Free surface effect on dune morphology and evolution

Our aim in this paper is to illustrate the importance of free water surface effects and sediment transport mode in the morphological evolution of sand dunes to upper stage plane beds. We have analyzed a large number of bed form data, 414 experiments from flumes and field, showing significantly different evolution of dune height and length in shallow (high Froude numbers) and in deep flows (low Froude numbers). In shallow flows, dune heights are observed to grow only in bed load dominant transport regime and start to decay for Suspension numbers greater than 1. Dunes in this case are not observed for Suspension number greater than 2.5. For low Froude numbers, dune heights continue to grow from bed load to suspended load dominant transport regime. Dunes in this case are not observed for Suspension number greater than 5. Furthermore, dunes in shallow flows reach significantly greater heights compared to dune heights in deep flows and dune lengths are generally larger in shallow flows

Effect of flow forecasting quality on benefits of reservoir operation - a case study for the Geheyan reservoir (China)

This paper presents a methodology to determine the effect of flow forecasting quality on the benefits of reservoir operation. The benefits are calculated in terms of the electricity generated, and the quality of the flow forecasting is defined in terms of lead time and accuracy of the forecasts. In order to determine such an effect, an optimization model for reservoir operation was developed which consists of two sub-models: a long-term (monthly) and a short-term (daily) optimization sub-model. A methodology was developed to couple these two sub-models, so that both short-term benefits (time span in the order of the flow forecasting lead time) and long-term benefits (one year) were considered and balanced. Both sub-models use Discretized Dynamic Programming (DDP) as their optimization algorithms. The Geheyan reservoir on the Qingjiang River in China was taken as case study. Observed (from the 1997 hydrological year) and forecasted flow series were used to calculate the benefits. Forecasted flow series were created by adding noises to the observed series. Different magnitudes of noise reflected different levels of forecasting accuracies. The results reveal, first of all, a threshold lead time of 33 days, beyond which further extension of the forecasting lead time will not lead to a significant increase in benefits. Secondly, for lead times shorter than 33 days, a longer lead time will generally lead to a higher benefit. Thirdly, a perfect inflow forecasting with a lead time of 4 days will realize 87% of the theoretical maximum electricity generated in one year. Fourthly, for a certain lead time, more accurate forecasting leads to higher benefits. For inflow forecasting with a fixed lead time of 4 days and different forecasting accuracies, the benefits can increase by 5 to 9% compared to the actual operation results. It is concluded that the definition of the appropriate lead time will depend mainly on the physical conditions of the basin and on the characteristics of the reservoir. The derived threshold lead time (33 days) gives a theoretical upper limit for the extension of forecasting lead time. Criteria for the appropriate forecasting accuracy for a specific feasible lead-time should be defined from the benefit-accuracy relationship, starting from setting a preferred benefit level, in terms of percentage of the theoretical maximum. Inflow forecasting with a higher accuracy does not always increase the benefits, because these also depend on the operation strategies of the reservoir.\u

Getting grip on complex water issues? : a case study: Rotterdam Mainport, Appropriate Assessment Wadden Sea

In Dutch water systems many human interventions are carried out. These interventions are designed to achieve management goals, like increase protection against flooding, improve environmental quality and/or stimulate the national economy. Decision-makers involved in these kinds of plans have to deal with ecological effects, physical effects, economic costs and benefits and technical feasibility. Furthermore, they operate within a complex web of interactions between policy, regulations, and social and political processes. This PhD-project aims at assisting decision-makers in constructing assessments of the impact of future human interventions in water systems

A simple morphodynamic model for sand banks and large-scale sand pits subject to asymetrical tides

We extend existing knowledge on theoretical growth characteristics of tidal sand banks by including asymmetrical tides with an M0, M2 and M4-constituent, thus allowing for migration. Furthermore, in the context of the continuously increasing demand on the Dutch sand market, we show that creating a large-scale offshore sand pit has long-term morphological implications, both for the pit itself and the surrounding area. The pit deepens, while around it a sand bank pattern emerges, spreading at a constant rate of the order of tens to hundred metres per year

Optimization of offshore wind farm power cable routing

Up to now methods to optimize cable route layout are only based on a flat seabed and do not take the seabed dynamics into account (Jenkins et al., 2013; Morelissen et al., 2003). The result of this approach is that power cable coverage is not guaranteed over wind farm design lifetime. Cable optimization is mainly executed based on shortest routes instead of cost reduction over the entire design lifespan. The aim of this research is to develop a Matlab based tool, which optimizes power cable route design based on expected morphological behaviour in the design lifetime of an offshore wind farm

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