Assessing dike safety is of key interest to societies in low-lying areas, but results can be implausible when, for example, they contradict the observed performance of the dike. To improve credibility, load monitoring data can be incorporated using reliability updating techniques. This paper investigated the role of load variations in reliability updating and assessing credible failure probabilities. It was found that the impact of reliability updating increases when load variations are small, as a large contribution to failure probabilities comes from relatively frequent load levels, of which the conditional failure probabilities are reduced most through reliability updating. Moreover, a credibility check was introduced for dikes that have been stable for decades, where load levels with return periods of up to 10 years are not expected to contribute more than 50% to the failure probability, indicating an imbalance between load variation and strength uncertainty. This imbalance occurs when the inverse gradient of the fragility curve exceeds 1.5 times the decimate height of the load. Many Dutch dikes, including canal dikes and dikes along the large lakes and delta regions, have small decimate heights. For these dikes, strength uncertainties must be sufficiently small to obtain credible failure probability estimates.

Flood risk is of key interest to societies in low-lying areas such as the Netherlands, where a wide variety of dikes protect people and property from flooding. This thesis focuses on canal dikes, located along drainage canals in polders, mainly in the northern and western regions of the Netherlands. Safety assessment outcomes and recent dike failures indicate that inner-slope instability is a key failure mechanism affecting canal-dike safety, occurring under both very dry and very wet conditions. Assessing inner-slope instability remains challenging due to large uncertainties in geotechnical and geohydrological parameters. The aim of this thesis is to enhance the geohydrological understanding of canal dikes through monitoring and observation-based analysis, thereby improving the assessment of their stability and the safety of dike systems as a whole. Two unique datasets were established: a multiyear detailed monitoring series of soil moisture levels and hydraulic heads at ten canal dikes, and a nationwide collection of over one hundred hydraulic head time series from about 50 monitoring sites.

The detailed monitoring series included two extremely dry summers (2020 and 2022). Hydraulic heads at inner-slope and toe monitoring points showed strong seasonal variation. During winter, conditions were near saturation, while during dry summers heads dropped down to nearly 2 m, despite outside water levels remaining nearly constant. Non-hydrostatic hydraulic head levels were observed within dike bodies, conditions often not accounted for in safety assessments for drought situations. The precipitation deficit proved the most reliable meteorological drought indicator, outperforming the standardised drought indicators (SPEI and SPI).

Analysing the nationwide dataset with time-series models, a non-linear model performed best, resulting in 35 reliable time-series models. Four clusters of dikes were identified, differentiated by response times, defined as the time required for 95% of an impulse's influence to dissipate. Longer response times cause peak heads to occur later in the winter. Peak head statistics indicated a that extreme heads are close to yearly occuring heads, with a median decimate height of 15 cm and a range of 5 to 50 cm. By 2100, extreme peak heads are expected to occur between three times less frequently and eight times more frequently, depending on the climate scenario and the type of canal dike.

The time-series models were used to hindcast 60 years of hydraulic head levels and estimate spatial dependencies, quantified using the length-effect factor of peak heads. At the polder scale, variation in head responses can increase the factor by a factor of four to five, whereas spatial weather variability can double it. At the scale of the entire canal-dike system, the length-effect of peak heads is dominated by spatial weather variability, increasing from about 6 for annually occurring heads to around 40 for heads with an exceedance frequency of 1/100 per year.

The impact of small load variations on reliability updating was analysed. The impact of reliability updating increases as load variations decrease, regardless of the prior failure probability. The two canal dikes, with decimate heights below 10 cm, benefited most from reliability updating, whereas updated failure probabilities of the river dikes changed only slightly. A credibility check was introduced: when uncertainty in dike strength is more than 1.5 times larger than the variation in loads, estimated failure probabilities are not credible. Reducing uncertainties in dike strength, through monitoring with reliability updating or more detailed soil strength data, leads to more credible failure probability estimates.

Managing the water and flood risk in low-lying polder regions depends on the performance of canal dikes. This performance is influenced by hydraulic heads, which can peak due to heavy rainfall, affecting their stability and potentially inducing dike breaches. Variations in head responses and head statistics are relevant for regional flood risk analysis of canal dike systems. This study examined the dynamics of peak heads in canal dikes on a national scale using time series models calibrated on a unique dataset of head observations across the dike system. Various model structures were evaluated, and a non-linear model performed the best. These models were used to simulate 30 years of head time series representing current and future climate scenarios. Subsequently, dike clusters were identified based on the coincidence of peak heads, allowing for the identification of dikes where peaks are caused by (dis)similar types of rainfall events. The differences and similarities in peak head response between dikes and identified clusters were related to physical dike characteristics. While the subsurface material and dike width appeared to influence the head response variation of clusters, their presence across multiple clusters indicates that they do not yield a definitive outcome. Moreover, peak head statistics across various dikes indicated that extreme and yearly occurring load conditions are relatively close to each other, with a median decimate height of only 15 cm. With climate change driving higher winter precipitation and summer evaporation, head statistics are changing. By 2100, extreme peak heads are expected to occur between 3 times less and 8 times more frequently, depending on the climate scenario and the type of canal dike.

Canal dikes in low-lying polders, as well as in other regions worldwide, are critical infrastructure for flood protection and water management. The subsurface water conditions can cause dike failures during excessive rainfall and prolonged periods of drought. There is a lack of multi-year monitoring of subsurface water conditions in canal dikes and an insufficient understanding of their geohydrological behavior. This study provides and analyses a novel multiyear data set of soil moisture and hydraulic heads (from February 2020 until March 2023) from a monitoring network covering various canal dikes with different characteristics in the western Netherlands. The data, including two extremely dry summers, highlight the impact of meteorological variations on the subsurface water conditions. Non-hydrostatic hydraulic head levels were observed during droughts that can be detrimental to dike stability and that are often not accounted for in safety assessments for drought situations. The effectiveness of various meteorological drought indicators applied to subsurface water conditions was evaluated: the precipitation deficit is the most reliable measure and outperforms the standardized drought indicators (SPEI and SPI). The drought recovery of dikes was analyzed to understand seasonal transitions and the sequence of different failure mechanisms, during dry and wet situations. This analysis also reveals differences between meteorological, soil moisture, and groundwater droughts, highlighting soil's storage capacity after drought and the limitations of meteorological drought indicators as proxies for soil moisture and groundwater. The insights from this study enhance assessments, inspection procedures and the identification of weak spots of dikes and other earthworks of infrastructure.

The management of water and flood risk levels in low-lying polder regions depends on the performance of canal dikes. Heavy rainfall can lead to peak hydraulic heads within the dikes affecting their stability, which can induce dike breaches. Variations in head responses and head statistics are both relevant for regional flood risk analysis of canal dike systems. This study examined the dynamics of peak heads in canal dikes on a national scale using time series models calibrated on observed heads. Various model structures are evaluated and a nonlinear model performed the best. These models were used to simulate long-term head time series. Subsequently, dike clusters were identified based on the coincidence of peak heads, allowing for the identification of dikes where peaks are caused by (dis)similar types of rainfall events. The differences and similarities in peak head response were related to physical dike characteristics. While no single significant relationship emerged, the soil type combined with the width of the dike appears to be important factors influencing the variation in head responses. However, the presence of the same soil type and dike widths in multiple clusters indicates that these characteristics do not yield a definitive outcome for the head response. Moreover, peak head statistics across various dikes were derived and indicated that extreme and yearly load conditions are relatively close to each other, with a median decimation height of only 15 centimeters. The head statistics are affected by climate change, characterized by increasing winter precipitation and summer evaporation. By 2100, extreme peak heads are expected to occur between 3 times less and 8 times more frequently, depending on the climate scenario and the type of canal dike.

In July of 2021, large areas in the catchment of the Meuse River in Belgium, the Netherlands and Germany were affected by extreme rainfall and floods. This paper presents the hydraulic and morphological data that were collected during and after the flood. The data were analysed to understand the hydraulic and morphological functioning of the Meuse River in the Netherlands during the flood event. The data showed that measured peak discharges in the upstream part of the Meuse and regional tributaries were the highest ever recorded. However, as the flood had a very short duration, peak attenuation played an important role, resulting in discharges and water levels in downstream reaches that were lower than during previous floods. Furthermore, the implementation of river widening and floodplain lowering measures as part of the Meuse Works programme contributed to a reduction in peak water levels along the Meuse. The analysis also showed that flood forecasts in the upstream part of the Meuse in the Netherlands depended heavily on rainfall forecasts and rainfall-runoff modelling and underestimated the peak water levels up to 36 hours before the flood actually peaked. Further downstream, the lead time increases and forecasts are based on discharge levels that are measured in upstream parts of the catchments. This results in more accurate estimates. The floods have also resulted in unprecedented morphological changes. The armour layer in the riverbed of the ‘Common Meuse’, consisting of very coarse gravel, was mobilised and layers of fine sand quickly eroded. This resulted in multiple scour holes with depths of 3 to 15m, especially in a reach which was hardly or not at all widened in the room for the river programme called Meuse Works. In this reach, the flow velocities were high and even higher than prior to the Meuse Works.

In juli 2021 zijn grote delen van Limburg getroffen door hevige regenval en overstromingen. Ook delen van België en Duitsland overstroomden met zeer veel schade en verlies aan mensenlevens tot gevolg. Dit betrof een extreme en ongeëvenaarde gebeurtenis met enorme impact. Daarom is naar aanleiding van de overstromingen deze verkenning uitgevoerd om een eerste stap te maken om beschikbare informatie over deze gebeurtenis te verzamelen en analyseren. Het onderzoek is uitgevoerd door een breed consortium (TU Delft, Deltares, HKV Lijn in Water, VU Amsterdam, Universiteit Utrecht, KNMI, WUR, Erasmus MC en Universiteit Twente) in opdracht van het Expertise Netwerk Waterveiligheid (ENW). Een overstroming heeft effect op de hele maatschappij. Daarom zijn niet alleen hydrologische en civieltechnische onderwerpen beschouwd, maar ook de maatschappelijke gevolgen van overstromingen, de crisisrespons en de gezondheidseffecten.

Contributors (in alphabetical order):
Nathalie Asselman (Deltares), Hermjan Barneveld (HKV / Wageningen UR), Jules Beersma (KNMI), Eline Boelee (Deltares), Wouter Botzen (VU Amsterdam), Eefke Copper (TU Delft), Dim Coumou (KNMI), Karin de Bruijn (Deltares), Anniek de Jong (Deltares), Jurjen de Jong (Deltares), Hans de Moel (VU Amsterdam), Ferdinand Diermanse (Deltares), Astrid Fischer (Evides) , Gert-Jan Geerling (Deltares), Marie-Louise Geurts (WML), Rob Groenland (KNMI), Mark Hegnauer (Deltares), Bas Jonkman (TU Delft), Nicole Jungermann (KNMI), Frans Klijn (Deltares), Andre Koelewijn (Deltares), Matthijs Kok (HKV / TU Delft), Elco Koks (VU Amsterdam), Bas Kolen (HKV / TU Delft), Marion Koopmans (Erasmus MC), Laurens Leunge (Deltares), Hans Middelkoop (Utrecht University), Roelof Moll (TU Delft), Jaap Mos (Dunea), Sjoukje Philip (KNMI), Gerbert Pleijter (HKV), Joost Pol (HKV / TU Delft), Stephan Rikkert (TU Delft), Guus Rongen (TU Delft), Rinus Scheele (KNMI), Julius Schlumberger (TU Delft), Peter Siegmund (KNMI), Kymo Slager (Deltares), Frederiek Sperna Weiland (Deltares), Bart Strijker (HKV / TU Delft), Henk v.d. Brink (KNMI), Janko van Beek (Erasmus MC), Marion van den Bulk (TU Delft), Bart van den Hurk (Deltares), Tim van Emmerik (Wageningen UR), Kees van Ginkel (VU Amsterdam / Deltares), Mick van Haren (TU Delft), Margreet van Marle (Deltares), Malou van Schaijk (TU Delft), Dennis Wagenaar (Nanyang TU), Davide Wüthrich (TU Delft)