FIRST , two disturbing videos
the first below is from Montreal (& not AP) – I’ll guarantee no one has ever considered much less done an energy survey (more E expended in just operating lights than possibly ‘produced’ by the plants, NTM the heating, labor, other inputs and the massive embedded energy content of the facility and equipment –
the second example is AP from London – again just Bunny Breakfast and Slug-Chow.
In both cases I ‘commented, “Stupid fkn monkeys. Lettuce is not food. Unless you are a rodent or a slug … which is likely.”
WRT length of your sands beds, here some factors to incorporate:
1. The top surface (both before forming furrows and in the bottom of the finished furrows) should be level from side to side and also from end to end (and thereby from corner to diagonal corner). Since water ALWAYS seeks ‘its’ own level’, the easiest way to accomplish this is to block the drainage outlet, flood the beds to saturation and slightly more, and then adjust the sand (and furrow) surfaces to be consistent with the water level. Note, all sand will settle some what during the first one to three saturation/drainage cycles. Some sand more so than others and not necessarily uniformly along the length and/or width. Therefore, it is suggested that one allow the bulk of this settling to occur before finalizing the surface preparations.
2. Because the top surface is level but the bottom of the bed is sloped down toward the outlet (drain) end, then the longer the bed length is, the greater the change of sand depth will be from one end to the other. The 30 to 33 cm depth suggestion is meant to reflect average depth at the shallow (uphill) end. The longer the bed is, the deeper the mean depth will become at the drainage (downhill) end.
3. To accomplish complete drainage of the bed (strongly advised), the longer the bed length is, then the more drainage assistance in the form of a longitudinal drainage pipe/tile is advised. As long as the bottom plane is sloped toward the exit, then beds of 3 to 4 meters in length may not require (but may still benefit from) drainage lines. Beds longer than 4 meters will probably benefit from drainage assisancet. Beds of 6 or more meters in length WILL require drainage assist provided by drain pipe/tile.
4. Beds of from 1 to 1.5 meter in width would require a single central drainage line. If beds are wider than this (for whatever reason you resist our recommendation), then two or more parallel drainage lines spaced approximately 2 meters apart (as many as needed) is suggested.
5. When using drainage lines, these need (do) not penetrate the end wall or cap of the bed. Merely terminate the lines into a small mound of pea gravel covering the drain line exit and also covering the the bottom slit (exit) of the drain end wall. This pea gravel covers the slit that is cut into the liner at the bottom of the end wall (NOT cut into the bottom plane of the bed).