Runoff through property line wall

[RTson]

I'm a civil engineer working on a residential project. The property is in an old area, so the property drains from the street at the front to the back yard. There is an existing 8' wide by 12" high opening in the block wall at the rear property corner for drainage.

The client will be demolishing the existing home and building a larger home, which results in more runoff generated by the site, so we need to restrict the outlet to ensure we do not increase the peak runoff going through the wall opening.

Does anyone know how to model flow through a rectangular opening at the base of a block wall? Is it a broad crested weir? Or just look at it as an open channel? If we look at it that way, flow depth is about 2-3 inches, which is well below the top of the opening. But the geometry doesn't seem like the kinds of weirs you see in textbooks.

 

[MintJulep]

So you're building a dam?

 

[bimr]

Don't understand why you need to model. You can measure the flow with a bucket and stop watch.

Replacing the house on this property may increase the flow to the backyards of the properties with lower elevations. How will you resolve that?

 

[psmart]

Remember that if you restrict the flow, you will have to detain the excess volume. You are essentially creating a detention pond. Do you have the space/volume to accommodate the excess? The required volume could be considerable. At some point it may also overflow (in undesirable directions) around the sides of the wall.