Compost is way too rich for Semps. in anything but a very small amount as a top dressing.
Tim this is a great thread.
The pot of Purple Dazzler went on a huge journey towards decline this past winter. It was not sitting in full rain so was not getting too wet. I decided to re pot it. When I got all the plants out the soil seemed to have shrunk and when I dumped it out it was like a concrete rock. The plants had almost no roots. But not to worry.....they are replanted in 3 5/8 pots and some in bigger pots and all are now doing well. I wonder if this plant is an especially heavy feeder for a Semp???
All soil in beds and especially containers needs to be replenished at least every 2 years and the native soil where you live Tim is very rich and heavy. Excellent for the Dahlias and nursery crops which grow so well there, but not good for Semps and other alpines. I use a mix of composted bark, coir and pumice. This is a free draining mix that the Semps. and sedums really love. It's close to what they have in their native habitats, (bits of rock, twigs & leaves that get stuck in crevices in the rocks of the Alps etc) Sand is for desert plants not alpines and if you were to add sand to your existing soil I am pretty sure you would have concrete since your natural soil contains some clay.
It would be a huge job to redo your entire riverbed all at once but taking it a bit at a time is do able. Even one plant at a time will get it done eventually and will result in less mess in your yard. Best of luck with your new project.
