I have been asked to bid a fairly big job for two retaining walls, one in front & one in back of a a home. The slope is fairly steep & the owner just wants a small, perhaps 2' retaining wall with mulch over the area. With the slope of the area I'm concerned that the mulch will run off in the rain. Is there something I can put under the mulch that will allow the water to run off, and allow the mulch to remain intact? The walls are approxamately 75' long each. Any help from someone more experienced than I will be greatly appreciated.