Deterministic modeling of the impact of underground structures on urban groundwater temperature
Graphical abstract
Introduction
Half of the world's population now lives in cities. This phenomenon of urbanization is such that this proportion will reach 70% before 2050 (Un-Habitat, 2008). Land constraints lead to the vertical development of urban areas, with the construction of ever-deeper structures (Bobylev, 2009) (e.g., subways, building foundations, underground carparks). In parallel, the urban subsoil is now recognized as a space rich in resources: available water, geomaterials and geothermal energy (Li et al., 2013b, a), which play a vital role in ensuring sustainable territorial development (Goel et al., 2012), but for which regulations remain wanting (Foster and Garduño, 2013). This results in a lack of coordination and planning in the exploitation of this space, illustrated by conflicts over use (Bobylev, 2009).
In particular, the resilience of groundwater resources has become a major issue. On the one hand, 40% of the water distributed in the water supply networks of Europe comes from urban aquifers (Eiswirth et al., 2004), while on the other hand, geothermal energy is now considered as a strategic urban resource (Lund et al., 2011), (Herbert et al., 2013) since the European Council made a commitment to reduce greenhouse gas emissions by 20% by 2020 (European Commission, 2009). In parallel, underground structures involving impervious elements or pumping and re-injection devices, can impact groundwater flow (Attard et al., 2016c). Impervious structures can act as an obstacle to the flow (Epting et al., 2008), (Pujades et al., 2012). Draining structures can result in a fragmentation of urban flow systems (Attard et al., 2016b). In addition, underground structures can impact groundwater quality (Chae et al., 2008), (Attard et al., 2016a) and temperature (Hu et al., 2008), (Epting and Huggenberger, 2013), (Ferguson and Woodbury, 2004).
Regarding groundwater temperature, the heat island effect on groundwater due to urbanization has been clearly observed in many cities around the world (e.g. Zhu, K., et al., 2010, Taniguchi, M., et al., 2009, Menberg, K., et al., 2013a). In particular, Menberg et al. (2013b and Benz et al. (2015) used an analytical heat flux model and a GIS approach to demonstrate that underground structures makes up a significant share of the total anthropogenic heat flux into urban aquifers. In addition to that, Ampofo et al. (2004) used a mathematical model written with an engineering equation solver to investigate the heat load of an underground railway. The authors showed that heat absorbed by the earth surrounding a subway can reach 30% of the total heat generated by the structure. Finally, Epting and Huggenberger (2013) used a deterministic modeling approach to assess the thermal potential natural state and the present state of the groundwater body of Basel (Switzerland). The heat-transport model showed a major influence of deep underground structures on groundwater temperature. On the one hand, the total anthropogenic heat flux of a city into groundwater provides a potentially sustainable geothermal resource (Benz et al., 2015). However, according to (Hähnlein et al., 2013) the shallow geothermal system is only sustainable if the generated energy is mainly renewable energy. Consequently, the total anthropogenic heat flux should be quantified (Benz et al., 2015) and extracted in areas where groundwater temperatures are high (Allen, A., et al., 2003, Revesz, A., et al., 2016). On the other hand, groundwater cooling systems can be disturbed where temperatures are increased by anthropogenic heat flux Epting et al. (2013).
Understanding the thermally affected zone (TAZ) of both urban underground structures and groundwater heat pump systems (GWHPS) is important as it could facilitate the management of urban underground development and geothermal exploitation. Since the TAZ of GWHPS has been described thoroughly in previous works (Lo Russo et al., 2012), (Lo Russo et al., 2014), (Sciacovelli et al., 2014), the aim of this paper is to quantify the TAZ of two common underground structures on urban groundwater temperature: (1) impervious deep foundations, and (2) a structure with a drainage and re-injection system. In addition, the cumulative impact of these structures was assessed at the scale of the city of Lyon (France). The consequences on the geothermal potential of urban aquifers are discussed regarding the TAZ and the heat flow generated by underground structures.
Section snippets
Study site
The area of the city of Lyon (France) (GPS coordinates: 45.75∘ N/4.85∘ E) was chosen to study the impact of underground structures on groundwater temperature. This city has a great potential for urban underground development in the light of the criteria proposed in (Li et al., 2013b) (i.e. subsurface geotechnical quality, groundwater quality, geothermal energy, geomaterial quality, urban population, population density and GDP per capita). In practice, this potential is reflected by the economic
Individual impacts of impervious and draining structures
First, the impacts of two design techniques, (1) an impervious structure and (2) a draining structure, on groundwater temperature were simulated in a generic 1 km2 bi-layered aquifer with hydraulic and material properties similar to that of the Lyon urban aquifer. This generic case was assumed to reproduce an urban aquifer recharged by precipitations and network losses, and discharging into a river downstream. As described in (Youngs, 1990) in form of analytical results, this configuration leads
Individual impacts of the impervious and draining structures - scenarios 1 and 2
Before running the transient 20-year simulation, the initial situations of scenarios 1 and 2 were simulated in steady state. In both cases, the temperature ranges from 15 °C at the upper surface of the aquifer (i.e. Z = 0 m), to 18 °C at the lower surface of the aquifer (i.e. Z = − 150 m). In the following, the results dealing with temperature disturbances were presented relatively to these initial situations.
The horizontal and vertical distribution of temperature disturbances in scenarios 1 and 2 were
Summary and conclusions
The first aim of this paper was to assess the TAZ of two common underground structures design techniques on urban groundwater under unconfined conditions. The second aim was to assess the cumulative TAZ of deep underground structures at the scale of an urban area. A deterministic 3D modeling approach was used and three simulations were run under average thermal conditions: (1) the impervious structure scenario, (2) the draining structure scenario, and (3) the cumulative impact scenario at the
Acknowledgements
The authors thank the Ministère de l'Écologie, du Développement Durable et de l'Énergie (the French Ministry of Ecology, Sustainable Development and Energy) for its financial support.
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