Development of a low-cost water vehicle for surveying river bed elevation and chemo-physical changes along the river course

Measuring open-channel hydraulics is crucial, for example, for deriving discharges from stage observations, estimating travel times for pollutant plumes, and assessing riverbed dynamics. State-of-the-art surveying approaches are typically conducted along predefined cross-sections of the river course, either manually using a flow meter or with instrumented boats. The latter are technologically advanced platforms equipped with electric propulsion, ADCP sensors, and high-precision RTK-GPS and may cost several tens of thousands of US dollars (or euros). To fill data gaps between cross-sections, surveys often rely on longitudinal boat campaigns, which are generally feasible only in larger streams without hydraulic barriers.

To support water authorities with limited budgets, particularly to survey smaller streams, we developed MONIKA, a low-cost surveying catamaran. In accordance with its acronym, MONIKA is comprised of three primary functions: MO - Monitoring (continuous tracking of water parameters), NI - Navigation (movement along and across the stream), and KA -  Kartography (mapping of the riverbed morphology). The platform is equipped with a castable sonar, GPS, and two CTD (Conductivity-Temperature-Depth) dataloggers. As an additional payload, a commercial high-precision inclination sensor is deployed to monitor the water surface slope. All data-processing steps are implemented in an object-oriented framework within an open-source Python package.

After extensive testing and design optimization, the engine-less boat can be deployed in two operational modes: (1) bank-guided operation using an aluminum rod and snap hook, and (2) free-floating operation in which the boat is retrieved with a net installed at the downstream end of the study reach. The free-floating mode is particularly suited for surveying riverbed slope, as it avoids operator-induced interference with inclination measurements.

As an initial application, MONIKA, was deployed at two sections of the Spree River (Germany) to support the placement of new sampling stations downstream of a river confluence. MONIKA was used to determine the minimum downstream distance required for complete mixing. Future applications will extend this approach to open-channel surveys in small rivers, with a particular emphasis on data-scarce catchments.