Discharge characteristics for trapezoidal compound labyrinth

Replacement weirs are commonly used to pass changed or higher than design storm flows. Such a situation could arise due to climate change or urbanization impacts. Labyrinth weirs generally have higher discharge capacities and therefore are used for this purpose. Trapezoidal
compound labyrinth weir is proposed in this study. The proposed compound weir acts as a staged labyrinth weir, where the trapezoidal section is utilized to satisfy design discharge requirements and functions as a compound weir during high flow events. However, there is a lack of sufficient information for the hydraulic design of compound labyrinth weirs. The main goal of this study was to investigate the efficiency of discharge of trapezoidal compound labyrinth weirs. Laboratory experiments were conducted in a rectangular laboratory flume. The flow characteristics for eight physical models with sidewall angles (α) ranging from 6° to 90° with half round crest were studied, and head-discharge relationships were assessed. The measured discharges were compared with predicted values using a modified form of the weir equation derived from energy conservation. The error percentage of discharge was calculated between the calculated discharge and the observed discharge when discharge particularly was separated through the notches on the weir. The results of this study showed that the compound discharge coefficient (Cdc) is increased initially with low H't/P' values due to the nappe interference of fallingjets being not so severe, then decreases with increasing H't/P' for all sidewall angles because of the increasing collision of fallingjets. The empirical equations were generated by curve-fitting for rang sidewall angles 6° to 90° and half round crest. These equations were validated for 0.05 ≤ H't/P'< ∼0.75-0.9. The error percentage was as large as 15% for α= 20° when discharge particularly was separated to the notches of weir (low stage).