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The Netherlands needs to increase its number of houses to accommodate its growing population. This will result in the construction of houses in flood-prone areas and in presently agricultural areas. At the same time, extreme water levels in the sea and large lakes, as well as precipitation during storms are expected to increase. Therefore it is necessary to gain insight in the extent and intensity of flooding (both “wateroverlast” and “overstromingen” in Dutch) due to extreme precipitation scenarios and in areas with a land use that is changing from agricultural to low and high-density urban areas. At present, SFINCS is already capable of computing so-called compound flooding, the combined effect of flooding due to rain, sea and rivers, whereby a simplified description of infiltration into the subsoil is taken into account. At the moment the model is not able to compute the effect of storm drains and sewer systems. This could be done with more detailed and complex software that is already exists, but these are computationally very expensive, and are less suitable to run a large number of scenarios in order to explore the solution space. In this project, we aim to develop functionality that enables faster flood assessment for extreme rainfall events and changing land use scenarios. We will investigate the benefits of incorporating a drainage formulation in SFINCS itself or develop a modular modelling framework that combines the strengths of SFINCS and a coupled open-source 1D urban model like D-Hydro 1D. We will develop a BMI (Basic Model Interface) coupling between the two model components, test it for a simple hypothetical case and UK EA benchmark test cases, and validate it for a number of urban areas in the Netherlands and abroad such as Zwolle, Jacksonville (Florida, USA) and a Danish city.
Work Package 2 – Flooding in riverine areas with steep slopes
The SFINCS model was originally developed for use in coastal zones where gradients (slopes) in the topography are small. However, a number of market parties have expressed interest in applying the model in more upland riverine areas where gradients in the river bathymetry and topographical slopes are larger. These steeper slopes strictly violate the mild-slope assumptions of the SFINCS formulations. However, this is the case with many other models that are still applied in these environments. In this project the aim is to assess and test the applicability of SFINCS in upland riverine cases and where needed make adjustments in the formulations. We will also assess if more hydrological processes like water retention need to be taken into account, or if modelled by an external dedicated hydrological model like Wflow – where in the watershed to best switch from the hydrological model to the hydrodynamic SFINCS model. We will test the model for a number of rivers such as the Vecht river in the Netherlands, the Kolding river in Denmark and the Upper French Broad River in the USA, which was subject to large flooding due to Hurricane Helene. The outcome of this topic is to have a better understanding of and guidance on the applicability of SFINCS in rivers.
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Coastal flooding is caused by tides, surges, and wave processes. However, wave-driven flooding is not taken into account in existing software when modelling large spatial scales because of the computational demand, or are only taken into account approximately using empirical formulations. However, especially on steeper coasts, the wave-driven contribution to flooding can be equal or larger than the contribution of storm surge. This is expected to be the case for the Dutch BES and CAR-islands. In his PhD work, Tim Leijnse developed a method to compute the wave effects in a computationally efficient way, implemented it in SFINCS and validated it for one case study in the USA. This functionality will be tested and validated on a wider variety of coastal types. Partners will suggest case studies and do the assessment. Results might indicate that further modifications of the wave module may be needed for application on steeper and coral-reef lined coasts, which will be explored in this project. The outcome of this project is more insight on the applicability of SFINCS to compute wave-driven flooding in different environments.
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Lowland areas such as the Netherlands, but also Louisiana and Bangladesh, are protected by levees. Breaching of levees and subsequent flooding has devastating effects such as loss of life and damage to structures and agricultural land. Based on existing empirical formulations, dike breaching can be incorporated into SFINCS as an external discharge source. This theoretical concept has been previously applied abroad, in cases in Denmark and Bangladesh. To make this wider applicable and much easier to use, we will develop a dike breaching module in the SFINCS model itself. Thereby, we intend to compare different discharge and breaking formulations, and apply it in a Dutch situation. Partners can compare the speed and accuracy to existing tools in the Netherlands. The advantage of this functionality is that SFINCS enables fast simulations, making it possible to explore the effects of uncertainties in breach location, width, depth, and timing—rather than being limited to just one or a few scenarios, as is currently the case.
Results
The expected end result is a new version of the SFINCS software with the four new and/or improved functionalities which will be made available to the general public. With this new version it will be possible to compute compound (combined) flooding due to tides, surges, waves, rivers and direct rainfall, and will include formulations to take the effect of storm drains in urban areas into account. The software will be more complete in the physics and faster than existing software and still be applicable for large spatial and temporal scales including coastal, upland river and diked areas.
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