Hydraulic grade lines in urban stormwater systems – do they predict surcharge conditions?

Abstract

The 1977 edition of Australian Rainfall and Runoff introduced the concept of Hydraulic Grade Lines (HGLs) as a measure of the surcharge potential for piped or underground urban stormwater drainage systems. This concept was included also in the 1987 edition of Australian Rainfall and Runoff. As a result of these publications, many authorities vested with management of urban stormwater drainage systems use the HGL to locate potential surcharges from their underground stormwater drainage systems. This acceptance of the importance of HGLs for the prediction of surcharge conditions in underground stormwater drainage systems, however, has been without serious consideration of the hydraulics of underground stormwater systems.Presented in this paper is a review of the hydraulics of underground stormwater systems. Of particular interest is the importance of the time variant nature of flows in stormwater systems and how this influences the determination of Hydraulic Grade Lines. Resulting from this review is a delineation of those situations where the use of a Hydraulic Grade Line is inappropriate and unlikely to lead to a more efficient drainage system.