overflow, the excess flow of storm-water may be diverted by an
arrangement similar to that shown in Fig. 18. [Footnote: PLATE
IV] In this case calculations must be made to ascertain the
depth at which the sewage will flow in the pipes at the time it
is diluted to the required extent; this gives the level of the
lip of the diverting plate. The ordinary sewage flow will pass
steadily along the invert of the sewer under the plate until it
rises up to that height, when the opening becomes a submerged
orifice, and its discharging capacity becomes less than when
the sewage was flowing freely. This restricts the flow of the
sewage, and causes it to head up on the upper side of the
overflow in an endeavour to force through the orifice the same
quantity as is flowing in the sewer, but as it rises the
velocity carries the upper layer of the water forward up the
diverting plate and thence into the storm overflow drain A deep
channel is desirable, so as to govern the direction of flow at
the time the overflow is in action. The diverting trough is
movable, and its height above the invert can be increased
easily, as may be necessary from time to time. With this
arrangement the storm-water can easily be screened before it is
allowed to pass out by fixing an inclined screen in the
position shown in Fig. 18. [Footnote: PLATE IV] It is loose, as
is the trough, and both can be lifted out when it is desired to
have access to the invert of the sewer. The screen is self-
cleansing, as any floating matter which may be washed against
it does not stop on it and reduce its discharging capacity, but
is gradually drawn down by the flow of the sewage towards the
diverting plate under which it will be carried. The heavier
matter in the sewage which flows along the invert will pass
under the plate and be carried through to the outfall works,