Characteristic reference areas for estimating flow resistance of natural foliated vegetation

Reliable estimation of vegetative flow resistance calls for physically sound and readily measurable plant properties. Laboratory flume investigations were conducted to examine four reference area properties in relation to the drag, reconfiguration, and flow resistance of foliated Black Poplar twigs. The experiments were novel in that three characteristic reference areas (leaf area AL, frontal projected area under flow AP, and still-air frontal projected area A0) as well as the foliage–stem reference area ratio (AL/AS) were evaluated. The drag forces were simultaneously measured for up to eight specimens in a plant stand at both partly and just submerged conditions. Due to the high AL/AS of the twigs, leaves contributed 74–98% of the total drag at mean velocities of 0.1–0.9 m/s. Both the partly and just submerged poplars had similar AP and drag per characteristic reference area. Thus, the derived parameter values could be used to estimate the friction factors of the poplar stands at low to just submerged conditions, with each of the three characteristic reference areas providing satisfactory estimates. The flow resistance estimation with AL may be further improved by using AL/AS as a secondary area parameter to take into account the share of the stem to the total drag. Comparison to literature data on other deciduous species suggested that the foliage–stem reference area ratio was an essential property for explaining the between-species variation in AP and flow resistance per AL.