Spatial variability in river bed porosity determined by nuclear density gauging: A case study from a French gravel‐bed river

Sedimentology, vol. 69, pp. 823–844

Abstract

Abstract Porosity is one of the key properties of fluvial sediments. It is defined as the ratio of pore volume to total volume. In river science, porosity is often assumed to be spatially constant, which might be a gross simplification of reality. Ignoring the spatial variations in porosity can cause errors in morphological, ecological, hydrological, hydrogeological and sedimentological applications. Although detailed information about spatial porosity variations can be obtained from porosity measurements at field sites, such information has never been collected where these variations might be important. In this study, field porosity measurements were carried out to quantify the magnitude of the spatial porosity variation for four different sedimentological environments of a braided river: a confluence, a tributary delta, a braid bar and a secondary channel. A nuclear density gauge was used for the measurement of porosity. The nuclear density gauge proved to be a time‐saving and labour‐saving technique that produces accurate porosity values with a root mean square error of 0.03. The four sedimentological environments showed significant differences in porosity, with mean porosity being lower for confluence and bar than for delta and secondary channel. Semi‐variogram analysis showed the absence of any spatial correlation in porosity for distances beyond 4 m. This shows that distance cannot be used as a parameter for porosity extrapolation in a fluvial system unless the extrapolation distance is less than 4 m. At least eight measurements of porosity are required to obtain a reliable estimate of mean porosity in a sedimentary environment, i.e. with uncertainty <0.03. Although grain size characteristics were found to have a significant impact on porosity, the relationships between these parameters and porosity were not very strong in this study. The unique porosity dataset, presented in this article, provides a valuable source of information for researchers and river managers.

Authors 4

1. Mina Tabesh corresponding Aachen Institute of Hydraulic Engineering and Water Resources Management

   Federal Institute of Hydrology · RWTH Aachen University

   Affiliation as printed

   Department of Fluvial Morphology, Sediment Dynamics and Management Federal Institute of Hydrology Am Mainzer Tor 1 56068 Koblenz Germany

   Institute of Hydraulic Engineering and Water Resources Management RWTH Aachen University Mies‐van‐der‐Rohe‐Straße 17 52074 Aachen Germany

   Department of Fluvial Morphology, Sediment Dynamics and Management, Federal Institute of Hydrology, Am Mainzer Tor 1, 56068 Koblenz, Germany

2. Stefan Vollmer

   Federal Institute of Hydrology

   Affiliation as printed

   Department of Fluvial Morphology, Sediment Dynamics and Management Federal Institute of Hydrology Am Mainzer Tor 1 56068 Koblenz Germany

   Department of Fluvial Morphology, Sediment Dynamics and Management, Federal Institute of Hydrology, Am Mainzer Tor 1, 56068 Koblenz, Germany

3. Holger Schüttrumpf Aachen Institute of Hydraulic Engineering and Water Resources Management

   RWTH Aachen University

   Affiliation as printed

   Institute of Hydraulic Engineering and Water Resources Management RWTH Aachen University Mies‐van‐der‐Rohe‐Straße 17 52074 Aachen Germany

4. Roy M. Frings

   Ministry of Infrastructure and Water Management · Rijkswaterstaat

   Affiliation as printed

   Rijkswaterstaat Zuid‐Nederland Ministry of Infrastructure and Water Management Avenue Ceramique 125 6221 KV Maastricht Netherlands

   Rijkswaterstaat Zuid-Nederland, Ministry of Infrastructure and Water Management, Avenue Ceramique 125, 6221 KV Maastricht, Netherlands