RAINFALL INLET CALCULATION 3

[sudu]

I am designing a rainfall inlet for a given area.
i have calculated Q for the given drainage area using rational method
Q = cia
for designing the length of rainfall inlet the fhwa manual gives the equation
LT = KT Q ^0.42 *SL^0.3 (1/(n*Se)^0.6
now my question is the
Q in the above equation is to be taken from the total Q found out using rational method or
i have to calculate it separately for gutter flow

 

[gbam]

Sudu -
I am assuming this is for a pavement inlet (curb opening or grate). The flow rate that one uses to design inlets is the flow calculated at that concentration point. The procedure for sizing the the inlet will be based on your total flow. The spread of the runoff in the street will be broken down into gutter flow (frontal) and side flow. If your standard curb inlets are not as large as you calculate then the procedure will compute bypass flow. FHWA has a computer program "Visual Urban" that automates the computation as described in FHWA's HEC22.

 

[sudu]

hi...thanks a ton for ur reply...
but my problem is not in calculating the inlet length or capacity...
i am just confused with the Q that is to be entered in doing so
for ex
my problem states that
my area contributing to runoff is 12 acres
my runoff calculated from Q = cia reads 72 cfs..
now from here on how do i go on to calculate the gutter flow
or as u said the flow in the concentration point?

can u plz tell me the gutter flow i need to use

if i calculate the inlet length based on my Q of 72 cfs
i get required inlets as 12-15' Inlets where as the current site condition are 2-10' inlets and 1-5' inlets
which absolutely makes no sense to me

 

[DrFluffenstuff]

I'm not sure if I totally understand what you're asking, but if you cut a cross section of the road then input that into Manning's equation using the maximum permissible depth of flow (curb height?) to determine the street flow capacity.

 

[gbam]

Sudu;
A overview map/sketch would be helpfull. Are the inlets you are proposing in a sag or on-grade? Have you reviewed the exisitng drainage analysis for the exisitng inlets? Once you do, ask yourself:
1. How does the flow rates compare?
2. Did the existing analysis miss contributing area?
3. Did you add too much area?
4. Evaluate your approach and confirm that you are on the right track.

Back to your original question:
You can use Manning's equation to compute the flow within the street for your entire discharge then back out the frontal and side flow manually. Review the HEC-22 manual and you should be Ok.

It sounds as if you are computing a discharge for a larger storm than typical street drainage. Check with your reviewing agency on the design criteria. Typically we use a 10-Year storm for on-site/pavement drainage analysis. Or, you may have significant "off-site" area contributing to the roadway. If possible, you can use an area - off roadway inlet to capture some of the runoff before it enters the street.

 

[beej67]

Sudu,

Hate to be the bearer of bad news, but 12 acres of runoff is far, far too much runoff to catch safely in a gutter pan, at least in my experience. Your road is going to flood out in every storm.

You need to restrict your inlets as much as possible to catching street drainage only, and any offsite drainage needs to be caught in headwalls or pedestal top inlets out in the right of way someplace before it gets into your road.

Any more than one to two acres of runoff in the gutter is too much here in Georgia, depending on municipal regulations.




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