Elaboration of a geological and hydraulic mapping project of infiltrability potential on the Aix-Marseille Provence Metropole (SE, France)

The Aix Marseille Provence metropolitan area experiences rapid urbanization that reinforces the need for infrastructure and implies considerable sealing of the substratum This region is a typical arid and Mediterranean environment where rain precipitation can be exceptionally catastrophic. This two factors creates runoff, overflow and flooding in the urban area. One solution to manage the flooding and overflow is to allow more water to penetrate into the soil, by removing the impermeable and anthropic materials where the geological substratum is naturally able to infiltrate the water.

Usually, standard parameters such as: topography, drainage density and hydrological balances, are used to estimate runoff and indirectly find the infiltrability values and ultimately tackle infiltration problematics. These approaches are informatic and mathematics-based that work in a small, delimited and homogeneous area. To integrate this problematics to large scale and heterogenous systems, reservoir geology concepts such as geomorphology, uncertainties of scale change processes or structural geology can be addressed. Therefore, this project aims to understand the geological processes that controls the infiltration potential in the geological substratum and its spatial distribution for the purpose of creating an infiltrability map of the Aix Marseille metropolis.

The goal of this study is to develop a method for predicting the infiltration capacity on a large scale and heterogenous area including urban zone. This involves acquiring local observational data points which classify rock outcrops in 4 “hydraulic types” (HT) defined as follows: HT-1 represents impermeable rocks or soils, where no infiltration is possible; HT-2 represents thin soils with variable porosity and permeability; HT-3 describes rocks with low to very high matrix porosity influenced by clay matrix presence and variable permeability; HT-4 describes rocks with fractures and/or karst networks with low to very high permeability depending on fracture/cavity density, with variable porosity. With the geolocated data points, a map is created on QGIS (a Geographic Information System free software) in order to up-scale the hydraulic types over a larger scale grid by spatial interpolation.

For an even acquisition area, geological heterogeneity and accessibility of outcrops determines the data number needed to upscale hydraulic types. This approach is well-known in reservoir geology and this large-scale project is the opportunity to apply the methodology to  hydrogeology field.

Additionally, to address the lack of visibility of outcrops, subsurface data (shallow well data from the BRGM, Bureau of Geological and Mining Research) will be combined with field observations. Furthermore, a calibration of this method will be required to quantify and to establish thresholds within the Hydraulic Types classification. This project will ultimately provide specific values for infiltration capacity and facilitate flood risk management without having to use complex and costly technologies.

 

Keywords : SIG mapping, infiltration, runoff, geological substratum, stratigraphy, structural geology, heterogeneity, precipitation, de-sealing, available water