Hydrological criteria to assess changes of flow dynamic in urban impacted catchments

Abstract

With regards to the implementation of the EU Water Framework Directive various assessment thresholds exist for morphological, biochemical and hydro-morphological parameters for sustainable water management. Besides the knowledge about the importance of hydrology for habitat conditions and biota respectively, there exists still a lack in consideration of changes of flow dynamics due to human impacts in current water protection. Natural flow variability maintains the integrity of aquatic ecosystems. In general alteration of flow dynamics result in changes of duration, magnitude and frequency of flows. Especially urban rivers can be crucial impacted by a rapid and short-term runoff response, mainly due to the impact of combined sewer and storm water overflows. The variability in flow dynamics makes the definition of fixed hydrological assessment indices difficult. There is the need of further knowledge in which ways flow dynamic is changed by human impacts and which hydrological parameters are suitable to capture these changes. In this study the hydrological assessment criteria flow acceleration, peak discharge and discharge dosage are determined with the focus to assess the quantitative impact on urban rivers. Dependencies of these hydrological parameters are studied within comprehensive statistical analyses with long-term data sets. The hydrological parameters were compared on impacted and un-impacted sites. The influence of land use, influences of discharge data resolution and scale dependencies respectively were investigated. Additionally, probability analyses and calculations of recurrence intervals were conducted for all three parameters. Using those three, sensitive hydrological parameters a hydrological approach is given, which considers the natural hydrograph as target value for a hydrological assessment and captures the whole spectrum of possible changes of the flood hydrograph. Furthermore, the suggested parameters are easily to determine from discharge time series and hence, widely available for management decisions. The study is a contribution for an improved implementation of hydrological assessment indices and capturing flow dynamic variability additionally to qualitative aspects within an ecologically meaningful assessment according to the EU Water Framework Directive.