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This section discusses the uncertainties in the technical core system of the urban drainage system. The internal future drivers of the system for hydraulic performance and treatment performance are analyzed.

The hydraulic principles of urban drainage system are mainly understood. Indeed, the hydraulic functionality is complex and is influenced by several internal and external factors. A consequence of the complex system’s behavior is that the frequency of design rainfall does not necessarily correspond with the resulting design flood frequency. In other words, a rainfall event with a recurrence time of five years does not necessarily result in flooding with a return period of five years. Because of the complexity of the system, there are unsystematic variances (Sieker et al.

2006a). Despite this, hydrodynamic effect models could represent the hydraulic performance of specific urban drainage systems, for which the consequences could be analyzed with a high degree of accuracy. Hence, Hauger et al. (2003) concludes that the hydraulic processes of urban drainage system are well understood, despite the complex system internal interaction.

Nevertheless, some uncertain future drivers will affect the performance of the technical core system.

Uncertainties for the hydraulic performance can occur during the operation process of the system.

During operation, there is the danger of failure—bottlenecks in ditches caused by waste, plants, sediments, etc. Such bottlenecks occur accidentally, but the danger could be reduced through regular control and maintenance. Furthermore, there are problems with the collimation (siltation) of infiltration facilities which seal themselves. This process adversely affects the infiltrate rate of the facilities. The collimation problems could be reduced by constructional measures (like sedimentation facilities or infiltration via the vegetated topsoil) or with management measures (like restricting the use of the infiltration areas to prevent the compaction of the soil and regular maintenance) (Sieker et al. 2009). Uncertainties for hydraulic performance can therefore be reduced significantly by regular control and maintenance. However, uncertain future drivers may also influence the quality of the maintenance itself. The standards for maintenance depend on the business model for the operation of the urban drainage system (Kluge & Libbe 2006). For example, if a sustainable urban drainage system is operated and maintained by several private landlords, it is uncertain if the proper maintenance is guaranteed because of missing technical expertise; and if private companies operate sustainable urban drainage systems, there is the danger that the maintenance will focus on economic objectives, thereby reducing precautionary maintenance (Ashley et al. 2007). Nonetheless, the public will likely expect that the maintenance is guaranteed in urban drainage systems, which are operated by utilities and interested in the long-term and reliable operation of the system (Ashley et al. 2007). Hence, further uncertainties additionally surround the future development of maintenance, the danger of failures in urban drainage systems, and the resulting consequences.

The uncertainties about the quality of maintenance in the future will pose consequences for the hydraulic performance of the sustainable urban drainage system. These uncertainties could be described by scenarios analysis, though they are structural uncertainties that will not be reduced.

The development occurs in a medium to long-term period.

Additional uncertainties for the treatment facilities of sustainable urban drainage systems are described in this section. Unlike the quantitative management, the qualitative management of sustainable urban drainage systems is affected by numerous uncertain future drivers.

 Treatment performance: According to Scholes et al. (2008b), the effectiveness of treatment

facilities depends on several local conditions in addition to the design characteristics of the specific facility, so that a generalized pollutant percentage removal rate is a mostly not valid indicator for performance. There are therefore uncertainties within the pollutant percentage removal rates of the treatment facilities of sustainable urban drainage systems. For example.

Scholes et al. (2008a) claim that different types of treatment facilities like ponds and wetlands must to be predicted in order to investigate the internal design factors that influence plan pollutant removal rate, like geometry bathymetry, hydraulic retention times, sediment water column, etc. However, there are also treatment mechanisms like the passage of the vegetated soil, with well-understood functionality and several experience-based verifications (Sieker et al. 2009). Nonetheless, uncertainties will remain concerning the technical understanding of the performance of several treatment facilities within sustainable urban drainage systems.

 Maintenance: To guarantee the reliability of performance, maintenance of the treatment facilities in sustainable urban drainage systems is required. The pollutant percentage removal rate is also influenced by the quality of the maintenance. As already presented above, the quality of maintenance is affected by several socio-economic drivers and hence is associated with future uncertainties.

 Technological development: Because of the long operational life span of urban drainage systems, it is possible that an available treatment technology will change during lifetime of the system. The development and implementation of technological innovation is characterized by several uncertain future drivers (Scholes et al. 2006; Evans et. al 2004a; Pinnekamp et al.

2008). Different from quantitative management facilities, which are already sophisticated, for treatment facilities, distinct technological improvements are possible. This is illustrated by the fact that in the present technical literature, only minor innovations are made for quantitative

management, but several innovations for the treatment of urban runoff are discussed. As examples, filter systems for gullies (Sommer 2004) and strategies for the retrofitting of wetlands (Ellis et al. 2003) are both mentioned as new treatment approaches. The invention and implementation of innovations depend on several socio-economic drivers, so the ability of a society to innovate corresponds with general economic development and the available funds for research. Furthermore, ability depends on the open-mindedness of the society toward change. There are huge uncertainties, even including incertitutede, looming over the innovations for treatment facilities of sustainable urban drainage systems and also regarding how such innovations will be implemented in future.

The quality of the maintenance of treatment facilities is, as described, associated with structural uncertainties. Furthermore, there is uncertainty in terms of the uncertainty itself, as it is presently unknown if the uncertainties associated with the treatment facilities are reducible uncertainties, caused by inadequate knowledge, or structural and not reducible uncertainties, caused by complex treatment processes. Moreover, there is incertitude about the future development of technological innovations. Thus the treatment facilities of sustainable urban drainage systems are clearly confronted with high uncertainties.