This soil was very rocky. We were able to screen some of the material that came out of it and use the larger rocks for drainage rubble. The smaller stuff was used later on for filling the earthbags for the stemwall.
The easiest way to make sure that the trench drains towards the exit is to pour some water into it. This way you will quickly find out where there are still some high spots in your trench.
Now the trench is almost full with rubble. Notice the black landscape cloth that will easily let water through, but will block silt from entering the trench. The longer you trench will stay free from smaller sand and silt, the longer it will drain well.
The rubble trench gets filled up to the top and then you can start to built the stem wall.