How to maintain moisture and have good drainage in a elevated, raised veg garden?
How to Prepare a Elevated Raised Veg Garden
Certified GKH Gardening Expert
It is always a good idea to add stones, whether it be gravel, or things like GrowStone (which is puffed recycled silica glass) to the bottom 3 or 4 inches before you add a rich, organic soil with compost. This will ensure that your bed maintains proper drainage.
Here is an article for further information: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/best-soil-raised-garden-beds.htm