Effect of Flow Rate on Appearance and Acoustics of Waterfalls

Waterfalls are special landscape elements. With increasing worldwide water use, not least for sustainable hydropower, the number of conflicts between water withdrawal and preservation of waterfalls is increasing. A recent method to determine residual flow releases, by taking into account the duration of the water abstraction and the visual appearance of waterfalls, has now been expanded to account for the variation of waterfall acoustics (sound level) with changing discharges. Furthermore, an extensive criteria checklist to determine the value of a waterfall is now available. Based on photographs as well as sound and discharge measurements at seven Swiss waterfalls of different morphological types, the effect of the discharge on the falls' appearance and acoustics has been systematically analyzed. Noticeable changes in these two parameters vary with both the discharge and the morphology of the waterfall, where sound level changes are less pronounced than changes in visual effects. At low flows the visual and acoustic variations are more distinct than at high flows. The less flow paths can be activated (typically for free-falling waterfalls), the more the relative change (in relation to the respective median discharge) is pronounced with decreasing flow for both the falls' appearance and acoustics. If a critical discharge is exceeded, any further increase in discharge is hardly perceivable. The more flow paths can be activated (typically for cascading waterfalls), the more the relative changes are gradual both in appearance and acoustics, so that several critical discharges are discerned. To determine residual flow releases, a critical discharge should be identified for those activated flow paths that result in the typical waterfall's appearance. The proposed method can be applied to determine this critical discharge to appraise the extent of a planned water withdrawal on the waterfall's appearance and acoustics. This extent is then compared to the value of the waterfall in a matrix, from which the consequences of the water abstraction become apparent.