Practical method for flow velocity measurements in ungauged sections

The river flow forecasting represents a crucial point to employ for improving a management policy addressed to the right use of water resources as well as for conjugating prevention and defence actions against environmental degradation. Indeed, for that the European directive 2000/60/CEE states the relevance of the monitoring and control activities as support to define the whole of protection measures to adopt for achieving the “good state” of the water body. Thus, the quantitative monitoring of rivers is an essential step for environmental purposes both for addressing middle and long term surveillance and control activities, and for civil protection purposes in terms of flood forecasting and risk mitigation. In such cases, a focal point is mainly represented by a continuous updating of flow depth/discharge ratio in each gauged section. This update relies on intense field activities not to easily carry out because of several factors mainly related to weather conditions and high costs. Thus, the aim of the present work is the development and implementation of quick and efficient sampling methods for discharge assessment in order to reduce the data acquisition time and processing maintaining a good level of accuracy and precision. This is of great interest in the chain of environmental monitoring for both gauged and ungauged river sections. In according with ISO rules, the choice of measurement verticals and velocity points has been derived by an optimization tolerance analysis proposing sampling procedures based on a set of discrete velocity measures collected following a fixed geometric pattern and grid (verticals and points). The backbone of the proposed methodology deals with the rationale that in a river flow the large amount of the moving fluid volume (discharge) is generally concentrated in about 30% up to 50% of the whole cross section and in this area the maximum velocity usually occurs. Three main verticals (i.e., placed at 1/3, ½ and 2/3 of the total observed width) are fixed for the field measurement considering the points velocities in 30% of the area below the free surface. Thus, the assessment of maximum and average velocities is available and the entropy velocity profile can be derived. The reliability of such method has been tested to gauged river sites located along the Alzette natural channel, in Grand-Duchy of Luxembourg. The results show a good “technical” agreement among observed and estimated water discharges with errors not exceeding 25%, overall.