Many cities of the Northern Hemisphere are covered by low permeable clay tills, which pose a challenge for stormwater infiltration practices. However, clay tills are amongst the most heterogeneous types of sediments and hydraulic conductivities can vary by several orders of magnitude. This PhD study was initiated with the objective to test and evaluate if the hydraulic performance of stormwater infiltration systems can be significantly improved if the site-specific geological heterogeneity is incorporated into the design and siting of such systems. The assessment is based on different field investigations on two typical Danish clay till sites, and one modeling study with the integrated surface water and groundwater model HydroGeoSphere.
The saturated hydraulic conductivity (Ksat) is the most critical soil physical parameter when it comes to sizing stormwater infiltration systems. In the first study, different field methods for Ksat estimation, the double ring infiltrometer, the Guelph permeameter and falling head infiltration tests in a small excavation, were compared and evaluated for their capability to return realistic Ksat values in tills. The double ring infiltrometer and the Guelph permeameter represent suitable methods for sizing stormwater infiltration systems if measurements are combined with geological knowledge from maps of near-surface deposits and borehole descriptions. If space allows, the more invasive infiltration tests in a small excavation are recommended, because measurements are less influenced by the local hydraulic heterogeneity and therefore give the most realistic values.
In a second study, non-invasive geoelectrical surveys were compared with data from invasive geological methods, including borehole descriptions, one description of a large excavation and near-surface spear auger mapping. The data returned a significant correlation of geoelectrical and spear auger mapped surface sediments. Furthermore, a highly permeable oxidized fracture-zone at greater depths was apparent on the 2D geoelectrical profiles. The successful identification of highly permeable zones in clay tills has potential for improving the hydraulic performance of stormwater infiltration systems.
The third study was an assessment of the influence of small-scale soil physical features in clay tills on the hydraulic performance of stormwater infiltration systems based on a HydroGeoSphere model. The incorporation of soil characteristics below and above the CaCO3 boundary into simulated soil columns showed an increase in infiltration capacities by a factor of 22 compared to homogenous clay. Infiltration showed a further increase of 8% if vertical tectonic fractures were included and by another 61% if biopores like earthworm burrows were also added. A comparison of HydroGeoSphere infiltration hydrographs with a simple soakaway model (Roldin et al. 2012b) showed that the exclusion of small-scale soil physical features may result in significant underestimation of the hydraulic performance of stormwater infiltration systems. Models employing standard soil physical parameters should be used with care as they do not always realistically describe site-specific hydrologic properties.
A fourth study showed that the hydraulic performance of infiltration trenches was increased by a factor of two, when spear auger mapping and geoelectrical surveying were used to position the trenches.