effective storm overflows is a matter of supreme importance.
Not only is it necessary for them to be constructed in well-
considered positions, but they must be effective in action. A
weir constructed along one side of a manhole and parallel to
the sewer is rarely efficient, as in times of storm the liquid
in the sewer travels at a considerable velocity, and the
greater portion of it, which should be diverted, rushes past
the weir and continues to flow in the sewer; and if, as is
frequently the case, it is desirable that the overflowing
liquid should be screened, and vertical bars are fixed on the
weir for the purpose, they block the outlet and render the
overflow practically useless.

Leap weir overflows are theoretically most suitable for
separating the excess flow during times of storm, but in
practice they rarely prove satisfactory. This is not the fault
of the system, but is, in the majority of the cases, if not
all, due to defective designing. The general arrangement of a
leap weir overflow is shown in Fig. 17. In normal circumstances
the sewage flowing along the pipe A falls down the ramp, and
thence along the sewer B; when the flow is increased during
storms the sewage from A shoots out from the end of the pipe
into the trough C, and thence along the storm-water sewer D. In
order that it should be effective the first step is to
ascertain accurately the gradient of the sewer above the
proposed overflow, then, the size being known, it is easy to
calculate the velocity of flow for the varying depths of sewage
corresponding with minimum flow, average dry weather flow,
maximum dry weather flow, and six times the dry weather flow.
The natural curve which the sewage would follow in its downward