A hydropeaking event means a short-term discharge fluctuation caused by power plant operation. It consists of two phenomena, the "peak discharge", an artificially increased discharge, and the "base discharge" as a subsequent decrease in discharge, which can alternate several times a day. Hydropeaking operation mode as a hydrological pressure strongly alters the natural discharge dynamics of a river and may cause a major impact on the ecological functionality. In contrast to other alterations and their consequences, it usually influences comparatively long water stretches.

Both the release of peak discharges and the strong fluctuations in the water level have a significant impact on fish, benthic invertebrates and other organisms in and next to the water. For example for fish larvae and juvenile fish there is, among other things, the risk of being drifted (increase in the flow rate) or stranding (rapid reduction of the wetted area, which can lead to a decimation of fish populations). Individual growth might be lowered by a reduction of natural food availability (eg macrozoobenthos) or reduced water temperature or be reduced by changing the water temperature. The knowledge to develop sustainable solutions to minimize the effects on aquatic ecology is currently not sufficient and requires further research into the interrelationships.

There are various technical options for reducing hydropeaking related run-off fluctuations and their consequential effects on the water biocenoses in terms of environmentally friendly use / mode of operation; e.g. retention basins, peak diversion, river morphological measures and changes in the operation mode of the power plants. The rehabilitation of hydropeaking impacted water bodies is an essential part of an ecologically oriented, sustainable water management for the production and maintenance of the good condition or good ecological potential according the Water Framework Directive (WFD) and is therefore very urgent in Austria.

Schwallexperimente an der HyTEC

The original task of the HyTEC facility is to investigate the ecological effects of artificially generated flow fluctuations, called hydropeaking LINK. Using the following example, the sequence of a simulated single hydropeaking event in the test channels is explained. In order to simulate such an event, a constant base flow (or low flow), is discharged in one of the two experiment channels, in the present example, 25 l/s were selected for the base flow (picture). After a certain period of time, the discharge is constantly increased (the so-called upramping phase) until a peakflow of 125 l/s is given. This represents the so-called peak phase,and causes  and increase of flow velocity water depth and width (figure). This peak flow is maintained for a certain period of time before the discharge rate is constantly reduced to the original base flow (25 l / s). This is called the downramping phase These five phases, base flow – upramping – peak flow – downramping – base flow characterize a complete single peak event.

Based on the design of the channels, it is possible to create different structures such as a flat gravel bank, bays or pothole structures etc., and to test their influence on possible effects of hydropeaking.

The following figure shows the different flow velocities (shown in color) as well as water depths and widths for a homogenous flattended gravel bank with five percent lateral slope in a cross sectional view at 25 l/s and 125 l/s discharge.

The corresponding heights in the ground plan are shown in the following illustrations.