Determination of flow resistance parameters of Blackberry for hydraulic modeling considering plant flexibility

Riparian vegetation essentially influences the flow resistance and therefore reliable plant parameters are necessary for hydraulic modeling. This paper presents results from an experimental study designed to determine species-specific resistance parameters of blackberry plants (Rubus armeniacus Focke) for the use in the flow resistance approaches accounting for plant flexibility of Järvelä (2004) and Västilä & Järvelä (2014). Both approaches are briefly described and presently available species-specific parameters for various species are summarized. Flume experiments to measure the drag force to velocity relationship using foliated and defoliated blackberry twigs collected from nature are described, and the collected data are used to derive the species-specific parameters. The variability of parameter values for individual specimen are shown and briefly discussed. Repeated tests with a specimen carried out with increasing and decreasing velocity steps, respectively, led to the formulation of a hypothesis for a vegetated hysteresis effect. The derived mean values fit into the range of the presently available values for other species so that they can be used to extend the database for both approaches.