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

A parameterization of flow separation over subaqueous dunes

Flow separation plays a key role in the development of dunes, and modeling the complicated flow behavior inside the flow separation zone requires much computational effort. To make a first step toward modeling dune development at reasonable temporal and spatial scales, a parameterization of the shape of the flow separation zone over two-dimensional dunes is proposed herein, in order to avoid modeling the complex flow inside the flow separation zone. Flow separation behind dunes, with an angle-of-repose slip face, is characterized by a large circulating leeside eddy, where a separation streamline forms the upper boundary of the recirculating eddy. Experimental data of turbulent flow over two-dimensional subaqueous bed forms are used to parameterize this separation streamline. The bed forms have various heights and height to length ratios, and a wide range of flow conditions is analyzed. This paper shows that the shape of the flow separation zone can be approximated by a third-order polynomial as a function of the distance away from the flow separation point. The coefficients of the polynomial can be estimated, independent of flow conditions, on the basis of bed form shape at the flow separation point and a constant angle of the separation streamline at the flow reattachment point. \ud \u

Participatory plant breeding: a way to arrive at better-adapted onion varieties

The search for varieties that are better adapted to organic farming is a current topic in the organic sector. Breeding programmes specific for organic agriculture should solve this problem. Collaborating with organic farmers in such programmes, particularly in the selection process, can potentially result in varieties better adapted to their needs. Here, we assume that organic farmers' perceptive of plant health is broader than that of conventional breeders. Two organic onion farmers and one conventional onion breeder were monitored in their selection activities in 2004 and 2005 in order to verify whether and in which way this broader view on plant health contributes to improvement of organic varieties. They made selections by positive mass selection in three segregating populations under organic conditions. The monitoring showed that the organic farmers selected in the field for earliness and downy mildew and after storage for bulb characteristics. The conventional breeder selected only after storage. Farmers and breeder applied identical selection directions for bulb traits as a round shape, better hardness and skin firmness. This resulted in smaller bulbs in the breeders’ populations, while the bulbs in the farmer populations were bigger than in the original population. In 2006 and 2007 the new onion populations will be compared with each other and the original populations to determine the selection response

The effect of sediment transport models on simulating river dune dynamics

River dunes, dynamic bedforms in the river bed, limit navigable depths during low flows and increase bed roughness. To predict the navigable depth or where maintenance dredging is needed, a numerical dune development model can be a powerful tool. To study the effect of sediment transport on dune shape and propagation, four different sediment transport models were applied in an existing dune development model. Each sediment transport model was able to simulate dune propagation, while only sediment transport models based on the shear stress reshaped the river dunes. The tested sediment transport models can simulate dune celerity similar to observations and realistic, though different, dune shapes for low and median discharges. Implementation of a gravitational bed slope effect combined with a critical shear stress results in low angle dunes, which are representative for river dunes during low river flows. Sediment transport models with spatial relaxation, also result in low angle dunes. However, the relaxation parameters need to be redefined for low flow situation to prevent transition to upper stage plane bed at too low flow velocities. Further analysis of the resulting dune shapes shows that the sediment transport model determines the dune shape in terms of slope angles, while the dune height is related to the total transport capacity

Sustainable organic plant breeding: Final report - a vision, choices, consequences and steps

In general, the characteristics of organic varieties - and by extension of organic plant breeding - differ from that of conventional breeding systems and conventional varieties. Realising an organic plant breeding system and subsequently steering it to meet changing demands is no less than a mammoth task. The many actions to be undertaken can be divided into short-term commercial and scientific activities, and longer or long-term commercial and scientific activities. Action must be taken in the short-term to ensure adequate quantities of organically propagated plants and seed. This is vital in consideration of Regulation 2092/91/EC which states that, as of 1 January 2000, all propagating material used in organic production must be of organic origin. Additional measures are needed to accelerate the development of organically propagated varieties. Within the breeding sector, variety groups should be established to streamline communication in the chain. Variety groups should have a large contingent of farmers, as well as representatives from the trade branch and breeders. Members should communicate intensively with each other, share experiences, and participate in trials and variety assessments. Questions, wishes and bottlenecks could be recorded by variety groups and passed on to other parties in the chain. The practical details of the plant health concept which is at the basis of organic breeding must be worked out (operationalised). This will require scientific research, for example on: root development and mineral absorption efficiency weed suppressive capacity in situ versus ex situ maintenance resistance breeding in combination with cultivation measures seed-transmitted diseases adaptive capacity alternatives for growth stimulants, silver nitrate and silver thiosulfate in the cultivation of cucumbers and pickles Such research should be carried out by academic institutions (such as Wageningen University and Research Centre) in collaboration with Louis Bolk Institute, Stichting Zaadgoed and private companies. A platform should be established to make an inventory of problems and priorities and to develop research proposals. Farmers could contribute their ideas to the platform through the variety groups. Conclusion A plant breeding system for organic production should be based on the organic concept of plant health and on the organic position on chain relationships. As the total land area under organic production is still relatively small, it is unlikely that commercial breeders will make large investments to develop organic breeding programmes without financial support from other parties, i.e. the government. In this early stage, it is vital that the government provides generous funding and plays an active enabling role. We hope that the action plan to stimulate organic plant breeding, as requested by Parliament, will dovetail with the activities described above

The effect of building geometry on aeolian deposition patterns:a field experiment

Worldwide, beaches are popular for recreation. As a result, people have built (holiday) houses, restaurants and other buildings at the land-sea interface. These buildings affect the wind field and wind-driven sand transport in their surroundings, thereby shaping the development of the beachdune system. Currently, society is facing a growing demand for buildings on the beach, but at the same time it lacks knowledge on the effect these buildings have. Therefore, we aim to understand the effect of buildings on the beach-dune interface. In this contribution, we present the results of a field experiment that, as a first step, documents the effect of building geometry on the size of aeolian deposition patterns around buildings in an open sandflat setting.<br/

Modeling the Cross‐Sectional Dynamics of Tidal Sandbanks in Sediment‐Scarce Conditions

Tidal sandbanks are large-scale dynamic bedforms that consist of sandy sediment. They have been observed in shallow seas with varying sediment supply, including sediment-scarce environments like the Flemish Banks, Zeeland Banks, and Norfolk Banks. However, we do not yet understand how scarcity affects sandbank evolution. Therefore, we have developed an idealized nonlinear process-based model with the aim of studying cross-sectional shape and migration under sediment-scarce conditions. Scarcity is included through a non-erodible layer from which no sediment can be entrained. Our results show that bank height and width decrease when the sediment budget decreases. The bank height is more sensitive to scarcity than bank width. Furthermore, sand scarcity decreases (and may even reverse) bank asymmetry and increases migration rate when a residual current is present. The migration rate attains a maximum for a specific sediment budget, which is controlled by the ratio of the cross-sectional area of the sandbank and the tide-averaged sediment flux. Our findings show that sandbank dynamics are strongly affected by scarcity, which is critical for seas with receding sediment stocks (e.g., through extraction)