Estimating the long-term evolution of river bed levels using hydrometric data

Jérôme Le Coz,Graeme Smart,Benoît Camenen,Benjamin Renard,Murray Hicks,Valentin Mansanarez,Michel Lang

doi:10.1051/e3sconf/20184006003

Jérôme Le Coz, Graeme Smart + Show 5 more

Open Access

https://doi.org/10.1051/e3sconf/20184006003

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Abstract

The stage-discharge measurements and rating curves accumulated over decades at hydrometric stations are a valuable source of information on the long-term evolution of river bed levels. However, the methodology to extract meaningful geomorphic information from such hydrometric data is not straightforward. We introduce an original method to estimate the parameters of successive rating curves by Bayesian analysis in sequence. These parameters reflect the physical properties of the channel features that control the stage-discharge relation: low-flow riffles, main channel, floodway (bars), floodplain, etc. The dates of rating changes are assumed to be known in existing hydrometric records. The uncertainty interval of each parameter is estimated, assuming, however, that no rating change has been ignored by the station manager. It is thus possible to clearly distinguish overall trends of the channel bed level from the local evolution of riffles and to evaluate whether the observed temporal changes are significant compared to the estimation uncertainties.

Highlights

The multi-decadal evolution of the geometry of river beds is only rarely monitored, typically with topographic surveys
The stage-discharge measurements and rating curves accumulated over decades at hydrometric stations are a valuable source of information on the long-term evolution of river bed levels
The choice of the reference flow is important because it determines the nature of the morphological feature the evolution of which is monitored: for a low flow rate, the stage will generally be determined by the width and height of a local cross-section, whereas for an intermediate flow it will be representative of the average level of the main channel

Summary

Introduction

The multi-decadal evolution of the geometry of river beds is only rarely monitored, typically with topographic surveys (cross-sections, longitudinal talweg or water-surface profiles) Even when such data exist, their analysis poses problems of spatial and temporal representativeness: even when cross-sections are surveyed at the same location, they may not be representative; talweg is mobile and not fully representative of the average bottom level; water-surface profiles are measured for different flows, etc. This work is a development of the BaRatin method [1] available for the Bayesian estimation of unique stage-discharge rating curves These parameters no longer depend on a reference flow to be fixed, but are linked to the various hydraulic controls identified: low-flow control riffle, main channel uniform flow control, floodway (bars), floodplain, etc. The method is illustrated with two unstable rivers in New Zealand, each subject to different natural or anthropogenic pressures

Morphogenic changes in hydraulic controls

Bayesian estimation of rating curves

Application

Conclusions and perspectives