Gravel depth

[Spreader]

Hi, I've got some tropica plant substrate 10/15mm in depth and I'm wondering how much gravel can be placed on top as regards to depth. I'd like quite a steep slope towards the rear corners of the tank. Also would I need to place any sort of substrate retainers to stop any landslides.
Many thanks

 

[alto]

Tropica videos just place the same 1-2 cm Growth Substrate beneath various gravels & depths of gravel, though there are a few journals where I've seen a second layer of the GS applied in areas of deeply banked gravels ... depending on your gravel type you may have some leaching into the water column
I just went with an even layer of GS.
Substrate retainers depends on your degree of slope & type of gravel (soil) & hardscape.