Chapter 3 : Reservoir models - KU Leuven

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3.4 Sensitivity analysis The main sewer system characteristics can be easily implemented in a reservoir model and the effect can be assessed on the long term. This makes a reservoir model an excellent tool to perform sensitivity analyses on the parameters and to search for optimised parameters. The main parameters that can have an influence on the conception of a sewer system are the throughflow capacity and the storage in the sewer system. Therefore, graphs can be made on which the overflow parameters are represented as a function of these two parameters. As demonstrated in the previous paragraphs, not only the maximum throughflow and storage are important, but also the instantaneous relationship between both. Therefore, the two most extreme instantaneous relationships are investigated, as there are the linear relationship and the constant throughflow. All other relationships are situated somewhere in between. Another important sewer system parameter is the concentration time. This parameter influences the peak shift and the smoothing of the hydrographs, so that it can also strongly influence the overflow parameters. Therefore, this analysis is also performed for different concentration times. In this analysis no dynamic storage or storage above the weir is included, as these parameters often only influence the shape of the overflow events. Finally, the runoff model can influence the results. For this analysis a runoff model with a depression storage of 2 mm and a constant evaporation of 0.1 mm/h is assumed. The studied overflow parameters are mainly overflow frequency and volume, but also overflow durations, mean and peak overflow discharge. The rainfall input used is the historical series of Uccle for the period 1967-1993. In figure 3.31 the overflow frequency is visualised for a wide range of possible values for the maximum storage in a sewer system and the maximum throughflow from the system, using a linear relationship between instantaneous storage and throughflow. In the graphs the maximum throughflow is not presented on the horizontal axis, but the overcapacity. The overcapacity is the maximum throughflow minus the dry weather flow. In this way these graphs can be used independently of the dry weather flow. Both maximum storage and overcapacity are presented relative to the contributing area, so that the graphs are representative for any size of catchment. In figure 3.32 the overflow frequency is visualised using a constant relationship between storage and overcapacity. In both figures 3.31 and 3.32 a concentration time of 60 minutes is used. It can be seen from these figures that the type of relationship between storage and throughflow has a large influence on the predicted overflow frequency. In these graphs the overflow frequency is defined as the number of days with an overflow event. If the overcapacity is low, the overflow frequency is strongly influenced by the maximum storage in the system. Or the other way around, to achieve a specific overflow frequency much more storage is necessary at low overcapacities. Analogously, graphs can be made for the percentage of rain water that is spilled via the overflows as is shown in the figures 3.33 and 3.34. 3.36 The influence of rainfall and model simplification on combined sewer system design
Figure 3.31 : Overflow frequencies calculated with a single linear reservoir model (concentration time = 60 minutes). Chapter 3 : Reservoir models 3.37
Page 1 and 2: Chapter 3 : Reservoir models 3.1 Hi
Page 3 and 4: In this figure 3.3 the constant thr
Page 5 and 6: 3.2.2 Concentration time As was alr
Page 7 and 8: (implemented) concentration time of
Page 9 and 10: In figure 3.7 the storage in the sy
Page 11 and 12: 200 175 150 125 100 75 50 25 0 thro
Page 13 and 14: The influence of the simplification
Page 15 and 16: Figure 3.14 : Longitudinal profile
Page 17 and 18: 1.2 throughflow (m 3 /s) 1 0.8 0.6
Page 19 and 20: Also for combined sewer systems the
Page 21 and 22: 3.2.4 Runoff models After the calib
Page 23 and 24: instantaneous runoff coefficient 1
Page 25 and 26: Other more detailed runoff models c
Page 27 and 28: 3.3 The reservoir modelling system
Page 29 and 30: If the concentration time is not ta
Page 31 and 32: 200 throughflow Q (l/s) 175 real re
Page 33 and 34: 3.3.3 Other features The reservoir
Page 35: For emission modelling it is very i
Page 39 and 40: Figure 3.33 : Overflow volumes (% o
Page 41 and 42: Not only the storage, the throughfl
Page 43 and 44: As the overflow events are strongly
Page 45 and 46: 16 peak overflow discharge (mm/h) 1

Chapter 3 : Reservoir models - KU Leuven

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