I have a problem wih my gutters overflowing and I wonder if it is because of the weight of water in the two sides of the downpipes. Essentially I have 90mm downpipes which go underground into 100mm pipes which then come back above ground and empty into a water tank. I am finding that my downpipes are overflowing and at the same time the water is only flowing slowly into the tank. The top of the tank is about 1.5m below the top of the downpipes. So the question is if you have a U shaped pipe where each side is a different diameter will the level of water in both side be the same or does the weight of water in the larger side slightly overcome the smaller?