Determination of hydraulic parameters for instream flow assessments

Abstract

AbstractDetermination of the hydraulic parameters of a stream is the first stage in the instream flow incremental method (IFIM). Two different modellings have been applied to simulate the hydraulic conditions (i.e. depth and velocity) of the Orne river at Rabodanges in order to assess the changes in fish habitat with the flow. One process consists of representing the reach examined by a few sections (the pilot sections), the other is a conventional flow model. The accuracy of both types of model has been calculated by comparison with depth and velocity data available for four different rates of flow. As regards the flow model, the accuracy of depth calculations is average (16 per cent) but decreases for low rates of flow. Velocity values are less accurate (24 per cent). Concerning the pilot sections method, the degree of accuracy of velocities should be improved (80 per cent) but the accuracy of depths is interesting (7 per cent). The representation of the flow conditions the accuracy of per cent usable area (PUA). For brown trout juveniles and fry the accuracy on PUA is of the same order of magnitude as the degree of accuracy of depth. The inaccuracy on velocities due to the method is not totally reflected in the PUA values, the probability‐of‐use curves of the species considered can act as a filter for errors linked to the method followed for assessing hydraulic parameters.