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Catch basin-freeboard-concept

Catch basin-freeboard-concept

(OP)
I recently read the following: "check the quantity of freeboard below the [catch]basin hood." In this context, is "freeboard" to be understood as the available holding capacity (of a particular catch basin) before the water it collects reaches the level of the overflow line?  

RE: Catch basin-freeboard-concept

my interpretation is that the freeboard in a CB is the vertical distance from the calculated water surface in the catch basin to the lip of the gutter or to the grate.

RE: Catch basin-freeboard-concept

(OP)
Thank you CVG for your response. I do agree with you, but that would be the definition of freeboard. Please note, however, that the quoted language is from an RFP. And within the context of that statement it seems to imply something else.

RE: Catch basin-freeboard-concept

freeboard does not have a "quantity" although it has a "distance".

it sounds as if whoever wrote the rfp didn't fully understand the definition of freeboard...

RE: Catch basin-freeboard-concept

(OP)
Yes, it looks that way.
Or, it may well be that the definition is "expanded" to mean volume. That is, once the vertical distance--freeboard--is measured or determined (the rpf deals with existing CBs), a volume can be calculated (the holding capacity before it reaches the flowing) for each CB.The sum of those volumes of all CBs evaluated will give you a total holding capacity of water collected before the water collected is conveyed. In a sense, freeboard although a length, it implies a volume; because that lenght may associated with 2 other dimensions of a 3D-structure (CB may be squared, rectangular or circular; freeboard in a channel, the "volume" would be width X length, and so on...)
Thank you so much for your input.

RE: Catch basin-freeboard-concept

catch basins that I am familiar with do not store any water before discharging. They begin to discharge as soon as any water enters the basin. perhaps you are thinking of some other type of catch basin with a large sump below the outlet pipe.

RE: Catch basin-freeboard-concept

In open channel flow, I believe the freeboard is the vertical distance from the maximum water surface to the top of channel walls (to handle surge?).

RE: Catch basin-freeboard-concept

(OP)
cvg--The CBs I am dealing with are installed in large cities. And from the bottom of these (there are various types) basins to the bottom lip of the pipe (usually 12" dia. pipe) that connects the CB to a nearby sewer, the minimum design height is 3 feet (4 feet is typical). As with time CBs will fill (debris, etc.) that height is reduced, thus reducing that available volume--holding capacity. These CBs require maintanence; they are opened and cleaned.

cntw1953--you are correct, in open channel flow that is the freeboard.  

RE: Catch basin-freeboard-concept

I too work on storm drains in "large cities", they rarely if ever contain sumps and so the term freeboard to describe the depth of the sump is completely foreign to me. Standard designs for the cities I have worked with do not allow a sump.

RE: Catch basin-freeboard-concept

yes, that is exactly what I mean by a sump.

RE: Catch basin-freeboard-concept

(OP)
I think that maybe the RFP is after all correct. The purpose of a freeboard is to manage a "surge" in whatever structure they're used. If you think about it, a "small" rainstorm may not bring about enough water that would reach the point of discharge (pipe) of the catch basins I am dealing with and therefore what the rfp calls the "quantity of freeboard" (calculated as the following volume: height from bottom of CB to bottom of pipe x area of cb) is in fact handling the surge. If it continues raining, however, there will be an overspill (as the freeboard will not be able to handle the surge) and the water will rise until discharge commences through the discharge pipe. So in that sense, the "quantity of freeboard" is that volume.

This has been a good exercise and I thank you for it,

RE: Catch basin-freeboard-concept

If the RFP is active can't ask for a clarification?

RE: Catch basin-freeboard-concept

(OP)
The RFP is active; I'm getting ready to respond to it and will ask for clarification. Thank you so much.

RE: Catch basin-freeboard-concept

I think that this value would depend on the storm, pipe size in and out, and the outlet capacity.  

If you get into a serious backwater condition where head has to be generated in the catch basin for more water to flow through the downstream pipe, the "freeboard" will not only be much less at any particular catch basin, but also lowered at any catch basin upstream due to the increased tailwater head at the downstream catch basin.

H*().  Now I'm confused!  

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Catch basin-freeboard-concept

(OP)
msquared48--please see that my initial questions was related to the following quote: "check the quantity of freeboard below the [catch]basin hood" (from an RFP) listed as one of the tasks of the selected consultant.

Now, please take a look at the attached sketch as it does represent well the catch basins in consideration, and then let me know what you think.
 

RE: Catch basin-freeboard-concept

I do not think it is referring to the volume rather I beleive its the depth or vertical distance of freeboard.  Have you requested a clarification from the agency?

RE: Catch basin-freeboard-concept

(OP)
When the CBs are inspected there may not be standing water inside them; also these CBs are standard size, so to have that vertical distance measured (the actual available; from top of accumulated fill/non-flotating debris) will in fact give you a volume. But you are right, better to have the agency clarify it. We'll do that soon. Thank you.

RE: Catch basin-freeboard-concept

OK.  Looks like you may be referring to how much material the cb can accumulate before the "freeboard" that allows the water to pass will be cut off.   

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Catch basin-freeboard-concept

WOW!!! I thought we were all civil engineers.  I am sure you guys are all employed as well!!! Think of the word "FREE BOARD" is basically the amount of clearance left before a structure is prone to handle surges(I believe). I saw one post that was even close to the definition. I can't believe you guys are employed.  Must be nice to be professionals and not know what Free Board is. The young civils have such a hard road ahead!!! Move over or get out of the way LOLOLOL!!!

RE: Catch basin-freeboard-concept

(OP)
diggerman,

You are stating a definition of "freeboard." That, can be found easily. The "understanding" of freeboard is what has been discussed here in the context that was used (see above),in the language quoted from an RFP. Also see the attached image.
What is your understanding?

On a separate note, I do hope that everyone is employed. These are difficult times for many, not just for engineers, and my understanding of this venues is that these are "intelligent work forums for enginering professionals." With that in mind, you now have, again, the opportunity to help others with your knowledge and expertise.  

RE: Catch basin-freeboard-concept

Very True LCid! We are actually going to be removing an existing bridge (very small one) because the freeboard is too low. Due to the overwhelming amount of developing in our area, the increased impermeable surfaces have forced run-off into local wetlands and has increased water levels surrounding the bridge. So..freeboard is very important.  I'm not in design yet, but know the consiquences of not allowing enough clearance for structures around fluctuating watersheds.  

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