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LCid (Civil/Environmental) (OP)
18 Dec 09 11:56
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?  
cvg (Civil/Environmental)
18 Dec 09 13:10
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.
LCid (Civil/Environmental) (OP)
18 Dec 09 13:17
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.
cvg (Civil/Environmental)
18 Dec 09 13:29
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...
LCid (Civil/Environmental) (OP)
18 Dec 09 13:46
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.
cvg (Civil/Environmental)
18 Dec 09 13:50
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.
cntw1953 (Civil/Environmental)
18 Dec 09 13:59
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?).
LCid (Civil/Environmental) (OP)
18 Dec 09 14:21
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.  
cvg (Civil/Environmental)
18 Dec 09 15:10
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.
LCid (Civil/Environmental) (OP)
18 Dec 09 15:27
Actually, I did not use the word "sump" to describe the volume from the bottom of the CB to the bottom of the discharge pipe.
Please take a look at the attached file.
LCid (Civil/Environmental) (OP)
18 Dec 09 15:32
cvg--Please take a look at this file. It's more clear than the previous one. Let me know what you think. Thank you.
cvg (Civil/Environmental)
18 Dec 09 16:11
yes, that is exactly what I mean by a sump.
cvg (Civil/Environmental)
18 Dec 09 16:40
LCid (Civil/Environmental) (OP)
18 Dec 09 16:41
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,
LCid (Civil/Environmental) (OP)
18 Dec 09 17:11
I see;very good. That is a different type of CB. Please take a look at the one attached.
 
gbam (Civil/Environmental)
18 Dec 09 17:14
If the RFP is active can't ask for a clarification?
LCid (Civil/Environmental) (OP)
19 Dec 09 9:18
The RFP is active; I'm getting ready to respond to it and will ask for clarification. Thank you so much.
msquared48 (Structural)
19 Dec 09 17:43
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

LCid (Civil/Environmental) (OP)
21 Dec 09 15:00
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.
 
gbam (Civil/Environmental)
21 Dec 09 15:10
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?
LCid (Civil/Environmental) (OP)
21 Dec 09 15:20
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.
msquared48 (Structural)
21 Dec 09 21:37
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

diggerman (Civil/Environmental)
28 Dec 09 13:49
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!!!
LCid (Civil/Environmental) (OP)
28 Dec 09 15:08
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.  
diggerman (Civil/Environmental)
28 Dec 09 15:46
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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