A conceptual model of check dam hydraulics for gully control: ef?ciency, optimal spacing and relation with step-pools

摘要

There is little information in scienti?c literature regarding the modi?cations induced by check dam systems in ?ow regimes within restored gully reaches, despite it be-ing a crucial issue for the design of gully restoration mea-sures. Here, we develop a conceptual model to classify ?ow regimes in straight rectangular channels for initial and dam-?lling conditions as well as a method of estimating ef?ciency in order to provide design guidelines. The model integrates several previous mathematical approaches for assessing the main processes involved (hydraulic jump, impact ?ow, grad-ually varied ?ows). Ten main classi?cations of ?ow regimes were identi?ed, producing similar results when compared with the IBER model. An interval for optimal energy dis-sipation (ODI) was observed when the steepness factor c was plotted against the design number (DN, ratio between the height and the product of slope and critical depth). The ODI was characterized by maximum energy dissipation and total in?uence conditions. Our ?ndings support the hypothe-sis of a maximum ?ow resistance principle valid for a range of spacing rather than for a unique con?guration. A value of c = 1 and DN ~ 100 was found to economically meet the ODI conditions throughout the different sedimentation stages of the structure. When our model was applied using the same parameters to the range typical of step-pool systems, the pre-dicted results fell within a similar region to that observed in ?eld experiments. The conceptual model helps to explain the spacing frequency distribution as well as the often-cited trend to lower c for increasing slopes in step-pool systems. This reinforces the hypothesis of a close link between sta-ble con?gurations of step-pool units and man-made interven-tions through check dams.