<p>Increasing urbanization and climate-change-related measures have resulted in a growing demand for knowledge of the subsurface beneath cities and urban water management. Yet, knowledge of urban subsurfaces is not well documented and the urban anthropogenic geology's impact on groundwater is poorly understood. This study examines the impact of urban geology on the water balance and the dynamics of shallow groundwater at city-scale. &#160;</p><p>An integrated surface-subsurface hydrological model was developed based on the MIKE SHE code for an urban domain in Odense, Denmark, covering an area of 10 km<sup>2</sup>. In addition to basic hydrological processes, the model included urban processes in the form of overland drainage based on the degree of paved area, perimeter drains around major buildings, subsurface drainage, leakage from the sewer system, and groundwater abstraction. Three geological models were tested as input to the hydrological model. The hydrological models were run with two different horizontal resolutions, respectively a grid size of 10x10 and 50x50 m. The three geological models varied in complexity and representation of the near-surface urban geology: (1) V0, the base model, represented the layered regional geology beneath the urban area. (2) V1, a revised version of V0, included a representation of subsurface infrastructure; road and railroad base and embankment material, basements, and utility trenches. (3) V2, a revised version of V1, in addition included data from shallow geotechnical boreholes, yielding a representation of local areas with fill material. The urban near-surface geology in V1 and V2 were represented in a voxel model with sand/clay fraction classes. All versions of the hydrological model were calibrated based on the same setup, objective functions, and a calibration dataset consisting of 53 hydraulic head time-series and stream discharge observations within the model domain.</p><p>The results showed that the heterogeneity was smoothened when the hydrological model included a complex near-surface urban geology in a 50x50 m grid size and thus an effect of the urban geology was not reflected in the simulated head or the water balance. Meanwhile, the near-surface complexity in the V1 and V2 models led to a better model performance in terms of mean error and annual amplitude error, when the hydrological model had a 10x10 m grid size, which is closer to the scale of the heterogeneities.</p><p>The study illustrates that the manmade urban geology, in terms of subsurface obstacles and utility trenches, impacts shallow groundwater dynamics and flow paths. Moreover, it documents that it is possible to represent heterogeneous urban geology in a city-scale model, given the data is available. The results suggest that to simulate the effect of urban geology on shallow groundwater the computational grid needs to be of a size that can resolve the main subsurface infrastructures. In conclusion, a representation of the urban near-terrain geology improves the simulation of shallow groundwater and thus provides a better basis for urban planning, water management, and transport modeling.</p>
Abstract. This study examines the impact of urban geology and spatial discretization on the simulation of shallow groundwater levels and flow paths at the city scale. The study uses an integrated hydrological model based on the MIKE SHE code that couples surface water and 3D groundwater simulations with a leaky sewer system. The effect of the geological configuration was analyzed by applying three geological models to an otherwise identical hydrological model. The effect of spatial discretization was examined by using two different horizontal discretizations for the hydrological models of 50 and 10 m, respectively. The impact of the geological configuration and spatial discretization was analyzed based on model calibration, simulations of high water levels, and particle tracking. The results show that a representation of the subsurface infrastructure, and near-terrain soil types, in the geological model impacts the simulation of the high water levels when the hydrological model is simulated in a 10 m discretization. This was detectable even though the difference between the geological models only occurs in 7 % of the volume of the geological models. When the hydrological model was run in a 50 m horizontal discretization, the impact of the urban geology on the high water levels was smoothed out. Results from particle tracking show that representing the subsurface infrastructure in the hydrological model changed the particles' flow paths and travel time to sinks in both the 50 and 10 m horizontal discretization of the hydrological model. It caused less recharge to deeper aquifers and increased the percentage of particles flowing to saturated-zone drains and leaky sewer pipes. In conclusion, the results indicate that even though the subsurface infrastructure and fill material only occupy a small fraction of the shallow geology, it affects the simulation of local water levels and substantially alters the flow paths. The comparison of the spatial discretization demonstrates that, to simulate this effect, the spatial discretization needs to be of a scale that represents the local variability in the shallow urban geology.
Abstract. This study examines the impact of urban geology and spatial resolution on the simulation of shallow groundwater levels and flows at the city scale. The study uses an integrated hydrological model based on the MIKE SHE code that couples surface water and 3D groundwater simulations with a leaky sewer system. The effect of geological configuration was analyzed by applying three geological models to an otherwise identical hydrological model. The effect of spatial resolution was examined by using two different horizontal grid sizes in the hydrological model, respectively 50 m and 10 m. The impact of the geological configuration and spatial resolution was analyzed based on model calibration, simulations of high-water levels, and particle tracking. The results show that a representation of the subsurface infrastructure, and near terrain soil types, in the geological model impacts the simulation of the high-water levels when the hydrogeological model is simulated in 10 m resolution. This was detectable even though the difference between the geological models only occurs in 7 % of the volume of the geological models. When the hydrological model was run in 50 m horizontal resolution, the impact of the urban geology on the high-water levels was smeared out. Results from particle tracking show that representing the subsurface infrastructure in the hydrological model changed the particles’ flow path and travel time to sinks, both in the 50 m and 10 m horizontal resolution of the hydrological model. It caused less recharge to deeper aquifers and increased the percentage of particles flowing to saturated zone drains and leaky sewer pipes. In conclusion, the results indicate that even though the subsurface infrastructure and fill material only occupy a small fraction of the shallow geology, it affects the simulation of local water levels and substantially alters the flow paths. The comparison of the spatial resolution demonstrates that to simulate this effect the spatial resolution needs to be of a scale that represents the local variability of the shallow urban geology.
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