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The Geul is a 60 km long stream. Its source is at about 350 m above mean sea level. To forecast flooding and to assess the effect of measures, it is essential to monitor both the discharge peak and the duration of elevated discharge. Especially during high-water conditions, discharge cannot be monitored accurately at existing monitoring stations. A challenge is that the Geul is morphodynamically active. During high-water the bed level may change substantially in parts of the Geul, making point-based discharge measurements within cross sections complicated. Additionally, the valley inundates at higher discharges, which complicates monitoring the discharge over the full width of the inundated valley. Finally, part of the Geul valley has protected Natura2000 status, which means that restrictions apply for interventions in the stream bed and maintenance. Waterschap Limburg aims to implement or improve a measurement station where discharge can be monitored continuously, even under extreme high-water conditions. In addition to the requirements for the mentioned challenges, it is necessary to limit the error in discharge measurement. We conclude that a solution that meets all requirements is not feasible. However, we propose two improvements for existing monitoring stations: 1 At Sippenaeken, just 800 m upstream of the Dutch-Belgian border, we propose deploying flow velocity meters at two levels under the bridge. By applying the index-velocity method using the flow velocity measurements, the uncertainty during medium to high discharge conditions will be reduced substantially with respect to the discharge that is currently obtained from a stage-discharge relation.
2 At Hommerich, we recommend adding a radar surface flow velocity measurement to the existing station. This provides reference measurements for the stage-discharge relation during high discharge conditions, verifies camera-derived flow velocity, and operates reliably day and night, enhancing measurement continuity.
For each solution, one should perform additional on-site discharge measurements from a boat at a regular basis, particularly during higher discharge events. Furthermore, station maintenance and data validation are required on a regular basis.