Modeling the establishment of preferential flow during

Modeling the establishment of preferential flow during infiltration in a heterogeneous glaciofluvial deposit Erij Ben Slimene(1,2)*, Laurent Lassabatere (1), Thierry Winiarski (1), Remy Gourdon(2)

1 – Context and objective

EGU2015-5503

Due to its rapid movement, preferential flow in the vadose zone allows much faster contaminant transport and creates significant consequences for ground-water quality.  This study deals with flow modeling during the infiltration phase. In particular, we want to point out numerically the worst conditions with regards to preferential flow, in terms of initial hydric conditions (initial water contents) and imposed flow rates.

4 – Results: Influence of material heterogeneity and top flux on preferential flow

2 – Profile: sedimentological and schematic description The study site is located on the glaciofluvial deposit of the east of Lyon, France.

The results of modeling volumetric water content and velocity vectors (Fig.2.) and the free boundary flux (Fig. 3.) are presented as a function of time for 2 imposed surface fluxes applied to a uniform profile (bimodal Gravel) and a heterogeneous profile (the 4 materials) Low Velocity

High Velocity

Water Content - th[-], Min=0.068, Max=0.159

Uniform Profile 0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230

where θ is the volumetric water content, x, y, and z spatial coordinates, h the water pressure (2) head, t time, and K the unsaturated hydraulic conductivity tensor The model was built using the 2D representation of the lithofacies .  Fine Element Mesh: Triangular elements; Max. number of FE-mesh nodes: 200000; Mesh stretched in x-z directions with stretching factor FS = 1; Boundary conditions: Bottom BC : Free drainage; Top BC: Atmospheric Boundary;  Initial conditions: A pre-drainage phase of 168h was modeled. The final pressure map was imported as a new initial condition; The hydrodynamic parameters of each hydrofacies (Tab.1) were implemented in HYDRUS 2D. θr θs α =1/hg n Ks Material (-) (m3m-3) (m3m-3) (m-1) (m h-1) Tab. 1. Hydrodynamic 0.013 0.337 20.5 2.92 3.52 Sand parameters for Van 0.037 0.274 4.74 2.40 0.551 Gravel Genuchten - Mualem 0.032 0.226 0.840 2.71 0.0432 Model Bimodal Gravel 0.360 111.6 2.70 360 Gravel free Matrix 0.020


0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230


0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230


0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230

0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230



t = 30h t = 15h t = 10h t = 5h t = 0h

Modeling water flow was performed using the generalized Richards equation

0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230

Heterogeneous Profile

4

25

Uniform Profile

Uniform Profile

20 Heterogeneous Profile

0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230

0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230



0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230

0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230



0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230


3

15

2

10

1

5

0

0


0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230


0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230

0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230

0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230

0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230





0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230

0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230

0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230

0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230





t = 30h

3 – Modeling : Parameters and numerical options

t = 75h t = 38h t = 25h t = 13h t = 0h

Fig. 1. Lithofacies description (up) and Material distribution in HYDRUS (down)

(1)

Uniform Profile

High Velocity

Q [mm/h]

Sand Gravel free Matrix

t = 75h

bimodal sandy Gravel Gravel


Q [mm/h] 5

Low Velocity

Fig. 2. Evolution of the volumetric water content as a function of time and velocity vectors distribution at steady state, calculated with HYDRUS for 2 imposed surface fluxes 5 mm/h (a) and 25 mm/h (b);  Low surface flux (5mm/h): heterogeneous velocity vectors distribution in the case of heterogeneous profile.  High surface flux (25mm/h): the wetting front can be assimilated to a plug flow. When the steady state is established, the velocity vector field is uniform for both profiles

0 10 20 30 40 50 60 70 Time [h]

0

5

10 15 20 Time [h]

25

30

Fig. 3. Water Flux at the lower boundary for an imposed surface flux of 5 mm/h (left) Water flux at the lower boundary for an imposed surface flux of 25 mm/h (right); data for the uniform profile (blue line), data for the heterogeneous profile (red line)  At low velocity, water contents remain low, especially in the most porous/draining materials (Sand and gravel-free Matrix) due to their low water retention capacities  lower permeability  Flow deviations in their interfaces with the predominant bimodal Gravel  a significant regionalization of flows

5 – Conclusions Numerical modeling permitted pointing out the existence of preferential flow paths associated with the sedimentary heterogeneity of the glaciofluvial deposit. Under a lower surface flux, Sand’s lens and gravel-free Matrix were the sources of capillary barrier effects, leading to a funneled flow in the layer above the interface between the materials. *Corresponding author : [email protected] - (1) University of Lyon, LEHNA UMR 5023 CNRS ENTPE UCBL, Lyon, France; (2) University of Lyon, LGCIE INSA Lyon, France

Modeling the establishment of preferential flow during infiltration in a heterogeneous glaciofluvial deposit Erij Ben Slimene(1,2)*, Laurent Lassabatere (1), Thierry Winiarski (1), Remy Gourdon(2)

1 – Context and objective

EGU2015-5503

Due to its rapid movement, preferential flow in the vadose zone allows much faster contaminant transport and creates significant consequences for ground-water quality.  This study deals with flow modeling during the infiltration phase. In particular, we want to point out numerically the worst conditions with regards to preferential flow, in terms of initial hydric conditions (initial water contents) and imposed flow rates.

2 – Profile: sedimentological and schematic description

4 – Results: Influence of material heterogeneity and top flux on preferential flow

The study site is located on the glaciofluvial deposit of the east of Lyon, France.

The results of modeling volumetric water content and velocity vectors (Fig.2.) and the free boundary flux (Fig. 3.) are presented as a function of time for 2 imposed surface fluxes applied to a uniform profile (bimodal Gravel) and a heterogeneous profile (the 4 materials) Low Velocity

High Velocity


Uniform Profile 0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230

Modeling water flow was performed using the generalized Richards equation

(2)

where θ is the volumetric water content, x, y, and z spatial coordinates, h the water pressure head, t time, and K the unsaturated hydraulic conductivity tensor

The model was built using the 2D representation of the lithofacies .  Fine Element Mesh: Triangular elements; Max. number of FE-mesh nodes: 200000; Mesh stretched in x-z directions with stretching factor FS = 1; Boundary conditions: Bottom BC : Free drainage; Top BC: Atmospheric Boundary;  Initial conditions: A pre-drainage phase of 168h was modeled. The final pressure map was imported as a new initial condition; The hydrodynamic parameters of each hydrofacies (Tab.1) were implemented in HYDRUS 2D. θr θs α =1/hg n Ks Material (-) (m3m-3) (m3m-3) (m-1) (m h-1) 0.013 0.337 20.5 2.92 3.52 Tab. 1. Hydrodynamic Sand 0.037 0.274 4.74 2.40 0.551 Gravel parameters for Van Genuchten - Mualem Bimodal Gravel 0.032 0.226 0.840 2.71 0.0432 Model 0.360 111.6 2.70 360 Gravel free Matrix 0.020


0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230


0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230


0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230

0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230



t = 30h t = 15h t = 10h t = 5h t = 0h

3 – Modeling : Parameters and numerical options

0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230


4

Uniform Profile

High Velocity

Q [mm/h]

25 Uniform Profile

20

Heterogeneous Profile 0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230

0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230



0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230

0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230



0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230


3

15

2

10

1

5

0

0


0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230


0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230

0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230

0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230

0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230





0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230

0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230

0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230

0.000 0.021 0.042 0.063 0.084 0.105 0.125 0.146 0.167 0.188 0.209 0.230





t = 30h

Fig. 1. Lithofacies description (up) and Material distribution in HYDRUS (down)

(1)

Uniform Profile

Gravel free Matrix

t = 75h t = 38h t = 25h t = 13h t = 0h

Sand

t = 75h

bimodal sandy Gravel Gravel


Q [mm/h] 5

Low Velocity

Fig. 2. Evolution of the volumetric water content as a function of time and velocity vectors distribution at steady state, calculated with HYDRUS for 2 imposed surface fluxes 5 mm/h (a) and 25 mm/h (b);  Low surface flux (5mm/h): heterogeneous velocity vectors distribution in the case of heterogeneous profile.  High surface flux (25mm/h): the wetting front can be assimilated to a plug flow. When the steady state is established, the velocity vector field is uniform for both profiles

0 10 20 30 40 50 60 70 Time [h]

0

5

10 15 20 Time [h]

25

30

Fig. 3. Water Flux at the lower boundary for an imposed surface flux of 5 mm/h (left) Water flux at the lower boundary for an imposed surface flux of 25 mm/h (right); data for the uniform profile (blue line), data for the heterogeneous profile (red line)  At low velocity, water contents remain low, especially in the most porous/draining materials (Sand and gravel-free Matrix) due to their low water retention capacities  lower permeability  Flow deviations in their interfaces with the predominant bimodal Gravel  a significant regionalization of flows

5 – Conclusions Numerical modeling permitted pointing out the existence of preferential flow paths associated with the sedimentary heterogeneity of the glaciofluvial deposit. Under a lower surface flux, Sand’s lens and gravel-free Matrix were the sources of capillary barrier effects, leading to a funneled flow in the layer above the interface between the materials. *Corresponding author : [email protected] - (1) University of Lyon, LEHNA UMR 5023 CNRS ENTPE UCBL, Lyon, France; (2) University of Lyon, LGCIE INSA Lyon, France

Context and objective version1 • Due to its rapid movement, preferential flow in the vadose zone allows much faster contaminant transport and creates significant consequences for ground-water quality. This study deals with flow modeling during the infiltration phase. In particular, we want to point out numerically the worst conditions with regards to preferential flow, in terms of initial hydric conditions (initial water contents) and imposed flow rates. version2 • Preferential flow in the unsaturated zone is a dominant influence in many problems of infiltration, recharge, contaminant transport and ecohydrology because it commonly generates high-speed, high volume flow with minimum exposure to solid earth materials. This study deals with flow modeling during the infiltration phase. In particular, we want to point out numerically the worst conditions with regards to preferential flow, in terms of initial hydric conditions (initial water contents) and imposed flow rates.