Verification of Urban Drainage System Designs by Means of SWMM5 and Climate Change Scenarios

Dirk Muschalla, Valentin Gamerith, Jonas Olsson and Günter Gruber

ABSTRACT

The Linz catchment is situated in Linz, Austria at the Danube River and covers approximately 900 km² in total. The area of downtown Linz with mainly combined sewers and 39 neighbor communes with combined and separate systems are drained to one central waste water treatment plant (WWTP). Several CSOs and CSO tanks are installed in the combined sewer system. The primary clarifiers at the WWTP are also used as CSO tanks during rainfall.

Historical rainfall data from downtown Linz was available for the period from 1993 to 2006. For the scenario analysis four predicted future rainfall time series were available. They were obtained by downscaling a future global climate model projection representing an intermediate level of future greenhouse gas and covering the period 1961-2100. In a first step, the global projection was dynamically downscaled to a 50×50 km grid over Europe. In a second step, the regional results were further downscaled to local scale. Future local changes in the frequency distribution of precipitation intensities between periods 1993-2006 and 2079-2092 were estimated on a seasonal basis. The final predicted time series was obtained by applying the estimated changes to the historical time series.

Two versions of the downscaling approach were applied. In the first the future frequency is identical to the historical, which is common Delta Change practice. In the second also estimated frequency changes were transferred to the historical time series.

An aggregated model of the Linz catchment and sewer system was available in SWMM5. In total 43 CSOs were included in the model. State-of-the art global sensitivity analysis and multi-objective optimisation methods were applied for model testing and calibration. The calibrated model was then used as basis for the following evaluations.

The predicted rainfall series generally show a decrease of rainfall intensities in the summer period and an increase in winter. High precipitation intensities and total precipitation generally increase.

The requirements as defined in the Austrian Regelblatt 19 guideline are met for both dissolved and particulate pollutants for the historical and for all so far available predicted time series.

The total overflow volume is increased by approximately 21 – 23%. For the most important CSO structures overflow volume increase in a range between 10 to 60%.

The significant increases of the overflow volumes for single structures could locally seriously affect the water quality of the receiving waters.

Knowledge of the CSOs that are sensitive to climate change can help to find proper mitigation strategies and measures in time for these areas to prevent critical water quality conditions.


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