Notched weir wall modeling question

[Sathre]

What is the correct way to model a notched weir wall? I have a 0.25’ by 1ft tall notch in my weir wall. The wall is 6 ft wide (inside diameter of the structure). I received a review comment from the governing unit in charge instructing that the weir width should be modeled as 5.75ft as .25 ft is already accounted for in the weir notch. The change does not affect the model at all but which is actually the correct way to model the wall?

 

[Twinkie]

Does the 6' diameter wall even come into play?

It is a 6-foot wide weir when that comes into play, not 5.75'

 

[psmart]

If the WSE comes above the top of the wall, you need to be careful to avoid "overlap" between the v-notch and the top of the wall. Reducing the length of the wall weir by 0.25 feet is only a partial solution, because there will still be overlap due to the flared sides of the vee.

One solution is to use the full 6' length of the wall, and set the Rise for the v-notch to 1 foot. This effectively makes the v-notch an orifice at high heads, with the additional flow area handled only by the 6' weir.

Another option is to use a custom weir/orifice, which basically allows you to draw the entire structure (notch plus wall) using a single outlet device.

For further details and illustrations please see

http://www.hydrocad.net/weir2.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Sathre]

Thanks for the information. I attached an image of the weir I was using (weir wall in an outlet structure). I am using 10.00-15 version of HydroCAD right now so if I am reading the compound weir link you sent correctly then the program already accounts for the adjusted width I think. As I said, not a big deal, it doesn't affect the model I'm just curious as to the correct way to model the feature (5.75' or 6').

 

[psmart]

As I explained in my previous post, using 5.75 feet is not entirely correct, because the top of the vee will continue to flare outwards at a constant angle and will overlap part of the 5.75 foot crest.

I suggest that you use 6 feet AND set the rise on the v-notch to 1 foot. This will eliminate any overlap in the flow area.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[psmart]

Sorry, I was assuming this was a vee notch. If it's a rectangular notch, then you can avoid the overlap by reducing the length of the wall weir by the notch width.

A custom weir, as shown in your screen shot, is also a good way to go.


Peter Smart
HydroCAD Software

http://www.hydrocad.net