I have installed a retaining wall alongside of an existing wall near to a boundary fence. The tradie suggest to fill them with drainage gravel and it’s good to go. I wanted to ask the Workshop community here if there is a cheaper option to fill the gap Or any other ideas that I could use?
It is 1meter high and 12 m long with a 12cm gap to fill. I intend to lay turf over it.
Here is a glimpse of it... Apologies for the sad state of my lawn.


Another option is, if leave it as it is and put a drainage channel on the top and put weed at and turf of top? Will that be enough?
Thanks in advance!