Impact of the dam breach mechanisms and characteristics on dam break flood wave and flooded area. Application in Tsiknias dam, Lesvos Island, Greece

This study aims to investigate the impact of the dam breach mechanisms and characteristics on the flood wave effluxing through the breach. The failure mechanisms, the geometric characteristics, and the development time of the breach are estimated with the five empirical equations provided by HEC-RAS. The flood routing is simulated with the HEC–RAS 2D model using the Eulerian-Lagrangian Shallow Water equations (SWE-ELM), derived from St. Venant differential equations. A finite volume grid was utilized to discretize the St. Venant equations. Then, the routing of the flood wave through the inundated area is performed by estimating the flood depth and the flow velocities for every wet cell of the grid for a given period. As a case study, a hydraulic 2D routing model was built to examine various breach scenarios of the Tsiknias dam, an earthen dam with a central clay core located northeast of Kalloni town of Lesvos Island. The dam is equipped with an Ogee Crest safety spillway which has a length of 50 m. The results show that the most unfavorable scenario of dam breach due to soil piping is the one that uses the Froelich 1995a equation. In this scenario, the maximum peak outflow from the breach is Q_peak =8225.5 m³/s and Kalloni town is inundated within 40 minutes after the breach. This time window is essential for the evacuation plan of the citizens in the case of a dam breach. Inside Kalloni town, the flood depths range from 2 to 5 m, whereas the flow velocities reach up to 6 m/s, which implies that the flood wave generated by the breach may inflict major damage to the infrastructure of Kalloni town. In Kalloni Bay, outside of the town, the flood depths reach up to 2 m and the flow velocities up to 3 m/s.