The generation of offshore tidal sand banks and sand waves

A simple morphological model is considered which describes the interaction between a tidal flow and an erodible bed in a shallow sea. The basic state of this model describes a spatially uniform tide over a flat bottom where the flow vector is represented as a tidal ellipse. The linear stability of this solution is analysed with respect to bed form perturbations. Results are presented for both a uni-directional and circular tide. In the former case the wave-length and the orientation of the fastest growing bed mode agree well with those of tidal sand banks. However, this model only predicts the growth of large-scale sand ridges. With a simplified numerical model we tentatively show that the effects of secondary currents on the sediment transport trigger the formation of instabilities at an essentially smaller scale, viz, sand waves. Another limitation of a model with uni-directional tides is that no selective modes found are the first to become unstable if the model parameters are varied. In the case of a circular tide, critical model parameters are found below which the basic state is stable. We conclude that this provides a starting point for the development of a weakly non-linear analysis, which will yield information on the amplitude behaviour of marginally growing bed forms

Building with Nature: Mainstreaming the Concept

Keynote Lecture

Objectifying Building with Nature strategies: Towards scale-resolving policies

By definition, Building with Nature solutions utilise services provided by the natural system and/or provide new opportunities to that system. As a consequence, such solutions are sensitive to the status of, and interact with the surrounding system. A thorough understanding of the ambient natural system is therefore necessary to meet the required specifications and to realise the potential interactions with that system. In order to be adopted beyond the pilot scale, the potential impact of multiple BwN solutions on the natural and societal systems of a region need to be established. This requires a ‘reality check’ of the effectiveness of multiple, regional-scale applications in terms of social and environmental costs and benefits. Reality checking will help establish the upscaling potential of a certain BwN measure when addressing a larger-scale issue. Conversely, it might reveal to what extent specific smaller-scale measures are suitable in light of larger regional-scale issues. This paper presents a stepwise method to approach a reality check on BwN solutions, based on the Frame of Reference method described in a companion paper (de Vries et al., 2020), and illustrates its use by two example cases. The examples show that a successful pilot project is not always a guarantee of wider applicability and that a broader application may involve dilemmas concerning environment, policy and legislation

ABLooM: Location behaviour, spatial patterns, and agent-based modelling

This paper presents an Agent-based LOcation Model (ABLOoM). ABLOoM simulates the location decisions of two main types of agents, namely households and firms. The model contains multiple interactions that are crucial in understanding land use changes, such as interactions of agents with other agents, of agents with their environment and of agents with emerged patterns. In order to understand the mechanisms that are at the basis of land use changes and the formation of land use patterns, ABLOoM allows us to study human behaviour at the microlevel in a spatial context. The models, which include economic theory, aspects of complexity theory and decision rules, show that it is possible to generate macrolevel land use patterns from microlevel spatial decision rules

Intermediate Scale Coastal Behaviour: Measurement, Modelling And Prediction

LONG-TERM GOAL: Our overall goal is to achieve a better understanding and better predictions of coastal behaviour at intermediate (event/season/year/decade) scales. We aim to bring together researchers from Europe and North America to gain the best possible benefit from developments in field observation, theory and numerical modelling.Award #: N00014-97-1-079

Evolution of marine morphodynamic modelling: Time for 3-D? In: Proceedings of 'New challenges for North sea research, Hamburg 1996

The question whether it is time for marine morphodynamic models to take into account the 3-D nature of water and sediment motion is discussed starting from practical demand and the state-of-the-art in basic knowledge and model development. This leads to the observation that there is a clear practical demand, and that suitable software environments and some 3-D model components exist, but that the basic knowledge on 3-D morphodynamic processes and the necessary know-how for utilizing these models are still insufficient. Hence it is concluded that there is an urgent need for modelling-oriented research on 3-D marine morphodynamics, but that it is too early to develop modelling facilities for practical application