# On the Origin and Distribution of Internal Erosion Signatures in the Floodplain Protected by River Dikes

Laurence Girolami, Stéphane Bonelli, Jean-michel Carozza, Edouardo-jovick

Fogueng-Wafo, Jules Burgat, Naïm Chaouch, Rémi Valois

► To cite this version:

Laurence Girolami, Stéphane Bonelli, Jean-michel Carozza, Edouardo-jovick Fogueng-Wafo, Jules Burgat, et al. On the Origin and Distribution of Internal Erosion Signatures in the Floodplain Protected by River Dikes. Journal of Flood Risk Management, 2025, 18 (3), pp.e70104. 10.1111/jfr3.70104. hal-04535464

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https://hal.inrae.fr/hal-04535464v1

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ORIGINAL ARTICLE

OPEN ACCESS

# On the Origin and Distribution of Internal Erosion Signatures in the Floodplain Protected by River Dikes

Laurence Girolami<fnref n="1" />,² | Stéphane Bonelli<fnref n="1" /> ID | Jean-Michel Carozza³ | Edouardo-Jovick Fogueng-Wafo<fnref n="1" /> | Jules Burgat<fnref n="1" /> |
Naïm Chaouch<fnref n="1" /> | Rémi Valois⁴

**Correspondence:** Laurence Girolami (laurence.girolami@inrae.fr)

**Received:** 10 January 2025 | **Revised:** 30 May 2025 | **Accepted:** 7 July 2025

**Funding:** This research was supported by the Water department of INRAE (Aqua department, Pari Scientifique), by the Region Centre-Val de Loire (contribution of Academic Initiative Project RHEFLEXES/201900134935), by Aix-Marseille University (ECCOREV/ITEM 2023 PALERO Project Relationships between paleo-channels and internal erosion processes around protective dikes), by the Direction Générale de la Prévention des Risques (DGPR) and by the Water and Environment INRAE Carnot Institute.

**Keywords:** alluvial terraces | flood protection dikes | floodplain architecture | internal erosion | paleochannels | paleovalleys | sand boils | sinkholes

## ABSTRACT

The subsoils of river dikes are often composed of highly permeable and low-density river sediments. Thus, erosion signatures (leaks, sand boils, sinkholes) can appear in the protected floodplain during floods, highlighting the development of hydromorphodynamic phenomena below the surface, which may harm the safety of the dike system. A multiscale methodology is deployed to understand and analyze the influence of floodplain architecture in terms of geological formations on the appearance of local erosion signatures. Particular attention is paid to the morphology of paleovalleys and paleochannels in order to image the subsurface in terms of substrate types and interfaces using geophysical methods. This information makes it possible to propose internal erosion scenarios. Application to a study area in the South of France (the Agly dike system) leads to new results. The classical backward erosion piping scheme is not relevant to explain the observed sand boils, as they are mainly caused by the suffusion-type internal erosion process. Suffusion and contact erosion appear to be the origin of sinkholes. The distribution of these signatures appears to be directly related to the shape and dimensions of the paleovalley and paleochannels, as well as to the presence of a low-permeability topsoil.

## 1 | Introduction

The subsoil beneath river dikes is a critical factor in their stability and performance during flood events. The constitution of this subsoil generally reflects the geological history and sedimentation patterns of river valleys. Coarse soils (sand and gravel) are typically found beneath river dikes in valleys that have been incised and subsequently filled with sediment under specific geological and fluvial conditions.

The subsoils beneath these dikes and in the protected floodplain are then made up of highly permeable, coarse-grained fluvial deposits. They are often affected by underseepage and internal erosion processes during flood periods. The surface manifestation of these underlying phenomena usually takes the form of leaks, sand boils, and sinkholes developed at various scales, from a few centimeters to a few tens of meters, all along the dikes and not too far from them (Semmens and Zhou 2019; Marchi et al. 2021; Girolami et al. 2023). However,

This is an open access article under the terms of the [Creative Commons Attribution License](https://creativecommons.org/licenses/by/4.0/), which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2025 The Author(s). *Journal of Flood Risk Management* published by Chartered Institution of Water and Environmental Management and John Wiley & Sons Ltd.

<footnote n="1">¹INRAE Aix-Marseille Université UMR RECOVER, Aix-en-Provence, France | ²GéHCO, Campus Grandmont, Université de Tours, Tours, France | ³LIENS, CNRS-UMR, Department of Human and Social Sciences, La Rochelle University, La Rochelle, France | ⁴INRAE Université D'avignon UMR EMMAH, Avignon, France</footnote>
in situ observations are still lacking to track the spatio-
temporal evolution of the various processes involved at differ-
ent scales, so that the question of their origin and distribution
is not well understood or reliably described in the literature
(Bonelli 2012, 2013; Van et al. 2022). Despite the importance
of this topic in terms of risk analysis, there is still a lack of
studies devoted to mapping flood risks associated with arte-
sian conditions and topsoil uplifts in the protected floodplain
(Julinek et al. 2020; Michelazzo et al. 2018).

The safety of a dike is influenced by small-scale defects and requires a good understanding of internal flows (Camici et al. 2017; Ceccato and Simonini 2023). The location of leaks, sand boils, and sinkholes is significantly influenced by the nature of the geological formations, in terms of depth and extent of the permeable layers formed by the presence of paleovalleys or paleochannels (Kolb 1975; Wolff 2002; Pazzi et al. 2018; Semmens and Zhou 2019). A correct analysis of the flow beneath the dike requires a description of the subsurface geology. Geophysical methods provide this type of information and are used for dike systems (CIRIA 2013; USBR 2019; Dezert et al. 2019). Here we consider the following situation: the paleovalley contains a layer of coarse and permeable soils at the surface, covering a marly base of low permeability, the latter resting on the bedrock. Interfaces between paleovalley topography and paleochannel fill and marly bedrock are investigated. Contrasts are more likely to be marked in terms of electrical properties than elastic moduli. For these reasons, electrical and electromagnetic methods prove more relevant. This combination is particularly effective for mapping superficial decompaction layers (Valois et al. 2011). Combined with sediment cores analyzed in the laboratory, the electrical measurements can then be converted into a geological map at a depth of several tens of meters (Mouhri et al. 2013; Chavez et al. 2022). Hydraulic conductivities can then be determined using conventional geotechnical correlations based on effective grain size.

The aim of this study is to take a step toward establishing a link,
beyond the state of the art (CIRIA 2013; USBR 2019), between
sedimentary architecture (geometry of the layers, nature of con-
tacts between layers and granulometric contrasts) and the asso-
ciated consequences in terms of leaks, sand boils, and sinkholes.
The study area is located in the South of France and concerns
the Agly river dike system, where numerous leaks, sand boils,
and sinkholes have been observed (Tourment et al. 2018; Van
et al. 2022; Zwanenburg et al. 2018). Using two classical geo-
physical methods (Frequency Domain Electromagnetic (FDEM)
and Electrical Resistivity Tomography (ERT)), the results ob-
tained by Girolami et al. (2023) highlight that it is not enough
to conclude the existence of backward erosion piping when a
sand boil is observed. The possibility of suffusion and contact
erosion must also be considered. This has been confirmed by
numerical finite-element modeling (Bonelli and Girolami 2025;
Deng et al. 2025). This analysis is pursued here with a multiscale
approach to support conclusions on the origin of the signatures
while obtaining new results on their distribution.

The article is organized as follows: Section 2 covers the geolog-
ical and geomorphological story of the River Agly, including a
description of the stratigraphic architecture of the river and its
alluvial plain. Section 3 deals with erosion signatures around

dikes. Section 4 describes the results of the observation of the
subsoils supporting the Agly dikes, using both frequency domain
electromagnetism (FDEM) and electrical resistivity tomography
(ERT), as well as examination of soil cores. Section 5 presents
the four basic situations explaining the origin and distribution
of erosion signatures in the floodplain protected by river dikes,
in the case of a dike located on a paleovalley containing a layer
of coarse and permeable sandy gravels on the surface. The con-
clusion is the last section.

## 2 | The Geological and Geomorphological History of the Agly River

When assessing the performance of a river dike system during
a flood, it is crucial to consider the characteristics of the sub-
stratum beneath the dike and in the protected floodplain. This
requires understanding and describing the morphological orga-
nization and stratigraphic architecture of the river and its allu-
vial plain, with reference to their geomorphological evolution.
This preliminary step, although crucial, is only briefly men-
tioned in the state of practice (CIRIA 2013; USBR 2019).

During the last glacial period, global sea level dropped by more
than 100m as vast amounts of water were locked in ice sheets.
As the climate warmed and the ice melted, sea level rose again,
eventually returning to its current level. According to well-
described mechanisms (Dalrymple et al. 1994; Wang et al. 2019),
the classical incised valley models describe how rivers carve deep
valleys into coastal plains or continental shelves during periods
of sea level fall, depositing coarse, fluvial sediments like gravels
and sands in high-energy environments. When sea level rises
again, these valleys are drowned and filled with finer estuarine
or marine sediments such as silts and muds, which accumulate
in lower-energy settings like tidal flats and lagoons. This results
in a paleo-valley stratigraphic sequence composed of coarse flu-
vial deposits, overlain by fine-grained estuarine or marine sed-
iments. However, this conceptual scheme is highly variable and
depends on regional geological, geomorphological, and climatic
conditions, and only partially applies to the River Agly.

The Agly is a medium-sized river with a catchment area of 980km², with an average flow of 7m³/s, up to 2100m³/s during flood periods. The river drains the northern part of the Roussillon basin, an 850km² triangular sedimentary basin open on its eastern side to the Mediterranean Sea. Geomorphologically, the basin is divided into two main units (Figure 1): a narrow alluvial plain upstream, up to 2km wide and 35m above sea level; a coastal alluvial delta downstream, almost 6km wide and reaching the sea.

The stratigraphy of the Agly River paleovalley reflects a com-
plex “compound” sequence shaped by repeated Quaternary sea-
level and incisions/filling cycles (Labaune et al. 2010; Tesson
et al. 2011). At depth, the valley fill alternates between high-
energy fluvial gravels (deposited during glacial lowstands) and
coastal muddy silts (accumulated during interglacial high-
stands), forming stacked depositional sequences. However, in
the basin, coarse deposits like gravels and sands dominate the
surface and shallow subsurface, with fine silts and clays largely
absent or preserved only discontinuously at depth.
<figure><img src="image_3.png" /><figcaption>**FIGURE 1** | Geological organisation of the Salanque alluvial plain, modified from Fonteilles et al. (1993) and Carozza et al. (2013). The study area is located approximately 8 km from the sea, upstream the town of Claira.</figcaption></figure>

This atypical pattern results from repeated incision phases, lateral migration of the valley, and erosive truncation of the upper fine-grained layers during subsequent lowstands. The result is a stratigraphy where high-energy fluvial deposits prevail at the surface, contrasting with classical incised valley models, while deeper sections retain the cyclical interplay of coarse and fine facies influenced by sea-level fluctuations.

Schematically, the stratigraphy of the Agly paleo-valley is as follows (Figure 2): a surface layer of coarse, permeable soils (Holocene sand and gravel), above a layer of fine, low-permeability soils (Pliocene sandy marls). The presence of coarse alluvial deposits at the ground surface is typical of braided river systems. This coarse layer reaches a maximum depth of nearly 60 m at the current coastline, while narrowing upstream to a depth of about 25 m at the study area near Claira (Figure 2), then decreases rapidly and disappears near Rivesaltes, where it gives way to a classic relief in stratigraphic staged terraces.

The broadening of deposits, from a confined continental plain to a wide coastal fan, is associated with modification of fluvial dynamics, dominated by defluviations, crevasses, and overflow. The chronology of these defluviations is well documented for the last two millennia (Carozza et al. 2013). The fluvial dynamics processes can isolate paleochannels and crevasse outcrops in the alluvial plain through thick sinuous or rectilinear layers of cobbles, gravels, and sands. These permeable layers can be considered as the result of bank failure processes (Friend et al. 1979). The accumulation of coarse sandy-gravel deposits near the riverbed led to the formation of natural dikes, such that today Agly channel is perched at

a few meters above its alluvial plain. This characteristic is clearly visible downstream from Claira, where the banks of the Agly are raised 9 m above sea level, while the plain rises to 7 m above sea level (Figure 2).

The case of the Agly River, where coarse sediments (sand and gravel) are found at the surface rather than fine estuarine/marine sediments, is an exception to the typical incised valley fill model. Three characteristics should be considered when assessing the performance of the Agly dike system during a flood: the presence of coarse and permeable deposits on the surface, beneath dikes, and in the protected floodplain; the presence of several paleochannels in the protected floodplain; the fact that the Agly River is perched above its alluvial plain.

These morphological features represent an important vulnerability factor during floods. The last two characteristics are more concerned with flood risk analysis and are not within the scope of this study, which only concerns the potential role of the coarse layers which form the substrate of dikes.

## 3 | Hazardous Erosion Signatures Around Agly River Dikes

Erosion of the subsoil beneath and around river dikes has been a common phenomenon in many rivers for at least a few decades, for example Mississippi and Ohio rivers in 1991 (Morton and Olson 2015), Mississippi river in 1993 (Li et al. 1996), Po river in 2018 (Marchi et al. 2021), among others (Van et al. 2022; Zwanenburg et al. 2018). On the Agly River, erosion signatures
(leaks, sand boils, and sinkholes) appear during floods or periods of high water levels, as during the repeated floods of March 2013, November 2014, October 2018, January 2020, April 2020 (Tourment et al. 2018; Van et al. 2022; Zwanenburg et al. 2018).

Sand boils are conical in shape, around 30cm in diameter and have been observed along the North and South banks, usually next to the leaks, around 100m from the dike toe (Figure 3). Otherwise, sinkholes have an approximately ellipsoidal shape and a flat base about 0.5–3m long and were mainly observed near the dike toe or about 60m away in the transverse direction. Figure 4 shows the signatures of erosion observed on the North bank after the flood of March 6, 2013.

The first embankment system, dedicated to the stabilization of the riverbed, dates back to the early 14th century (Carozza and Puig 2011). Despite significant reinforcements carried out after

the exceptional floods of 1936, 1940, and 1942, no coherent system was proposed until the 1970s. At that time, protecting dikes of 12.5 km long, 2.5 m high, and 8–12 m wide were built from 1969 to 1974 along the approximately 65 m wide river to protect 30,000 people from the risks incurred by floods.

The dike embankment consists of silty sand and sandy loam, while a two-meter-deep drainage spur was built downstream after the flood event of March 6, 2013. From 1977 to 2020, the dike system was exposed to 11 floods. The 1999 flood event caused a breach on the North bank, causing 35 casualties, with a peak flow of about 2110 m³/s⁻¹, while that of the 2013 flood event caused a breach on the South bank, with a peak flow of about 970 m³/s⁻¹. In both cases, evidence of internal erosion in the subsoil was observed. The historical floods and these two breaches are documented (Tourment et al. 2018; Van et al. 2022; Zwanenburg et al. 2018).

<figure><img src="image_4.png" /><figcaption>**FIGURE 2** | Schematic cross-section of the Salanque alluvial plain, adapted and modified from Carozza (2015). The profile of the Agly channel is shown upstream and downstream of the study area, located northwest of Claira. Sediment cores are shown in the valley (Tesson et al. 2005).</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**FIGURE 3** | Erosion signatures observed in the floodplain of the Agly dikes (Pyrénées-Orientales, France) after flood events: Sand boils (a), sinkholes (b, c, d). Pictures made in 2012, 2013, and 2014 (INRAE).</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**FIGURE 4** | Presentation of the study area: The diked riverbed and the protected plains bordering it, on which erosion signatures (leaks, sand boils, sinkholes) observed on the North bank after the flood event of March 6, 2013 were reported. The white lines represent the position of the electrical profiles (ERT) exposed in this study. Examples of sinkholes and sand boils observed along the profiles are illustrated (INRAE pictures made in 2022 and 2013).</figcaption></figure>

Despite the fact that these internal erosion processes have been observed for a long time along river dikes, the scientific community still lacks a general vision, with an observation of the system at different scales. At large scales (i.e., 13 km dike system scale and millennium scale), historical records, sedimentary drilling, and large-scale measurements allow for the mapping of fluvial morphology with alluvial deposits and bordered terraces (including paleochannels) along protected plains, which may constitute areas of preferential erosion. At the medium scale (i.e., at the dike section scale and at the flood scale), field observations and numerical modeling allow for the identification of internal hydromorphodynamic processes in areas mainly affected by floods. At the small scale (i.e., at the erosion signature scale and at the scale of physical processes of fluid/grain coupling), laboratory experiments carried out under controlled conditions allow for a quantitative description of the physical mechanisms involved.

This work focuses on large- and medium-scale analyses of the river morphology and subsoils with the aim of first describing the origin and distribution of internal erosion signatures that affect the protected plains during floods. The geomorphological analyses cover an area of 13 km long (following the embankment system), 8 km wide, and 60 m deep. Both geophysical and geo-mechanical investigations cover an area of 1 km long and up to 1 km wide (following the most frequently observed erosion signatures) for a depth of up to 45 m. The study area is recognized as one of the most affected during floods (Figure 3). Erosion

signatures, observed after the major flood event of March 6, 2013, have been reported and expose a borderline location (from 0 to 80 m from the dike toe), with a specific spatial distribution that alternates between leaks, sand boils, and sinkholes potentially guided by the morphology and topography of the field. The repeated appearance of these signatures after each flood event may suggest the possible presence of superficial and highly permeable materials beneath this area. This was confirmed by a first observation of the subsoil (Girolami et al. 2023). Based on these preliminary results, the analysis is further developed in this work.

# 4 | Observation of Subsurface Soils Supporting Agly Dikes

## 4.1 | Imaging and Sampling Methods

Frequency domain electromagnetism (FDEM) is relevant for mapping lateral variations in apparent electrical conductivity in sub-surfaces of protected floodplains. Measurements were performed with a Geonics EM31 equipment (McNeill, 1980; Frischknecht et al. 1991), with an operating frequency of 9.8 kHz. The instrument is made with a magnetic dipole transmitter and a coplanar magnetic dipole receiver with an intercoil spacing of 3.66 m. The vertical dipole configuration was chosen to maintain a satisfactory sensitivity for depths ranging from 0 to 6 m deep (depending on soil type), although the highest sensitivity corresponds to a depth
of around 1.8 m below the surface (indicative values corresponding to a homogeneous environment). The survey was carried out along parallel profiles on a 2.5 × 2.5 m grid.

Electrical resistivity tomography (ERT) is relevant for mapping vertical variations in electrical conductivity or resistivity (Dahlin, 2001), while capturing reliable depths of layer interfaces when the materials have sufficiently high contrasts, such as sandy marls and gravelly sands. Different equipment was used: SYSCAL Pro (IRIS instruments) for the 470 m long P1 and P3 profiles with an electrode spacing of 5 m (measurements performed in spring 2024); ABEM SAS4000 for the 125–160 m long P2 and P4-P7 profiles with an electrode spacing of 1 m (measurements performed in spring 2023). The apparent conductivity/resistivity inversion was performed with the ResIPY software (Blanchy et al. 2020), which provides 2D conductivity/resistivity sections with an investigation depth of approximately 10 times the electrode spacing.

Sediment cores were collected over the period from 22 to 29 April 2013, following the major flood of 6 March. The instrument used (Hydrofore 1200) consists of a mechanically welded chassis mounted on a fixed track of 6.5 m long and 2.5 m wide. The motorization includes hydraulic pumps coupled to a Perkins diesel engine of 125HP at 2300 rpm. The investigation depth ranged from 5 to 15 m deep, which allows the different layers located under the river dikes to be properly investigated and the conductivity measurements to be reliably interpreted.

## 4.2 | Surface and Cross-Section Mapping

In order to have an overview of the situation, we carried out a FDEM surface mapping of the electrical conductivity of the soil, averaged over the first 6 m beneath the surface, while covering an area of approximately 1 km along the dikes and up to 1 km in the transverse direction, except in vineyard fields where no measurements were taken due to the presence of numerous metal stakes every meter (Figure 5).

The zones of lowest conductivity ($10^{-2}$ S.m$^{-1}$) are shown in brown in Figure 5 and may correspond to the coarsest and more permeable materials. The zones of highest conductivity ($2.5 \times 10^{-2}$ S.m$^{-1}$) are shown in dark blue and may correspond to the finest and less permeable materials (Figure 5). At a first glance, it appears that the Agly dikes are bordered by large permeable zones up to 400 m wide and at least 6 m thick, throughout the explored section of approximately 1 km long. On the South bank, these layers are delimited by thick layers of fine and less permeable soils, while materials of intermediate grain-size, shown in green, predominate on the North bank. The plotting of the position of the erosion signatures on Figure 6 coincides with the delimitation of these permeable layers which seem to control their presence and even their distribution in the protected plains.

The location of the soil cores (4 on the South bank; 5 on the North bank) is shown in Figure 6. Examination of the sediment cores, mainly sampled near the dike toe, reveals that the most permeable materials (shown in brown) consist mainly of gravel with a sandy matrix supported by cobbles, or even pebbles, and represent channel deposits that can thicken up to 15 m below the dikes in some locations (see sample S4 in Figure 6). The coarsest

deposits appear to be located near the edges of the dike while the finest are deposited toward the alluvial terraces. Conversely, the finest and least permeable materials (shown in dark blue) consist of sandy marls, clayey silts to silty clays, and represent a deep impermeable substrate with an approximate depth of 12.20 m at point S1 (Figure 6) or rising to the surface upstream, near the town of Rivesaltes, or downstream, further from the dikes. In between, the layers shown in green consist mainly of sandy silts to silty sands (see the top of samples S₁ to S₉ in Figure 6) and represent crevasse deposits overlying alluvial terraces and paleochannels. The presence of several paleochannels (Figures 1 and 5) in the northern part may suggest that the river was progressively displaced downward in the southern part.

In order to explore in depth the geometry of the sediment reservoir (i.e., channel deposits) beneath river dikes and the depth of the paleovalley that delimits the interface between the sandy-gravelly sediments and the marly substrate, we carried out in 2024 two ERT profiles across the embankment system, which was made possible by the river drought.

The profiles, whose position is reported in Figure 4 (see P1 and P3), 470 m long and 50 m deep, are shown in Figure 7. The low conductivity (or high resistivity) areas are shown in brown, while the high conductivity (or low resistivity) areas are shown in dark blue, using the same color bar and scale as the FDEM surface mapping for comparison.

The alluvium is a few meters thick under the current river bed and thickens under the dikes up to the protected floodplain: this is due to the system of braided deposits. This permeable level under the dikes and in the protected floodplain extends to a depth of 35 m, roughly alternating between sandy silts, silty sands, coarse sands, and numerous gravels from the surface to the substrate. Beneath these layers, sandy marls form a substrate of very low permeability located at a depth of around 18–20 m below the river.

On the north-eastern flanks, the permeable layer rises to the surface, approximately 25–35 m from the dike toe. On the southwestern flank, it extends into the protected floodplain for almost 200 m. A significant difference between the two profiles lies in the north-eastern protected floodplain. On profile P1, the permeable layer continues for approximately 50 m and is a few meters thick. On profile P3, the permeable layer does not extend into the protected floodplain. These permeable layers are covered by a layer of topsoil consisting of sandy silt approximately 1 m thick.

The combination of surface and vertical maps, obtained from granulometric analyses, allows the results to be interpreted as highlighting the geometry of the valley composed of approximately 200 m wide and 25 m deep at this location, dug in the marly substrate, then filled by alluvial sediments through three superimposed transgressive cycles. Once the valley was filled, episodes of defluviation formed alluvial terraces that spread out in the protected floodplains, thus containing paleochannels of a few meters thick oriented in the northeast direction toward the Leucate basin (Figure 1).

Repeated electrical profiles across the 125 m long and 20 m deep northern dike (P2 and P5 in Figure 4) allow for more accurately mapping the longitudinal geometry and base of the paleovalley
<figure><img src="image_7.png" /><figcaption>**FIGURE 5** | FDEM surface map of the soils electrical conductivity averaged on the first 6 m of depth of the study area. The erosion signatures of March 6, 2013 are also reported on this map.</figcaption></figure>

in areas frequently affected by erosion, while a 164 m long and 20 m deep longitudinal profile along the dike (P4 on Figure 4) allows to map its geometry in the protected floodplain. As shown in the profile of the Salanque Figure 2, the paleovalley is slightly narrower and shallower upstream and becomes deeper and wider downstream, which is also highlighted by the longitudinal profile P4 (Figure 8) whose thickness varies from 8 to 12 m downstream in the protected plain (15 m from the dike toe). Figure 8 shows photographs of the sedimentary borehole located on points P4 at a coarse level (with gravels and pebbles) at 5 m depth. These results suggest that the head of the watershed may have been affected upstream by successive erosion processes following past floods, which in turn progressively filled the valley located downstream with flood deposits. Local variations in valley geometry can also locally control the velocity fields of internal and surface flows during floods, thus playing a role in the distribution of erosion signatures along the protected plain. The reconstruction of a grid of profiles (taking P2, P5, P6, P7 located in Figure 4) allows to locate the limit of erosion processes

beyond which no more signatures are observed during floods (Figure 9).

More detailed profiles (Girolami et al. 2023) allow highlighting the presence of a thick permeable layer of at least 8 m deep covered by a thin cohesive layer of around 1 m thick and bordered by the marly substrate, under the sand boils zone. This configuration highlights the possibility of the formation of fluidization chimneys, thanks to the presence of a superficial heterogeneous cohesive material, from which leaks and then sand boils (when erosion of the underlying sandy material is initiated) can develop from the permeable and granular layers. During floods, internal flows propagate within the Holocene sediments, from the river to the protected plain, following the sandy-marly interface until reaching the surface. The marly substrate at the edge, located in the north-eastern direction, plays an important role in the distribution of the sand boils by forming an impermeable barrier to internal flows that slows their progression toward the bottom of the protected plain and preferentially guides them toward the
<figure><img src="image_8.png" /><figcaption>**FIGURE 6** | Interpretation of the FDEM surface map obtained from geomorphological analyses and sedimentary boreholes sampled in the study area (S1–9). The erosion signatures (leaks, sand boils, sinkholes) observed on the North bank after the flood of March 6, 2013 are represented respectively in dark blue, light blue, and red (North orientation and color scale are indicated in Figure 6).</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**FIGURE 7** | Vertical section of soil resistivity along profiles P1 (a) and P3 (b) and 5 m thick soil cores taken near the dike toe, in front of the berm. Profiles P1 and P3 across the whole diked bed where sand boils and sinkholes where reported on the North bank after the flood of March 6, 2013.</figcaption></figure>

surface. The heterogeneous surface layer, on the other hand, prevents homogeneous fluidization of the entire soil (and the formation of a suspension (Amin et al. 2021)) by preferentially driving the flow through permeable chimneys. Sand erosion can potentially occur during the flood peak when the drag force on

the particles is high enough to fluidize them and transport them to the surface to eventually form a conical deposit.

Conversely, the presence of a thick sandy cover layer about 3 m deep above the marly substrate several meters thick below the
<figure><img src="image_10.png" /><figcaption>**FIGURE 8** | (a1) Vertical section of soil conductivity/resistivity along profile P4, near and along the North dike where leaks, sand boils, and sinkholes were reported in 2013. (a2) 8.30 m thick sediment core taken near the dike toe, in front of the berm. (a3) Picture of the sediment sample (from the surface, upper left, to the base, lower right). The color scale is shown in Figure 8.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**FIGURE 9** | (a) Location and layout of four vertical soil conductivity/resistivity sections taken along the North bank (along profiles P2, P5, P6, P7) where sand boils and sinkholes were reported in 2013; (b) vertical soil conductivity/resistivity section along these profiles (P2, P5, P6, P7). The color scale is shown in Figure 8.</figcaption></figure>

sinkhole area (Figure 7b) may favor the superficial collapse of the low-density and permeable layers after the flood peak. As observed from the sediment samples, the collapse appears to remain superficial and does not affect the layers located below the permeable lenses. Here again, the presence of sand boils in the same region is favored by the presence of a sufficiently thick sandy layer covered by a cohesive soil that will allow the formation of preferential flow pipes from which leaks and fluidization chimneys will develop. Their location near the dike toe may suggest a maximum flow velocity at this location, high enough to initiate fluidization and erosion of the sand.

# 5 | Origin and Distribution of Internal Erosion Signatures in the Protected Floodplain

## 5.1 | Occurrence by Internal Erosion of Open-Framework Gravels in Sandy Gravels

Geophysical measurements reveal a non-tabular geometry of the interface between the highly permeable soil at the surface and the low permeable soil at depth. For a given geometry, in the case of a dike located above the paleovalley, the two main

parameters are the presence of a cohesive topsoil and the presence of paleochannels extending into the protected floodplain. Here we consider the case of a paleovalley containing a sandy gravel layer on the surface. If the dike system is subjected to a flood, the subsoil is subjected to seepage. Due to the high permeability of the sandy gravel layer, the pressure losses are low and the pore pressures are high.

In the absence of topsoil, a permeable subsurface layer can lead to slow flooding of the protected floodplain as the water table rises.

In the presence of low-permeability topsoil, high pore pressures result from the first flood in the appearance of artesian zones and uplift zones in the protected floodplain. In artesian zones, the slightest defect in the topsoil can lead to leakage in the protected floodplain: concentrated outflow of seepage water, also known as boils. Examples of a defect are a fluidized sand vein (heave) or a pre-existing vertical crack. In the uplift zones, the cohesive topsoil is subject to hydraulic failure (cracking).

The presence of defects in the topsoil is a triggering factor for sand boils and backward erosion. Since the soil beneath the
topsoil is sandy gravel, erosion is caused by suffusion, which appears to be selective backward erosion. This process has not yet been studied to our knowledge. We propose to call it backward erosion suffusion. This is a different process from that generally considered to explain sand boils and their consequences. The usual assumption is that the permeable layer is sand and that backward erosion creates a pipe under the topsoil. This is called backward erosion piping (van Beek et al. 2011; Bonelli 2013).

In all cases, the erosion of the sandy gravel causes the appearance of low-density areas, immediately under the dike and in some places under the topsoil layer, inside the permeable layer at the level of the reduction in the thickness of the sandy gravel layer if a paleochannel exists (Deng et al. 2025; Bonelli and Girolami 2025). This internal erosion actually transforms some areas of the sandy gravel into open-framework gravel. These are the red areas in Figure 10.

Open-framework gravel has a unimodal particle size distribution with a mean particle size greater than 2 mm (without sand), with a hydraulic conductivity above $10^{-2}$ m/s. Gravelly fluvial deposits contain open-framework gravel as planar and

cross-strata of varying scale, usually overlain by and resting on bimodal sandy gravel (Lunt and Bridge 2007).

The presence of open-framework gravel is an important factor in triggering internal erosion and sinkholes (Luo and Huang 2020; Fell et al. 2014): excessively high flow rates through open-framework gravel can cause adjacent sand layers to migrate into their large voids, through suffusion and contact erosion, creating local collapse zones. The settlement of the soil column above causes the appearance of sinkholes on the soil surface. Contact erosion is selective here, contrary to what is classically considered (Bonelli 2012, 2013): only the sand of the sandy gravel is eroded by the flow in the open-framework gravel. In addition, the appearance of an open-framework gravel zone within the sandy-gravel layer, at the level of the decrease in the thickness of this layer, can induce a surface signature. The flow can then be locally fast enough to initiate contact erosion of the sandy-marly support layer, which can also lead to settlement and the appearance of a sinkhole on the soil surface. These sinkholes can occur without the prior appearance of sand boils, contrary to what is often assumed (Tourment et al. 2018; Van et al. 2022; Zwanenburg et al. 2018).

<figure><img src="image_12.png" /><figcaption>**FIGURE 10** | The four basic situations explaining the origin and distribution of erosion signatures in the floodplain protected by river dikes. Case of a dike built on a paleovalley fill containing a layer of sandy gravel on the surface. On the left, development of open-framework gravelly zones (red), no defects in the topsoil layer (no sand boils). On the right, typology of appearance of the three surface signatures: Leaks, sand boils and sinkholes. The two influencing parameters are the presence of a topsoil and the presence of a paleochannel.</figcaption></figure>
## 5.2 | The Four Basic Situations Explaining Erosion Signatures

The two main hazards of a flood risk analysis are groundwater flooding by rising water table in the subsoil and flooding by dike breach. Four basic situations can be defined according to the presence or absence of a topsoil, and the presence or absence of a paleochannel. These four basic situations explain the origin and distribution of internal erosion signatures in the floodplain protected by river dikes, in the case of a dike located on a paleovalley fill containing a layer of sandy gravel on the surface (Figure 10). This general framework is deduced from the results of numerical simulations (Deng et al. 2025; Bonelli and Girolami 2025).

In the absence of topsoil (cases A and B), the water table rises during a flood above the ground surface, causing groundwater flooding of the protected floodplain. The uplift condition corresponds to the critical hydraulic gradient of Terzaghi and causes the fluidization of the sand at the dike toe (also known as heave and blowout, that is to say the uplift of the grains of a non-cohesive soil). The development of an open-framework gravel pipe under the embankment can initiate a localized erosion zone at the dike toe and cause the appearance of a sinkhole. When there is no topsoil, there can be no sand boils, as water dissipates more diffusely, reducing the localized pressure gradient needed to fluidize sand and form sand boils. In this case, if the soil is homogeneous and the stratigraphy is pendular, the most likely location for a sinkhole is the dike toe. This is illustrated in Figure 3.

If a low-permeability topsoil layer covers the sandy gravel, and there is no paleochannel (case C), the entire sandy gravel layer is saturated and under pressure. If a defect in the topsoil layer causes a leak to appear and triggers erosion (which requires that the size of the “crack” and the local upward gradient allow fines to be transported from the collector), a sand boil may appear, and possibly a sinkhole linked to this sand boil. This can occur anywhere between the dike and the end of the sandy gravel layer. It basically depends on the thickness of the topsoil. In addition, the most likely location is the place where the artesian pressure is highest, at the dike toe. The flow is confined in the 2D plane perpendicular to the embankment, and the 3D effect probably becomes very important. Thorough analysis using numerical modeling is necessary to provide orders of magnitude and clarify the influence of the 3D effect. In this context, it is necessary to obtain information regarding the three-dimensional description of the subsurface stratigraphy.

If a low-permeability topsoil layer covers the sandy gravel, and there is a paleochannel (case D), the upper part of the sandy gravel is likely to be subjected to significant groundwater flow. If a defect in the topsoil layer causes a leak to appear and triggers erosion, a sand boil may appear, and possibly a sinkhole linked to this sand boil. This can appear anywhere from the dike toe and over a distance that can only be assessed by numerical modeling. It basically depends on the thickness of the topsoil. In addition, the two most likely locations are the dike toe and the reduction in thickness of the sandy gravel layer, that is, the entrance area of the paleochannel.

An important result for case D is to be confirmed by numerical modeling: in the absence of a defect in the topsoil layer (no leakage or sand boil), the appearance of open-framework gravels can cause erosion of the surrounding soils (sandy gravels, sand or sandy marls) and induce a sinkhole. Two most likely locations are the dike toe and the entrance area of the paleochannel.

## 5.3 | Recommandations

The recommendations deduced from this work concern all types of studies relating to river dikes: the design of new structures, safety analyses, hazard studies, reinforcement projects, or projects to relocate existing dikes. These recommendations are illustrated in Figure 10. Important information to look for about the soils beneath the dike, and the subsurface in the protected floodplain, includes:

i. Stratigraphy, in terms of the presence of a coarse, higher-permeability soil layer, and the location and geometry of the interface with the less permeable layer below;

ii. The presence of topsoil in the protected floodplain, and its thickness;

iii. The presence of a paleochannel in the protected floodplain.

This work presents an example of a multiscale and multidisciplinary methodology to obtain this information: (i) the morphological organization and stratigraphic architecture of the river and its alluvial plain are inferred with reference to its geological and climatic histories; (ii) subsurface images are derived from geophysical techniques (ERT, FDEM); (iii) coring provides direct information on the different geological layers.

# 6 | Conclusion

When analyzing the flood risk of a floodplain protected by existing river dikes, it is essential to understand the causes of surface signatures (leaks, sand boils, sinkholes) observed during and immediately after the river flood. The conclusion of this study is that the distribution of these signatures is directly related to the shape and dimensions of the paleovalley and paleochannels, and to the presence of a low-permeability topsoil.

The key point is the multiscale and multidisciplinary character of the methodology. The description of the morphological organization and stratigraphic architecture of the river and its alluvial plain constitutes the first step, at large spatial scales. The observation of the subsoils supporting the dike system constitutes the second step. This involves locating, using geophysical methods, the interfaces of the coarse soil layer with high permeability and the supporting soils with low permeability. The correlation between the geophysical observables (e.g., electrical resistivity) and the quantities of geomechanical interest (hydraulic conductivity) requires information on existing soils, most often obtained locally with geotechnical methods (e.g., by coring). The analysis of scenarios showing the influence of the shape of the paleovalley and paleochannels on the location and development of internal erosion is the third step.
This methodology has been applied to the study area of the
Agly dike system, where numerous leaks, sand boils, and
sinkholes were observed. The combined use of FDEM and
ERT methods proves to be a fast and cost-effective solution to
continuously map the subsurface and capture shallow hori-
zontal information. In the case of a permeable layer consisting
of sandy gravel, the classical backward erosion piping scheme
is not relevant to explain the observed sand boils, since they
are mainly caused by a backward erosion suffusion process.
Both suffusion and contact erosion appear to be the origins
of sinkholes.

Acknowledgments

We thank the managers of the Agly river dikes (SMBVA, F. Nicoleau) for having provided authorizations for this work and for fruitful discussions. We thank the company DRIM for ERT results on P1 and P3 profiles, as part of a service order on behalf of INRAE. We thank the company Naga Geophysics for ERT results (raw data) on P2, P4, P6, P7 profiles, as part of a service order on behalf of INRAE. For further details on the geophysical methods deployed, please contact the authors of the paper.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are available from the
corresponding author upon reasonable request.

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