These approaches are designed to supplement the 1500 hydrometric stations currently operational in the UK (Marsh and Hannaford, 2008). There is a drive for greater ease of use of this kind of technology, being spurred on by the Internet of things approach (IoT) to create an easy-to-use framework that everyone can contribute to (Moreno et al., 2019). The methods by which rivers are monitored are ever evolving with new techniques such as particle image velocimetry (PIV) and acoustic Doppler current profiling (ADCP) measuring flow velocity, and ultrasonic depth meters (UDM) measuring stage (Kruger et al., 2016 Muste et al., 2004, 2008). Hydrometry-the measuring of components of the hydrological system such as river flow characteristics-is crucial in flood mitigation strategy and monitoring (Chacon-Hurtado et al., 2017). By demonstrating that sound can operate as a hydrometric tool, we suggest that sound monitoring could be used to provide cost-effective monitoring devices, either to detect relative change in a river or, after more research, a reliable stage measurement. Future work in examining the role of these roughness elements is required to understand the full extent of this technique. We introduce a filter that is capable of isolating a river's sound from other environmental sound. Our analysis has shown a positive relationship between an R 2 of 0.73 and 0.99 in all rivers, but requires careful site selection and data processing to achieve the best results. We use data from those sites collected during storms Ciara and Dennis to produce a relationship between this sound and river stage. On the basis of these findings, we collect audio recordings from six sites around the northeast of England, covering a range of flow conditions and different roughness elements, since 2019. We found that sound is controlled by roughness elements in the channel, such as a boulder or weir, which influences the sound produced. Data collected along a 500 m length of the River Washburn during July 2019 is used to determine what makes a site suitable for sound monitoring. We present a method to determine site suitability and the process of how to record and analyse sound. Previous published work has suggested that there might be a relationship between sound and river stage, but the analysis has been restricted to a narrow range of flow conditions and river morphologies. Measuring sub-aerial sound could provide a new, efficient way to continuously monitor river stage, without the need for in-stream infrastructure. The passive, ambient sound above the water from a river has previously untapped potential for determining flow characteristics such as stage.
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