For those of you who have experience with the above, have you ever had an issue with keeping materials (compost, soil, mulch, etc.) in place on the apex of these contours? I sheet mulched my entire front yard recently, which is on a slight slope, so I thought it would be nice to contour the edges along the street and beside my neighbors' driveway, so that materials wouldn't wash away in a heavy rain. I consulted folks who do this professionally about what to do and where. I dug a little trench about two feet from the edge and filled it with half-rotten wood, and flipped the sod to the downhill side. The shaping worked out just as I'd hoped, and I feel confident that it will both conserve rain water and help keep materials in place. The problem is, however, that I sheet mulched over everything, so now materials slide off of the cardboard hills. I'm sure eventually as the cardboard breaks down, this will be less of an issue, but for now it's effectively slippery, even though it's just a small bump of perhaps eight inches.
Must I purchase offsite stone and build a little wall downhill to keep the materials from washing away? Poke hundreds of little sticks into the downhill side to act as a little fence, like people use to maintain sand dunes? I have read about contours and hugelkultur so many times, and I don't recall anyone mentioning this problem. Any tips appreciated!