Abstract
Hydropower plants can profoundly modify the natural morphology and hydrology of rivers and can alter the functionality of habitats for fish living and reproduction. In particular, hydropeaking leads to rapid and frequent changes of the hydrodynamic conditions and it is crucial to ensure aquatic habitat quality is maintained as much as possible during these periods. One present method to determine mitigation measure (minimum and maximum flow, rate of change) is to perform hydraulic simulations in 1D or 2D in the range of flow variation and to evaluate habitat quality for fish with the microhabitat method. The hydraulic model calibration has to be conducted carefully since the model has to reproduce precisely the hydraulic conditions from low to high flow rates (up to several times the mean flow of the rivers). Within this range, the friction coefficient can evolve greatly because at low flows the size of roughness elements become comparable to the water depth.. This phenomenon is observed by performing the modelling and the calibration at 2 stations on a river in French Pyrenees with different cross section shapes, one with progressive overflowing of some banks with large blocks and the other one without such phenomena. Thanks to field measurements of water levels at low and high discharge, the calibration process has shown that the friction coefficient can be multiplied by 2 as a function of the discharge. The paper proposes a methodology to evaluate the most appropriate tool. As water depth is concerned, the 2D simulations (TELEMAC 2D) provide similar results to those obtained with 1D (HEC-RAS) because flow remains unidirectional. Then a sensitivity analysis is carried out to estimate the uncertainty on the fish habitat outputs for a fish species (brown trout in the present study) resulting from several widely used friction laws. These friction laws can lead to different conclusions about habitat suitability depending on the calibrated coefficient. Finally, to perform relevant habitat modelling, it is necessary to measure water levels at several discharges and to describe accurately the spatial variability of roughness height.
Highlights
Anthropic activities can either decrease or increase the natural discharge by withdrawal or hydropeaking respectively
The aquatic habitat depends on the hydrology and the discharge flowing in the river and the impact on ecosystem is a crucial issue for river management
The microhabitat method has been applied using 1D and 2D hydraulic models calibrated with several methods, corresponding to available tools used for engineering purposes
Summary
Anthropic activities can either decrease or increase the natural discharge by withdrawal or hydropeaking respectively. The aquatic habitat depends on the hydrology and the discharge flowing in the river and the impact on ecosystem is a crucial issue for river management. To be relevant the hydraulic model has to be coupled with biological model which indicates the suitability to hydraulic parameters of each fish species and life stages or activities (reproduction, alevins, juvenile, adult). This coupling is the base of the microhabitat method which is integrated in software such as EVHA [3]. The model has to be relevant for both low and high discharge
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