This is done by means of sluices
having self-acting valves, (or tide-gates,) opening outward, which will be
closed by the weight of the water when the tide rises against them, being
opened again by the pressure of the water from within, as soon the tide
falls below the level of the water inside of the bank.
The gates and sluices may be of wood or iron,--square or round. The best
would be galvanized iron pipes and valves; but a square wooden trunk,
closed with a heavy oak gate that fits closely against its outer end, and
moves freely on its hinges, will answer capitally well, if carefully and
strongly made. If the gate is of wood, it will be well to have it lie in a
slightly slanting position, so that its own weight will tend to keep it
closed when the tide first commences to rise above the floor, and might
trickle in, before it had acquired sufficient head to press the gate
against the end of the trunk.
As this outlet has to remove, in a short time, all of the water that is
delivered by the drains and ditches during several hours, it should, of
course, be considerably larger than would be required for a constantly
flowing drain from the same area; but the immense gates,--large enough for
a canal lock,--which are sometimes used for the drainage of a few acres of
marsh, are absurd. Not only are they useless, they are really
objectionable, inasmuch as the greater extent of their joints increases
the risk of leakage at the time of high water.
The channel for the outflow of the water may sometimes, with advantage, be
open to the top of the dyke or dam,--a canal instead of a trunk; but this
is rarely the better plan, and is only admissible where the discharge is
into a river or small bay, too small for the formation of high waves, as
these would be best received on the face of a well sodded, sloping bank.
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