Rainfall data for the design of sewer detention basins

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1 INTRODUCTION
Detention basins in sewer systems are used to reduce the runoff
peak flows downstream of the basins or to control the amount of
overflow of untreated sewage water to the recipient, or to reduce
the amount of pollution in the sewage water
Rainfall data for design of the size of a basin must be selected
to meet the aim of the basin. As a genera 1 rule basins are designed
to store a specified volume of water while sewer pipes are
designed to carry peak flows. The same rainfall data may not be
appropriate to use for the design of basins as for the design of
pipes. Other factors influencing the selection of rainfall data
are if the basin is located off-line or on- ine. Off-line basins
are located separately from the sewer system and the water enters
the basin via, for example, a weir. On-line basins are a part of
the hydraulic flow system where the water is flowing through the
basin and is detained by controling the outflow from the basin
by, for example, a nozzle, a weir, or a pump. This report describes
the selection of rainfall data for design of on-1 ine
basins with an outlet of a nozzle-type located at the bottom. The
permitted maximum outflow and the permitted maximum water depth
in the basin are limited and flooding is not permitted more
quently than stated by the design return period used.
Two different kinds of rainfall data can be used for the design
of detention basins:
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Design storms estimated from Intensity-Duration Frequency
(I-D-F) relationships or from historical rainfall data.
Historical storms or time series generated by statistical
methods.
A design storm is a rainfall which is developed for a certain
design return period, and the flow value which is calculated by
means of the storm is said to obtain the same return period as
the storm.