River flow is one of the most important and yet challenging hydrological variables to measure accurately. Traditionally, this has required significant resources of people, equipment and infrastructure, all present onsite. In times of flooding, accurate river flow measurements are critically important, but can be particularly difficult and potentially extremely hazardous to obtain.
Furthermore, in an age where the importance of water and the risks it poses have never been greater, many monitoring networks around the world are inadequate, declining, or falling into disrepair.
FluViSat (Fluvial Video from Satellite) is a one-year research project funded by the European Space Agency (ESA) and led by the UK Centre for Ecology & Hydrology (UKCEH) in collaboration with partners at Queensland Government in Australia and Luxembourg-based flood mapping specialists RSS Hydro. It has the potential to enable the accurate determination of flow rates in medium and large rivers anywhere on the planet, with no requirement for people or equipment to be on site, or even on the same continent!
The FluViSat concept combines several significant advances that distinguish it from many other satellite-based streamflow and flood mapping projects:
- Accuracy: The FluViSat methodology provides a direct measurement of the critical parameter of water flow speed, with the potential to greatly improve assessments of river discharge, and the magnitude of floodplain flows.
- Scale: By using very high resolution (sub-metre pixels) imagery, the method is applicable to even the relatively small rivers found in the UK.
- Timeliness: Using the commercial satellite technology, FluViSat can achieve multiple observations in a single day.
Key to achieving these three advances is the collaboration with commercial high-resolution satellite imagery provider, Planet Labs. Its 21-strong SkySat constellation has the potential to collect still or video imagery for any part of the Earth up to seven times a day, with pixel sizes as small as 0.5 metres.
The FluViSat methodology exploits this capability, combined with recently developed techniques for measuring water speeds using digital video imagery.
The basic principles are simple: the speed of visible features on the water’s surface are tracked using computer algorithms, to provide a map of water speed for the full width of the river. As any hydrologist knows, however, you cannot tell the total discharge of a river simply by looking at the speed of the water’s surface. You also need two other key pieces of information: first, the width and depth of the river channel and, second, the average speed of the water throughout the channel at that location. This information can be obtained before or after the FluViSat observation, from site surveys and remote sensing datasets.