The performance of syphonic rainwater outlets within gutters.

Throughout Europe, roof areas are commonly drained using a conventional gutter and
downspout system. These are usually large in volume and have the capacity to
discharge rainwater at high rates of flow. There is now increased awareness of
syphonic roof drainage systems. Historically, the definitive method for the design of
gutters within the United Kingdom is BS6367: 1983 British Code of Practice for the
Drainage of Roofs and Paved Areas. This publication clearly sets out the theoretical
model to predict the hydraulic performance of a gutter. In 2000, this Code was
superseded by BS EN 12056-3 which, shares a common theoretical basis and method
of derivation as BS 6367:1983. These codes do not specifically address syphonic
systems and currently there are no design criteria for such systems.
Hence, there is an urgent need for a better understanding of the hydraulic performance
of syphonic systems. This is particularly relevant to systems that are installed in
gutters that drain large industrial and commercial buildings.
The work reported in the thesis describes a series of experimental investigations that
were carried out to improve knowledge and understanding of the way in which
syphonic systems perform. Initially the study concentrated on the construction of a
full-scale experimental system to test the hydraulic performance of syphonic system
outlets located within a 600mm wide gutter. Tests were completed with single
(primary) outlets and primary outlets in combination with independent overflow
outlets (secondary outlets). The outlets were positioned at a number of different
locations along the length of the gutter and combinations of both primary and
secondary outlets were tested.
The thesis has concluded that the performance of syphonic rainwater systems is much
more complex than conventional roof drainage systems. Specific findings of the study
are:
• The application of existing theoretical models for the design of conventional
rainwater drainage systems should not be transferred to syphonic systems. An
additional factor of safety is required within the existing theoretical model.
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• The position of the outlet in the gutter has a significant influence on the depth
profile along the gutter length. Outlets located near to the gutter end resulted
in an increase in the depth profile of the outlet.
• When an independent overflow system (secondary system) was used in
conjunction with a number of primary outlets within a common gutter, it was
found that the overflow system, dominated the flow profile within the gutter.
• A method, based on dispersion of solutes, was developed to determine the
actual flow rate through each outlet of a syphonic system. It was shown that
the flow rate through each outlet of the system was not the same and that the
water level in the gutter was redistributed along the gutter length. This implies
that the negative pressure created in syphonic systems is not a limiting factor.
These findings have important design implications.
• A methodology to calculate the influence on water depth in any gutter and for
any outlet position has been established and is recommended as a basis for the
improved design of a syphonic system.