I was watching this [https://www.youtube.com/watch?v=YkR79oDAgOg&t=409s&ab_channel=PracticalEngineering](https://www.youtube.com/watch?v=YkR79oDAgOg&t=409s&ab_channel=PracticalEngineering) video which displays at 2:06 the equation for flowrate over a wier.
At 3:33 he mentions that the most efficient shape is to match the curve of the water, that being a ogee crested weir. I dont quite understand how this works, is air resistance greater than the friction caused by an ogee crested weir?
at 5:26 he shows how increasing the length of a wier also increases flow rate. My initial thought is that the flow upsteam should be the flow downstream (assuming no friction) so i don’t quite understand why increasing the area of which the water can fall over the crest affects it.