THEMATIC NETWORK ON SMALL HYDROELECTRIC PLANTS ENVIRONMENTAL GROUP RESERVED FLOW – SHORT CRITICAL REVIEW OF THE METHODS OF CALCULATION 1 SCOPE OF THE DOCUMENT This document has no scientific conceit. It mainly aims to give a quick and wide overview about the different methods proposed for reserved flow calculation both form the regulatory and research point of view. The overview itself gives the opportunity for pointing out merits and lacks of the main groups of methods, with the scope not to enter a infinite scientific discussion about the best method, but to stimulate a more general discussion on the role of reserved flow in river protection and on the global consequences of reserved flow on environment and on, more specifically, the chances of surviving to it of small hydroelectric plants. 2 FOREWORD The reserved flow world is a polytheistic world. Starting from the definition. We need to clarify what we mean as reserved flow from the early beginning. Unfortunately each national regulation has its own definition. Nevertheless all definitions emphasize the protection of the natural life in the river. “Minimum” or “reserved” flow must be distinguished by “guaranteed” flow, this last being referred to the release of water (from a specific point as a gate or a valve) in order to comply with specific obligations, regardless of the residual flow in the river downstream of the diversion works. In spite of the lack of simple and unique definitions, the objectives of assuring a minimum flow downstream of hydroelectric installations or other water abstractions is fairly clear. Anyway we must point out that the problem of a clear definition is important because the definition can greatly affect the value of the reserved flow itself. We still introduce the auxiliary concept of “dotation” corresponding to the artificially regulated flow rate at a certain time and in a certain cross section to guarantee a required amount of water in a different cross section of the same river.
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