Modelling transverse dunes

Transverse dunes appear in regions of mainly unidirectional wind and high sand availability. A dune model is extended to two‐dimensional calculation of the shear stress. It is applied to simulate dynamics and morphology of three‐dimensional transverse dunes. In the simulations they seem to reach translational invariance and do not stop growing. Hence, simulations of two‐dimensional dune ?elds have been performed. Characteristic laws were found for the time evolution of transverse dunes. Bagnold's law of the dune velocity is modi?ed and reproduced. The interaction between transverse dunes led to the interesting conclusion that small dunes can travel over bigger ones. Copyright © 2004 John Wiley & Sons, Ltd.     

Approaches to analyse and model changes in impacts: reply to discussions of “How to improve attribution of changes in drought and flood impacts”*

ABSTRACT

We thank the authors, Brunella Bonaccorso and Karsten Arnbjerg-Nielsen for their constructive contributions to the discussion about the attribution of changes in drought and flood impacts. We appreciate that they support our opinion, but in particular their additional new ideas on how to better understand changes in impacts. It is great that they challenge us to think a step further on how to foster the collection of long time series of data and how to use these to model and project changes. Here, we elaborate on the possibility to collect time series of data on hazard, exposure, vulnerability and impacts and how these could be used to improve e.g. socio-hydrological models for the development of future risk scenarios.     

Wetlands with controlled drainage and sub‐irrigation systems—modelling of the water balance

Over the past centuries, the agricultural use of wetlands in Central Europe has required interference with the natural wetland water balance. Often this has consisted of drainage measures alone. In low‐precipitation areas, it has also involved the operation of combined drainage and sub‐irrigation systems. Model studies conducted as part of planning processes, or with a view to finding out the impact of changing climate conditions on the water balance of wetlands, must take these facts into account. For this reason, a water balance model has been devised for wetlands whose water balance is governed by water resources management systems. It is based on the WBalMo model system. Special modules were integrated into WBalMo to calculate the water balance of wetland areas (WABI module) and to regulate inflow partitioning within the wetland (REGINF module). When calculating the water balance, the WABI module takes into account precipitation and potential evapotranspiration, groundwater levels below surface, soil types, land‐use classes, inflows via the running water system, and data for target water levels. It provides actual evapotranspiration, discharge into the running water system, and groundwater levels in the area. The example of the Spreewald, a major wetland area in north‐eastern Germany, was used to design and test the WBalMo Spreewald model. The comparison of measured and calculated water balance parameters of the wetland area confirms the suitability of the model for water balance studies in wetlands with complex water resources management systems. The results reveal the strong influence of water management on the water balance of such areas. The model system has proved to be excellently suited for planning and carrying out water management measures aimed at the sustainable development of wetlands. Furthermore, scenario analyses can be used to assess the impact of global change on the water balance of wetlands. Copyright © 2006 John Wiley & Sons, Ltd.     

Regional grain size variations in aeolian sediments along the transition between Tibetan highlands and north‐western Chinese deserts – the influence of geomorphological settings on aeolian transport pathways

Sediment distribution is investigated applying grain size analysis to 279 surface samples from the transitional zone between high mountains (Qilian Shan) and their arid forelands (Hexi Corridor) in north‐western China. Six main sediment types were classified. Medium scale (10³ m) geomorphological setting is carefully considered as it may play an important role concerning sediment supply and availability. A tripartite distribution of sedimentological landscape units along the mountain to foreland transition is evident. Aeolian sediments (e.g. loess and dune sands) are widespread. They are used to identify aeolian transport pathways. The mU/fS‐ratio (5–11 µm/48–70 µm) among primary loess opposes the two grain size fractions being most sensitive to varying accumulation conditions. The first fraction is attributed to long‐distance transport in high suspension clouds whereas the latter represents local transport in saltation mode. The ratio shows strong correlation with elevation (R² = 0.77). Thus, it indicates a relatively higher far‐traveled dust supply in mountainous areas (>3000 m above sea level [a.s.l.]) compared to the foreland. The contribution of westerlies to high mountain loess deposits is considered likely. Hereby, the influence of the geomorphological setting on grain size composition of aeolian sediments becomes apparent: the contribution from distant dust sources is ubiquitous in the study area. However, the far‐distance contribution may be reduced by the availability of fine sand provided in low topography settings. Plain foreland areas support fine sand deflation from supplying river beds, allowing the formation of sandy loess in foreland areas and intramontane basins. In contrast, high mountain topography inhibits strong sand deflation into loess deposits. Eastern parts of the Hexi Corridor show higher aeolian sand occurrence. In contrast, the western parts are dominated by gravel gobi surfaces. This is attributed to higher sand supply in eastern parts provided by the Badain Jaran Desert and fluvial storages as sand sources. Copyright © 2014 John Wiley & Sons, Ltd.     

How to improve attribution of changes in drought and flood impacts

ABSTRACT

For the development of sustainable, efficient risk management strategies for the hydrological extremes of droughts and floods, it is essential to understand the temporal changes of impacts, and their respective causes and interactions. In particular, little is known about changes in vulnerability and their influence on drought and flood impacts. We present a fictitious dialogue between two experts, one in droughts and the other in floods, showing that the main obstacles to scientific advancement in this area are both a lack of data and a lack of commonly accepted approaches. The drought and flood experts “discuss” available data and methods and we suggest a complementary approach. This approach consists of collecting a large number of single or multiple paired-event case studies from catchments around the world, undertaking detailed analyses of changes in impacts and drivers, and carrying out a comparative analysis. The advantages of this approach are that it allows detailed context- and location-specific assessments based on the paired-event analyses, and reveals general, transferable conclusions based on the comparative analysis of various case studies. Additionally, it is quite flexible in terms of data and can accommodate differences between floods and droughts.     

A GPS-Based Online Control and Alarm System

A global positioning system (GPS)-based online control and alarm system (GOCA) for monitoring of three-dimensional movements has been developed at the Karlsruhe University of Technology. The GOCA hardware consists of an array of GPS sensors and communication units to be placed in the monitoring area. The hardware-dependent control software communicates with the GPS sensors and provides the GPS baseline data and covariance information to the GOCA deformation analysis software. The GOCA center, which comprises both the control software and the GOCA software, may be linked – for example, over a long distance – to another personal computer (PC) that serves as a remote control station. GOCA is able to provide the full capabilities of classical deformation analysis online (with stations grouped into stable points and moving object points). Both types of points may be occupied either continuously or over short periods at different times. The object points are determined with respect to the stable points. A network adjustment is performed for each interval of data collection, and the coordinate and covariance information may optionally be transformed into a specific reference system (e. g., the building system). Unstable reference points are to be detected by statistical tests. The estimated object point time series are filtered with respect to gross errors using robust estimation techniques. Online filters are used to smooth the time series data of critical displacements and to predict other deformation functions. The time series data, as well as prediction results, are displayed graphically for each object point. An example concerning the online monitoring of a slag heap in a coal-mining area is included. ? 2000 John Wiley & Sons, Inc.