Long-Term Water Temperature Variations in a River Submitted to Hydropeaking

Downstream of seasonal reservoirs, rivers are subjected to discharge variations initiated by the periodic operation of the hydropower plant, including affected discharge magnitudes and gradients. Obviously, this technical regime interferes with the natural behavior of the local fauna, including fishes. In some cases, the fish population decreased presumably as a consequence of abrupt discharge and temperature variations. The first aspect is well documented in literature, whereas the second one remains vague. In fact, the release of cold water from low reservoir levels can strongly and abruptly perturb temperature-sensitive fishes living in the downstream river. Stress is generated due to the very short time scale of variations for both, discharge and temperature.

This paper presents the methodology and preliminary results of a study on long-term in-situ temperature measurements of a river submitted to hydropeaking. The thermopeaking characteristics are captured together with the operational regime and the meteorological conditions, as well as under the influence of two natural tributaries without hydropeaking.

The temperature measurements were taken along the Sarine River in central Switzerland between Rossens Dam and Maigrauge Dam (in the town of Fribourg). The related river reach is 15 km long and natural, i.e. without correction measures or power plants, situated in a wild gorge. The photo below gives an impression of the river characteristics. The discharge is supplied (1) by the residual-discharge turbine of Rossens Dam as 7 m³/s, and (2) periodically by the stream turbines as 70 m³/s. The turbined water initiates from a low elevation in the Lac de la Gruyère (at Rossens Dam, with a height of 83 m). Some distance downstream, two tributaries join the Sarine and add in total an average yearly discharge of some 18 m³/s. Overall, 20 temperature and pressure transmitters were installed along the measurement reach at suitable locations, recoding a value every 5 minutes for two years meanwhile. The pressures were measured to define whether a transmitter was in the water or on land (due to discharge variations and the bed development). The temperature development indicated two aspects, namely the influence of hydropeaking as well as that of the tributaries.