Spatial Variations in River-Bed Porosity - Stratification and Imbrication

河床由沉积物颗粒和充满水的孔隙组成。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DFG联合项目(2016-2019年)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。到目前为止，这些技术以前所未有的精度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，我们确定了颗粒大小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1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而意想不到的现象：现场测量的孔隙度值与实验室对同一沉积物样本测量的孔隙度值根本不同。这表明，与通常的假设不同，河流沉积的孔隙度受到沉积物颗粒在沉积物中所处位置的强烈影响，特别是在地质方面，受到层化和叠置效应的影响。揭示这些影响对于理解和预测砂砾河床河流的空间孔隙度变化是至关重要的。本文提出的研究目的是确定层化和叠置对孔隙度的影响，重点是河流相砂砾混合体。以下问题需要回答：层化和叠置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？这种影响与河流工程师的日常实践有什么关系？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，开发出层化和叠置程度的数学参数。然后，实验室实验和数值模拟将提供层化和叠置参数与孔隙度之间的数学关系。这一关系将根据将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项目的孔隙度预报器。对于实验室实验，将使用先进的创新技术进行数值模拟和现场测量，特别是摄影测量学、磁共振成像和基于物理的填充模型。最后，本文提出的研究对河流管理的好处将在德国尼德海因泥沙管理的案例研究中得到例证。

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.