spread calculation 9

[wuwo]

I'd like to know how to do the spread calculation.
I found the station of each inlet, then I calculate the overland runoff, at last the spread should be calculated. but how?

 

[francesca]

The manufacturer of the inlet should provide an equation for calculating spread. It is a function of road grade, cross slope and grate characteristics.

 

[wuwo]

It is almost impossible for me to get the equation from manufacturer. I'm not familiar with spread calculation. I wonder if I can use any formula about spread to do it?

 

[cvg]

spread is a function of flow rate and gutter/roadway geometry. Inlet capacity really doesn't figure into the equation. However, in order to calculate the peak flow rate in the gutters, you will need to first calculate the runoff, then the capacity of the inlets and finally estimate the amount of flow which bypasses each inlet.

 

[gbam]

As cvg stated you need to calcuate the runoff and then compute spread. "Visual Urban" can be downloaded from FHWA and used to compute the spread and inlet capture for the catchbasin. The link is below:

http://www.fhwa.dot.gov/engineering/hydraulics/software/softwaredetail.cfm#hy22_visual_urban

Good Luck!

 

[francesca]

Why would you specify a product from a manufacturer who won't let you calculate if it'll do the job?

You can find a similar shaped grate from a different manufacturer and use their equation. cvg picked up some of the aspects I missed; manning's n also features into it.

 

[RWF7437]

Just curious....why would you need or want to do such a calculation ?

good luck

 

[cvg]

RWF7437 - spread calculation is required by every agency I work with to show the width of flooding in the street and to check the dry lane width. Most agencies require a certain width to remain dry to provide all weather traffic flow along with emergency access. Usually a minimum of 1 dry lane in each direction for an arterial street. High speed highways require additional dry width for obvious safety reasons.

 

[RWF7437]

Thanks CVG.

In 47 years, I've never been asked to make such a calculation. Considering that flows calculated by the Rational Method, or any other method, are estimates of probabilities ( + or- 50% at best ?) and considering that most vehicles ( except tricycles ) can probably drive through 6 inches to a foot of water without encountering great difficulty, seems like a great waste of time.

Thanks for the information nevertheless.

good luck

 

[cvg]

granted, they can make it through a foot of water perhaps. However, I have noticed that even with just 1 or 2 inches of water, the travel speed of most vehicles drops way down to perhaps 10 - 15 miles per hour. In a large city, that can produce gridlock if it occurs near rush hour. Therefore, maintaining at least a couple of travel lanes that are totally dry goes a long way towards maintaining at least some traffic flow.

 

[RWF7437]

Hydraulic Speed Bumps ! What a concept

 

[gbam]

RWF7437 -- You may be on to something.

 

[TerryScan]

Spread caclulations attempt to insure that sufficient inlet spacing is provided.

While a vehicle 'can' drive through 6" of water; would you want to be traveling on wet pavement with no standing water...and then hit a spot of 3" depth while traveling at 30mph?

For a 2000 ft. street length...is a pair of basins at the sump sufficient? An extra pair either side of the sump? Is a pair every 200 ft. overkill? (this all depends on road cross slope and longitudinal slope and inlet efficiency of course).

While estimates of flow have margins of error, one can at least establish a standard for attempting to have a design which is mindful of safety with economy in mind as well.

 

[jonrpatrick]

I'll be honest, I am totally surprised at some of these responses: having never to have done them?? Wow!
I live in the Charlotte, NC area, and every municipality surrounding charlotte requires it. Of course, they all use the same standardized inlets, so they provide the formulas to use and the rainfall intensity to evaluate it for.
It's not hard- you just set up the spreadsheet and start 'plugging and chugging', but it's a major hassle.
Sounds like other parts of the country are much luckier.

 

[gbam]

One additional thing is that side inlet capture is based on spread. As well as curb opening length required to capture more flow. If 90% of the flow is spread over the lanes and you have a short inlet, the majority of r/o will bypass the inlet.

FHWA's HEC22 describes the capture analysis for inlets on-grade. Just follow the link I provied above to get to the website.