Answer: I apologize for the delay in answering your question.
Based on your description, the swale would not be a good location for a vegetable garden. Swales are normally placed to channel rain water and snow melt away from the building and toward an area where it can seep into the ground safely. It works as part of the overall drainage pattern devised for a given area and is part of a system -- for example, the runoff from the paved areas and lawns and roofs all has to go somewhere. It would not be wise to block this against the water flow! The size of the swale should have been calculated by a landscape architector other professional to contain the expected runoff in storm situations. If the capacity of the swale is reduced, the water will back up -- all too often in a location where it is not welcome and can be very damaging.
Another caution with regard to retaining walls is that they can collapse if too high and not properly anchored and drained from behind. I would suggest you consult with a professional landscape architect if you want to build terraces in that area. They should be able to tell you if it is feasible and how to do it safely.
Elsewhere, on a slope, it is often not necessary to make specifically raised beds as the slope provides excellent drainage in and of itself. The soil can be improved and a series of terraces developed however to make the garden perform better and to avoid erosion problems.
If possible, you might want to place your garden elsewhere if you have a spot that is more suitable.
In regard to raised beds, they can be built as simply or as complex as you have the skill and materials to build. In general a small rise of about a foot can be very effective in allowing you to create a good growing bed. The width is determined by your reach -- four feet is a typical width for this reason, it also happens to make good use of a standard length of lumber. You can also use rocks or even build the beds with bare soil sides rather than in a container. The depth of the bed is often based on the amount of material available to fill it, since the cubic yardage is rather daunting if there is a large square footage involved. Generally a mix of native soil and compost or other organic materials would be recommended. Additional amendments (eg nutrients or lime) would be added according to the results of basic soil tests.
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