Faculty of Civil Engineering, University of Belgrade
9th Urban Drainage Modelling Conference, Belgrade, Serbia pages:1-11
Urban Drainage Modelling Conference edition:9 location:Belgrade, Serbia date:3-7 September 2012
Urbanization significantly impacts flow regimes of river systems. Especially impervious areas, in combination with waste water treatment infrastructure, can exert several pressures on the hydrological cycle. It is thought that the pressure of urbanization on the river hydrology can be increased due to climate change as some climate change models predict more intense summer storms. Quantifying this possibly increased impact is however only feasible when long time series of urban fluxes are input as point sources into the hydraulic river model and combined with the simulation of long time series of catchment rainfall-runoff results. An increase in urban flux extremes does indeed not necessarily lead to an increase in river discharge extremes as the concentration times of both systems are different.
In this article it is investigated if the impact of urban fluxes on the river system of the Grote Nete catchment (Belgium) is expected to change significantly due to climate change. First the impact of Waste Water Treatment Plants (WWTPs) and Combined Sewer Overflows (CSOs) outflows on the river system for the current climatic conditions is determined by including the urban fluxes as point sources in a detailed, hydrodynamic river model and comparing this to the result of the simulation that does not include these urban point sources. An extreme value analysis is performed on the modeled peak flow discharges in the river. The rainfall and evapotranspiration time series are then perturbed by the quantile perturbation tool developed by Ntegeka and Willems (2009). Perturbed series are generated for four different climate scenarios namely high summer, high winter, mean and low. These scenarios aim to cover the range of impact results obtained if the full set of available climate models would be considered. All four scenarios are considered in this impact investigation to account for the overall uncertainty that is present in the regional and global climate change model projections that were used to develop the scenarios. The extreme value analysis is repeated for the four different climate change scenarios both with and without the urban fluxes as point sources.
The obtained extreme value distributions are analyzed for the changes in the 10-year river peak flows. Ten years is most commonly used as design return period for sewer systems. For this return period the peak flow extremes increase with 8% due to urbanization under current climatic conditions. This impact of urbanization is not changed significantly for the low, mean or high winter climate scenarios. However, for the high summer climate scenario, an increased impact of 20% is obtained. Under the scenario that more intense storms occur during the summer season, the increased overflow discharges from the urban sewer system lead to a more significant increase in peak flow extremes in the natural river system than compared to the current impact. The magnitude and direction of the impact of different climate change scenarios on the river peak flow extremes is more difficult to determine than the impact of urbanization. This is due to the high uncertainty in the future climate projections. The projected changes in river peak flow extremes vary between -15% to +45%. This high uncertainty makes it very difficult to derive adaptive measures to counteract the possible effects of climate change. Flexible designs that take the uncertainty into account will be required.