Hydraulic characteristics of siphonic roof drainage outlets

Rains, JP

Abstract

Siphonic roof drainage uses an innovative hydraulic design to increase the efficiency of
rainwater disposal. The roof outlet is a critical component of the system and not only
promotes and sustains priming of the system but also controls the depth of approach
flow. This thesis investigates the hydraulic characteristics of an idealised roof outlet to
improve outlet design.
A literature survey was carried out and found that only a limited number of studies have
been undertaken on single outlets on flat roofs. Therefore, the search was widened to
include related subjects and the information was used to develop a programme of
experimental testing.
An idealised outlet was tested within a physical hydraulic model across a range of
geometries to observe and measure the water flow rate, depth, pressure and the air flow.
The relationships between the key parameters are presented and compared with the
theoretical values. The study confirmed that a baffle plate is essential for stable priming
of the outlet and that the idealised geometry is sufficient to provide a stable and efficient
operation of the system. The maximum typical air flow rate during the priming sequence
was approximately a third of the water flow rate and occurred at less than half of the
maximum water flow rate. The outlet should be designed to ensure rapid and stable
priming by creating a turbulent, well-mixed flow at the roof outlet. The depth of
approach flow can be reduced by increasing the diameter of the baffle plate within
practical limits and by reducing the height of the baffle.
An empirical relationship was developed between the water radial velocity and the
pulsing frequency of entrained air and is presented in the form of a modified Strouhal
number. This relationship could be used to develop a non-intrusive flow measurement
system in the air-regulated regime of operation.
Tests were conducted on a small range of commercial outlets and the results compared
well with those from the idealised outlet. The conclusions drawn from the idealised
outlet would thus be useful in the development of commercial outlets.

• Thesis Type: Thesis
• Deposit Date: Aug 19, 2021
• Additional Information: Funders : Engineering and Physical Sciences Research Council (EPSRC)
• Award Date: Jan 1, 2008