Drag and reconfiguration of freshwater macrophytes

SUMMARY 1. Submerged freshwater macrophytes face large hydrodynamic forces in flowing waters in streams and on wave-swept lake shores and require morphological adaptations to reduce the drag and the physical damage. This experiment studied five species of freshwater macrophytes and strap-formed plastic leaves to test the predictions that: (i) increasing flexibility leads to greater reconfiguration and lower drag coefficients, (ii) flexible plants experience a steeper decline of drag coefficients with increasing water velocity than unflexible plants and (iii) plants mounted vertically on a horizontal substratum bend over in fast flow attaining a shielded position of low drag. 2. The results confirmed all three predictions. In fast flow, plants mounted upright on a horizontal platform gradually approached a position aligned with the flow, depending on their flexibility. In the range 8–50 cm s−1 the deflection followed an interspecific negative linear relationship between log (tangent Φ) and velocity, where Φ represents the shoot angle normal to the horizontal level. Above 50 cm s−1, further deflection was reduced perhaps by a combination of the elasticity and packing of shoots and the increasing lift generated by fast flow. 3. Drag coefficients of plants ranged between 0.01 and 0.1, typical of moderately to very streamlined objects. Drag coefficients declined log-log linearly at increasing velocity, following negative slopes between −0.67 and −1.24 (median: −1.0) because of reconfiguration and formation of a shielding canopy. Drag coefficients declined much less (median: −0.55) for plants floating freely in the streaming water and which were capable of changing their shape but unable to form a shielding canopy. Drag coefficients declined even less for relatively unflexible plastic leaves (−0.30 to −0.40), and they remained constant for stiff, bluff objects. 4. The experiments suggest that flow resistance of flexible, submerged macrophytes in natural streams may increase in direct proportion to water velocity because they form a shielding submerged canopy, and high water stages at peak flow may result in greater proportions of the water passing unimpeded above the canopy. In contrast, stiff amphibious and emergent reed plants should experience an increase of flow resistance with at least the square of velocity as reconfiguration is small and former aerial plant surfaces come into contact with the streaming water at higher water stages. Field experiments to test these predictions are urgently needed.