Hydraulic interaction between the above and below ground drainage systems via gully inlets

The primary objective of this study was to complete an experimental programme to better understand the hydraulic performance of typical individual types of gully inlets and systems used in practice by analysing the interaction of flow into and from typical gully systems by determining the head-discharge relationship of each system. Therefore, a full scale laboratory system comprising of a testing platform with an inlet tank and an outlet tank on both ends of the platform has been designed to mimic the hydraulic interaction between the above and below ground drainage system via gully inlets and the designated catchment area. Longitudinal slope was later incorporated onto the initially flat testing platform to represent different road conditions. Tests were completed with the flow in one direction to the gulley (intermediate tests) and from both tanks such that the flow to the gulley is in two directions (terminal tests). Surcharged condition was also tested where two flows were released into the system – a primary flow coming from the primary inlet and a secondary flow coming from an alternative inlet straight into the gully system itself. A gully pot manufactured by Milton Precast with a diameter of 375mm and 750mm nominal depth was used for this study and was tested over a range of flowrates of 0 – 50 l/s. Another variable studied was two different longitudinal slopes (SL). Two different types of grates with BS EN 124 loading class of C250 representing different hydraulic characteristics were also used and were tested for a range of surcharged and non-surcharged flow conditions. The interaction – expressed in terms of head-discharge relationship, was determined for the different gully systems and flow conditions tested. Based on the head-discharge relationship, a range of coefficient of discharge, Cd was established. Cd is known to be a function of many parameters and hence to examine how Cd changes, a dimensional analysis approach has been used. This is followed by a review of the application of different types of equations in an attempt to link the dimensionless terms and hence define a universal equation that describes the performance of the system for a range of conditions. This analysis has resulted in a number of significant findings, which have formed the conclusions to the thesis and may be used to inform the way in which these coefficients are represented in water industry standard software.