Vegetation effects on low flow water temperature small to the medium size urban stream Liesingbach

Urban streams are exposed to strong anthropogenic pressures and frequently exhibit extensive morphological modifications, including embankments, channel straightening, and elevated longitudinal slopes. Especially in urban areas hydraulic capacity and requirements on flood control are driving factors for channel geometry and morphology, leaving strongly limited lateral space availability. Despite these pressures, restoration has gained increasing attention also for urban streams. Hydraulic conditions change, roughness is getting higher, slope might be reduced. Such morphological alterations have a direct influence on hydraulic behavior. Frequently, a trade-off exists between riparian vegetation and hydraulic capacity, since flow capacity is generally highest where woody vegetation is absent from banks and adjacent zones. In urban stream systems thermal energy input during low flow situations is mainly connected to short wave radiation and to air temperature. Restoration works slow down the system, increasing hydraulic residence time. In consequence slower systems exhibit a higher energy input, which makes them more sensitive to heat stress. Riparian vegetation can buffer energy input via shading effects. When addressing the trade-off between the services provided by riparian vegetation and hydraulic capacity, linking hydraulic and thermal aspects could provide valuable insights. The research done in the INTERLAYER project dedicates the impact of riparian vegetation on water temperature during low flow conditions at the small to medium size urban river, Liesingbach. Based on a water temperature model the vegetation effects on water temperature through shade during a low flow period with high air temperature values are quantified and compared to a scenario in absence of riparian vegetation. A HEC-RAS water quality model was set up for 8 km along the urban flow path of the river Liesingbach using the full energy budget approach. The model was calibrated and validated by measured water temperature data for two river heat periods in two consecutive years. Vegetation data was collected by hemispherical photographs. The prevailing vegetation was tested against a total absence of vegetation. The results show that effects of shade through riparian vegetation were able to buffer of 0.7 to 1.3°C daily water temperature peaks. An impact on daily minimum values was not detected. Based on our results vegetation effects are not limited to dense riparian forests. Even single tree lines, or shrubby vegetation implemented through soil and water bioengineering techniques have remarkable influence on water temperature. Vegetation can clearly support ecological aspects during low flows. With the prevailing water temperature model, we created a tool to quantify the impacts and predict potential changes. Building on this integration of hydraulic and vegetation modelling will enable us to improve ecological outcomes during low-flow periods.