Flanders Hydraulics

Design-flood estimation for small fluvial catchments in the Benelux plus Germany region relies predominantly on simplified event-based methods, reflecting limited data availability and the absence of consistent regional guidelines. While such approaches are widely applied in practice, key modelling choices are often unguided, leading to large uncertainty and inconsistent protection levels across regions. This study investigates how basin similarity, catchment storage, and geology can be used to improve the robustness of event-based flood estimation in data-fragmented settings. This study integrates high-resolution observations from a network of experimental and operational basins with hydro-meteorological records. Event-scale hydrological signatures relevant to event-based modelling (time to peak, rising-limb characteristics, peak discharge, and runoff coefficients) are derived and analysed across simple antecedent moisture conditions (AMC) classes. Basin similarity is assessed using a combination of physical descriptors and observed event responses, and basins are grouped into response classes using clustering techniques. Leave-one-basin-out testing is applied to evaluate the transferability of response characteristics within and across classes. A parsimonious event-based rainfall–runoff model, representative of methods commonly used in design-flood studies, is then applied. Model parameters are constrained using empirical ranges derived from observed events. Model experiments systematically vary response-time formulations, AMC assumptions, and parameter sources (regional data, proxy basin, or class-based values) to quantify their influence on simulated peak discharges. Sensitivity and robustness are evaluated across storage and geology classes. Results show that uncertainty in event-based design floods is dominated by response-time and AMC assumptions, and that similarity-based parameter transfer can reduce uncertainty in storage-controlled catchments but performs poorly where storage contrasts are large or data are sparse. The findings provide empirical guidance on when event-based regionalization is best suited and highlight structural limitations of current practice. This work supports the development of more consistent, evidence-based design-flood guidelines for small basins in the Benelux region.

Tidal rivers and estuaries may experience high levels of suspended particulate matter (SPM), which impacts water quality and ecosystem functioning. The processes controlling the development of estuarine turbidity maxima (ETM) are fairly well understood. However, predicting the maximum SPM concentration in an estuary based on aggregated parameters (estuarine dimensions, river discharge, tidal range) remains, up to now, impossible without extensive in-situ measurements and/or numerical models. This study introduces an approach that links the strength of the ETM to the tidal, river, and morphological characteristics of a system. Using in-situ data from contrasting meso- to macro-tidal estuaries, we found a consistent pattern of maximum SPM concentrations within a two-dimensional parameter space. The resulting turbidity diagram reveals a high SPM hotspot in estuaries with specific forcing conditions, corresponding to intermediate relative tidal amplitudes and freshwater Froude numbers. This multi-site research advances our predictions of ETM intensity in tide-dominated estuaries, offering a straightforward method to explore potential turbidity trajectories under various human pressures.

Compacted subsoils affect vegetation growth and soil water balance. Numerical models help quantifying the hydrological impacts of subsoil compaction. These models are useful to evaluate measures that augment groundwater recharge in compacted soils and are important to guide proper water resource management under climate change in these soils. However, vegetation in these models is often parameterized using only limited field measurements or using relations between vegetation parameters and other variables. In this study, we show that uncertainties in vegetation parameters linked to transpiration (leaf area index [LAI]) and water uptake (root depth distribution) can significantly affect modeling outcomes. We used the HYDRUS-1D soil water flow model to simulate the water balance of experimental grass plots on the sandy soil of Belgium’s Campine Region. The compacted case has the compact subsoil at 40- to 55-cm depths while the non-compacted case underwent artificial decompaction. The models for each case were calibrated using soil moisture sensor data at two depths. We calibrated the soil water flow model for the compacted and non-compacted case considering three different vegetation scenarios that represent various reactions of canopy and root growth. Subsequently, we simulated soil water flow for different future climate scenarios. Our experiments reveal generally higher soil moisture content on the compacted case, suggesting subsoil compact layer’s role of promoting soil water accumulation above it. Moreover, the compacted case had lower LAI while the non-compacted case had deeper roots. Considering these canopy and root growths’ reactions in our models, results show that compaction does not always reduce deep percolation because of enhanced water uptake from the non-compacted case’s deeper roots. Therefore, while soil compaction affects both vegetation growth and soil water balance, this affected vegetation growth can further influence the water balance. Hydrological studies on (de-)compaction should dynamically incorporate vegetation growth above- and belowground under cases with compaction being present or absent. Thus, field evidence of vegetation growth and yield, often far lacking in compaction studies, is vital.

Flanders (Belgium) is expected to experience more severe drought and flooding events in face of climate change. Infiltration to increase groundwater recharge is therefore adopted as policy strategy to deal with both hydrological extremes. Stormwater provides an interesting water source for managed aquifer recharge, given the high urbanization and imperviousness level of the region. Furthermore, the historical ban on infiltration in groundwater protection zones for drinking water production has been removed to encourage infiltration practices. This could potentially enhance groundwater recharge in the groundwater abstraction zones, but concerns remain regarding the impacts on groundwater quality due to the potential contamination of stormwater with a wide range of pollutants originating from traffic, building materials, weed control and other more diffuse sources.Therefore, tools need to be developed to weigh out benefits of groundwater replenishment relative to potential groundwater quality risks. This research aims to contribute to the knowledge on the hydrological aspects of this quantity-quality balancing exercise by investigating the potential of stormwater managed aquifer recharge to replenish the groundwater system in Flemish groundwater protection zones. For this, potential stormwater volumes that could supply managed aquifer recharge are calculated and compared to the actual groundwater recharge and pumping volumes for drinking water production to assess the significance of this practice in protection zones.Results indicate a variable, but high stormwater infiltration potential in Flemish protection zones, providing up to 29% extra groundwater recharge in all protection zones combined. Furthermore, this practice could compensate up to 32% of abstracted phreatic drinking water volumes. Locally, the potential can be higher, reaching 100% in protection zones located in highly urbanized areas, including zones around the city of Leuven. Stormwater infiltration can therefore be considered as an important drought adaptation measure in Flemish protection zones, given the same order of magnitude of stormwater and pumping volumes in these areas. However, recent studies raise concern on the occurrence of organic micropollutants in stormwater and data in the Dutch and Flemish setting is insufficient. Therefore, additional research on occurrence and fate of these substances is needed.

Shoreline erosion presents a significant threat to coastal areas globally, highlighting the need for a thorough understanding of the underlying physical processes to formulate effective mitigation strategies (Luijendijk et al., 2018). Studies on shoreline evolution (Mutagi et al., 2022) often rely on empirical formulations to calculate longshore sediment transport induced solely by waves. However, the extent to which tides contribute to this transport, especially in meso- and macro-tidal coastal environments, remains poorly understood.This study, conducted as part of a research project (known as MOZES) funded by the Flemish government, aims to quantify the relative effects of tides on longshore sediment transport in the nearshore zone. To achieve this objective, an idealized model (known as Q2Dmorfo, Arriaga et al., 2017) and two complex numerical models (Scaldis-Coast and Flemco, Dujardin et al., 2023) are utilized. In the idealized model, wave- and tide-induced longshore sediment transports are computed using two analytical models inspired by the work of Longuet-Higgins (1970) and Southgate et al. (1989). The two complex models calculate longshore sediment transport by fully resolving the flow field, thereby considering tides, waves, and wind.Various scenarios are explored, forcing the models with tides and waves independently, as well as in combination. The study area selected for this research is the Belgian coastal zone, which is characterized by meso- to macrotidal conditions.ReferencesArriaga, J., Rutten, J., Ribas, F., Falqués, A., & Ruessink, G. (2017). Modeling the longterm diffusion and feeding capability of a mega-nourishment. Coastal Engineering, 121, 1 - 13. doi: 10.1016/j.coastaleng.2016.11.011Dujardin, A.; Houthuys, R.; Nnafie, A.; Röbke, B.; van der Werf, J.; de Swart, H.E.; Biernaux, V.; De Maerschalck, B.; Dan, S.; Verwaest, T. (2023). MOZES – Research on the Morphological Interaction between the Sea bottom and the Belgian Coastline: Working year 1. Version 4.0. FH Reports, 20_079_1. Flanders Hydraulics: AntwerpLonguet-Higgins, M. S. (1970). Longshore currents generated by obliquely incident sea waves: 1. Journal of Geophysical Research (1896-1977), 75 (33), 6778-6789. doi: 10.1029/JC075i033p06778.Luijendijk, A., Hagenaars, G., Ranasinghe, R., Baart, F., Donchyts, G., & Aarninkhof, S. (2018). The state   of the world’s beaches. Scientific reports, 8 (1), 6641.Mutagi, S., Yadav, A., Hiremath, C.G. (2022). Shoreline Change Model: A Review. In: Nandagiri, L., Narasimhan, M.C., Marathe, S., Dinesh, S. (eds) Sustainability Trends and Challenges in Civil Engineering. Lecture Notes in Civil Engineering, vol 162. Springer, Singapore. https://doi.org/10.1007/978-981-16-2826-9_64Southgate, H. N. (1989). A nearshore profile model of wave and tidal current interaction. Coastal Engineering, 13 (3), 219-245. doi: 10.1016/0378-3839(89)90050-1. Sedimentary Geology, 33(3), 195–216.

_ Flanders Hydraulics and the Maritime Technology Division of Ghent University have proposed a hybrid testing platform for the development and validation of motion control algorithms for autonomous navigation in shallow and confined water, which consists of towing tank facilities and simulators. Within this framework, the towing tank at Flanders Maritime Laboratory, Ostend, Belgium, featured in large size, tuneable water depth, and a state-of-the-art free-running system, a series of autonomous manoeuvering tests with increasing complexity are designed and carried out. These manoeuvres, including acceleration with heading keeping, path following, speed control, and their combinations, are enabled by implementing guidance and control algorithms on a physical ship model, namely the benchmark ship KVLCC2 at a scale of 1/75. The controllable environment and large space of the tank basin facilitate the upgrade of the existing algorithms and the development of new algorithms. The definition of the autonomous manoeuvres, experimental setup, and parameter settings are elaborated and the effect of implementing different guidance strategies is discussed. Recorded results demonstrate good agreement between the measurements and the expectations, and control accuracy is quantified with evaluation indices. In summary, the feasibility and flexibility of the testing platform and the effectiveness of the algorithms are proved through extensive experiments. Keywords guidance and control; free-running test; physical testing platform; path following; speed control

Low-lying countries typically have mildly-sloping beaches as part of their coastal defense system. Many countries in north-western Europe have coastal urban areas that rely on this type of defense system, which consists of a low-crested impermeable sea dike with a relatively short promenade, and a long (nourished) beach in front that acts as a very/extremely shallow foreshore as defined by Hofland et al. (2017). Along the cross-section of this hybrid beach-dike coastal defense system, storm waves are forced to undergo many transformation processes before they finally overtop the dike. These hydrodynamic processes include shoaling, sea-swell (SS) wave energy transfer to sub- (also infragravity or IG waves) and superharmonics via nonlinear wave-wave interactions, wave dissipation by breaking and bottom friction, reflection against the dike, wave run-up and overtopping on the dike, bore impact on a wall or building, and finally reflection back towards the sea interacting with incoming bores on the promenade. Due to breaking of the SS waves and growth of the IG waves on the shallow foreshore, the IG waves can become as important or even dominant at the toe of the dike (Hofland et al., 2017; Lashley et al., 2020), which influences the overtopping process (van Gent, 1999). Field measurements of all these processes at the same time are very challenging but necessary since field observations do not suffer from scale nor model effects. Field data are therefore crucial to evaluate design methodologies, which rely on physical and numerical modelling. This paper presents the field setup and the design features of the innovative research dike, unique in the world, including a performance analysis.