How reliable are inlet (hydraulic) models at representing stormwater flow?

It is common practice to base storm drainage network designs on gutter inlet capture information obtained from tests on hydraulic scale models. Growing interest in collection of gross pollution at inlets (‘source control’) is leading researchers to re-visit this domain of hydraulics and to question the reliability of scale models. Results from 0.4-scale and full-size models of a gutter inlet are compared in a laboratory study. Significant differences (up to 40%) were observed in flow captured, the small-scale models showing the lower values. This has important implications for pit spacing and the economics of drainage networks. Performance variation is considered to be the consequence of differences in zones of full and transition turbulence in the two model systems. This explanation is supported by differences being lower for high-flow conditions. These findings sound a warning to those using scale hydraulic models to seek solutions to the problem of ‘at source’ containment of gross pollution.