In de geschiedschrijving van ons land speelt water een cruciale rol en daarbij vooral het optreden van stormvloeden en overstromingen. Kees Slager beschrijft in zijn boek Watersnood zo’n vijftig stormvloeden over de laatste duizend jaar, die hij aanduidt als ramp.1 Hoewel niet eenduidig gedefinieerd, slaat ‘ramp’ op een overstroming waarbij (grote) schade ontstaat en/of waarbij mensenlevens verloren gaan. Ofschoon niet volledig gecorreleerd, is er een duidelijk verband tussen de maximale waterstand bij een stormvloed en de gevolgen daarvan voor de samenleving in de kustgebieden. Voor de geschiedschrijving kan enige ordening in de opgetreden stormvloeden zinnig zijn om daarmee een zo objectief mogelijk totaalbeeld te schetsen. Stormvloedstanden zijn dan het meest relevant voor een objectieve beschrijving en het middel bij uitstek om daar ordening in te brengen. Helaas ontbreken die gegevens grotendeels of volledig voor stormvloeden langer dan zo’n tweehonderd jaar geleden.

Een andere reden waarom het moeilijk is zo’n ordening aan te brengen, is het feit dat stormen niet over het hele land even heftig zijn. De ramp van 1 februari 1953 trof vooral Zuid-Holland, Zeeland en Noord-Brabant, terwijl de Kerstvloed van 25 december 1717 in ons land vooral een grote ramp veroorzaakte in Groningen en Friesland. Rangschikking van stormvloeden zal daarom altijd slechts gelden voor een beperkt gebied. In het volgende zullen wij proberen een rangschikking te maken van stormvloeden langs de kust van Zuid-Holland tussen Goedereede en Katwijk, omdat dit kan worden gezien als een redelijk homogeen gebied als het gaat om stormvloedstanden en omdat voor dit gebied enige waarnemingen van stormvloedstanden van vroegere stormvloeden bestaan.

Coastal and fluvial flood defences currently rely primarily on existing (grey) infrastructure such as dikes. However, coastal flood risk is expected to increase substantially in the near future. This requires ever increasing efforts to strengthen dikes. To aid these conventional methods, Nature-based Solutions (NbS) are increasingly proposed, such as coastal wetlands. Coastal wetlands have many ecological benefits, but also aid flood protection, especially tidal marshes. Tidal marshes protect the dikes behind them through wave attenuation and reduce flood damage if the dike is breached. Meanwhile, flood risk assessment relies on dike breach modelling to estimate the breach discharges for inundation simulations. Yet, how these foreshores (e.g., tidal marsh) affect dike breach development is largely unknown. For this reason, we experimentally explore how different foreshores affect the dike breaching process. In this study we performed a series of breach tests with a 1.5-1.9 m high model sand-dike with and without a foreshore. We tested two types of foreshores, an erodible sand and low-erodible clay layer, acting as proxies for a sandy beach and unvegetated tidal marsh. Because dike breach flow closely resembles weir flow, the standard weir equation applies, which is also frequently used in breach discharge models. The observed foreshore effects are qualitatively evaluated using this weir equation. Depending on foreshore stability, we find that foreshores affect breach hydrodynamics which alters the weir shape, leading to reduced breach width growth and ultimately limits the specific discharge.

Climate change leads to sea level rise worldwide, as well as increases in the intensity and frequency of tropical cyclones (TCs). Storm surge induced by TC’s, together with spring tides, threatens to cause failure of flood defenses, resulting in massive flooding in low-lying coastal areas. However, limited research has been done on the combined effects of the increasing intensity of TCs and sea level rise on the characteristics of coastal flooding due to the failure of sea dikes. This paper investigates the spatial variation of coastal flooding due to the failure of sea dikes subject to past and future TC climatology and sea level rise, via a case study of a low-lying deltaic city- Shanghai, China. Using a hydrodynamic model and a spectral wave model, storm tide and wave parameters were calculated as input for an empirical model of overtopping discharge rate. The results show that the change of storm climatology together with relative sea level rise (RSLR) largely exacerbates the coastal hazard for Shanghai in the future, in which RSLR is likely to have a larger effect than the TC climatology change on future coastal flooding in Shanghai. In addition, the coastal flood hazard will increase to a large extent in terms of the flood water volume for each corresponding given return period. The approach developed in this paper can also be utilized to investigate future flood risk for other low-lying coastal regions.

Global change amplifies coastal flood risks and motivates a paradigm shift towards nature-based coastal defence, where engineered structures are supplemented with coastal wetlands such as saltmarshes. Although experiments and models indicate that such natural defences can attenuate storm waves, there is still limited field evidence on how much they add safety to engineered structures during severe storms. Using well-documented historic data from the 1717 and 1953 flood disasters in Northwest Europe, we show that saltmarshes can reduce both the chance and impact of the breaching of engineered defences. Historic lessons also reveal a key but unrecognized natural flood defence mechanism: saltmarshes lower flood magnitude by confining breach size when engineered defences have failed, which is shown to be highly effective even with long-term sea level rise. These findings provide new insights into the mechanisms and benefits of nature-based mitigation of flood hazards, and should stimulate the development of novel safety designs that smartly harness different natural coastal defence functions.

Significant reduction of the rate of erosion of a sand bed is obtained when sand is mixed with a small amount of bentonite. In previous experiments this behaviour has already been shown for relatively low flow velocities. In this case, the erosion process is dominated by grain-by-grain erosion, which is characterised by low ratios of the erosion velocity and permeability (v_e/k<3). It is unknown whether these reductions in the erosion process also occur at relatively high flow velocities, where dilatancy-reduced erosion dominates (v_e/k>3). Experiments were executed in a tilting flume to investigate the erosion rate of the sand-bentonite mixtures. In 13 different tests, the dry volume percentage of the bentonite additive, the diameter of the sand particles and the depth-averaged flow velocity were varied. The depth-averaged flow velocities ranged from 1 to 2 m/s and all erosion tests were performed under supercritical flow conditions. The experiments show that the bentonite additive did not influence the strength characteristics of the sand however the permeability did decrease significantly. This proves that the significant decrease of the erosion rate was caused by the decrease of the permeability of the sand and that the test conditions were in the dilatancy-reduced regime.

Dikes constructed from sand generally have a sand core and clay layers on the slopes and the crest to protect the core against erosion. In extreme hydraulic conditions, several failure mechanisms can lead to destruction of the clay layers, exposing the sand core to water. When water overtops the dike and the protective cover of the land-side slope or the crest fails, water flows over the core and erodes the sand. The dike starts to breach, and eventually the land behind the dike is flooded. The rate at which the dimensions of the breach grow influences the rate of inundation of the polder. Reduction of the inundation rate may be achieved by retarding the breaching process. This may reduce the number of casualties, resulting in increasing safety for the inhabitants. In order to achieve, for instance, a safety level ten times higher, mortality has to decrease by a factor of 10. The breaching process can be retarded by reducing the erosion velocity of the sand core. Experiments were executed to investigate the effect on the erosion velocity of adding bentonite to sand. The results of these experiments showed a significant reduction of the permeability and erosion velocity of the sand–bentonite mixtures compared with those of pure sand. The effect of adding bentonite on the breaching process was investigated by applying the Bres model (breach erosion in sand dikes model) to a sand dike tested in a large-scale field experiment (Zwin'94). It was found that adding a small percentage of bentonite reduces the rate of breach growth and the inundation rate in the polder. For the Zwin’94 dike it was determined that 5·4% of bentonite is sufficient to reduce the inundation rate below a threshold value of 0·5 m/h, leading to a significant increase in safety.

Two regimes can be distinguished for the pickup flux of sand. At a Shields parameter of less than about 0.5 (corresponding with flow velocities of 0.5–1 m=s), the erosion process is dominated by the size and density of the grains (grain by grain pickup). At higher flow velocities, the bulk properties of the sand bed start to influence the erosion process. Dilative behavior results in the inflow of water to the sand bed, which reduces the pickup flux (dilatancy-reduced pickup) because of the shearing of layers of sand. A pickup function was recently developed for this regime, incorporating the effect of bulk properties, such as permeability and porosity, on the pickup. This function agrees well with data of previous erosion experiments in which the permeability and porosity of the sand bed were varied. However, these experiments just met the condition for dilatancy-reduced pickup. The flow velocity during these previous experiments was between 1 and 1.5 m=s, while the Shields parameter varied between 1 and 2. In order to validate this pickup function for dike breaching and jetting of sand, during which the flow velocity ranges within 5–50 m=s, erosion experiments were executed at higher flow velocities. These experiments were executed in an adapted closed flume of the slurry circuit of the Dredging Research Laboratory at the Delft University of Technology (DUT) at flow velocities of 2–6 m=s (Shields parameter is between 50 and 1,000). The results of these experiments are consistent with the theory of dilatancy-reduced pickup. The porosity of the sand bed influences the erosion process, especially at flow velocities of more than 4 m=s bulk property.

Within the frame work of the realisation of the ‘Sigmaplan’ for the river Schelde in Flanders (Belgium), a large-scale dike breaching experiment following overflow was held at Lillo (Antwerp) in 2012. The outcomes of the breach test serve to unveil the impact of a chosen breach growth model, to set application limits, to come up with guidelines for proper selection and usage of the model to be applied.

Breach growth models are used to predict the breach dimensions and to estimate the flow through the breach. All assessed models pretty well succeed in this. However, starting from various premises and taking into account a (limited) set of different breaching mechanisms, the use of today’s state-of-the-art breach growth models is not entirely trouble free

Deriving the bed shear stresses from hydrodynamic models in breach models is challenging due to the continuous changing hydraulic head over the breach in combination with horizontal and vertical flow contractions, and the continuous rapidly changing breach geometry. Three stages can be distinguished in breach flows. Stage 1 initiates when the embankment starts to overflow and is characterized by flows over the embankment crest and down the landside slope. Stage 2 initiates when the landside slope has retreated towards the waterside slope. The hydraulic head increases rapidly and the flow contracts both horizontally and vertically resulting in a fully 3-dimensional flow. During Stage 3 a full breach has developed and the flow contracts mainly horizontally. This paper presents a SWOT analysis of flow modelling methods applied in breach models to derive the bed shear stresses. The paper shows that for a number of cases analytical methods are more accurate than numerical methods due to the fact that they give a more accurate description of the shear stresses on the embankment surface, whereas for some numerical methods errors are found of the order of 50%.

The fluvial river is a kind of open system that can interact with its outside environments and give response to disturbance from outside on the earth. It can adjust itself to the disturbances outside the system and reflects new characteristics in the process of reaching a new equilibrium. The TGD (Three Gorges Dam) constructed at the Yangtze River Upstream was set up to operate in 2003. And it has changed the boundary conditions of the downstream reaches and has broken the long term equilibrium of the Yangtze River system. The reaches alter the river regime to response the disturbance from the TGD. In this paper, the case study Shashi Reach is selected to analyze the variations of the river regime in a river system. The discharge is becoming smoother without large peak or lower discharge for the regulation of the reservoir. The sediment diameter becomes coarser downstream of the TGD and the sediment transport rate decreases as the sediment concentration becomes lower, in spite of the sediment erosion along the reach downstream. The thalweg moves in plane dramatically to adjust itself to reach a new equilibrium. And the topography changes a lot since there are different sediment and flow conditions. The disturbed river system is in the process of reaching a new equilibrium.