Like others have said, bigger is better, a single 24" will flow more water than two 12's. 12" = 113 sq. in, 24" = 452 sq in.

The county had that problem here on one of their roads. They dug the culverts out. formed a big rectangle out of concrete for a culvert and put the road back over the top of it.

Also, what the culvert is made from will influence flow rate too. Smooth walls on a culvert will flow more than a heavily ribbed wall culvert.

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