What part am I looking for when I want to have a switch on the outside of my sump pump drainage pipe?

I live in Northern IL, Chicago area, and I am planning on running my sump pump drainage pipe, currently comes right out of house, underground but I am not going the 42 inches to get under the frost line, I will only go 18-24 inches underground. Reason being, I have a smaller yard and I need to have the pipe come out into the yard at some point to drain the excess that does not drain from the holes in the underground pipe. To alleviate the problem of pipes freezing and backing up, I am looking for a switch that I can put on the sump pump pipe that will allow me to drain either into the new pipe underground, during summer, and when switched over, during winter, it will drain straight out of the pipe onto the ground like it currently drains. Does anyone know if this exists and where I might be able to find it. I am sure it is available at the local hardware store, but would much rather know what I am looking for going in. Don't want to be talked into doing something different by their "expert" sales staff. A link to the part or a link to a picture of the part would be superb! Thanks in advance.