The idea of making a small hole on the flapper of the back flow check valve is an excellent and economical idea. Most people disagree by making different assumptions. That is, the time that is needed to drain the water held in the riser pipe when it is busy pumping water, compared to the time needed to completely drain the pipe when it stops working. When the pump is busy pumping water such as in a heavy rain, it stops (when the sump is emptied) and resumes (the sump is refilled) pumping in fast cycles. So, when it temporarily stops while the sump is being refilled, only a small amount of water sink down into the sump through that small hole. This does not really reduce the pump efficiency. After the last cycle of pumping (rain stops), there will be long time to allow the water to completely sink down into the sump, emptying the pipe. If the pump works non-stop, then the issue becomes irrelevant. The only 2nd thing that is needed is to pitch the 'horizontal' segment of the discharge pipe after the riser to the exterior. So in the winter the entire pipe stays empty, eliminating the possibility of being frozen. The other ways of preventing discharge pipes freeze are more complicated, unnecessarily and cost more. - a non-professional opinion. Since my house had this issue, I thought about this way. But the local code is somewhat different (Minnesota). I wish this way be adopted as the code to make the job easy and less costly.