Many enclosed former estuaries experience water quality problems. These problems are related to stratification, which inhibits the downward mixing of dissolved oxygen to the deep former estuarine channels and pits, leading to hypoxia. Former estuaries like Lake Veere and Grevelingenmeer have also seen events of massive fish mortality, presumably related to the wind-driven upwelling of deep water. To unravel these dynamics, we require more insight into the spatiotemporal behaviour of the stratification.

Stratification is currently measured through fortnightly temperature-salinity-oxygen transects with measurement points every few kilometres. These often do not capture all relevant dynamics, such as mixing and internal oscillations of isopycnals (e.g., tilting, seiching). Echosounding, which overcomes these issues, has been used extensively in the ocean to detect stratification but has seen limited use in lakes, where scatterers are often scarce. Impedance gradients by stratification are an established source of backscatter but are often overshadowed by other potential scatterers (i.e., air bubbles, biota, turbulence). The current generation of acoustic Doppler current profilers (ADCPs) allows for multi-frequency echosounding in combination with the usual velocity measurements, providing new opportunities to acoustically monitor stratification by covering a large frequency bandwidth.

In this study, we aimed to continuously monitor stratification and infer internal processes through narrowband multi-frequency echosounding. In the summer of 2024, we collected measurements with two up-looking ADCPs, three underwater moorings equipped with thermistor chains, and a high-resolution CTD casting instrument in the northwest of Lake Veere. The ADCPs measured backscatter at three frequencies: 250, 500, and 1000 kHz, a wide bandwidth that we expected to facilitate the distinction between backscatter mechanisms at a high spatial resolution. We compared direct temperature measurements by moorings, high-resolution conductivity-temperature-depth (CTD) casts, and frequency-dependent acoustic backscatter to infer backscatter mechanisms. We then used the acoustic backscatter to monitor stratification by tracking (gradient) maxima and compared acoustically derived thermocline heights to directly inferred thermocline heights from the temperature moorings. Finally, we used the monitored stratification to characterize internal seiching by combining data from several locations. To that end, we analysed the time series of thermocline heights using the continuous wavelet and wavelet coherence transforms to identify periods of internal seiching and validated those with current profiles measured by the ADCPs.

We observed stratification of the upper water column in the acoustic data during extended warm periods. The upper and lower layers were separated by a gradient in backscatter, with increased backscatter in the upper water column. This gradient coincided with the directly measured main thermocline, demonstrating the potential of monitoring the thermocline height through acoustic backscatter. We hypothesize that the large backscatter gradient (as opposed to a local backscatter maximum) was caused by a difference in phytoplankton concentration, which remained above the thermocline through buoyancy and bloomed during extended warm periods (end of July and August). Consequently, stratification was more visible in the acoustic data during these warm periods, despite not necessarily being strongest at those times. We did not observe impedance gradients due to stratification; instead, we attribute local backscatter maxima to suspended matter aggregating on isopycnals. As such, we continuously observed a secondary layer near the bed throughout June and July.

We computed phase velocities of internal seiching using CTD casts and estimated the internal seiching bandwidth between 1–10 hours. We found events of cross-lake and along-lake internal seiching—with respective periods of 4 and 6 hours—in thermocline heights through increased wavelet coherence and out-of-phase behaviour between measurement locations on opposing sides of the lake. Additionally, most thermocline height time series sporadically contained a daily component, which was likely forced by wind rather than heat input. The secondary layer near the bottom contained both a daily and semi-daily component, with the latter likely related to tidal forcing. We validated the observed internal seiching using velocity data to confirm that velocities above and below the thermocline were out-of-phase (i.e., reversed). Finally, we found that velocity shear was not a consistent indicator of thermocline height, but was often either concentrated at or bounded by the thermocline, thus providing a general measure of the average thermocline height.
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This report details the development of a Flood Early Warning System (FEWS) for the Tana Basin in Kenya, executed by a multidisciplinary team from the Delft University of Technology. Recognizing the Tana Basin’s vulnerability to flood risks, exacerbated by climatic variability, limited funds, and limited available data, the project proposes a model that combines computationally efficient hydrological and hydrodynamic modelling with robust stakeholder collaboration. The study area comprises the entire Tana Basin, with a specific focus on the flood-prone area near Garissa used for validation. The FEWS developed incorporates local and scientifi-cally derived knowledge to forecast floods, aiming to aid the transition from a technologically intermediate to a technologically advanced FEWS. Through an iterative process of model selection, validation, and stakeholder feedback, the system attempts to integrate the GR4J hydrological model in SuperflexPy and combines this with the Super Fast INundation of CoastS (SFINCS) model. Data sources include global remote sensing datasets like FABDEM & CHIRPS. Furthermore, it uses the water level gauge data provided by the Water Resource Authority of Kenya, as well as TAHMO weather station data.

The report concludes by reflecting on the modelling techniques for both the hydrological and hydrodynamic models and provides recommendations for the further development of a FEWS in the Tana Basin in Kenya. The implementation of the hydrological model was not able to propagate external flows through the network, making it poorly suited for use in the Tana Basin. The hydrodynamic model works decently well in flood conditions but overpredicts flooding during regular flow conditions. Recommendations on stakeholder engagements and data-sharing practices to foster a resilient flood management system in the Tana Basin include more comprehensive Memoranda of Understanding (MoU) and stricter adherence to the Disaster Risk Management Framework of the United Nations.

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Two control structures in the form of compound weirs were built near Pannerden and in the Hondsbroeksche Pleij. These compound weirs consist of multiple adjacent gates with individually configurable weir heights. To make optimal use of the flexibility of the structures, research must be done into how different compound weir configurations affect the stage-discharge relationship. Both perfect and imperfect flow are researched.
This thesis has the following research question: “How is the flow over a compound weir affected by the configuration of the individual weirs?”
The research on imperfect weirs was caried out by constructing two analytical models based on a combination of momentum- and energy conservation, and a combination of Carnot losses and energy conservation. This was done for two cases, one case in which the equations were solved explicitly by using the average weir height, and the second case in which the equations were solved by solving a system of equations (one for each gate). The research on perfect weirs was caried out by making use of the Rehbock formulation, again for both the individual weir heights and the average weir height. To validate the models and to research interaction between adjacent gates, experiments were carried out in a 3-meter-wide flume with a scale model of the control structure near Pannerden.
In the experiments it was found that in the case of perfect weir flow, the discharge coefficients hardly change for different configurations with the same average weir height. The recorded maximum change was 1.9%. The best performing model for perfect flow was the model based on the average weir height. In the case of imperfect flow, it was found that the discharge coefficient can vary up to 13%, so the configuration influences the discharge coefficient significantly. No unambiguous results were found on how certain configurations affect the discharge coefficient. The models for imperfect flow performed well for an average weir height of 10 cm, but not for an average weir height of 7.5 cm. The least performing model was the model based on individual gates with the momentum equation.
With PIV (Particle Image Velocimetry) it was found that lateral flow starts at a greater distance from the weir if the lateral travelling distance is bigger. Furthermore, the streamlines hit the weir at an increased angle if the transition between high and low weir heights was sudden instead of gradual.
Further research should focus on adding more forms of energy head loss to the analytical model. Focus should be on one specific average weir height and more data should be collected on fewer configurations to get more insight into the behaviour caused by specific properties of a configuration.
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