1" Wide Weir 1

[KatiLynSki]

I am currently reviewing some plans which propose the use of a 1" wide weir as the outlet structure of a detention pond in order to regulate peak flows to below pre-development conditions. Conceptually the thought of a 1" wide weir does not seem feasible.

The equation being used is Q=2.8*L*(h^1.5).

So if he wanted only 0.5 cfs to leave the pond for a 10 year storm, then the height of the 1 inch wide weir would be 1.66 feet.

In order to prevent the 1" wide weir from being clogged he is putting it inside of the outlet structure which has a trash rack on the inlet with bars spaced 1".

He is doing his whole detention pond design using the structure feature of the program Win-TR55. (Which I am finding very hard to follow.) Every time I try to recreate his design with PondPack or any other program it spits out errors at me.

Any help would be appreciated. I have met with the engineer multiple times already and he is adamant about his 1" wide weir being able to work.

-KD

 

[dik]

I've used 2" slots for controlled discharge... with a screen and inverted elbow to reduce the chances for clogging...

Dik

 

[LHA]

A trash rack with 1" screen? That will clog after the first mowing, or leaves falling. Guaranteed.

You probably can't prohibit it though. Check, some Ord's have prohibitions against control structures under a min size. But if not, I would require some tough O&M notes on the recorded documents.

Engineering is the practice of the art of science - Steve

 

[cvg]

how has he chosen C=2.8? I would think that C would be quite sensative and may also vary considerable based on the depth of flow. So his outlet rating curve may be a bit arbitrary. For higher flow depths, this sounds more like an orifice (with the top missing).

For small flows a sharp crested v-notch weir might be a better choice. These are highly accurate and often used for measuring flows. Won't tend to clog (as much) as a slot would.

 

[psmart]

As cvg suggests, using the weir equation for a 1" wide slot is questionable at best. The equation assumes uniform flow across the entire (1") length of the crest. In practice, end contractions will reduce the effective crest length, which will cause a major flow reduction in this case. The extent of the contractions will also depend on the exact geometry of the slot. The orifice equation is probably better suited to your situation. Of course, 1" is still an impractical width. Even if it doesn't block completely, the discharge will be unpredictable, making this a poor outlet choice for a finely-tuned detention pond.

 

[RWF7437]

Your time might be better spent reviewing the detention "standards" which give rise to such questions. If flows are as small as 0.5 cfs and control weirs are as tiny as 1-inch wide how much flood protection is really being accomplished ? On can only wonder.

good luck

 

[LHA]

I agree with RWF7437. I am both a reviewer and designer.

I would suggest the designer request a waiver of depeaking altogether. The justicfication would be both the obvious de-minimus impact this project is having (who is this 0.5 cfs during the 10-yr going to "flood" exactly?) and the obvious potential for problems any "control structure" which could address such depeaking would have. Then, after he requests a waiver, you can go on record as recommending it be granted, from an engineering perspective. The governing body might not grant the waiver, in which case, it looks like you are back to the ol' 1" weir/orifice/leaf catcher thingy. But at least you will feel you were diligent from the technical end of things.

Engineering is the practice of the art of science - Steve

 

[blueoak]

Who is responsible for maintaining the basin? If it the city then you need to get a variance, because your maintainence staff will be a tad upset.

If not, then just make sure the top is open for overflow conditions. The basin probably holds the whole 10 yr event if a 1" weir works. And even better you will get a 24 hr delay before it is released because the owner will have to send out a crew to clean it the day after it rains each and every time.

 

[Schopsy]

We have been kicking around putting a minimum orifice size in our municipalities ordinance to address situations such as this. 4" seems reasonable. If the peak flow from a 4" orifice is going to overwhelm the system downstream, we've got bigger problems that ought to be considered in a much broader context.

 

[geosavvy]

Yea, heh, that's what I was thinking too.

I'm not an expert, but it seems to be that if you only need a 1" weir then you are probably talking about a small area and very little runoff.

Is it not possible to just detain everything and let the storage volume simply percolate/evaporate? Maybe let the 25 or 50 year even pass through an emergency spillway, but detain the 10 year completely. I mean, how much could it increase your volume to get rid of a 1" weir? The storm even is probably over by the time that thing has dumped out a few buckets of water.

I know it will largely depend on the soils properties, but with such a small amount of water, I can't imagine you would need much more than a sandy silt to pull it off.

 

[cvg]

requirement here is to retain the entire 100-year, 2-hour storm. Drywells are required to drain the water within 36 hours. If dry-wells are not feasible (no clay layer or high groundwater level), then a bleed-off pipe is allowed and designed to bleed off the entire storm within the 36 hour period. Bleed lines which connect to a storm drain or to a street gutter are restricted to 1 cfs maximum.

Orifices are only allowed on detention basin facilities which are generally not allowed, except for larger regional facilities. Emergency spillway would be designed to pass the 100-year 24-hour storm and greater.