I'm currently digging out and levelling the area for my pool. The back yard has a slight grade away from the house such that the high side is about 12" - 14" higher than the low side (which is at grade). I'm leaving a 2' walkway around the pool. When I build the retaining wall, do I need to install a drain behind it or can I leave that step out in this case? Thinking that, since it's such a low wall and it's only going halfway around the pool, is it really necessary?