Department Urban Water Management
Urban Water Infrastructure Model (UWIM)
150 years ago the first sewers were built in Switzerland - the beginning of gravity sewerages triumphal procession in urban water management leading to a unique monopoly position. But today experts discuss more and more the sustainability of the present sewer structure
A variety of new sanitary systems are suggested promising to compensate disadvantages of the established system. But to compare and assess different alternatives their principal behavior needs to be characterized. Surprisingly central sewerage is rather bad characterized. Cost parameters, efficiency und effective life expectancy are among other parameters that are hardly known, but that would be necessary for a comprehensive assessment. Additionally it is rather difficult to receive long term sewerage data. Therefore the Urban Water Infrastructure Model (UWIM) was developed to identify general drivers influencing the characteristic of sewer systems and their costs quantitatively.
Final goal of the software and the model is the support of infrastructure development questions, such as transition management from traditional sewerage to decentralized wastewater treatment.
Urban Water Infrastructure Model (UWIM)
The urban water infrastructure model (UWIM) is a conceptual model that describes the water infrastructure of a settlement quantitatively in terms of generic input parameters, such as size of catchment area, number of buildings.
It contains of two main modules
- (i) the catchment area module, and
- (ii) the sewer construction cost module.
The catchment area module represents the core of UWIM. The output parameters are the total length of sewer pipes and the diameter distribution based on the number of house connections and the amount of water that needs to be drained.
Figure 1 shows the setup of the simplified catchment area that is used for calculations.
The computation of the diameter distribution uses a modified rational sequential Manning-Strickler approach for the peak flow caused by the run-off of the dimension rain in the drained area. The model predictions were compared with the detailed analysis of different sub-catchments. The data were directly taken from the sewer databases of different cities and compared with the model prediction. The results show that the model predicts the principal behavior of different sized catchment areas.
Knowing the total length and the diameter distribution the sewer construction cost module calculates constructions costs depending on depth in which the pipes are installed and complexity of construction, e.g. green field construction or integration below an existing street. Other cost relevant influences such as underground properties, pipe material, and construction and safety regulations are excluded for the sake of simplicity.
UWIM provides the basis for a detailed quantitative characterization of various drives influencing sewer systems and lays the foundation for dynamic modeling of sewer development, e.g. the growth of sewer systems associated with the growing settlements or the decay of sewers.
An implementation in R can be found here.
UWIM-Applications and recent projects
Economy of Scale
Sewer systems are the state of the art for wastewater handling. In Switzerland, more than 96% of the population is connected to centralised sewer networks. How large is the optimal size of a sewer network? Is bigger indeed cheaper? UWIM was used to answer this questions about the the ‘Economy of Scale’ for combined sewer networks.
The simulation results show that in Switzerland there is an apparent economy of scale for combined sewer systems. This is the result of two main opposing cost factors: (i) increased construction costs for larger sewer systems due to larger pipes, and (ii) higher population and building densities in larger towns decreasing specific costs per capita.
So far UWIM can only describe sewer systems in connected settlements. In reality several areas are connected by transport sewers and represent the catchment area of a waste water treatment plant. This urban sprawl influences the sewer system, but until know there is now convenient landscape measure to describe this correlation.
For dynamical modeling of a sewer system is a deterioration model fundamental. For Switzerland are no data available which describes the history of the replacement of the pipe sections, but the condition at present of the sewer system is known. The aim is the development of a deterioration model which can be calibrated with the available data.
Total Costs and their Dynamic Simulation
To compare different sewer systems or reconstruction strategies the total costs need to be calculated dynamically. Therefore the knowledge about life expectation of sewers has to be improved as it plays a major role in estimating capital and maintenance costs.