Details

Establishing an ideal discharge measurement station in the Gulp that meets all requirements specified by Waterschap Limburg (WL) is not feasible. Observations by WL have confirmed the challenges posed by seasonal variations in the level of the streambed and vegetation growth, making a stable stage-discharge curve implausible. Nonetheless, our work outlines a conceptual design that provides continuous discharge measurements along a stretch of the Gulp, from the Dorpsstraat to the wooden bridge in Slenaken. While not meeting all the imposed requirements, the conceptual design, using two measuring techniques, may provide a discharge estimate at the Gulp near Slenaken. Spanning approximately 21 kilometres with a catchment area of 46.7 square kilometres, including Belgian plateaus exceeding 300 meters above datum (NAP1) at the drainage divide, the Gulp flows into the Netherlands near Slenaken and converges with the Geul at Gulpen, with a bed level around 91 m NAP. The rapid response to rainfall in its upper reaches can result in flash floods, contrasting with a relatively more delayed response downstream in Gulpen. Both discharge peak and elevated discharge duration are therefore essential to obtain an accurate forecast of the flood extent and its impact in the valley. Existing monitoring stations in the Gulp cannot accurately monitor discharge during extreme conditions, particularly during high-water levels—a critical aspect for effective water management. Consequently, WL seeks to implement or enhance a station in the upstream part of the Gulp capable to measure discharge continuously, even under extreme high-water conditions. Implementation of a discharge station in the Gulp faces several challenges. When the Gulp overflows at higher discharge levels, accurately assessing discharge over the full width in the inundated valley presents a challenge. Additionally, the stream is morphological active. Especially during floods, this leads to significant bed level changes in large parts of the Gulp, complicating point-based discharge measurements within cross sections. A final challenge stems from the protected Natura2000 status of the Gulp valley, which places restrictions on streambed interventions and maintenance. To address these challenges, we have selected a location in Slenaken where the bed level is assumed to be rather stable since 2021. Our conceptual design for a discharge measurement station at this location employs a combination of two measurement techniques for two discharge regimes:
1 Low to medium discharge: Developing a stage-discharge curve for the existing water level sensor and incorporating a camera measurement system to measure surface velocity and to monitor bed profile and vegetation growth. The camera system offers adaptability to seasonal changes, ensuring stage-discharge curve accuracy.
2 Medium to high discharge: Installing a side-looking Doppler instrument under the Dorpsstraat bridge to measure flow velocity continuously and derive discharge using the index-velocity method.
For any 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.