Armouring influence on the estimate of instream flow

For defining instream flows, the so-called incremental methods based on the general formulation given by Bovee [1] have by now become established. This method requires the defining of four suitability indexes expressed as a function of the hydraulic characteristics of the current (velocity and depth) and the physical characteristics of the channel (armour and cover). The product of the water surface of the current in the reach of interest for these indexes provides a measurement of the habitat available for aquatic life (Weighted Usable Area, WUA). In the absence of other limiting factors (temperature, pollutants), the WUA is a function of flow. The influence of the armour, together with the cover, is taken into account especially as a parameter that measures the availability of food and shelter for the various stages of development of fish species. Only recently have some studies pointed out the influence on the suitability indexes of local velocities and their statistical distribution in a channel reach (Lamouroux et al, 1995 [2]). Local velocities are in fact often influenced by river bottom irregularities directly dependent on the armour, in low flow periods. This article gives a report of the results of an experimental campaign aimed at evaluating this influence. It is thought that the mean velocities measured, or estimated by means of permanent motion hydraulic simulations, do not give a satisfactory representation of the macroscopic turbulence of the current which has the greatest effect on fish species suitability. The work uses data kindly furnished by the Tiber River Basin Authority, which promoted the campaign within the larger framework of its activities [3, 4].