Estimation of the Performance of Stilling Basins under Climate Change Conditions: Experimental Study

In recent years, Iraq has experienced climate change that has led to increased drought cycles and prompted authorities to implement small-scale dam projects for water harvesting, particularly in remote areas. These changes have been characterized by irregular and unpredictable rainfall, ranging from heavy downpours to extremely low levels. This has made it difficult to determine effective design specifications for hydraulic structures and has sometimes led to failures in spillways and stilling basins. Flood discharges have also exceeded conventional design limits, reducing the energy dissipation efficiency of these basins and compromising flow safety. This study aims to improve the performance of stilling basins to cope with the increased discharges induced by climate change, through a series of experiments on a physical model of a Type III stilling basin (one of the United States Bureau of Reclamation (USBR) classifications; here defined as a basin with baffle blocks and an end sill designed to produce a strong hydraulic jump) in a small water harvesting dam in Iraq. The study included a standard basin and three modified models, with discharges between 20 and 130 m³/h, exceeding the design discharge of 60 m³/h. The results showed that the energy dissipation efficiency of the standard basin reached approximately 36% at design discharge, decreasing to 26% at higher discharges (a reduction of about 28%). The first modification (redistributing the baffle blocks) did not improve performance, and efficiency remained at 25–26%. The second modification (increasing the block height) increased the efficiency to 41% at design discharge (+14%) but decreased to 27% at higher discharges. The third modification (adding a second row of blocks) was the most effective, with an efficiency of approximately 43% at design discharge (+18%), and maintained over 29% at discharges between 100 and 130 m³/h, with more stable hydraulic performance, making it more suitable for flow conditions under climate change in Iraq.