My backyard sits lower than my neighbors to either side and is bordered in the back by a parking lot that does not have any drains. So when it rains my yard turns into a swamp and it usually takes several days to dry up. I have four oak trees bordering the back of the yard with a few roots above ground which in turn makes a bowl affect. What can I do to make my yard to not hold water and the tools I will need? I want to keep the oak trees. and would like to eventually lay flag stone to extend my patio into the yard. Can you help me turn my yard into a welcoming place?