Background
Fifty-six percent of the Netherlands’ land surface area is susceptible to flooding from the sea and rivers. This affects 11 Million people (out of a total of 17.6 Million, and 67% of the Gross National Product (CBS, 2022). Worldwide, 10% of the population (600 million) lives in coastal areas that are less than 10 m above sea level (McGranahan et al., 2007) and up to 4 % of global population is expected to be flooded annually in 2100 with 120 cm of global mean sea-level rise, with expected annual losses of 9% of global gross domestic product (Hinkel, 2014).
It is therefore essential that we can assess the flood impacts during extreme events of rainfall, river runoff and storm surge, as well as due to changes in these impacts due to changing land use, urbanization and climate change.
In order to assess the various scenarios of flooding and exposure, it is essential to have tools that can predict flooding in a computationally efficient way. Deltares with partners have built such a model called SFINCS which was originally intended for large-scale coastal areas. The model is developed open-source with executables shared publicly.
SFINCS
SFINCS (Super-Fast Inundation of CoastS) is a reduced-complexity model capable of simulating compound flooding with a high computational efficiency balanced with an adequate accuracy. In SFINCS a set of momentum and continuity equations are solved with a first order explicit scheme based on Bates et al. (2010). For more information see Leijnse et al. (2021): https://doi.org/10.1016/j.coastaleng.2020.103796
Compound flooding during tropical cyclones and other extreme events result in tremendous amounts of property damage and loss of life. Early warning systems and multi-hazard risk analysis can reduce these impacts. However, large numbers of computations need to be run in a probabilistic approach and in a short time due to uncertainties in the meteorological forcing. Current modelling approaches are either fast but too simple (bathtub approach) or models are very accurate but too slow (e.g. Delft3D, XBeach). SFINCS balances a high computational efficiency with adequate accuracy. Therefore the model is very appropriate to either to model a large stochastic set of scenarios, run the same model on a higher resolution or model larger scales. Thereby, one can use it as a quickscan tool to quickly test possible adaptation and mitigation measures to flooding. The scope of the model is therefore different than other models in Deltares’ modelling suite
TKI program
This project is part of the TKI subsidy program of the Dutch Ministry of Economic Affairs (where TKI is an abbreviation for ‘Topsector Knowledge and Innovation’). This program provides an incentive for knowledge institutes to work with the engineering community, as it provides a matching subsidy on top of in-kind and cash contributions from participants of a Joint-Industry-Project. This document describes the scope of the TKI SFINCS – DROMOS (Driving Research On MOdelling with SFINCS) project, the tasks to be undertaken by Deltares and the partners, but also financial practicalities and the way of collaborating. An important aspect of the TKI project will be community building: the community building with the consortium is considered an essential aspect which will inspire new developments and may form a start for further knowledge exchange.
Scope
Aims and priorities of the project
In this project we want to develop and test new functionalities in the SFINCS model so that users can assess and forecast flooding in urban areas with drainage systems, wave-driven flooding on coasts, flooding in riverine areas with steep slopes, and dike breaching. The end result is a new version of the reduced-complexity model SFINCS with which we can compute compound flooding due to tides, surges, waves, as well as river discharges and direct rainfall in urban areas as well as at larger regional scales, at a computational speed which is orders of magnitude faster than conventional software. In this project we bring together partners from government and industry, from the Netherlands and abroad with expertise and knowledge about flooding scenarios which may become the norm in the Netherlands. We will focus on four functionalities.
Work Package 1 – Flooding and drainage in urban areas
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.
Work Package 3 – Wave-driven flooding
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.
Work Package 4 – Breaching of clay dikes
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.
The four new functionalities will have been tested for a number of test cases that the project participants will bring to the project and execute, with reports showing the results, limitations and potential improvements to be made. Some of the cases will be taken up in the SFINCS testbed.
At the end of the TKI project, flood risk assessments can be conducted across a wide range of topographic types (coastal, riverine, urban) at large spatial scales (of 100s of kilometres) for surge, waves, rain and riverine drivers. Using its speed, a probabilistic approach allows many scenarios to run and to explore uncertainties in the meteorological and hydrodynamic forcing as well as the uncertainties in the response of dikes and drainage systems. In this way, the solution space for possible interventions such as flood mitigating measures can be explored.
The TKI project will also be a platform for users to interact. This will benefit all parties, not the least the Dutch consultants, who, with better knowledge of the physical context and improved software, will have an advantage in the international consultancy market. This interaction also benefits the quality of the software meeting market needs, the quality of the projects that will be executed using the software, and the long-term sustainability of SFINCS. We hope to establish this long term perspective through shared use of the software, and user conferences that are held annually (outside the proposed project).
