Impact of dredging and dumping on the stability of ebb–flood channel systems

The impact of dredging and dumping on the morphologic stability of the tidal channels is investigated using morphologic field observations for the Westerschelde estuary dating back to 1955. The results are used to verify the theoretical concept presented by Wang and Winterwerp (2001). This concept states that a critical threshold for the amount of sediment dumping exists above which a channel system in equilibrium may become unstable and degenerate. The value of this threshold amounts to 5–10% of the total sediment transport capacity. Verification of this concept using field observations is not straightforward as the morphology of tidal channel often changes as a result of both natural processes and human interferences, i.e. the channels are not in equilibrium. In addition, the morphological timescales associated with channel degeneration are large (decades to centuries). Verification of the theory thus requires a careful analysis of abundant morphological data and numerical modeling of sediment transports. The results of such analyses presented in this study confirm the existence and the approximate magnitude of the critical level for dumping that follows from theory. Refined guidelines are derived to use the theoretical concept as an engineering tool for the evaluation and design of strategies for dumping in estuarine multi-channel systems. In the absence of the required morphological data the indicative theoretical level of 5–10% can be used to obtain a first estimate of the dump capacity in two-channel systems.     

Improving Coal Quality Estimations with Geostatistics and Geophysical Logs

In most modern coal mines, there are many coal quality parameters that are measured on samples taken from boreholes. These data are used to generate spatial models of the coal quality parameters, typically using inverse distance as an interpolation method. At the same time, downhole geophysical logging of numerous additional boreholes is used to measure various physical properties but no coal quality samples are taken. The work presented in this paper uses two of the most important coal quality variables—ash and volatile matter—and assesses the efficacy of using a number of geostatistical interpolation methods to improve the accuracy of the interpolated models, including the use of auxiliary variables from geophysical logs. A multivariate spatial statistical analysis of ash, volatile matter and several auxiliary variables is used to establish a co-regionalization model that relates all of the variables as manifestations of an underlying geological characteristic. A case study of a coal mine in Queensland, Australia, is used to compare the interpolation methods of inverse distance to ordinary kriging, universal kriging, co-kriging, regression kriging and kriging with an external drift. The relative merits of these six methods are compared using the mean error and the root mean square error as measures of bias and accuracy. The study demonstrates that there is significant opportunity to improve the estimations of coal quality when using kriging with an external drift. The results show that when using the depth of a sample as an external drift variable there is a significant improvement in the accuracy of estimation for volatile matter, and when using wireline density logs as the drift variable there is improvement in the estimation of the in situ ash. The economic benefit of these findings is that cheaper proxies for coal quality parameters can significantly increase data density and the quality of estimations.

     

Dunefoot dynamics along the Dutch coast

The dynamics of the dunefoot along a 160 km portion of the Dutch coast has been investigated based on a data set of annual surveys dating back to as early as 1850. The linearly detrended (or residual) dunefoot positions comprise an alongshore uniform and an alongshore non‐uniform component. The former is expressed as 10 to 15 m of landward retreat along extensive (>10 km) stretches of coast during years with severe storm surges and as up to 5 m of seaward advance during years without significant storm activity. The latter, alongshore non‐uniform component is organized in sandwave‐like patterns, which may have a longevity of decades to up to the duration of the entire data set (150 years). Their wavelengths vary along the coast, from 3·5 to 10 km; migration rates are 0–200 m a^?1. Dunefoot sandwaves are shown to be the shoreward extensions of similar sandwave patterns in the beach position. The non‐uniform dunefoot behaviour constitutes at least 80 per cent of the total residual dunefoot dynamics, implying that along the Dutch coast residual dunefoot variability is controlled by temporal and spatial variability in beach characteristics, and not by storm‐induced uniform erosion. Various potential mechanisms causing beach sandwaves are discussed. Copyright © 2002 John Wiley & Sons, Ltd.