Implementation of Sewer Network Structures into Numerical Heat Transport Models via an Adaptive Surrogate Approach

Martin Binder & Jannis Epting

According to projections of the United Nations, more than two thirds of the world’s population will live in urban regions by the mid of the current century. Rapid urbanization is associated with negative impacts on urban groundwater availability, with deteriorating factors for both quantity and quality. Adequate management strategies are required to increase the resilience of cities and their ecosystems. Sophisticated numerical models, designed for simulating the water flow as well as solute and heat transport processes in the subsurface, are powerful and established tools supporting decision making and planning and, hence, the sustainable use of subsurface resources.

Besides quantifying the water volumes available for abstraction, understanding the current thermal state of the subsurface and groundwater resources, as well as potential changes in the light of ongoing climate change and urbanization, is essential. Models designed for this very specific task should include at least all major objects (e.g., underground car parks, tunnels, sewer networks) which thermally contribute to groundwater heat regimes. For instance, heat exchange between the subsurface and sewer systems (the latter conducting comparably warm, untreated wastewaters from households and industry to treatment plants) may significantly contribute to the so-called subsurface urban heat island effect as especially observed in densely populated areas, and should, therefore, be addressed in groundwater management models.

However, fully 3-D implementations of all subsurface objects, especially of sewer networks with hundreds of kilometers of pipes, are typically out of question when applying such numerical models, since it would be associated with large computational demands (due to local refinements of model meshes) and, most likely, with increasing numerical instabilities of such simulations.

To overcome this limitation, the focus of our current research is to evaluate the suitability of an adaptive surrogate method as illustrated in Figure 1. Our method is based on quantitatively transferring the expected thermal exchange rates between, for instance, sewer pipes and their surrounding area in a simplified form to the elements of an existing model mesh. For this, the thermal interactions to be implemented are numerically investigated at multiple levels of complexity (e.g., multiple pipe sizes and shapes), and at multiple scales.

Funding: This work was supported by a postdoc fellowship of the German Academic Exchange Service (DAAD) granted to main author M.B. (project “DAAD.SewerHeat”, PKZ 91724232). Further funding granted to co-author C.E. was contributed by the German Research Foundation (grant 499973567, project “ReCAp”).

Konzept_Kanalisationsnetze

Concept of the adaptive surrogate approach (example: sewer networks) using both steady-state (e.g., pipe locations and shapes) and transient datasets (e.g., temperature distributions and distance to groundwater level).