The problem of basic stage flood in Tacherting and investigation of countermeasures
For investigation of basic stage flood in Tacherting a 3-D Groundwater-model for Tacherting (large scale) and Wajon (small scale) was generated, which was linked to a 1-D hydraulic runoff-model of the river Alz.
The calibration of these models resulted from data of existing monitoring wells and from descriptions by residents about the water-levels in their cellars during the basic stage flood.
After calibration various scenarios of groundwater levels and overland flows under actual conditions (without any measures) were simulated. On the one hand for the actual happened event in June 2013, on the other hand for only the flood or the rainfall event separately. This shows that the water levels in Tacherting and around the meadows are mainly a result of the flood of the river Alz. On the whole the high groundwater levels can be traced back to a overlap of flood and heavy precipitation.
5 scenarios each for the large scale model and for the small scale model were simulated. Two very effective scenarios were formed for both Tacherting and Wajon. One consists of a drainage system around Tacherting and Wajon, though which groundwater is delivered to a bucket elevator, which finally pumps the water into the river Alz. For Tacherting the bucket elevator could be replaced by an inverted siphon which heads under the Alz-canal. The other possibility consists of an installation of a pump gallery in the local area of Tacherting and Wajon. In case of basic stage flood the can be put temporarily into operation by the technical assistance organisation (THW) or the fire brigade.
Both measures show in the simulation a lowering of the groundwater level of more than 50cm in both local areas, by what most houses are protected of harm by groundwater.
Therefore both variants are recommendable, however they are connected to highest costs for the necessary construction work and maintenance and repair.
Additionally a forecasting system based on the calibrated model in Mike SHE is presented, with that in case of floodwater in the river Alz and a rainfallevent the groundwater levels in Tacherting and Wajon can be calculated. Like this measures for minimizing harms can be taken promptly.
More information you can find here (in german).