I recently installed a 15'x 20' paver patio on 4" compacted gravel + 1" sand base with paver sand in the joints and I used treated ground contact 4x4's as a retainer. Because my backyard slopes so much I had to build up the back part of the patio 8" above ground level to keep the grade at 4" for the 15' length of the patio. My problem is that everytime I have a heavy rain the joint sand and even some of the base sand on the last row of pavers on the back and also the low end of the sides erodes (sand runoff). How is the best way to solve this? Please be specific with your answer.