Hau, joby. I will address these questions in the order you asked them.
You can locate a growing mound on gentle slopes by using the trench method, dig a trench, fill it with the hugel
wood cover with the dirt removed from the trench. These will work well when run on the contour.
The best method for controlling rain runoff is swales and
berms. A
swale is cut on contour, it is a shallow, curved bottom trench, wider than it is deep. The dirt you remove is placed down hill from the swale and forms the berm. On a grade such as you mention, I would make small ditches from one swale to the next alternating ends so the
water would move down hill in a zig-zag motion. Growing mounds (Hugels) can be used as the berms but they usually aren't. Ease of down hill access on a hugel is determined by how tall you make the hugel.
A terrace can be a good space for a growing mound. It mostly depends on your wants and needs. Hugels are very good at holding water for plant
roots, it is what they were designed to do.
When you make a growing mound, the technique used to build it is relevant to the plants you can grow on it. Standing wood on end makes for tighter packed wood and so will hold more water long term, but you don't want to plant really deep rooted plants for at least the first year. Horizontal wood takes more fill than the standing wood type, and the fill should be layered on as you stack the wood up to the height you desire. Both types should be planted with a cover crop as soon as you get the blanket (covering soil) on. It also helps all mounds to be watered as they are built. You can use most any decomposable item you want for the fill layers. I like to have bones and other meat type scrap near the bottom, it makes for less digging by critters at first.
All mounds will need to be "topped off" at least every fall or spring, depending on when you have mulch or
compost ready to put on as a top dressing. Since all mounds will settle, this is just a part of the nature of a growing mound.
If you find that you built a mound lower than you really wanted it, just add to it as you can. They can be finished mounds or works in progress, it's all up to you, the builder. My ancestors sometimes took up to a decade to get a single mound completed. There are also mounds the built as multi purpose mounds, some were simply "garbage mounds" where all the "trash" went to be buried, later on they would be planted and become a growing mound.
