Hydrocad and infiltration

[luklyz]

hi
This is great forum. I'm new to Hydrocad and i have a issue with a infiltration. I have a parcel 3.108 ac. in central Wisconsin. Wisconsin code stands that for high imperviousness sufficient runoff volume of the post- development infiltration volume shall be at least 60% of the pre-development infiltrations volume based on 1 and 2 year 24h peak runoff. My problem is for 1 and 2 year event my runoff is 0 i`m not discharging water from the site. i`m not sure how i can find out how much rain is infiltrate at 2 year storm. Any suggestion how to solve this. Thank you all

 

[psmart]

Perhaps I'm missing something, but if you have zero runoff from the 2-year event, then you would be infiltrating the entire 2-year depth, correct?


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[luklyz]

i thought about infiltration depth but my runoff depth is 0.

 

[Twinkie]

So you have a zero discharge from the site in the post development condition? Is this correct? Is this because of the Tc? Are you storing this volume somewhere? You just need to explain this better, if you have a 0 discharge from the site, there is nothing to infiltrate. Are you sure you have your inflow nodes setup correctly?

 

[beej67]

If your runoff node itself shows zero runoff, before it makes it to the stormwater management facility, you did something wrong in your model.

Most obvious error I can think of is you entered acres where the program expected square feet.


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

http://www.campbellcivil.com

 

[luklyz]

hi
Sorry for confusion. I have 0 cfs in pre development and i need to infiltrate 60% of pre in post conditions.

 

[psmart]

More details are still needed to answer your question.

Please complete your statement: "I need to infiltrate 60% of pre..."

60% of what? Please be precise.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Twinkie]

So your local regs state that you need to infiltrate 60% of the pre-development runoff volume, in the post development condition? What storm are we talking about here?

I think I understand what you're getting at, but I want to be sure. Let me just get this started and I'm sure Peter or beej will round out the edges.

Is your pre-development such that there is no runoff? 20 acre watershed, low CN, long Tc?? There just isn't any runoff because the design storm is too low for the watershed to produce any runoff, and HydroCAD is telling you this. So how do you infiltrate 60% of zero, because your pre-development site sure is infiltrating a lot of water, so we can't just pave the entire site and say this all works.

This seems to be one of those things you need to work out with your reviewer, and you may need to just do some math with a pencil and paper here.

Lets use that same 20-acre watershed, CN=31, Tc=65 mins, 2-year 24 hour storm = 3.30"/hr

Using the above 239,580 ft^3 is infiltrated across your site, 60% of which is 143,748 ft^3 which would be your infiltration requirement.

 

[luklyz]

Hi
Sorry for slow respond.
You are right with everything.
Everything has to be for 2 year event 2.90"/hr. The site is 3.108 ac, grassy field with flow path 330 ft (from SE to NW) CN=39, slope 1.3%, TC= 2.4 min. On the edge of the property there is a ditch (is going a round the property S to E to N). On the West there is a access road.
Can you tell me the formula you used?.
Also i would like to know, how i can enter the infiltrated volume in to the hydrocad to design underground infiltration device.
Thank you for your input

 

[Twinkie]

I think in the above I didn't carry out the volume across a 24-hour duration, but you get the jist.

But for you I would do:

(2.9/12 ft/hr)(24 hrs/day) = 5.8 ft/day * (3.108ac)(43560 sf) = 785,230 cf/day * 60/100 = 471,138 cf is your infiltration requirement.

 

[Twinkie]

I hit post too soon, but as for your infiltrated volume, you need to design some kind of BMP that will infiltrate that volume through a combination of soils conductivity, storage volume, and outlet control. Use the help menu on HydroCAD to learn how to enter the conductivity, and show that your discarded volume for the 2-year storm exceeds 47K cf ..... you're on your own for the actual basin design unless you want me to start billing you.

 

[psmart]

2.90"/hr would be a reasonable peak intensity for the 2-year event, but this is not sustained over the entire 24-hour storm, so multiplying by 24 hours will grossly overestimate the 24-hour depth.

I would expect the total 2-year 24-hour depth to be somewhere in the range of 2-5 inches, depending on the project location.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[gbam]

One can obtain the 24-hour precipitation depth from the NOAA website:

http://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html?bkmrk=az

Looking at the NOAA site the value that was provided 2.9" is the 2-yr 24-hr depth. not the intensity.

I suggest you take another look at your hydrology