Considerable wear takes place on the
ramp, which should, therefore, be constructed of blue
Staffordshire or other hard bricks. The ramp should terminate
in a stone block to resist the impact of the falling water, and
the stones which may be brought with it, which would crack
stoneware pipes if such were used.
In cases where it is not convenient to arrange a sudden drop in
the invert of the sewer as is required for a leap weir
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.
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