Simple flowmeter device for LID systems: From laboratory procedure to full-scale implementation

Low impact development (LID) systems provide sustainable solutions for flood risk mitigation. Given their relevant role in urban water management, accurate estimation of the outflow rate of such systems is crucial to assess their retention efficiency and the corresponding volumes discharged into the drainage network. Thus, the selection of an appropriate flowmeter device is necessary. Thus far, various flowmeter devices have been designed and calibrated for pipes, open channels, rivers, and irrigation systems, while only scarce and general information has been presented for the devices used in LID systems. The main objective of this study is to propose a new, simple, and easily replicable flowmeter device, which can be positioned in confined spaces, such as drain wells, for measuring a large outflow rate range of LID systems. To identify the most influential parameters on the discharge coefficient, the evolutionary polynomial regression multi-objective and multi-case strategy approaches were used for data mining. The results demonstrate that the ratio between the upstream head (h) on the weir crest and the weir width (b), namely the (h/d) parameter, significantly affects the discharge coefficient. Therefore, neglecting the Reynolds and Weber condition and h/d coefficient (where d is the weir height), a simple and accurate relationship of the discharge coefficient was obtained. Finally, to implement the laboratory findings to a full-scale green roof, the runoff collected by the developed device for a continuous period of three months was analysed. The findings demonstrate that the plotted hydrographs do not exhibit fluctuations and effectively interpret the small flow, at both the continuous and event scales.