(last checked by me 2021)
The main channel has training walls (also called revetments) which are blocks of rock/concrete which help to keep the tide flowing along the channel so maintaining depth. These are charted. In my experience they dry by approximately 2.5 metres at CD (lowest astronomical tide). Note that one wreck (the PEGU - charted just north of C6 red buoy and east of the training wall) does dry much more than 2.5 metres - see image here and more detail in Wrecks of Liverpool Bay Vol II.
The ALPHA buoy (red buoy) between Q12 and C2 marks the transition
from the Queen's Channel to the Crosby Channel in the main shipping
channel.
I use it as a convenient mark for a short cut going to/coming
from the north.
You will surprise the big ship navigators but save a few miles.
You are asked by Mersey VTS (channel 12) to inform them, if crossing the
training walls, - so they know that it is intentional.
Going north from ALPHA, you first cross the training wall (depth about 2.5 metres above Chart Datum) - the echo sounder shows quite a localised peak here. Then you cross the west end of Taylor's Bank which has a similar least depth. So at half tide (5.2 metres above CD), you have at least 2.5 metres depth on this short cut.
For a sonar image of crossing the wall, see here.
Depth (as metres above CD) from my survey (distances in metres near ALPHA buoy).
(last checked by me 2021)