Risk of entrapment at low-head dams

This study examines the dangerous countercurrent that can form at the downstream face of a low-head dam for certain flow conditions, utilizing computational fluid dynamics software and laboratory observations. The objective of this project was to identify easily measured parameters that can be used to accurately distinguish between the possible high-risk and low-risk states of flow. The research was carried out on two common low-head dam shapes (ogee-crested and flat-topped) and three dam heights (0.61, 1.52, and 3.05 m) with simulations performed for various upstream and downstream water depths. It was found that the combination of parameters (hu - hd)=P (named the risk factor) was capable of defining transition points between states of flow, and therefore the low-risk and high-risk conditions. With regard to the ogee-crested dam simulations, it was found that the most dangerous flow conditions occurred at risk factors between 0.09 and 0.80. For the flat-topped dams tested, the high-risk conditions occurred at risk factors ranging from 0.34-0.71. Countercurrent length was also examined, with rollers from the ogee dams ranging from 2.05-5.95P in length, and from 2.60-5.45P for flat-toppeddams. 2013 American Society of Civil Engineers.