Spatial Variations in River-Bed Porosity - Stratification and Imbrication

河床是由泥沙颗粒和充水孔隙组成的。孔隙含量（或孔隙度）决定了河床作为水生生物栖息地的适宜性、航道的稳定性、饮用水水库的淤积率以及古河流沉积物中石油和地下水的可开采量。尽管其具有经济和社会意义，但直到最近，人们对河流系统孔隙度的自然变化知之甚少。在DFG联合项目（2016-2019）中，我们解决了这一知识差距。首先，我们开发了测量孔隙率的创新技术。这些技术，迄今为止前所未有的准确性，然后被用来量化在微观，中观和宏观尺度的河床孔隙度的空间变化的幅度。此外，我们确定了粒度和颗粒形状对孔隙度的影响，并开发了一种新的基于物理的方法来预测孔隙度的空间变化。到目前为止，这项（正在进行的）研究产生了4篇同行评议的期刊论文和一篇博士论文（更多的后续）。这些新技术的高精度揭示了一个有趣的和意想不到的现象：在现场测量的孔隙度值是从根本上不同的孔隙度值在实验室测量的同一沉积物样品。这表明，与通常假设的不同，河流沉积物的孔隙度受到沉积物颗粒在沉积物中的定位方式的强烈影响，特别是在地质学方面，受到分层和叠瓦效应的影响。解开这些影响是必不可少的，以了解和预测砂卵石河床的空间孔隙度变化。这里提出的研究的目的是确定分层和叠瓦对孔隙度的影响，重点是河流砂砾石混合物。下面的问题是要回答的：分层和叠瓦作用如何影响孔隙度？这种影响有多大？如何在孔隙度预测中考虑这种影响？这种效应与河流工程师的日常实践有什么相关性？为了回答这些问题，首先将基于材料科学和地质学的理论原理开发分层和叠瓦程度的数学参数化。通过室内实验和数值模拟，建立了层状和叠瓦状参数与孔隙度之间的数学关系。这种关系将根据在法国收集的大量现场数据进行验证，并用于扩展我们当前DFG项目的孔隙度预测器。对于实验室实验，数值模拟和现场测量将使用先进的创新技术，特别是摄影测量，磁共振成像和基于物理的包装模型。最后，这里提出的研究对河流管理的好处将在德国下莱茵河的沉积物管理的案例研究中举例说明。

River beds are composed of sediment particles and water-filled pores. The pore content (or porosity) determines the suitability of the river bed as a habitat for aquatic organisms, the stability of navigation channels, the siltation rate of drinking-water reservoirs and the exploitable volume of oil and groundwater in ancient river deposits.Despite its economic and societal relevance, until recently, little was known about natural variations in porosity in fluvial systems. In a joint DFG Project (2016-2019) we addressed this knowledge gap. First, we developed innovative techniques to measure porosity. These techniques, with hitherto unprecedented accuracy, were then used to quantify the magnitude of spatial variations in river bed porosity at micro-, meso- and macroscale. Furthermore, we determined the effect of grain size and grain shape on porosity, and developed a new physics-based method to predict spatial variations in porosity. So far, this (ongoing) study resulted in 4 peer-reviewed journal papers and one PhD thesis (more following soon). The high precision of these new techniques revealed an interesting and unexpected phenomenon: porosity values measured in the field are fundamentally different from porosity values measured in the laboratory on the same sediment sample. This shows, that – other than often assumed - the porosity of fluvial deposits is strongly affected by the way in which sediment grains are positioned in sediment deposits, especially – in geological terms – by stratification and imbrication effects. Unravelling these effects is essential to understand and predict spatial porosity variations in sand-gravel bed rivers. The objective of the research proposed here is to determine the effect of stratification and imbrication on porosity, focusing on fluvial sand-gravel mixtures. The following questions are to be answered: How do stratification and imbrication affect porosity? How big is this influence? How can this effect be accounted for in porosity predictions? What is the relevance of this effect for a river engineer’s daily practice?To answer these questions, first a mathematical parametrization of the degree of stratification and imbrication will be developed based on theoretical principles from material sciences and geology. Laboratory experiments and numerical simulations then will provide a mathematical relation between the stratification and imbrication parameters and porosity. This relation will be validated against extensive field data to be collected in France and used to extend the porosity predictor from our current DFG project. For laboratory experiments, numerical simulations and field measurements use will be made of advanced, innovative techniques, specifically photogrammetry, magnetic resonance imaging and physics-based packing models. Finally, the benefits to river management of the study proposed here will be exemplified in a case-study of sediment management in the German Niederrhein.