Drop structures are the most common hydraulic structure used in irrigation systems and water distribution and wastewater collection networks. While the channel slope is less than the natural ground slope, a drop is used to decrease the elevation of the channel bottom. Drops also dissipate a significant portion of the upstream flow kinetic energy. By applying the netting dissipators over the vertical drops namely netting drop type dissipators, the head loss increases significantly. By increasing or decreasing the tail-water depth downstream of the netting dissipators, the flow characteristics amend thereby changing the flow regimes downstream of the drop. The main objective of the present study is to examine the effects of changing the tail-water depth on the flow regimes and the hydraulic characteristics of flow downstream of vertical drops equipped with the netting dissipators. Experimental investigation was performed to study the flow regimes and their limitations and the changes in the flow hydraulics due to the variation of the tail-water depth in netting drop type dissipators. Three different drop heights and eight different netting dissipators with certain grid dimensions were used. The relative critical depth Y c / h (where Y c is the critical depth and h is the vertical drop height) was considered between 0.188 and 0.42. In each series of tests, for a constant flow discharge, the tail-water depth downstream of the drop structure was changed gradually in eight steps and the flow regimes, conditions and hydraulic characteristics such as; depth and length of the pool downstream of the drop and mixing length of the turbulent flow region were measured in detail. Accordingly, the limitations of flow regimes were specified and empirical equations were obtained to estimate these limitations. Also empirical equations were presented to estimate the hydraulic characteristics of flow inside the pool of a vertical drop equipped with netting dissipators. Overall, results indicate that changing the tail-water depth downstream of the netting drop type dissipator tends to generation of two main flow regimes namely bubbly impinging jet flow regime and surface flow regime. However the surface flow regime itself divides into three sub-regimes including bubbly flow regime, surface wave flow regime and surface jet flow regime. Furthermore, increasing the tail-water depth results in increasing the pool depth, whereas, the pool length and the length of the turbulent mixing flow region decrease significantly. Keyword: Flow regime, Hydraulic parameters, Netting dissipator, Tail-water, Vertical drop.
