# Morphometric analyses of a deltaic strandplain inferred from LiDAR data and old maps: A contribution to the coastal risk mapping of the Tiber Delta (Italy)

Justine Sarrau, Ferréol Salomon

► To cite this version:

Justine Sarrau, Ferréol Salomon. Morphometric analyses of a deltaic strandplain inferred from LiDAR data and old maps: A contribution to the coastal risk mapping of the Tiber Delta (Italy). Journal of Geographical Sciences, 2025, 35 (10), pp.2229-2247. 10.1007/s11442-025-2410-8. hal-05425896

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# Morphometric analyses of a deltaic strandplain inferred from LiDAR data and old maps: a contribution to the coastal risk mapping of the Tiber Delta (Italy)

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Justine Sarrau¹, Ferréol Salomon²

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¹Justine SARRAU, PhD Geography and GIS, Department of Geography and Urban Sustainability, United Arab Emirates University, Sheik Khalifa Bin Zayed Street, Asharej, Al Ain, Abu Dhabi, United Arab Emirates - justine.sarrau.pro@gmail.com - corresponding author

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²Ferréol SALOMON, Laboratoire Image Ville Environnement (UMR 7362), Centre National de la Recherche Scientifique (CNRS) and Université de Strasbourg, 3 Rue de L'Argonne, 67000 Strasbourg, France - ferreol.salomon@live-cnrs.unistra.fr

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## Highlights:

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- Chrono-morphological analysis of the strandplain of the Tiber Delta;

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- Height variations are associated with sedimentary processes, neotectonics, sediment compaction and human activities;

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- Long-term subsidence is identified in the area of the international airport of Rome.

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## Abstract:

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Morphologies of deltaic strandplains are the result of multiple sedimentary dynamics interacting with climate, neotectonic and anthropic impacts. They record long-term evolution of coastal areas but also reveal past and present hazards that can be essential to better predict risks in urbanised deltas. This paper aims to identify the effect of a long-term evolution of the prograded plain of the Tiber Delta in using current ground surface variations and geohistorical data. This study applies GIS (Geographic Information System) tools to LiDAR (Light Detection and Ranging)-derived DEM (Digital Elevation
Model) data combined with stratigraphical data, aerial photography interpretation and old maps. The main outcome shows areas of subsidence are primarily located in the central part of the Tiber Delta.
Lower heights at the river mouth are due to subsidence and specific lower morphologies associated with fast progradation phases, while similar low heights just north in the area of the international airport of Rome are due to subsidence alone. Subsidence under the airport is associated with the compaction of the silty clay infill of the Late Pleistocene Tiber Valley mostly deposited during the Holocene transgressive period. Only observed in the airport area, the presence of washover fans and the large extent of paleolagoons reveal the kind of risks that are increased by this subsidence. This study demonstrates that the densely urbanised central part of the Tiber Delta faces higher risks of marine submersion and coastal flooding considering ongoing relative sea level change. It also contributes to show the importance to better characterise past coastal morphologies to identify areas prone to subsidence.

**Keywords:** river delta, chrono-morphometry, LiDAR, subsidence, relative sea level change

**Figure captions:**

Figure 1. – Study area and DEM derived from LiDAR data of the outer plain of the Tiber Delta.

Figure 2. – Selected area and dataset.

Figure 3. – Extraction of the altitudes for the urban and non-urban areas in each beach system and proportion of urban area for each beach ridge.

Figure 4. – Synthetic view of the different areas identified in the Tiber Delta – data assessment and range of heights in each area.

Figure 5. – Relationship between heights of the beach ridges and TDS sediment thickness.

Figure 6. – Focuses on the DEM: the river mouths (1), the western part of Area 4 (2), the area of Ostia (3), and the southern part of the Tiber Delta (4 and 5).

Figure 7. – Synoptic view of the main results of the study in terms of hazard, vulnerability and risk. NB: The subsidence for the Tiber Delta for the future 100 years is not modelled, exposure to coastal flooding and submersion is only based on the current state of the height extracted from LiDAR data
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# 1. INTRODUCTION

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River deltas are sensitive environments exposed to climate change and sea level fluctuations (Nguyen and Woodroffe, 2016; Da Lio and Tosi, 2019; Das et al., 2020). They host a significant proportion of the global population due to their fertile soils, connectivity to inland and maritime transport, and offer ecosystem services such as biodiversity luxuriance (Nicholls et al., 2020). However, processes such as erosion, sedimentation or subsidence triggered by natural and anthropic factors compromise human settlements and activities in deltaic plains.

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Deltaic plains are landforms related to a river mouth gaining terrestrial ground on a body of water. River deltas "cap underlying deposits with subaqueous extensions" (see definition in Syvitski et al., 2022). The presence of these underlying deposits is inherent to river deltas and explain subsidence due to compaction in such contexts (Stanley and Warne, 1993; Stanley and Corwin, 2013). While subsidence is a natural phenomenon, anthropic activities contribute to increase sinking rates of deltaic plains across the world (Minderhoud et al., 2015; Syvitski et al., 2009). Vertical instability associated with deltaic plain subsidence is of major concern for coastal populations since it increases the risk of submersion. Today, the risk of coastal submersion is primarily due to an increase of the sea level rise associated with climate change and it is enhanced in deltaic areas where sea level rise is combined with fast subsidence (Dunn and Minderhoud, 2022).

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Similar to other deltas, the Tiber Delta, located downstream of Rome, is also affected by accelerated subsidence (Bozzano et al., 2018; Delgado Blasco et al., 2019; Manassero and Dominijanni, 2010). Italian authorities are particularly aware of the threat of subsidence since an important population lives there (e.g. Lido di Ostia, Fiumicino, Focene, Fregene) and major infrastructure, such as the international airport, is located in the Tiber Delta. Today, the Tiber Delta is one of the most urbanised Mediterranean deltas.

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Recent studies revealed accelerated subsidence below Rome (Stramondo et al., 2008) and urban areas in the Tiber Delta (Bozzano et al., 2018; Delgado Blasco et al., 2019) using Interferometric Synthetic Aperture Radar (InSAR). They demonstrated the link between built structures, their date of
construction, the type of underlying deposits and the rate of subsidence. Notably, a link between the palaeolagoon of Maccarese, composed of fine sediments, in the inner delta plain (Figure 1 and 2) and the accelerated subsidence under new buildings (Bozzano et al., 2018) was observed.

Many studies showed the importance of reconstructing long-term deltaic evolution to better understand current subsidence (e.g. Nile Delta: Stanley and Warne 1993; Stanley and Corwin 2013; Po delta: Teatini et al., 2011; Rhine Delta: van Asselen et al., 2018; Mekong delta: Zoccarato et al., 2018; Mississippi Delta : Törnqvist et al. 2008). In such studies, geomorphological units of the deltas, chrono-stratigraphies and sediment composition are essential to locate areas prone to subsidence, but few surface morphologies analyses are conducted. This paper aims to contribute mapping coastal areas affected by vertical variations, specifically subsidence, mainly considering surface morphologies in using LiDAR (Light Detection and Ranging)-derived DEM (Digital Elevation Model) data combined with stratigraphical data, aerial photography and old maps.

In parallel, current subsidence is generally quantified and monitored with multi-temporal approaches for the last years or decades using GPS (Fabris et al., 2014; Wolstencroft et al., 2014) or InSAR data (Da Lio and Tosi, 2019; Fiaschi et al., 2018; Higgins, 2016). However, GPS data only allow to interpolate elevation values and can be difficult to implement on certain types of soils as well as on wide areas with infrequent levelling surveys and low density of the controlled points. Additionally, the uncertainty of the measurement of InSAR still needs improvement and only applies to built areas (Gesch, 2009; Higgins, 2016). In contrast, we propose here to use LiDAR data collected during a single period focusing on non-built-up areas. Stratigraphical data, aerial photography and old maps contribute to better constrain the interpretation and the consequence of the subsidence.

This study focuses on the outer delta plain or progradational plain of the Tiber Delta located along the coast. This coastal plain is composed of beach ridges (Figure 1). In the Tiber Delta, these beach ridges are aeolian sandy beach ridges, which means that they cannot directly be used as sea level indicators (Tamura, 2012). The lower parts are wave-built facies formed in relation to the sea level, and the upper parts are aeolian facies forming dunes. Large areas across different beach ridge systems from different periods will be studied to identify sections of the outer delta plain affected by subsidence. Fluvial, coastal, and aeolian dynamics will first be considered to explain surface topography and after
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vertical mobility will be considered. In addition, complementary information from 1940s aerial photographs, old maps, and chronostratigraphies will be used to support our hypotheses. Ultimately, this paper will discuss risks related to the subsidence along the coast of the Tiber Delta.

<figure><img src="image_2.png" /></figure>

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<caption>Figure 1: Study area and DEM derived from LiDAR data of the outer plain of the Tiber Delta.</caption>
## 2. GEOGRAPHICAL AND GEOLOGICAL CONTEXT

The Tiber River has a course of 405 km and drains a watershed of 17375 km². It accumulates sediments at its mouth and forms a delta composed of a deltaic plain of c. 150 km² (emerged part of the delta) and the subaqueous delta of c. 500 km². The deltaic plain is located 25 km south-west of Rome (Figure 1) and can be separated into two parts: the inner delta plain towards the east, which was originally occupied by large lagoons now reclaimed; and the outer delta plain to the west, a strandplain plain characterised by a succession of beach ridges (Bellotti et al., 2007, 1995). The Tiber Delta is dominated by waves with two longshore drifts going to the north and the south on each side of the main river mouth. The river separates into two branches at Capo Due Rami a few kilometres upstream of the two river mouths. The main branch is called the Fiumara and the secondary branch is called the Fiumicino (Figure 1). The latter was initially a canal excavated during the Roman period (Keay et al., 2005).

Around 20000 years ago, the sea level was almost 130 m below the current sea level (Lambeck et al., 2014), with a coastline a few kilometres westwards (Bellotti et al., 1995). The Tiber was then flowing in an incised valley in the northern part of the current river delta (Bellotti et al., 2007; Milli et al., 2013). The upper part of the coarse fluvial material at the bottom of this palaeo-valley was deposited most probably after the Last Glacial Maximum (18000-12000 cal BP) and is now located between 80 and 60 m below the surface (Bellotti et al. 1995; Milli et al. 2016 - Lowstand System Tract). From 16000 cal BP to 7-6000 cal BP, the sea level rose quickly and then more slowly for the last 6000 years (Lambeck et al., 2010; Vacchi et al., 2016). The Tiber Delta was affected by transgression during the period of quick sea level rise and the palaeo-valley was filled with fine estuarine and then shelf deposits (See Pesce Luna Core in Milli et al. 2016 - Transgressive System Tract). During the last 6000-7000 years, the Tiber Delta started to build beach ridges and to prograde to the west. The stratigraphy of the progradation is characterised by a coarsening upward stratigraphy from prodeltaic silty clays to sandy shoreface deposits (Milli et al., 2016; Salomon et al., 2020).

Researchers proposed different phases of progradation of the Tiber Delta at different timescales since 7000-6000 cal. BP (Bellotti et al., 2018; Bersani and Moretti, 2008; Bicket et al., 2009; Giraudi,
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2004; Salomon, 2013). However, all observed an acceleration of the progradation during the last 4000 years. It includes strong pulses of progradation occurring in the 8th -5th c. BCE (Salomon et al., 2018) and in the 15th - 16th c. CE (Bersani and Moretti, 2008; Le Gall, 1953). For the last decades, the coastline of the Tiber Delta retreated mainly due to sediment starvation at the river mouths due to dams built upstream in the watershed (Bellotti and De Luca, 1979). Today, the deltaic plain hosts important urban areas such as Lido di Ostia and Fiumicino but also essential infrastructures for the city of Rome including its international airport (Leonardo da Vinci) and its main exhibition centre (Fiera di Roma).

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The beach ridges of the Tiber Delta are aeolian sandy beach ridges according to the classification of Tamura (2012). Wave-built facies are located below the dunes. Bicket (2010) studied the characteristics of the sand dunes in the southern part of the Tiber Delta and dated their formations. Notably, he discusses the formation of the dunes in relation to palaeoclimatic factors. He observed that progradation phases occurred not only during cooler/wetter periods, while the formation of dunes are not easy to attribute to specific palaeoclimatic conditions.

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Glacial Isostatic Adjustment (GIA) cause a slow subsidence in the Tiber Delta (Lambeck et al., 2011). Regional data also suggest an uplift due to isostasy of the Apennines chain and volcanic activity associated with the Sabatini Mountains to the north and the Alban Hills to the south, while it is not clear how much it now affects the Tiber Delta (De Rita et al., 1994; Ferranti et al., 2006; Karner et al., 2001; Mantovani et al., 2009). The Tiber Delta itself is affected by local uplift or subsidence due to active faults under the delta (Bigi et al., 2014; Ciotoli et al., 2016; Marra et al., 2019). In addition, the delta shows surface deformation controlled by sediment compaction leading to subsidence. Using Advanced Satellite Synthetic Aperture Radar Interferometry (A-DInSAR), Bozzano et al. (2018) studied subsidence in the Tiber Delta focusing on the load of built areas. They demonstrated that built areas above the palaeolagoons of Maccarese and Ostia, as well as the river mouths were particularly affected by subsidence. In parallel, Sentinel-1 SNAP-StaMPS Persistent Scatterer Interferometry showed similar results (Delgado Blasco et al., 2019). Higher values were observed under some tracks of the international airport (max. value of -14.8 mm/yr; avg. -6.6 mm/yr along the track), along the new construction of the Rome Fiumicino highway, Via Portuensis (-19 mm/yr max, avg. = -7 mm/yr) and at the river mouths of the Tiber Delta (-9 mm/yr) (Delgado Blasco et al., 2019).

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# 3. METHODOLOGY

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## 3.1. Dataset

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This study aims to identify long-term effects of subsidence on surface topography.

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Consequently, LiDAR data are the main dataset used in this work. Two LiDAR datasets from 2011

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were provided by Italian authorities (Ministero dell'Ambiente e della Sicurezza Energetica - Geoportale

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Nazionale (ex Ministero dell'Ambiente e della Tutela del Territorio e del Mare)) and cover the outer

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and inner parts of the delta. We used the DEMs derived from the LiDAR data generated by Italian

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authorities. The two DEMs have a spatial resolution of 0.96 m (between 41.570N, 12.089E : 41.979N,

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12.550E, origin in 41.979N, 12.089E - EPSG 3857) and 1.92 m (between 41.560N, 12.080E : 41.960N,

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12.539E, origin in 41.960N, 12.080E - EPSG 3857). For this study, we combined the two datasets into

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a single DEM with 1.92 m resolution.

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Land use data were used to differentiate urban from non-urban areas in the DEM. We used the

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Urban Atlas from 2012 (https://land.copernicus.eu/local/urban-atlas/urban-atlas-2012) since this is

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chronologically the closest dataset to the LiDAR data (2011) and the most accurate.

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To better understand the distribution of the heights in the DEM, we used a chronology of the

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progradational phases of the Tiber Delta. Beach ridge systems and their dates are an update of Giraudi

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(2004) and Bicket (2010) proposed by Salomon (2013). In addition, an interpolated model of the Tiber

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Depositional Sequence (TDS) built using sedimentary cores was also added to the GIS database (Milli

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et al., 2013). The TDS corresponds to the Late Quaternary Tiber River deltaic succession, e.g. the last

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120 ka (Milli et al., 2013). The lower limit of the TDS is at a maximum 80 m under the surface of the

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Tiber delta plain. In the Pesce Luna Core located west of the international airport of Rome, the TDS

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starts between 48 and 52 m deep. At the bottom (48-52 m below surface), it is composed of pebbles,

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then silty clay (48 to 18 m b.s.) and sand in the upper part (0-18 m b.s.) (Milli et al., 2013). Most of the

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sequence is dated to the last 13000 years (0-48 m b.s.). Detailed cross-sections between sedimentary

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cores in the Tiber Delta are proposed by Bellotti et al. (2007) and Milli et al. (2013, 2016).
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Finally, complementary information was extracted from Royal Air Force (RAF) aerial photographs dated to 1944 (Aerofototeca Nazionale - ICCD, Rome) and old maps of the Tiber Delta (Amenduni, 1884). Washover fan morphologies were drawn from the aerial photographs and the palaeolagoon extents were digitised from old maps.

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<figure><img src="image_3.png" /><figcaption>Figure 2: Selected area and dataset.</figcaption></figure>

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## 3.2. Heights extraction from the DEM

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Identifying areas of subsidence in the long term is a rather complex task. Lower areas in deltaic areas can be explained by many factors. In the Tiber Delta, lower heights are mainly observed in the inner delta plain and relate to palaeolagoons reclaimed at the end of the 19th / beginning of the 20th century. Important subsidence affected and still affects the inner delta plain (Bozzano et al. 2018, Delgado Blasco et al. 2019), but its effect combines with the morphologies of the palaeolagoons. Consequently, subsidence cannot be identified in the inner delta plain using a DEM only. In contrast, identifying areas affected by subsidence in the outer delta plain is more realistic. Consequently, we select and cut the DEM of the outer delta plain to conduct this study (Figures 1 and 2). A progradational
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plain composed of beach ridges adjusts to base levels, e.g. the sea level for the Tiber Delta. Lower
heights are likely to be associated to lower sea levels or subsidence. Nevertheless, cautious
interpretations of aeolian sandy beach ridges with complementary information are needed to
differentiate the effects of sedimentological processes from the vertical variations.

Urbanised areas do not reflect the heights of the natural delta that we analyse in this study (e.g. coastal, fluvial, aeolian, neotectonics). We created two DEMs of the outer delta plain using the Urban Atlas from 2012. We clipped the outer delta plain DEM using the Urban Atlas in differentiating urban areas (e.g. built areas) from non-urban areas (e.g. arable lands, forests, herbaceous vegetation associations, land without current use, open spaces with little or no vegetation, pastures, permanent crops, water and wetlands).

East to west profiles were created following a grid to cover the outer delta plain from north to south. The profiles were extracted using parallel lines separated by 0.883 m for both the “Urban DEM” and the “Non-urban DEM” (Figure 3, see the map). They revealed the morphologies of the ridges and swales, but evidence related to subsidence are difficult to observe at such a scale. Consequently, we extracted heights of the profiles to points and grouped them using different frameworks: (1) coherent areas succeeding from north to south numbered from 1 to 8 (Figure 3 and 4); (2) beach ridges systems formed during 7 periods of time (Figure 3); and (3) sediment thickness of the TDS between 15 to 80 m (Figure 5).

The polygons of the beach ridge systems (Giraudi, 2004; Bicket, 2010; Salomon, 2013) and the sediment thicknesses of the TDS (Bellotti et al., 2007; Milli et al., 2013) are issued from regularly updated studies according to the new chrono-stratigraphies published every few years.

The 8 areas from north to south of the Tiber Delta were delineated in this study. They were separated using the average heights of the north-south profile in each beach ridge system presented in Figure 3 – see limits of the 8 areas with the dotted blue lines. It appears that most limits are related to secondary rivers and marine-lagoon channels or canals (Rio Tre Denari, Arrone River, Collettore Generale – Acque Alte, Canale Dei Pescatore). For the central part between the Collettore Generale and the Canale Dei Pescatore, we grouped the river mouths area including the Fiumara and Fiuimicino distributaries (Area 5) and separated it from the area of the airport of Rome (Area 4).
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The distribution of heights associated to the 8 areas of the non-urban DEM are presented in **Figure 4** together with data assessment (preserved extent of the outer delta plain and proportion of the areas not urbanised).

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### 3.3. Complementary data extraction

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#### 3.3.1. Extent of the 19th century palaeolagoons

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Based on two maps from Amenduni (1884) showing the delta before the reclamations (**Figure 2**), we digitised the extent of the palaeolagoons of the Tiber River with water levels at different altitudes: +0.4 m and +0.8 m according to the Tiber Delta sea level defined in 1884 (**Figure 7**). Importantly, they reveal the location and the extent of lower height areas of the Tiber Delta 140 years ago.

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#### 3.3.2. Location of the palaeo-washover fans

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Washover fans or overwashes are morphologies formed during storms along a sedimentary coast. They are composed of a breach through an active foredune or a beach ridge in erosion and by a delta-like feature or fan. We systematically digitised them in the entire progradational plain of the Tiber Delta using the RAF aerial maps of 1944 (Figure 7 – Geohistorical data and TDS map).

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## 4. ANALYSIS

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### 4.1. General trends in the non-urban DEM

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The prograded plain of the Tiber extends over 38 km from north to south. The width of the central part of the outer delta plain is the most developed with a protrusion towards the sea, while its width reduces towards the northern and the southern margins (Figure 4 – Data assessment). Additionally, urbanised areas are important in the central part of the delta and decrease towards the margin. Non-urbanised areas are over 80% towards the margins (Areas 2, 7 and 8) and only ca. 50 % for Areas 4 (international airport), 5 (Fiumicino) and 6 (Ostia) (Figure 4 – Data assessment). These two trends (e.g. width of the plain and urbanisation) relatively balance the representativity of the values extracted from the non-urban DEM in the central and margin areas.
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Data displayed in **Figures 3 and 4** show that the heights in the central part of the Tiber Delta are lower than on the northern and southern margins. The subdivision into 8 areas from north to south of the outer plain of the Tiber Delta reveals lower median heights towards the centre of the delta and the river mouths (Areas 4 and 5 are between 1.2 and 1.3 m a.s.l.) and higher median heights towards the deltaic margins (e.g. 3.2 m in Area 1 to the north and 3.9 m in Area 8 to the south) (Figure 4). It corresponds to a difference of ca. 2 m between the median heights near the river mouth and the margins.

While the two margins of the Tiber Delta show higher altitude compared to the central part of the delta, the northern and southern parts of the outer delta plain display significant differences. The range of the height values is much larger in Areas 6 and 7 to the south (ca. 4-5 m) compared to Areas 2 and 3 to the north (ca. 4 m - Figure 4). The range of heights is similar for margin areas 1 and 8 (ca. 6 m). In addition, while median heights are higher to the south, they also increase faster towards the margin.

<figure><img src="image_4.png" /></figure>

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<caption>Figure 3: Extraction of the altitudes for the urban and non-urban areas in each beach system and proportion of urban areas for each beach ridge.</caption>
<figure><img src="image_5.png" /><figcaption>Figure 4: Synthetic view of the different areas identified in the Tiber Delta – data assessment and range of heights in each area.</figcaption></figure>

From east to west, corresponding to the direction of the deltaic progradation, the average height values of the beach ridge systems decrease. The oldest beach ridge system (4000-800 BCE) has an average height of 3 m a.s.l. The most recent beach ridge systems (2011-1750 CE and 1750-1650 CE) are at 2.5 m a.s.l. In this trend, the beach ridge system 800 BCE-400 CE (2.5 m a.s.l.) and 1650-1400 CE (2 m a.s.l.) demonstrate abnormal low values.

Amplitude of the heights from the cross-sections are generally low (<2 m). However, the range of the height values increase towards the south and especially for the beach ridge systems dated after 1400 CE. These variations can reach 9 m, but mostly 4 m.

## 4.2. Comparisons between the surface heights in the non-urban-DEM and the TDS

Surface heights from the non-urban DEM were put in relation to the TDS thicknesses of the Tiber Delta (Figure 5). Deeper TDS sequences are likely to have longer units of fine-grained deposits (Milli et al., 2013) and generate more subsidence by compaction. The most important TDS thicknesses
290 under the deltaic plain are located in a filled palaeo-valley passing under the international airport of 291 Rome. The TDS reaches 80 m thick. Under the Isola Sacra between the Fiumara and Fiumicino 292 distributaries, the TDS is mostly around 40-45 m thick, while lower thicknesses are reconstructed 293 towards the south of the Tiber Delta (15-35 m thick). We separated the analyses of these three groups 294 from north to south in Figure 5.

295 The areas with deeper TDS are mostly covered by the international airport (Figure 5.1). 296 Consequently, the non-urban DEM is patchy for the beach ridge systems that developed before 1400 297 CE in Area 4. The non-urban DEM for the 4000-8000 BCE and 1400-400 CE is located mostly on 298 higher grounds developing towards the end of these progradational phases to the west. The two higher 299 ridges are particularly visible in the northern part of the outer delta plain in Figure 1 (see also profile 300 AA' on the same figure). However, except the beach ridge system dated to 1400-1650 CE that does not 301 show any trends, lower heights in all beach ridge systems are located above thicker TDS and higher 302 heights are above shorter TDS. Slope values obtained for TDS between 80 and 50 m vary positively 303 between 1.11% and 1.71%.

304 In Area 5, the Fiumicino is above a TDS of 40 m and the Fiumara above a TDS of 45 m. No 305 trends can be observed in this area except for 2 values of TDS. Nevertheless, the graphs show decreasing 306 heights westwards.

307 TDS between 15 and 35 m are mainly located south of the Fiumara. Additionally, thicker TDS 308 are westwards and reduce towards the east. Height values from the non-urban DEM are higher above 309 deeper TDS and lower above shorter TDS.
(1) Subsidence driven by sediment compaction?

Area 4 mainly

<figure><img src="image_6.png" /><figcaption>Range of heights in each beach ridge package in relation to the underlying TDS thickness</figcaption></figure>

(2) River mouth dynamics and subsidence?

Area 5 mainly

TDS thicknesses
of 45 and 40 m
are located mainly
at the river mouth of the Tiber

We cannot observe the effect of
subsidence alone in this area.
However, we can observe
a combined effect of subsidence
with a fast progradation
on the heights.

<figure><img src="image_7.png" /></figure>

(3) Aeolian dynamics and uplift

Areas 6, 7 and 8 mainly

TDS thicknesses
from 15 to 35 m
are located mainly
towards the south in
our dataset

The trends observed for
each sediment thickness
follows the same
north-south and east-west
patterns than
the general data
presented in Figure 4

<figure><img src="image_8.png" /></figure>

Sattar and Saleman, 2024

<caption>Figure 5: Relationship between heights of the beach ridges and TDS sediment thickness.</caption>
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## 4.3. Complementary observation from 1940s aerial photographs and old maps

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Geohistorical data digitised from 19th century old maps and RAF aerial photographs from

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1944 are reported in **Figure 7**.

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Palaeolagoons are mainly observed in the inner deltaic plain, e.g. palaeolagoon of Ostia to the south and Maccarese to the north. Palaeolagoons are observed in the outer delta plain only west of the Isola Sacra, in the Roman harbour of Portus and under the area now occupied by the international airport of Rome. A palaeolagoon at + 0.4 m in 1884 is also observed on the right side of the Tiber river mouth of the Fiumara. It corresponds to the low height values also observed on the DEM (Figure 6-A).

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Palaeolagoons are also observed at + 0.4 m in the 800-4000 BCE beach ridges and at + 0.8 m in the 800 BCE-400 CE beach ridges under the airport. These parts were not visible on **Figure 5-1** due to the airport infrastructure. However, these observations from old maps correspond to the results obtained to the west on the DEM (Figures 5-1 and 6.2), e.g. lower surficial heights are above thicker TDS.

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We visually interpreted all aerial photographs from 1944 to identify washover fans and digitised them. All washover fans are located in Area 4 (Figure 7).

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## 4.4. Urbanisation distribution

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Urbanisation mainly affects the most recent beach ridges. It represents 62% of the last beach ridges formed during the last 250 years while the oldest beach ridge only reaches 18,1%. From north to south, urbanisation is around 50% of the zones at the mouth of the delta and drops under 10% at the margins. It highlights that anthropic development is mostly new and focused on the mouth of the delta.

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<figure><img src="image_9.png" /><figcaption>Figure 6: Focuses on the DEM: the river mouths (1), the western part of Area 4 (2), the area of Ostia (3), and the southern part of the Tiber Delta (4 and 5).</figcaption></figure>

# 5. DISCUSSION

## 5.1. Geomorphological interpretations

The interpretation of the heights of the progradation plain is particularly complex. They can be explained by considering two main interpretative systems: sedimentological processes either fluvial, coastal or aeolian, and vertical instability either subsidence or uplift. However, it can be challenging to identify the role of one or the other in explaining general altitudinal variations. In the following parts, we will first try to unravel the role of the land use, dune formation, fluvio-coastal dynamics, and second, aim to identify possible areas affected by subsidence.

### 5.1.1. A prograded plain of aeolian sandy ridges

Higher altitudes towards the north and the south of the outer plain of the Tiber Delta could be related to more developed aeolian dunes and/or to uplift. Sedimentological and geochronological studies conducted by Bicket (2010) reveal important dunes in the southern area. Unfortunately, the limit between the dune and coastal deposits is not stated clearly. Coastal ridges formed in relation to the mean sea levels and the swash waves (Tamura, 2012), but aeolian morphologies such as dunes can reach up
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to ca. 8 m a.s.l. to the south. The differences of heights between the northern (higher values) and southern (lower values) deltaic margins in **Figure 4** may partly be due to the land use. The southern part of the outer delta plain hosts more forest areas including the Natural Park of Casterporziano (Riserva Naturale Statale di Casterporziano). Vegetation probably grew on beach ridges since the formation of the foredune and increased sediment traps on the dunes (Bicket, 2010). Additionally, forests contribute to better preserving beach ridges (Bicket, 2010). **Figure 3** shows important and quick height changes in beach ridge systems dated to the last centuries to the south (from 1400 to 2011). In contrast, agricultural activities conducted throughout the outer delta plain, but particularly to the north contribute to smooth beach ridge features. **Figure 6-3** shows no beach ridges south of the archaeological site of Ostia (agricultural zone probably since the Roman period) and better preserved beach ridges to the west under the forest.

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Considering east-west sections across the outer delta plain, we can observe a general decrease of the heights from inland (mean value around 3 m) to the modern coastline (mean value around 2.5 m) (Figure 3). This is common in a prograded plain of aeolian sandy beaches (Tamura, 2012). Older beach ridges were formed adjusting to lower sea level (Lambeck et al., 2010), but dunes could have continued to grow through a longer period of time compared to the more recent beach ridge dunes. It is possible that regional uplift could have contributed to the rise in the heights (Ferranti et al., 2006). Here again, a continuous uplift would have risen the older beach ridges more. Nevertheless, a systematic analysis of the beach ridges using palaeoenvironmental techniques would be necessary to clarify the height of the limit between coastal and aeolian sediments.

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Increased aeolian activity and bigger dune formation due to modern infrastructure is also observed in the southern part of the Tiber Delta. **Figures 6-4 and 6-5** demonstrate that the coastal road (SP 601 - Via Litoranea) contributes to the remobilisation of aeolian sediments and creates higher dunes close to it.

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In **Figure 3**, beach ridge systems dated to 800 BCE-400 CE and 1400-1650 CE demonstrate abnormal decrease of the mean heights respectively at 2.5 m and 2 m a.s.l. These two periods correspond to periods of accelerated progradation in the Tiber Delta (1st millennium BCE progradation: Salomon et al. 2018; 16th century progradation: Bersani and Moretti 2008). Two complementary hypotheses can

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be proposed. First, accelerated progradation formed larger and deeper swales (Bellotti et al., 2001 for the Ombrone Delta), and dunes hardly developed on the ridges separated by these swales. Second, fast sedimentation and progradation were prone to accelerated primary subsidence (Teatini et al., 2011 for the Po delta).

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These height variations in the outer delta plain seem to not correlate with potential rise and fall of the sea level for the last 6000 years sometimes hypothesised along the Tyrrhenian Coast (Biserni and van Geel, 2005) or eventual changes in wave regimes. Processes related to river mouth progradation, dune formation and uplift/subsidence are the most likely to explain these height changes between beach ridge systems considering north-south and east-west height variations. More developed interpretations would be possible in conducting systematic drilling campaign, sedimentological identification of the aeolian and coastal sand deposits and delineation of the depth of the changes. These results open a new set of questions for conducting more detailed geological and geomorphological studies.

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### 5.1.2. Evidence of subsidence

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In the central part of the Tiber Delta, the heights of the outer plain are of lower values, especially in Areas 4 and 5. Area 5 includes the river mouths of the Tiber Delta, e.g. the Fiumara to the south and the Fiumicino to the north. Lower heights near the Fiumara river mouth can be attributed to quick river mouth progradation, deeper swales, lower ridges as well as subsequent primary subsidence as already mentioned. In contrast, Area 4 is located away from the river mouth area and lower height values cannot be explained by the quick progradation.

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It is suggested that subsidence due to sediment compaction is the main factor explaining lower heights in Area 4. We tested this hypothesis in considering that the subsidence was related to the thickness of the TDS and especially the clayey and silty parts deposited during the last 13000 years (Milli et al., 2013). It can be observed that the palaeochannels of the Late Pleistocene were incised in a valley right under Area 4. TDS reaches 50 to 80 m under the surface of Area 4. DEM pixels with lower height values are located on thicker TDS, except for the 1650-1400 CE beach ridge system (Figure 5.1). Slopes between 1.0 and 1.7 % were observed for the median values of heights in relation to the TDS

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sediment thicknesses between 50 and 80 m. The absence of trends observed in the 1650-1400 CE beach ridge system is probably due to the few values available. The long north-south landing runway of the international airport of Rome covers most of this beach ridge in Area 4.

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Under Area 5, the TDS is only around 40-45 m deep. There is not enough data diversity to observe an effect of the thickness on the surficial height values. We mainly observe that older beach ridge systems are higher than most recent ones probably due to longer time of sedimentation inland, either due to floodplain or aeolian deposition, or of regional/local uplift.

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Finally, the TDS thickness of 35 to 15 m was mainly reconstructed for the southern part of the Tiber Delta (Milli et al., 2013). TDS thickness of 35 m is in the north and decrease to the south to 15 m. Similar to the observation made in Figure 4, DEM heights increase towards the south due to a combined effect of aeolian dynamic and uplift. The TDS was not available for the entirety of the area due to the location of the sedimentary core. Northern and southern areas could bring valuable information for the interpretation. Conducting a new drilling campaign to the north and the south could help to better understand the height variations in relation to the sediment thickness and composition.

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To conclude, lower surface heights values observed in Areas 4 and 5 are either due to subsidence alone (Area 4 in Figure 4 and especially Zone B in Figures 6 and 7) or a combination of subsidence with river mouth morphologies formed during fast progradational phases (Area 5 in Figure 4 and especially Zone A and C in Figures 6 and 7). Unfortunately, we cannot provide absolute values of subsidence in the long term considering that all heights values of the Tiber Delta are relative. New drillings with high resolution chrono-stratigraphies would probably help to better constrain the chronology.

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At the scale of the delta, Solari et al. (2018) and Delgado Blasco et al. (2019) identified areas with stronger subsidence in the palaeolagoon areas (Maccarese to the north and Ostia to the south) and at the river mouth between the Fiumara and the Fiumicino areas, and all of this in relation to new load due to new structures. They mainly focused on short-term subsidence in urban areas. In contrast, our study is focusing on non-urban areas affected by long-term subsidence. In this sense, the two different approaches are complementary.

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<figure><img src="image_10.png" /><figcaption>Figure 7: Synoptic view of the main results of the study in terms of hazard, vulnerability and risk. NB: The subsidence for the Tiber Delta for the future 100 years is not modelled, exposure to coastal flooding and submersion is only based on the current state of the height extracted from LiDAR data.</figcaption></figure>

## 5.2. Subsidence and coastal risks assessment in the long term

We identified two types of hazards that could affect the lower coastal heights along the coast of the Tiber Delta: submergence due to relative sea level rise and coastal flooding due to storms during short-term events.

Considering the current surficial state and without modelling ongoing subsidence, ca. 25% of areas 4 and 5 of the height values are already under the threat of the worst-case scenario of sea level rise SSP5-8.5 m proposed by the IPCC (2022) (Figure 7). A higher percentage would be reached with modelled subsidence of the Tiber Delta for the future. SSP5-8.5 m scenario proposes an excess warming of 4°C and a mean sea level rise of 0.63 to 1.01 m.
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The systematic record of the washover fans on aerial photographs of the 1940s reveals they were primarily located in Area 4, the area also affected by subsidence. These washover fans were observed from the oldest to the more recent beach ridge systems. Similar washover fans could also have existed in Area 5, but erosional phases after progradational phases could have erased evidence. However, the results demonstrate that coastal flooding exposure is closely related to the heights of the coast and that the threshold is constrained within heights of 1 m, e.g. between a median area of 1.3 m (Area 4) and 2.2 m (Area 3). The sea level rise of the 21st c. will clearly expand the area affected by coastal flooding due to storms and increase the risk for the central part of the delta.

Located near Rome, the Tiber Delta is particularly affected by urban sprawl. It also hosts important infrastructure for the capital of Italy, such as its international airport, an intermodal infrastructure, and its main exhibition centre. Deformation of the surface due to subsidence will continue to affect the tracks of the international airport of Rome (Manassero and Dominijanni, 2010).

In addition, urbanisation mostly affects the areas along the coastline. Almost 60% of the last beach ridge systems (1750-2012 and 1650-1750) is urbanised (Figure 3). Urbanisation mostly developed near the river mouths of the Tiber River in Areas 4, 5 and 6, with around 50% of the areas urbanised.

Areas the most exposed to submergence and coastal flooding in the 21st c. are also the areas the most populated and urbanised in the Tiber Delta (Areas 4 and 5). By extension, all urban areas along the coast should be more exposed to storms. The risk related to submergence and coastal flooding is also combined to the erosional risks affecting the coast of the Tiber Delta for almost 50 years (Bellotti and De Luca, 1979).

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# 6. CONCLUSION
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To conclude, the height variations visible in the Tiber delta strandplain can be explained by complex interplays of processes including fluvial, coastal and aeolian dynamics, neotectonics, sediment compaction and human activities. However, more detailed interpretations of the sedimentary processes or neotectonics would be possible in conducting additional geomorphological fieldwork and palaeoenvironmental analyses. Nevertheless, areas of subsidence were possible to identify by crossing different datasets including LiDAR data, aerial photography, old maps and available stratigraphies of the Tiber Delta.

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Consequently, the outer plain of the Tiber Delta was studied to participate in finding areas of subsidence. To do this, we used a chrono-morphological approach on a DEM analysed by GIS techniques to understand its altitudinal variations considering north to south areas, beach ridge systems and the sediment compaction potential derived from the TDS.

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Areas at the mouths of the delta and near the international airport of Rome display the lowest heights. They are related to a progradational plain developed over thick Holocene stratigraphies prone to compaction (Area 5 but mostly Area 4) and to quick progradational phases (8th – 5th c. BCE and 15th -16th c. CE) (Area 5). In the first case, the interpretation in terms of subsidence is straightforward. Regarding the second case, strong and accelerated subsidence right after the deposition at the river mouth is possible since sediments were deposited quickly and not stabilised. However, these low values can also be due to coastal morphologies formed during the quick river mouth progradation. To better understand the height variations, further geomorphological studies would be necessary. Notably, it would need new core drilling to characterise sediments, especially between wave-built and aeolian facies and better chronological data from deep cores to constrain the timing of the subsidence on Areas 4 and 5.

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Evidencing areas of relative subsidence also led to understand how the delta is a sensitive place to storms and relative sea level rise. Projections show that globally driven sea level rise will have an impact on low altitude areas located around the mouths of the delta as they are highly anthropized. In addition, urban sprawl influences its vulnerability especially along the coastline. A future analysis about the potential impact of future sea level rise or storms applied with local scenarios could help further understand the exposure of the delta to these types of risks.
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497 7. Acknowledgements

498 We would like to thank Pierre-Alexis Herrault and Grégoire Skupinski for their advice and help in
499 developing the work in this study. We would also like to express our gratitude to Leah Holguin for
500 proofreading the English text.

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502 8. Authors Contributions Statement

503 Conceptualization, Methodology, Software, Formal analysis, Writing - Original Draft, Visualization

504 Conceptualization, Methodology, Writing - Original Draft, Visualization, Supervision, Project
505 administration, Funding acquisition

506 9. Data Availability

507 The data that support the findings of this study are openly available and posted in the trusted repository
508 Geoportale Nazionale from the Ministero dell'Ambiente e della Sicurezza Energetica at
509 http://www.pcn.minambiente.it/mattm/procedura-richiesta-dati-lidar-e-interferometrici-ps/. This
510 repository does not issue DOIs.

511 10. Bibliography

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