Creature pit: retracting bridge that they are dumped onto and another above it to prevent fling-out before dumping the payload into the real pit...
Or just use a floor grate, which just drops items without flinging them. Not sure about hatch covers, but I think they're the same way.
Also, how are you getting the stuff onto the bridges? I'd like to try this myself.
For my experiments I constructed floors over the bridges, then designated stockpiles on those floors. When objects were placed in the stockpiles I designated them forbidden. Once the stockpiles were full I cleared the stockpile designation (leaving the forbidden objects) then carefully deconstructed the floors, causing the contents of the stockpiles and the materials the floors were made from to drop onto the bridges.
There's a much easier way, just designate a garbage zone over a hole in the ceiling above the bridge and dump the items in question.
I figured out a way to make a retracting bridge conveyor, same general principle as the hall full of retracting bridges, but each bridge is a z level lower than the previous, so it goes downhill.
Side view:
# - wall
_ - floor
D - door
B - retracting bridge, all hooked to the same repeater/lever
= - floor grate (keeps things from being thrown back into the original room)
_<_=_________________
#xDB #############
#x_DB ############
#x__DB ###########
#x___DB ##########
#x____DB #########
Goes MUCH faster if the hallway is more than one tile wide, because an item is flung in a randomly chosen direction, and if there's a wall in the way it simply won't move.
representative level, top down:
#########
####X####
####D####
##BBBBB##
## ##
#########
You can still have the input be a single tile pit, though, like this:
First four levels, including access floor:
a - lever linked to all the bridges
b - lever linked to safety grate
To use, drop the stones on the grate down onto the 1x1 bridge by opening the grate, if you have not already done so, and then close the grate. Then put lever 'a' on repeat for a while. Alternately, instead of a grate, just drop the objects more than 3 z levels (actually, I think a single extra level would do it), and they won't come back out, allowing you to add more cargo while it's running.
z0
#########
#+++>+++#
#+++++++#
#+++=+++#
#++a+b++#
#########
z-1
#########
####X####
####D####
####B####
### ###
#########
z-2
#########
####X####
####+####
####D####
###BBB###
## ##
#########
z-3
#########
####X####
####+####
####+####
####D####
##BBBBB##
## ##
#########
You can also make the bridges more than one tile long, which decreases the speed of object flow a bit, but moves it further horizontally for every z level dropped.
I leave the first level one tile long because it's narrow, and already tends to hang on to stuff for a while.
z0
#########
#+++>+++#
#+++++++#
#+++=+++#
#++a+b++#
#########
z-1
#########
####X####
####D####
####B####
### ###
#########
z-2
#########
####X####
####+####
####D####
###BBB###
###BBB###
## ##
#########
z-3
#########
####X####
####+####
####+####
####+####
####D####
##BBBBB##
##BBBBB##
## ##
#########
I haven't had time to try bridges 3 or more tiles long, but I think it would work, at reduced speed.
Because cargo is thrown down a z level, animals that end up in this may sustain injuries. Throwing my livestock down a 10 level one got lots of grey wounds, though many were completely uninjured, with a red spine wound on a cavy.
Also, you should lock the access doors. While d-b-d-ing some stone in my food stockpiles, and accidentally marked some booze barrels. One of my dwarves came in an access door to have a drink, and while he wasn't injured or even thrown down a level (just stunned), he did hold the door open long enough for it to get some rocks thrown into it, trapping it open. From then on, rocks continued to get leak out that door instead of continuing down.
Objects do sometimes get thrown back up a z level, but the general trend in the flow is forwards. I originally designed it with alternating bridges hooked to different activators, and floor hatches between each level (also hooked to alternating activators), so that no back flow could take place, but it turned out to be unnecessary and a waste of time/mechanisms, and made it quite difficult to automate. I still think there is some merit to that set up, as I suspect you could run it in reverse to get a (slow, prone to backflow) escalator.
As someone said earlier, in this sort of system you can not predict how long it will take any given object to come out the other end, but if you add plenty, and keep adding more, you should get a more or less continuous flow, though it takes a long time to empty once you stop adding cargo.
Also, note that these bridges are retracting, not raising. Raising bridges would destroy pretty much all the stuff you want moved.
Ultimately, this will pretty much never be less work than just hauling whatever it is that needs moving, and thus is just a stupid dwarf trick.