The long-term sediment demand of the Dutch coast is integral to the current Dutch Coastal Flood and Erosion Risk Management policy. The Coastal Genesis 2 research programme was initiated to address the sustainability of this policy under sea level rise by focusing on key uncertainties in the conceptual model of the sediment demand of the Dutch coast. The substantive scientific contributions of the Coastal Genesis 2 research programme are analysed in this paper by applying an output-outcome-impact framework. The direct outputs of the programme are categorised in terms of the knowledge types of a 5-element framework, namely measurement data, simulation models, system understanding, conceptual models, and policy and practice. The research outcomes arise from the interactions of these knowledge types. Our analysis of these outcomes highlights that synthesising new scientific insights into shared conceptual models is critical to achieving impact in policy and practice. In the Dutch situation, a new shared conceptual model of the long-term sediment demand enabled the development of four potential nourishment strategies aiming to meet the strategic goals of the Coastal Flood and Erosion Risk Management policy on a timescale up to 20 years. In 2021, the Minister of Infrastructure and Water Management officially articulated her intention to adopt the advised nourishment strategy from 2024 onwards. This represents a lasting impact of the Coastal Genesis 2 research programme in policy and practice. Further, the insight regarding the pivotal role of shared conceptual models as intermediary between science, policy and practice may prove useful in the design of future research programmes aiming to influence policy.@en

Dutch coastal policy aims for a safe, economically strong and attractive coast. This is achieved by maintaining the part of the coast that support these functions; the coastal foundation. The coastal foundation is maintained by means of sand nourishments. Up to now, it has been assumed that net transports across the coastal foundation's offshore boundary at the 20 m depth contour are negligibly small. In the framework of the Coastal Genesis 2.0 program we investigated sand transports across this boundary and across other depth contours at the lower shoreface. This paper presents a computationally efficient approach to compute the annual sand transport rates at the Dutch lower shoreface. It is based on the 3D Dutch Continental Shelf Model with Flexible Mesh (3D DCSM-FM), a wave transformation tool and a 1DV sand transport module. We validate the hydrodynamic input against field measurements and present flow, wave and sand transport computations for the years 2013–2017. Our computations show that the net annual sand transport rates along the Dutch coast are determined by peak tidal velocities (and asymmetry thereof), density driven residual flows, wind driven residual flows and waves. The annual mean alongshore transports vary along the continuous 20 m depth contour. The computed total cross-shore transports are onshore directed over the continuous 20 m, 18 m and 16 m depth contours and increase with decreasing water depth. The effect of density difference and wind on the 3D structure of the flow and on the sand transports cannot be neglected along the Dutch lower shoreface. Our computations show that excluding the effect of density results in a significant decrease of the onshore directed transports. Also switching off wind largely counteracts this effect. The net cross-shore transport is determined by a delicate balance between gross onshore and offshore transports, where wave conditions are important. We show an example for Scheveningen where the net cross-shore transport is onshore directed when including all wave conditions but would be offshore directed when excluding waves higher than 3.5 m. In contrast, at Callantsoog the highest waves contribute more to the offshore directed transports. These results suggest that storm conditions play an important role in the magnitude and direction of the net annual transport rates at the lower shoreface.

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