Determining the flood areas initially involved mirroring the water levels of a 100-year flood in order to derive a water surface. Subsequently, this information was collated with the current Digital Terrain Model (ATKIS® DTM) of the Senate Department for Urban Development and Housing. To differentiate between flooded areas and bodies of water, the Gewässerkarte (Map of Bodies of Water) was superimposed on this data.
The water levels of a 100-year flood were determined using several methods. This required, on the one hand, a joint approach with the State of Brandenburg, as some bodies of water flow from Brandenburg to Berlin and as the Havel flows back from Berlin into Brandenburg. On the other hand, this involved adapting the methodology to suit the natural conditions and data availability. The chapters focussing on the individual flood areas detail the methodological approaches taken. Table 1 provides an overview of all methods and references to further studies. Please refer to the respective studies for a more detailed description of each method.
Müggelspree and Gosener Wiesen flood area
Berlin’s Müggelspree and Gosen Canal are located in the backwater areas of the Mühlendamm impoundment. The water levels are mainly controlled by the weirs and locks at the Mühlendamm lock, the Kleinmachnow lock and the Oberschleuse (upper lock). Due to the impoundment’s control system and its large retention capacity, the annual flow throughput and water levels are not always directly linked. Flood damage does not necessarily have to be linked to abnormally high Spree inflows. Previsouly, control was adjusted depending on the situation and other premises.
In the process of defining flood areas, comprehensive investigations were carried out into the potential influence of purposeful weir control, aiming at minimizing the risk of adverse flood effects. Using the hydro-numerical model GERRIS/HYDRAX of the German Federal Institute of Hydrology and one-dimensional non-stationary calculations, the influence of controlling water levels during flooding was investigated for three severe floods in 1975, 1994 and 2011. Control strategies were developed in collaboration with the Wasserstrassen- und Schifffahrtsamt Berlin (Waterways and Navigation Authority) based on past events and taking into account existing objectives and restrictions: in the event of flooding, flood damage in settlement areas is to be minimized, damage to timber building supports is to be prevented by lowering the water level in the weir of the Mühlendamm lock as required and navigation is to be continued for as long as possible.
The results demonstrate that the effects of flood discharge from the Spree can be reduced by controlling the weirs accordingly. The spatially distributed water levels shown in the flood area maps of a 100-year flood are based on the water level gradient. An administrative agreement was concluded between the Generaldirektion Wasserstraßen und Schifffahrt (General Directorate Waterways and Navigation) as the authority overseeing the Berlin Wasserstraßen und Schifffahrtamt (Waterways and Navigation Authority) (which operates weirs and locks) and the Senate Department in order to decrease adverse effects of flooding by proactively controlling the water levels of the Berlin impoundment.
The results of the State of Brandenburg were taken over for the Gosener Wiesen area, as, in the event of flooding, the border between Berlin and Brandenburg is flooded, too, and the developed method does not cover this area (IWU 2015).
Lower Havel / Lower Spree flood area
The Lower Havel / Lower Spree flood area is within the responsibility of the Brandenburg impoundment. The strategy was adapted to that of the State of Brandenburg in order to guarantee a methodically uniform approach for the Brandenburg impoundment. For the period from 1964-2013, a statistical analysis of water level outliers was carried out for seven gauges (Charlottenburg Unterpegel (UP, Downstream Gauge), Sophienwerder, Spandau UP (Downstream Gauge), Freybrücke / Tiefwerder, Pfaueninsel, Potsdam Abz. (Outer District) and Potsdam Lange Brücke). These flood levels serve as supporting points for the water level gradient of a 100-year flood. The water levels were derived by linear interpolation of the supporting points taking into account the gradient change caused by different flows and cross-sections. The flood area was divided into flow-through (Lower Havel I flood area) and impounded (Lower Havel II flood area) sections in order to standardize specific exceptions to use restriction based on hydraulic conditions (IWU 2014).
Erpe, Panke and Tegeler Fließ flood areas
The methodology for determining the Erpe, Panke and Tegeler Fließ flood areas is fundamentally the same. In order to determine the flow rates for a 100-year flood, a hydrological precipitation runoff model was developed for the corresponding catchment area, taking into account the relevant runoff-forming factors such as land use, topography, soil conditions, impervious soil coverage as well as influences of management and rainwater discharges. This model was calibrated and verified using precipitation and climate data as well as recorded runoffs. The runoff measurements were then used as input variables for the hydraulic model to calculate water levels and flow conditions. One-dimensional non-stationary models were used. The hydraulic models are mainly based on geometric data on flow cross-sections, flow conditions and roughness. For this purpose, cross-sections used during measuring using DTMs were extended to include foreshore areas. The hydraulic model was also calibrated and verified referring to the existing water level hydrographs and water levels recorded in the measuring process. Using this model, the water levels for a 100-year flood were calculated (IPS 2009, IPS 2013 and Koenzen et al. 2011).