Roof Infiltration Hydrology 5

[Quest101]

Hello,

I would like to design an infiltration trench to infiltrate roof drainage only, in order to reduce my site impervious and thus volume discharge, since there is not much room for swm structures on site.

If I use the rational equation to get a volume.
How would I go about that? Would my Tc be 5 min? If so, would I multiply my Q by 60*5 to get the total Volume needed in my trench ?

Or if I use the modified rational equation how would I get my total elapsed time to ultimately get my required volume ?

Thank you very much!

 

[RWF7437]

You can't get a hydrograph using the Rational Method and therefor you can't get a volume. You can get an imaginary hydrograph using the so called Modified Rational Method.

Can't recommend either.

 

[psmart]

As RWF suggests, you really need a full runoff hydrograph (plus other details) to determine the required volume.

A simplistic solution would be to retain the entire runoff voume from the roof, such as the 24-hour depth. But the storage will be oversized if you don't make allowances for real-time infiltration and any permitted overflow. And that requires a full hydrograph analysis.

But even this calculation depends on the event (duration and return period) you are trying to handle. More information is needed to answer your question...


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[DMcGrath]

I would think that for a roof, volume of rain in = volume of stormwater out. Mine works that way most of the time! As psmart mentions, you could oversize it for the whole volume, but in reality that volume arrives over time, thus the need for hydrograph analysis.

What might be more difficult than the design is the sales pitch to the local government to get it approved.

 

[cvg]

no hydrograph is needed for volume calcs and I would recommend holding the entire storm and discounting real time infiltration or overflow. Runoff volume equals total storm precipitation times drainage area times runoff coefficient. For roofs, C = 1.

however for the gutter or downspout design you might want to check the peak flow rate. that will require a very small time of concentration. for a sloped roof, 5 minutes may be way to long. perhaps it is only 1 or 2 minutes maximum.

If you want to check the runoff for only partial storage of the roof runoff, then add in the volume of storage into your rainfall - runoff model which should use a hydrograph and this will reduce the net runoff and perhaps will also reduce the peak runoff from your site.

 

[saruman26]

Let's simplify.

Does the municipality or state have a target treatment volume, i.e. 2" of runoff? If so, then this is easy.

To determine the volume of runoff to be managed, multiply the roof area (square feet) by the rainfall depth (in inches), then multiply by 1/12 to covert to cubic feet.

If this number is relatively small and you can readily construct an underground chamber for percolation that fits within land constraints and isn't too expensive, then design an infiltration system with this much capacity. Don't forget to provide an acceptable overflow method.

If this number is large, there is not enough land area, or the cost is too high, then a hydrograph (TR-55, also called SCS or Curve Number method) is the way to go. Assume a 5 minute Tc, as this is the minimum for computations. Create the hydrograph in an appropriate design software (Hydraflow Hydrographs, HydroCAD, etc). Then preliminarily size an underground infiltration trench comprised of the storage media you plan to use. Don't forget to add the soil permeability rate as an outlet within the basin design.

Route the hydrograph through the estimated detention basin and see if it works. Multiple iterations may be required before adequate storage capacity is achieved to store runoff while it infiltrates.

Hope this helps. It really isn't as bad as it sounds.

 

[RWF7437]

http://www.stormh2o.com/july-august-2006/urban-hydrology-methods.aspx

 

[Quest101]

Thank you all for your help !

I'll use the rainfall and multiply it by the area of my roofs and store that volume in my trench and see how long it takes to dewater, not more than 72 hours I understand. I will use 1.0 in/hr.

It appears that I only have to control all storms up to the 5 year event.

Saruman, I don't see how to create a hydrograph using the 5 min tc.
I guess you would have to break the hydrograph into made up Qs throughtout the 5 min duration, from beginning to end of hydograph?

 

[RWF7437]

Is this "Local Detention" run amok ?

 

[TerryScan]

Be sure to do actual infiltration tests on site to determine your design rate.

 

[peaksmill]

RWF7437; Thanks for the link to the article. I think that there should be a requirement that everyone that wants to join the storm/flood forums has to read that article first. The more you learn about hydrology the more you realize that this segment of civil engineering is just a bunch of “voodoo science” unless projects are designed based on statistical analysis of actual storms. This is of course, not practical on 99% of all development projects.

 

[Quest101]

Hello all again,

RWF7437, not sure what you mean by a mok.
Thank you for the link !

TerryScan, yes I will ultimately wait for the actual infiltration rates to design the trenches. We are designing them to handle de 100 yr event, 7 in/hr.

 

[RWF7437]

A long answer to Quest101 and Peaksmill's questions may be found at

http://www.pdhonline.org.

A very abbreviated version of the basic idea is:

"(Course H131)is based on a belief that stormwater detention, stream restoration, groundwater recharge, multi purpose public land use and similar community goals are most fairly and efficiently achieved through a system of carefully planned, designed, and publicly owned and operated, regional detention basins.

"... this course ... follow(s) a step by step approach to the preliminary design of such a regional detention facility. Once designed, this detention basin (is) subjected to a series of possible storms in a effort to estimate how well it will perform over its usefull life."

 

[RWF7437]

here's the .pdf