Author(s): Gems Bernhard; Nemmert Johannes; Rutschmann Peter
Keywords: 3D modelling; Flood protection; REYNOLDS-equations; Turbulence models 1
Abstract: In view of the flood disasters in recent years flood protection in urban areas has become a top-ranking subject. In August 2005 an extreme flood with a 270 years return period occurred at the Inn River endangering the historic centre of Innsbruck, Austria. Overtopping of the dams almost occurred and in such a case a disastrous damage would have to be experienced. In a first step a detailed analysis of the registered floods at the gauging station Inn/Innsbruck was carried out. Gauging data were available for the last 55 years. The analysis showed that the 2005-flood had a statistical return period of 197 to 268 years, depending on the distribution functions used. Three-dimensional numerical simulations were then carried out with the commercial software FLOW-3D_R_ using a very detailed terrain model, resulting from conventional survey and laser scanning and including the exact geometry of all bridges crossing the river. A calibration run with the 2005-flood was first performed to adjust the roughness parameter in the numerical model in a way that the results best fitted the observations. Then additional runs with two decisively higher discharges, resulting in major flooding of the city centre of Innsbruck, were carried out. These flooding scenarios represented floods with a 1290 years and a 4830 years return period respectively. The computations resulted in flood maps indicating the inundated areas with water depths and fluid velocities. The present paper describes the advantages and difficulties of a fully three-dimensional and extremely detailed numerical simulation of flooding scenarios for the city of Innsbruck. The results proved the advantage and even necessity of a three-dimensional approach. It could clearly be shown that the bottle necks in terms of flooding are the bridges and that the correct simulation of the flow close to the bridges clearly requires a three-dimensional approach. Once the flow runs under pressure at a bridge an instantenous backwater effect occurs which results in a water level increase of about 80 centimetres damping slowly out further upstream. The present computations also show that nowadays computer power is the limiting factor. The computational domain was subdivided into more than 6 million grid cells with a minimum size of 1. 25: 1. 25: 0. 25 metres. Therefore simulating one and a half hours of the observed flood peak required about 6 weeks of CPU time on a fast Intel based two-processor PC. The results of the present computations have been used to optimise and adjust flood control measures and to provide detailed defense plans for authorities.