I live in California, in an HOA. My home is next to a drainage ditch. The water channels under the street I live on were plugged up in 2004, when a big rainstorm hit us in Victorville. I was flooded in the yard because a wall of water came over the street and reentered the drainage ditch from my property. A street facing my property was the second source of water, that flooded my home itself. Water flowed down my driveway, into my garage, creating a pool of water 3' deep in there, and 5" inside the home. You have to take into consideration the way the water drainage system is maintained, and other factors such as other sources of water flow. Pray that the retaining wall is higher than the water flow level in a storm.