I'm re routing my rain gutter under a walkway using a pcv pipe? but sprinklers found sprinklers on both side of the walkway. I've already dug 3 ft. at the downspout side. I still have to measure the pitch to the other end of the drain. Will the water back up with that deep of a slope? I have corrugated drain pipes on the sides of my house installed by landscaper also leading to end of my front lawn and so are my neighbors. Been severe drought in CA. Just the last couple days we had rain that's how I noticed water pooling where the downspout is. Thanks.