Spatial Variations in River-Bed Porosity - Stratification and Imbrication

河床是由泥沙颗粒和充满水的孔隙组成的。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、饮用水水库的淤积速率以及古河流沉积物中石油和地下水的可采储量。尽管它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。在一个联合DFG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度的创新技术。这些技术，迄今为止前所未有的准确性，然后被用来量化河床孔隙度在微观，中观和宏观尺度上的空间变化幅度。此外，我们确定了颗粒尺寸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）研究已经发表了4篇同行评议的期刊论文和一篇博士论文（很快会有更多）。这些新技术的高精度揭示了一个有趣且意想不到的现象：在相同的沉积物样品上，现场测量的孔隙度值与实验室测量的孔隙度值根本不同。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——从地质学角度来说——受到分层和叠瓦作用的影响。解开这些影响对于理解和预测砂砾河床的空间孔隙度变化至关重要。本文提出的研究目的是确定分层和叠瓦作用对孔隙度的影响，重点研究河流砂砾石混合物。需要回答以下问题：层理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解释这种影响呢？这种效应与河工的日常实践有何关联？为了回答这些问题，首先将根据材料科学和地质学的理论原理建立分层和瓦叠程度的数学参数化。然后，实验室实验和数值模拟将提供分层和瓦叠参数与孔隙度之间的数学关系。该关系将通过在法国收集的大量现场数据进行验证，并用于扩展目前DFG项目的孔隙度预测。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，本文提出的研究对河流管理的好处将以德国尼德莱茵河沉积物管理的案例研究为例。

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.