Debris is actually what is created when you use the Obstacle Course 'production' run at the Orcs Rally Point. If you hover over the 'goods' it creates you will see the name there
"The Orcs Rally Point is where you will send the Orcs to have fun. Sending them to the Spare Time Activities will produce Armament
, If they join a Wild Party you will have a lot of Orc Dung
to clean up, the Obstacle Course will make the building full of Debris
and after a Raid they will bring a huge Loot
."