Testing a theoretically-based overland flow resistance law by Emmett's database

Abstract

The main aim of this paper was to test a recently theoretically deduced flow resistance equation, based on a power-velocity profile, using a wide database of available measurements carried out in laboratory and field experimental runs with overland flow under simulated rainfall. In comparison with previous calibrations and validations of this theoretically deduced flow resistance equation, the used database by Emmett is characterized by a wide range of rainfall intensities (from 79.2 to 303.5 mm h−1 for laboratory runs and from 178.3 to 215.9 mm h−1 for field investigations) and bed slopes (from 0.33 to 17% for laboratory runs and from 2.9 to 33.2% for field investigations). For the first time, the wide Emmett's database of laboratory runs carried out by an overland flow subjected to rainfall in a rough-bed-flume was used for testing the theoretical resistance law. Furthermore, the seven investigated field vegetated plots allowed to broaden the range of examined subcritical flows and rainfall intensity. Laboratory data were divided into two datasets (calibration and testing), and the overland flow measurements on a smooth bed by Yoon and Wenzel were added to the testing dataset to assess the applicability of the calibrated equation for different bed roughness conditions. The available laboratory measurements of flow velocity, water depth, hydraulic cross-section area, wetted perimeter and bed slope allowed both to calibrate and test the relationship between the velocity profile parameter Γ, the bed slope, the flow Froude number, and the rainfall Reynolds number, which is a dimensionless group representing the effect of rainfall intensity on flow resistance. The field data were used to calibrate the same flow resistance law and evaluate the effect of different types of vegetation, adapting the relationship obtained by Nicosia et al. The developed analysis showed that: i) the theoretical flow resistance equation gave an accurate estimate of the Darcy–Weisbach friction factor for overland flow under simulated rainfall on rough bed both in laboratory and in field condition; ii) the flow resistance increased with rainfall intensity for the investigated laminar overland flow; iii) the flow resistance relationship is able to take into account different vegetation types characterized by different roughness.