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.

Coastal flood risk is expected to increase due to climate change and population
growth. Much of our coastlines is protected by “grey” infrastructure such as a dike.
Dike maintenance and strengthening requires ever increasing capital and space,
putting their economic viability in question. To combat this trend, more
sustainable alternatives are explored, also known as Nature based Solutions. A
promising option has shown to be tidal marshes. Tidal marshes are coastal
wetlands with high ecological and economic value. Also, they protect dikes
through wave attenuation and in case of a dike breach reduce its development.
However, the effectiveness of a tidal marsh on reducing dike breach development
rates highly depends on the stability of the tidal marsh itself. Not much is known
about the stability of a tidal marsh under dike breach conditions, which are
accompanied with flow velocities that can reach 4–5ms−1. In this study we tested
the vegetation response and erodibility of a mature tidal marsh, in-situ, under high
flow velocities ( > 0.5ms−1). Our results confirm that tidal marshes similar to the
one tested in this study are highly erosion resistant with low erodibility. More
research is necessary to confirm this for tidal marshes with different soil and
vegetation properties. For tidal marshes similar to what is tested thus far, erosion
under dike breach conditions is negligible and other erosion mechanisms such as
headcut erosion probably dominate the erosion process.

Coastal flood risk is expected to increase substantially in the near future. Main drivers are climate induced sea level rise, increased storm surge and land subsidence. Meanwhile, land subsidence compounds to increased extreme water levels as more land is susceptible to flooding. Without coastal defense or adaptation 50% more people are exposed to flooding than present day (Kireczi et al., 2020).

Coastal regions are currently primarily protected by hard (grey) flood defenses such as storm surge barriers, seawall, dikes and dunes. Periodically, strengthening of these grey structures is necessary to comply with current or updated safety standards. For dikes, conventional strengthening methods are crest heightening or (base) widening. However, these methods have structural and financial limits. Instead, more sustainable methods are explored in which nature also plays a larger role. These solutions are known as Nature based Solutions (NbS).

For flood protection, tidal marshes have gained great interest as a Nature based Solution in the past two decades. Tidal marshes provide a lot of ecosystem services (Barbier et al., 2011). One such service is flood protection, attributed to wave attenuation (Vuik et al, 2016). A secondary effect is flood impact reduction (Zhu et al., 2020) due to the high elevation of tidal marshes limiting the inflow to the breach. Secondly, the tidal marsh can act as a sill in front of the breach when water levels drop below the tidal marsh level.

To quantify the effect of tidal marshes on flood impact the breaching process in combination with a tidal marsh (or foreshore in general) needs to be understood. In this study we performed a large-scale physical dike experiment where we breached a dike seven times. Three tests are done without a sediment layer in front of the dike (no foreshore), two with a sandy layer (sandy beach) and two with a clay layer (tidal marsh without vegetation). From the experiments we gain insight into differences in the dike breaching process with and without an erodible sediment layer in front of the dike

Coastal defences such as dikes are increasingly pressured by climate change. Increasing storm surge, extreme rainfall and periods of draught requires evermore strengthening of dikes to maintain flood risk standards. Conventional dike strengthening (i.e., heightening and/or widening) will be either structurally or financially unfeasible. Therefor, engineers are exploring other, more sustainable, methods to ensure future flood safety. A promising method is incorporating tidal marshes in the coastal defence system. Tidal marshes reduce dike loads by wave attenuation, increase bio diversity and ecology and under the right circumstances are able to grow with sea level rise. Moreover, in case of dike failure, resulting in a dike breach and inundation of the hinterland, tidal marshes have been shown to reduce breach erosion rates. This reduction positively affects flood risk. However, in order to quantitatively estimate the effect, dike breach models need to also model tidal marsh erosion. In this study we tested a mature tidal marsh, in-situ, in winter conditions under high flow velocities (up to 2.5 m/s) to measure the erosion and estimate erodibility. We measured little erosion, order millimeters after a cumulative 2-2.5 hours. Small-scale experiments, such as the Jet Erosion Test, showed high resistance to erosion (85-140 Pa) and large varying erodibility (6.5-45 cm3/N·s). By estimating the shear stresses acting on the soil during the experiment we compare the data with the small-scale results. The comparison gives insight in whether the small-scale experiment results can be accurately translated to full-scale erosion. Also, the experiment showed which (erosion) mechanisms are important for tidal marshes during a dike breach.

Nature-based strategies, such as wave attenuation by tidal marshes, are increasingly proposed as a complement to mitigate the risks of failure of engineered flood defense structures such as levees. However, recent analysis of historic coastal storms revealed smaller dike breach dimensions if there were natural, high tidal marshes in front of the dikes. Since tidal marshes naturally only experience weak flow velocities (~0-0.3 ms^-1 during normal spring tides), we lack direct observations on the stability of tidal marsh sediments and vegetation under extreme flow velocities (order of several ms^-1) as may occur when a dike behind a marsh breaches. As a first approximation, the stability of a tidal marsh sediment bed and winter-state vegetation under high flow velocities were tested in a flume. Marsh monoliths were excavated from Phragmites australis marshes in front of a dike along the Scheldt estuary (Dutch-Belgian border area) and installed in a 10 m long flume test section. Both sediment bed and vegetation responses were quantified over 6 experimental runs under high flow velocities up to 1.75 ms^-1 and water depth up to 0.35 m for 2 hours. These tests showed that even after a cumulative 12 hours exposure to high flow velocities, erosion was limited to as little as a few millimeters. Manual removal of the aboveground vegetation did not enhance the erosion either. Present findings may be related to the strongly consolidated, clay- and silt-rich sediment and P. australis root system in this experiment. During the flow exposure, the P. australis stems were strongly bent by the water flow, but the majority of all shoots recovered rapidly when the flow had stopped. Although present results may not be blindly extrapolated to all other marsh types, they do provide a strong first indication that marshes can remain stable under high flow conditions, and confirm the potential of well-developed tidal marshes as a valuable extra natural barrier reducing flood discharges towards the hinterland, following a dike breach. These outcomes promote the consideration to implement tidal marshes as part of the overall flood defense and to rethink dike strengthening in the future.