Modeling of a tile drainage network in glacial clayey tills

G. De Schepper, R. Therrien, JC. Refsgaard

In the proceedings of: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Groundwater-Surface Water Interactions II

ABSTRACT: Tile drainage is a common agricultural practice in poorly drained production fields to guarantee the productivity of crops and to reduce flooding risks. The impact of shallow tile drainage networks on groundwater flow patterns and associated nutrients transport from the surface needs to be quantified for adequate agricultural management. A challenge is to represent tile drainage networks in numerical models, from field scale to catchment scale, while accounting for the influence of subsurface heterogeneities on flow and transport. A numerical model of a field scale tile drained area (5 ha) has been developed with the fully integrated HydroGeoSphere model for the Lillebæk catchment, which is located on Funen Island, Denmark. The Lillebæk catchment includes various tile drainage networks that are monitored regularly, one of those have been selected to build a model. The catchment subsurface is made of Quaternary deposits which consist of a local sandy aquifer with upper and lower clayey till units, confining the aquifer in the upper part. The main modeling objective is to assess the influence of tile drains on the water flow pattern within the confining clayey till unit and on the nitrate reduction zone depth, also known as the redox-interface, while accounting for local geological heterogeneities. Using the national-scale geological model for Denmark combined with available local data, a hydrogeological model at field scale has been generated. A proper representation of the tile drains geometry is essential to calibrate and validate the water flow model associated with nitrate transport. HydroGeoSphere can represent drains directly into a model as one-dimensional features, which however requires a very fine mesh discretization that limits the size of the simulation domain. Because of this limitation, we are testing an alternate approach where the tile drainage network and surrounding porous medium are represented by a dual-continuum formulation, similar to that used to represent fractured porous media, for example. Validating this drain representation method in a model should allow us to work at larger scale and thus to model the entire tile drained catchment without having to implement each and every drain, which would induce a very high number of model elements leading to time-consuming simulations. The main idea leading the development of such a model is to test the discrete drains vs. the dual-continuum through different numerical aspects, and to evaluate the answer of those aspects to a high precipitation event on a period of about 5 days. This contribution will focus on some preliminary simulations designed to validate the dual-continuum approach to represent tile drainage networks. These simulations are also designed to demonstrate the applicability of the approach for 3D variably-saturated flow modelling at the field scale, accounting for local geological heterogeneities.

Please include this code when submitting a data update: GEO2013_574

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