Narrowboat/Canal Boat Hire for Barge Holidays at Mercia Marina, Staffordshire, UK 

Locks: Innovation, form and function

The problems facing the engineers designing the first waterways were:

  • Conserving water
  • Restricting currents within safe limits
  • Maintaining depth of water

Prior to the invention of locks water depth could only be
maintained by the use of dams, which would then have to be partially
broken to allow a boat passage, either with or against a strong
current, which would have been dangerous and hard work. 
The Pound Lock
 
The pound lock in many modified forms is the basic solution used to
overcome these problems.The lock consists of a central chamber not
much longer than the size of the largest boat to be moved, with gates
at each end to control the water. Each gate has paddles which can be
lifted to allow the lock to be filled or emptied as required. 
The usual staring position for using a lock is with both gates closed
and all paddles closed. At this stage not water is passing through the
lock and the level of the water in the lock is stable.
 The water level in the chamber then has to be adjusted to match the
level of the canal the boat is floating on prior to entering the lock.
If the boat is going downstream this means filling the chamber by
lifting paddles in the top (upstream gate). In this instance, with the
chamber full, the water level will now match that of the canal filling
the lock, with the lower gates now holding back a considerable amount
of pressure to maintain the chambers water level above that of the
lower canal level.
The top gates however can be easily opened to allow the boat to enter
the chamber as the water levels either side are balanced.
With the boat now in the chamber the top gate is now shut and its
paddles closed.
The water level in the chamber now has to be reduced to that of the
lower canal by opening paddles in the lower gates which drain the
chamber.
Once water levels are balanced either side of the lower gates the
gates can be opened to allow the boat to leave the chamber.
At this stage the top gates are maintaining the level of the top
canal, thus controlling current, maintaining water depth and
conserving water. 
A boat moving upstream reverses the process. 
Usually excess water in the higher canal is drained off by weirs
create a stream that bypasses the lock for water that would otherwise
cause the upper canal to flood, though in some cases lock gates are
designed so excess water passes straight over the top of them. 
Other information:
http://dspace.dial.pipex.com/town/square/gd86/locks.htm
http://www.canalhttp://www.pigpen.org.uk/locks.htmjunction.com/canal/l
ock.htm 
http://www.pigpen.org.uk/locks.htm