Sampling sufficiency for determining hydraulic habitat diversity

Abstract

The geometry and hydrodynamics of river reaches are key ecohydraulic descriptors. Statistics of water depth and velocity measurements are usually taken as proxies for habitat suitability in rivers. However, little is known about the sufficiency of data to produce effective and representative results. In this research, 19 reaches with differences in terms of discharge, river width, substrate, reach length, cross-section spacing and geomorphology are investigated. Measurements of flow depth and velocity were taken at multiple, equally spaced cross-sections along each reach. Data were sub-sampled using different methodologies and analysed each time. The sets of sub-sampled data were then compared with those calculated with the full data set from a reach. The focus was put towards the hydro-morphological index of diversity (HMID), a combination of the classical ecohydraulic variables flow depth and velocity. It represents the spatial variability of hydraulic habitats in a reach. The results point out that, with a well-defined sampling strategy, 100 measurement points lead to a good estimation of the HMID value in a reach, if more than eight measurement points are taken per cross-section. For geomorphologies with small complexity or when the analysis only includes the estimation of mean flow depth or mean flow velocity, this number can be decreased according to the results presented here. These findings help both, aquatic ecologists and engineers to estimate their data reliability for hydraulic field measurements in a river reach and are herein discussed taking into account the different studied morphologies.

Keywords:

• Field data acquisition
• ecohydraulics
• river restoration
• geomorphology
• hydro-morphological index of diversity

Acknowledgements

Pierre Bourqui, Jonas Durand-Gasselin, Anthony Maître and Laura Vigne contributed in the framework of their master thesis to the field data collection. Elena Battisacco and Diego Tonolla helped during the field campaigns in the Sarine River. Constructive comments from Nicolas Lamouroux and two anonymous reviewers increased the quality of this article.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This research project was part of the National Research Programme "Energy Turnaround" (NRP 70, www.nrp70.ch) of the Swiss National Science Foundation (SNSF, Project No. 153972). The first author has a complementary funding by the Swiss Federal Office of Energy (SFOE, Project No. SI/501673-01501673-01).