method that is accepted by your reviewing agency; otherwise, you will have to justify what method you use.

 

[SMIAH]

Please don't compute a tc for a 7500 sq feet projet.

Modified rational formula.

 

[civilman72]

Don't forget about runoff from off-site tributary basins.

 

[azukarita]

Can I use Seelye Chart to calculate Tc?

SMIAH: why do you recommend not to calculate Tc for the project and use modified rational instead?

 

[SMIAH]

The site's too small as cvg pointed out.

You could do a detention analysis, for example, with:

Peak flows for Pre-developpment
A = 7500 sq feet (?)
C = 0.15 (Woods) or whatever the site was/is.
I = x inches/hour based on a duration of 5-10-15-20 min for NJ

Peak flows for Post-developpment
A = 7500 sq feet
C = 0.40 (residential single family) or whatever the site is.
I = x inches/hour based on a duration of 5-10-15-20 min for NJ

Vp = Qp * D - Qo *((D + tc)/2)

Vd = Detention volume
D = Duration of the storm
Qp = Runoff peak for this duration
Qo = Allowable peak flow (e.g. pre-developpement peak flow)

Iterative process for different duration/time of concentration

 

[beej67]

I was recently forced by a reviewing agency to produce Tc calculations for an urbanized redevelopment site where the maximum travel length of any watershed was 300 feet. By SCS method.

/shrug

I'm also working on a project right now where the reviewer is demanding I do a 100 year routing model of the roof of an existing building, to determine whether or not the 100 year storm surcharges the roof so much that it overtops the parapets and bypasses our underground vault. I'd say that's a first.

Sometimes reviewers want funny things, and it's just easier to give it to them than argue with them.

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

http://www.campbellcivil.com

 

[LincolnPE]

Just FYI>>

Sheet flow is the most sensitive component of the overall Tc calculation. Be very careful with the slope you choose and Manning's n used for Sheet Flow. Sheet flow length should be no more than 100-150 feet.

Most municipalities specify a minimum Tc of 5-10 minutes.

 

[LincolnPE]

I chose to use the NRCS Methodology and I am using the WinTR-55 sofware to run the calculations.

The NRCS methodology is not really applicable to an area this small. You really have to know what you are doing, you need to be more experienced, and be able to evaluate whether the results make any sense or not. You can actually do more harm than good here trying to apply the NRCS method on this small site.

Use the Rational Method, this is the standard practice criterion for these small watershed evaluations.