# Alluvial aquifer modeling with MODFLOW in the case of a well-field facing global change (AUVERWATCH Project)

Jordan Labbe, Hélène Celle, Pierre Nevers, Victor Klaba, Jean-Luc Devidal, Marie-Laure Clauzet, Gilles Mailhot

► To cite this version:

Jordan Labbe, Hélène Celle, Pierre Nevers, Victor Klaba, Jean-Luc Devidal, et al. Alluvial aquifer modeling with MODFLOW in the case of a well-field facing global change (AUVERWATCH Project). The International Association of Hydrogeologists, IAH2023, Sep 2023, Cap Town, South Africa. hal-04274181

HAL Id: hal-04274181

https://hal.science/hal-04274181v1

Submitted on 7 Nov 2023

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Distributed under a Creative Commons CC0 1.0 - Universal - International License
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UNIVERSITÉ
Clermont Auvergne

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# Alluvial aquifer modelling with MODFLOW in the case of a well-field facing global change

(AUVERWATCH project)

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Cap Town, September 21st 2023

**Jordan Labbe**, Hélène Celle, Pierre Nevers, Victor Klaba, Jean-Luc Devidal, Marie-Laure Clauzet, Gilles Mailhot

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Distinctive trends at the watershed scale

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Water abstractions ($Q_W$) over the alluvial hydrosystem

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**The Allier River:** known for its recurrent and intense summer droughts (Q<10m³/s)

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Labbe et al., (2023)

**The dam of Naussac:**

* Located in the upstream watershed

* 185 Mm³ of stored water

* Supports the Allier River low-flows (from June to October)

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But the winter rainfall is less and less abundant and the dam is hardly refilled....

...Considering the regional importance of the Allier alluvial hydrosystem for irrigation, industries and domestic supply – Water stakeholders are wondering:

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The Naussac dam affected by drought

https://www.zoomdici.fr

How extreme low-flows affect groundwater levels of an exploited alluvial aquifer?
Taking the case of a municipal well-field:

<figure><img src="image_15.png" /><figcaption>Geologic map of the study area (BRGM)</figcaption></figure>

The Val d'Allier well-field:

➤ 71 pumping wells

➤ 8 millions of m³ per year for the
100,000 inhab. of the Clermont-
Ferrand municipality

➤ On the recent Allier River
terrace (Fy-z)
Taking the case of a municipal well-field:

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Groundwater Vistas 8 (MODFLOW-2005)

Model based on:

Geological investigations

Groundwater/River levels monitoring

ERT surveys

<figure><img src="image_18.png" /><figcaption>Imaging the interface between marls and alluvium</figcaption></figure>
Groundwater Vistas 8 (MODFLOW-2005)

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Calibrate the model in natural flow conditions (no change in hydraulic heads over time)

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Allocate the hydraulic conductivities (Kx, Ky) over the modeled area
Groundwater Vistas 8 (MODFLOW-2005)

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Calibrate the model in forced flow conditions (hydraulic heads may vary over time)

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Allocate the specific yield $S_y$ over the modeled area
West Model grid extent East

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Row 110

Constant head on
southern border

General Head
Boundary (GHB) on
eastern border

West-East profile of the water-table level

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1) Normal flow conditions

$Q_{RIVER} = 30\text{m}^3\text{/s}$

Pumping schedule:

12h pumping

12h recovery
West Model grid extent East

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Row 110

General Head
Boundary (GHB) on
eastern border

Constant head on
southern border

## West-East profile of the water-table level

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2) Extreme low-flow conditions

$Q_{RIVER} = 2m^3/s$

Pumping schedule:

12h pumping

12h recovery
West Model grid extent East

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Left bank:
no-flow
area

Row 110

General Head
Boundary (GHB) on
eastern border

River
Boundary on
western border

Constant head on
southern border

West-East profile of the water-table level

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For 2 months:

2) Extreme low-flow conditions
Q<sub>RIVER</sub> = 2m<sup>3</sup>/s

Pumping schedule:
20h pumping
4h recovery

(to anticipate higher domestic
water needs)
* Low-flows are a major issue for connected hydrosystems such as River/alluvial aquifers

* Extreme low-flows in the River also affect the groundwater levels (static & drawdown levels are decreasing)

* Could be a threat for drinking water production

* Provide more accurate answers to water stakeholders and to give them some tools to anticipate future potential water crisis

### Limits:

* The model still needs to be improved:

i) it is calibrated in transient conditions on a daily basis,

ii) it needs to be calibrated and validated on a monthly time-scale to consider the alternance of low/high flow periods
Thank you for your attention

Contact: jordan.labbe@doctorant.uca.fr

This work is part of the AUVERWATCH Project (SNO H+, IR OZCAR, eLTER)

We thank the Agence de l'Eau Loire-Bretagne and the I-SITE CAP20-25 for
funding this project