Historical and Recent Environmental Changes of the Ombrone Delta (Central Italy)

Journal of Coastal Research, Special Issue No. 61, 2011

344

Journal of Coastal Research SI 61 344-352 MCRR3-2010 Conference Proceedings 2011

Historical and Recent Environmental Changes of the Ombrone

Delta (Central Italy)

C Tarragoni1, P Bellotti2, L Davoli3, B.M Petronio4 and M Pietroletti5

ABSTRACT

TARRAGONI, C., BELLOTTI, P., DAVOLI, L., PETRONIO, B.M and PIETROLETTI, M., 2011. Historical

and Recent Environmental Changes of the Ombrone Delta (Central Italy). In: Micallef, A. (ed.), MCRR3-2010

Conference Proceedings, Journal of Coastal Research, Special Issue, No. 61, pp. 344-352. Grosseto, Tuscany,

Italy, ISSN 0749-0208.

Reconstruction of shoreline variations in the area of the Ombrone delta apex has been achieved by comparing

aerial photos acquired in 1995, 1998, 2004 and 2006. This reconstruction allowed to define the recent evolution

of the Ombrone delta area in terms of erosional and progradation rates. Comparison with older erosional and

progradation rates derived from Bellotti et al. (1999a) evidences increasing erosional rates along the delta apex

during the investigated interval. Moreover, it has been possible to identify the importance of mouth bar growth as

a recurrent mechanism for confining narrow sea stretches. These results have been used as reference for inferring

the genesis of a set of four older beach ridges and a lake reported in historical maps. According to previous

studies, the modern ponds (locally known as “chiari”) are remnants of the old lake observed in historical maps.

Based on the morphological analysis and reconstruction herein proposed the origin of the modern Chiaro Grande

pond and Chiaro del Porciatti ponds is independent from that of the old lake, and likely reflects the growth of

bars offshore the Ombrone River delta. Lake avulsion during main flood events, however, may have represented

an additional contribute to the sourcing of the ponds, as supported by the evidence of washover breaks that

connect the ponds with the area at their back where the lake existed until the end of 19th century.

ADDITIONAL INDEX WORDS: Ombrone River delta, shorelines evolution, ponds.

INTRODUCTION

The Ombrone River delta, on the southern Tuscany coast

(Fig. 1A), is located within the Grosseto plain, an alluvial

plain filled by plio-quaternary deposits.

The Grosseto plain is confined by coastal mountain belts

extending down to Castiglione della Pescaia, to the north, and

by Monti dell’Uccellina to the south.

DOI: 10.2112/SI61-001.1 received XX; accepted XX

©Coastal Education & Research Foundation 2011

The Ombrone River enters the southern part of the Grosseto

plain with a meandering course and becomes more linear and

gently arched toward southwest, 8 km from the river mouth.

North of the Ombrone River, the Emissario San Rocco

Channel crosses the plain along an E-W direction that

becomes ENE-WSW in its final part (ca. 6.5 km). The

Ombrone River delta (Fig. 1C) is a wave dominated delta

(GALLOWAY, 1975). It is characterised by a single distributary

and by a cuspate strand plain with small coastal basin

bordered by different beach ridges (Innocenti & Pranzini,

1993). These coastal basins are locally named “chiari”.

1Dipartimento di Scienze della Terra,

Università degli Studi di Roma

“La Sapienza”. P.le A. Moro,

5 – 00185, Rome, Italy

Tel:+390649914823

[email protected]





Tel:+390649914822,

[email protected]

www.cerf-jcr.org





Tel: +390649914802

[email protected]

4Dipartimento di Chimica,




Tel: +390649913723

[email protected]

5ISPRA (Istituto Superiore Protezione

e Ricerca Ambientale).

Via Curtatore, 3,

00185 – Rome, Italy

Tel: +390650074220

[email protected]

Historical and Recent Environmental Changes of the Ombrone Delta (Central Italy)

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Figure1. Location of the study area (A and B), close-up of the Grosseto plain and Ombrone delta (C) and detail of the delta apex (D).

This work illustrates the results of a morphological study

conducted on the Ombrone delta apex, between Emissario San

Rocco and Monti dell’Uccellina (Fig. 1C); in addition to the

morphological study, some chemical analyses have been

performed on water samples from the ponds.

The study area shows marked morphological differences

between the left and right wings. The former is characterized

by closely-spaced beach ridges, abruptly bending WSW; sand

dunes separated by narrow depressions are also present near

the river mouth. The right wing shows higher and more

continuous beach ridges that bend more gently toward the

river mouth; here the sand dunes are separated by wider and

deeper depressions compared to the left wing, some of which

host ponds (chiari; Fig. 1D). The beach ridges are interpreted

as preserved ancient shorelines. Their temporal evolution is

reconstructed on the basis of historical maps, modern aerial

photos of the area and limited bathymetric data, in order to

determine the related genesis of the ponds. Regarding

chemical analyses, two set of superficial water samples have

been collected in order to study seasonal changes of the

following parameters: biochemical oxygen demand (BOD),

Cl-, NO3- and PO4

- content.

Chemical oxygen demand (COD) was not determined

because of the high Cl- content (ranging from 13 to 24 g/L). In

fact, COD determination is based on oxidation of organic and

inorganic compounds present in the water with K2Cr2O7 in

H2SO4 medium and Cl- content must be < 1 g/L.

METHODS AND MATERIALS

Morphological analysis was carried out using

aereophotographs (ALISUD flight, 1995) and available

satellite images (http://maps.live.it, 1998; Google Earth, 2004

and http://www.visual.paginegialle.it, 2006), historical maps

(Archivio di Stato, 1823, 1832, 1851, 1871 e 1883; tavoletta

IGM - Bocca d’Ombrone, 1929, 1985), single beam

echosounding maps (Istituto Idrografico della Marina, 1977),

bathymetric and topographic profiles cross-strike to the

shoreline (E. Pranzini, 2006, unpublished data) and newly

acquired single beam echosounding data from the ponds.

Shoreline variation has been studied using 23 images of the

investigated coastal area derived from

http://www.visual.paginegialle.it and 14 images from

http://maps.live.it.

Tarragoni et al.

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These images have been imported in Google Earth for

digitalization of the shoreline. Vector data relative to the 1998

(maps.live), 2004 (Google Earth) and 2006 (Pagine Gialle)

shorelines were then saved as .kml files and used for

comparison with the vector data of the 1995 shoreline

position.

Also available data on older shorelines (between 1929 and

1995) were taken into account for comparison aimed at

detecting significant changes in shoreline trends (Fig. 4).

A sediment core retrieved from Chiaro Grande pond allowed

to sample 35 cm of sand; however no chronological

information could be obtained. No core was retrieved from the

two ponds of the Chiaro del Porciatti because of the extensive

presence of vegetation.

In addition to the above data, chemical analyses on water

samples from the ponds have been performed as well. The

oxygen dissolved in water was determined according to the

Winkler method (L. Winkler, 1888). The method is based on a

set of chemical reaction in which the dissolved oxygen is

converted to a chemical equivalent quantity of iodine that is

determined iodometrically.

The oxygen saturation percentage was calculated as ratio

between the measured oxygen and the oxygen solubility in

water, taking into account the temperature-oxygen solubility

relationship (Table 1).

Table1: Oxygen solubility at different water temperatures.

Temperature- Oxygen Solubility Relationship

Temperature (°C) Oxygen Solubility (mg/L)

0 14.6

5 12.8

10 11.3

15 10.2

20 9.2

25 8.6

100 0

Biochemical oxygen demand (BOD) was determined by

testing the difference in oxygen content in two identical water

samples. The dissolved oxygen of one sample is determined

immediately, whereas the second sample is stored in the

absence of light at 20 °C and the residual oxygen is

determined after five days. The resulting value (in milligrams

per liter) is an indicator of the relative organic strength of

sewage or other polluted waters.

Orthophosphate determination was carried out

photometrically (λ=710 nm) after the reaction between

orthophosphate ions and molybdate ions in concentrated

sulphuric acid and the reduction of the molybdophosphoric

acid obtained with ascorbic acid.

Nitrate concentration was determined photometrically

(λ=552 nm) after the reaction between nitrate ions and a

benzoic acid derivative in concentrated sulphuric acid.

Cl- determination was carried out using the Mohr titration

method (Mohr, 1877): water sample (pH 7-8) was titrated

with 0.1 M AgNO3 solution against K2Cr2O7 indicator.

DATA ANALYSIS

Shoreline variations

The comparison of four shoreline positions recording their

migration during a 12 year interval (spanning between 1995

and 2006) allows to define coastal erosion and progradation

and estimate their rates.

For the left wing the comparison “step by step” (1995-1998,

1998-2004 and 2004-2006) underlines the increasing of the

shoreline trait evidencing erosion; the left wing appears stable

during the 2004-2006 interval, also due to the presence of a

rigid barrier at the apical area.

For the left wing the same comparison shows the increasing

of the shoreline trait evidencing stability and erosion; the

shoreline trait evidencing progradation is shorter.

Overall, the comparison between 1995 and 2006 images

highlights the retreat of the shoreline of the apical area and the

enlarging of the wings: greater part of the eroded sediment

from the apex is variably distributed along the wings, with a

preferential northward dispersion (Figs. 2 and 3).

Figure2. Estimates of shoreline variations in the Ombrone delta area

identified by different colours. Stable areas in light grey, retreating or

prograding areas in dark grey and black, respectively. The same

colour code is used for the pie charts expressing percentages (A-B-C-

D refer to the distinct sectors reported in Tab. 2).


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Figure3. View of the delta apex with the successive shorelines

highlighted.

The values of accretion/erosion rates have been compared

with the similar values calculated for the 1973-1995 interval.

In addition, records of shoreline variations after 1929 (Fig. 4)

illustrate erosion, at variable rates, of different sectors of the

Ombrone River delta apex (Bellotti et alii, 1999a,b; E.

Pranzini, 2001).

Figure4. Shoreline variations along the Ombrone delta apex. Notice

the symmetrical retreat of both the left and right wings, with the

exception of the 2004 and 2006 trend in which retreat is asymmetric,

with the 2006 shoreline clearly retreating only on the right wing.

For the right wing, the 2004-2006 interval is characterized

by higher erosion rates in the apex area (B in Table2)

compared to the 1998-2004, 1995-1998 and 1973-1995

intervals, and by lower progradation rates of the sector

between Emissario S. Rocco and Chiaro del Porciatti ponds

(A in Table. 2). The sector between the Chiaro del Porciatti

ponds and the Ombrone mouth shows erosion during the

1973-1995 interval (-8 m/a), accretion during the 1995-1998

interval (5 m/a), overall stability during the 1998-2004

interval (-0.5 m/a) and intense erosion during the 2004-2006

interval (-14 m/a).

The left wing is dominated by overall erosion, with the

exception of a trait close to the delta apex where an artificial

barrier protects the shoreline from erosion. In the area

between Marina di Alberese and Torre Colle Lungo high

progradation rates are concomitant with marked erosion of the

apex (1995-1998), likely cannibalizing the sediment there

eroded; the 1998-2004 and 2004-2006 intervals show

variations comparable with the expected seasonal variations;

therefore the shoreline may be assumed as stable.

Lake and beach ridge evolution

The definition of the beach ridges in the area of the

Ombrone River delta is often complicated because of the

intense anthropic activity, particularly in the case of those

farther retreating from the coast (likely predating the Late

Middle Age; Bravetti & Pranzini, 1987; Innocenti & Pranzini,

1993; Pranzini 1994 a, b).

Younger beach ridges have a more evident morphological

expression and allow a better definition of their overall

geometry, thickness and genetic relationship with the

Ombrone River delta. The genesis and timing of the

succession of beach ridges within the evolution of the

Ombrone delta was based on geoarcheological findings

(Innocenti & Pranzini, 1993; Bellotti et alii, 1999, 2004).

Table 2: Variation of the erosion/accretion rates in four reference

sectors of the study area. Values from the left wing (reported in C and

D) are affected by the presence of a relict road acting as a barrier

that hampers erosion of the shoreline.

Erosio

n/accre

tion

rates

(m/a)

A – From

Emissario

San Rocco

to Chiaro

del

Porciatti

ponds

B – From

Chiaro del

Porciatti

ponds to

Ombrone

mouth

C – From

Ombrone

mouth to

Marina di

Alberese

D – From

Marina di

Alberese

to Torre

Colle

Lungo

1973 –

1995 0.5 -8 -6 0.2

1995 –

1998 5 5 -8 2

1998 –

2004 2 -0.5 -4 - -

2004 -

2006 -0.5 -14 - - - -

Ponds

The data acquired during the single beam survey allowed a

detailed definition only of the morphobathymetry of the

Chiaro Grande pond (Fig. 1D), whereas vegetation mats

hampered data acquisition in the two ponds of the Chiaro del

Porciatti. Consequently, a DEM (TARRAGONI et alii, 2009;

Tarragoni et al.

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348

VECTOR, 2010) could be produced only for the Chiaro

Grande pond (Figure. 5). The results show a mean bathymetry

of -3.5 m for all ponds, with the maximum depths of -4 and -

4.5 m within two main depressions of the Chiaro Grande

pond. It is noteworthy that the surveys took place after the wet

season. In fact, the two ponds of the Chiaro del Porciatti

where connected by an ephemeral channel and also the area

landward of the Chiaro Grande pond appeared submerged by

a 30-40 cm water table.

Figure5. DEM of the Chiaro Grande pond (left) showing the

elongated shape of the deeper depression and the shallower branching

(maximum depth of 70 cm). The bathymetric map (right, above)

shows the distribution and shape of the deepest depressions.

Chemical analysis

Two sets of superficial water samples have been collected:

one in April and October 2007 from the Chiaro Grande

system, consisting of the three little lakes (Figure. 6), and one

in October 2007 and April 2008 from the Chiaro del Porciatti.

On all samples temperature and pH have been measured in

loco, whereas biochemical oxygen demand (BOD), Cl-, NO3-

and PO4- content have been determined after one day.

The pH values, ranging from 7.3 to 8.6 are higher in April,

whereas water temperature (from 18 to 27 °C) is higher in

October.

BOD values from the Chiaro del Porciatti range between 0.6

and 1.6 mg/L in April, and between 0.4 and 2.1 mg/L in

October; whereas those from the Chiaro Grande system are

higher, ranging between 1.64 and 4.91 mg/L in April, and

between 0.95 and 8.00 mg/L in October, also showing higher

variability. The Chiaro Grande shows higher nitrate content

(Graph. 1 and 2) and lower phosphate content (between 0.005

and 0.390 mg/L in April and between 0.170 and 0.560 in

October) than Chiaro del Porciatti (between 0.250 and 0.640

mg/L in April and between 0.220 and 1.220 in October).

Regarding the oxygen dissolved in water, it should be

measured in the field immediately after collection, since the

delay between sample collections and testing may result in an

alteration in oxygen content. Alternatively, one can fix on-site

the dissolved oxygen in the collection bottle, filled completely

with water, by adding just below the surface of the bottle a

series of reagents (manganese sulphate and alkali-iodide-azide

reagent).

Figure6. Sampling location of Chiaro Grande System and Chiaro del

Porciatti.

Graph 1. Nitrate concentration of Chiaro Grande system.

Graph 2. Nitrate concentration of Chiaro del Porciatti.

If oxygen is present, a brownish-orange cloud of precipitate

or flock will appear. The second part of the determination

could be made in laboratory.

DISCUSSION

Based on the review of existing data and integration with

newly acquired bathymetric data we propose a re-


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349

interpretation of the coastal evolution along the area of the

Ombrone delta during the past tow century.

Shoreline variations

Comparison of four shoreline positions recording their

migration during a 12 year interval (spanning between 1995

and 2006) allows to define coastal erosion and progradation

and estimate their rates. The results, reported in Table 2, are in

accordance with the fact that the sediment eroded from the

apex is largely dispersed toward the north. Dominant

northward sediment dispersal is also supported by the

following evidences:

• the area undergoing accretion on the right wing

(north of Emissario San Rocco) is greater than the

area eroded from the same wing, thus implying a

surplus of sediment likely supplied from the south

(Fig. 2),

• the occasional formation of bars showing shore-

parallel elongation toward north. For example, the

1995 spit extended continuously from the left to the

right wing, before it was cut at the river mouth (Fig.

3),

• a submerged bar extending northward from the left

wing to the right wing, with shore-parallel elongation

(Fig. 9).

Lake and beach ridge evolution

The analysis of historical maps has provided evidence of a

lake located behind the XVII – XVIII century beach ridge

(Innocenti and Pranzini 1993). This evidence is further

supported by the present day morphology of the area that

hosted the lake, which is markedly flat and is still periodically

drowned under a water table up to 30-40 cm high. According

to Bellotti et alii, (2004), that identified remnants of the lake

on historical maps, the lake underwent progressive shrinking

during the 20th century, by this giving rise to the ponds.

However, based on the revision of the historical maps, and

their comparative analysis with present day images (Google

Earth), we suggest that the ponds originated independently

from the lake evolution.

In figure 7A an historical map (1823) of the Ombrone delta

is made partially transparent and superimposed on a modern

image from Google Earth of the same area for comparison:

the Ombrone River course in the two maps coincides and the

position of the lake behind the XVII – XVIII century beach

ridge can be compared with that of the Chiaro Grande pond

(evidenced in white); details are shown in plain view and

oblique view close-ups (Figures. 7B and 7C, respectively).

The lake is confined seaward by the beach ridge indicated as

“a” in Figure 7C; this beach ridge is present in the 1823, 1832,

1851, 1871 and 1883 maps; in the latter only it shows several

morphological breaks, likely formed during washover events

(Fig. 8); this evidence suggests that no other beach ridge

existed in front of beach ridge “a” at least until 1883.

The shape of the lake is likely related to washover events

(Bellotti et alii, 1999) through the western rim of the pond,

which, in the 1883 map shows a less regular trend compared

to other traits of the pond rim (Bellotti et alii, 2004). These

washover events denote an increased erosional effect despite

the wet period at the end of the 19th century which should

have enhanced progradation; this effect reflects primarily the

impact of the construction of the La Steccaia dam.

Figure7. A) Comparison between the 1823 historical map and a

modern image (Google Earth) of the Ombrone delta; the Chiaro

Grande pond is evidenced in white. B) Detail highlighting the position

of the lake (white dashed line) with respect to the present day Chiaro

Grande pond; C) oblique view of the area of the lake and position of

the ancient beach ridges. The beach ridges are those dated by

Innocenti and Pranzini (1993).

Moreover, similar washover breaks are absent or of very

limited extent along beach ridges “b”, “c” and “d” (Fig. 7C).

Therefore, the origin of the “b”, “c” and “d” beach ridges and

the ponds occurred during 1883 – 1929 interval (the 1929 map

being the first map reporting the ponds) and thus

independently from the evolution of the lake. Indeed, between

the end of 19th and the beginning of 20th century the growth of

a spit or a bar determined the confinement of a narrow sea

stretch that became the present day Chiaro Grande pond;

beach ridges “c” and “d” formed before 1929.

The infill of the lake consists of a regular, tabular sediment

sheet likely deposited by fluvial floods; during these events,

the existing washover breaks may have worked as draining

channels connecting with the ponds (Fig. 8); evidence of a

similar connection is reported from the 1929 map by Bellotti

et alii (2004). This mechanism may also account for the origin

of Chiaro del Porciatti ponds: water spills during flood events

are conveyed within the previously formed washover breaks,

further eroding and incising the external beach ridges “b”, “c”

and “d” (Fig. 8).

The progressive reduction of the ponds during the last

century is likely the effect of the increasing anthropic impact,

although the evidence of their progressive reduction may be

biased by the effect of seasonal changes in their extent with

respect to the time of their survey.

Tarragoni et al.

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350

Figure8. View of the study area in 1998. A: Chiaro del Porciatti

ponds; B: Chiaro Grande pond; C: recent-most pond; D: spit and

submerged part of a bar extending northward from the delta apex; E:

smaller spit on the right wing of the delta apex. Also sediment plumes

indicating northward dispersal can be observed.

Ponds

As remarked above, the Chiaro Grande pond formed after

the growth of a spit that confined a narrow sea stretch. A

modern analogue of this mechanism has been identified ca.

600 m offshore the Ombrone River mouth; here, the DEM of

the submerged delta area shows the presence of a bar that

delimits landward an area with maximum depth of 3.5-4 m

(Figure. 9).

Figure9. Detail of the right wing showing the ponds and the

succession of beach ridges (a-d). Also the washover channels

connecting the ponds with the lake are indicated (white arrows).

A pond would form if the bar could grow vertically above

sea level isolating the stretch of sea behind it. A similar

morphological feature has been recognized also 200 m

offshore the Ombrone River mouth, on the basis of

unpublished bathymetric profiles acquired during 2006

(courtesy of Prof. E. Pranzini, University of Firenze).

An example of the stage during which such bar emerges

(becoming a spit) is provided by a satellite image of the

Ombrone delta in 1995 (Fig. 3D).

The spit extends along-strike to the Ombrone River mouth,

from the left wing apex to the right one. Concomitant to the

isolation of the stretch of sea behind it, the river mouth is

forced to move ca. 500 m northward, resulting in a 90° bend

in the final trait of the river.

A further step in the evolution from submerged bar and spit

to pond formation can be observed in a more recent (1998)

satellite image of the same area (C in Fig. 10), where a new

coastal pond forms immediately south and seaward of the

Chiaro Grande pond.

Closer to the river mouth also the growth of a new spit

extending northward can be observed (D in Fig. 10). The 2004

image shows that the 1998 pond has been replaced by a new

one located further north (compare Fig. 8 and Fig. 10); also

this latter pond was no longer present in 2006Further evidence

of the high dynamism of this coastal area, as described above,

derives from field work carried out during 2007, 2008 and

2009. Coastal ponds observed during the 2007 and 2008

surveys appeared of different size, the latter being

significantly smaller. Also the shoreline near the Chiaro

Grande pond changed from regular and linear, in 2007, to

markedly concave in the 2008. Finally, in 2009 the shoreline

returned to a position very similar to that of 2007 and a new

coastal pond formed. Similar changes have also been observed

during shorter (seasonal) time intervals.

Figure10. DEM of the submerged delta derived from single beam

echosounding data. The submerged bar extending from south to

north around the Ombrone River mouth defines a partially confined

proximal area, interpreted as an embryonic stage of ponds

formation.

Chemical Analysis

In all samples Cl- content is very high (13-24 g/L) probably

due to marine washover, groundwater infiltration and

evaporation; such values cannot be considered typical of

neither fresh nor marine water.

Oxygen content is higher in April, likely reflecting a

prolonged interval of high productivity and low pollution;

similarly, the percentage of oxygen saturation values (Graph.

3 and 4) are higher in spring than in autumn.

In particular, the values during the spring interval are almost

all higher than 100% (100-220%) in the Chiaro Grande

system, and between 80-110% in the Chiaro del Porciatti

ponds. These values indicate that the extent of the

photosynthetic process is different in the two sites. Also


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351

values during autumn are different, resulting lower than 50%

in the Chiaro Grande system and between 50-80% in the

Chiaro del Porciatti ponds.

Graph 3. Oxygen saturation percentage of the Chiaro Grande system.

Graph 4. Oxygen saturation percentage of the Chiaro del Porciatti

ponds.

In general, oxygen saturation percentage of Chiaro Grande

system respect that of Chiaro dei Porciatti is higher in spring

and smaller in autumn, with the exception of the values of

water samples coming from stations a, b and c. These stations

are located in a small lake surrounded by beach deposits

deprived of any vegetation (Fig. 6).

These differences can be explained considering the different

extent of the photosynthetic activity related to the presence of

vegetation. Where the photosynthetic activity is higher, a

greater amount of organic matter is present. Consequently, the

oxygen consumption for the organic matter transformation is

higher, in accordance with BOD values reported above; thus,

given the persistence of biologically degradable organic

compounds, the amount of oxygen present in the water is

expected to further decrease with time.

Differences regarding NO3- and PO4

- concentrations are

observed between the Chiaro Grande system and the Chiaro

del Porciatti ponds. The NO3- content in the Chiaro Grande

system is greater than in the Chiaro del Porciatti ponds

(Graph. 1 and 2), whereas PO4- concentrations show an

opposite behavior, probably due to the different use of the

surrounding land: as pasture in the first case and as farm land

in the last.

CONCLUSIONS

We propose an interpretation for the evolution of the

Ombrone River delta apex during the last two centuries. It is

known that this area was characterised by an overall

progradation trend since the 15th century Renaissance period;

this trend inverted in the late 19th century, following the

building of La Steccaia dam (between 1875 and 1879), with

consequent landward migration of the shoreline.

Erosion processes included washover events capable of

breaching the beach ridges containing a lake reported in

historical maps until 1883. Only during a limited interval

between 19th and 20th century a progradation trend was re-

established, during which the ponds originated. Later on

(second half of 20th century), a renewed erosional phase

determined the concomitant retreat of the Ombrone delta apex

and limited advance of the wings.

The morphological study in the area of the Ombrone delta

apex, by means of comparing historical maps and aerial

images, allowed:

• Evaluate shoreline retreat during the 1995-2006

interval and evidence the progressive erosion of the

delta apex at increasing rates.

• Recognize that the ponds formed independently from

the progressive shrinking of a pre-existing lake; this

lake was located behind the beach ridge dated 17th-

18th century; therefore the ponds in front of it likely

formed in a later interval encompassed between 1883

(date of the recent-most map reporting the lake) and

1929 (date of the first map reporting the ponds).

• Propose a mechanism for the origin of the Chiaro

Grande pond based on the closure of a narrow sea

stretch consequent to the emersion of a bar.

• Propose a mechanism for the origin of the Chiaro del

Porciatti ponds that takes into account fluvial flood

events and avulsion through washover breaks; the

average depth of 3.5-4 m of the ponds is in

accordance with the erosional potential of flood

events.

In addition to the above morphological study, chemical

analysis on samples from the Chiaro Grande and Chiaro del

Porciatti ponds have been performed. These revealed greater

production of organic matter in the Chiaro Grande system,

related to a higher concentration of NO3- that likely reflects

farming activity in the surrounding land.

These results show the relevance of reconstructing shoreline

evolution and coastal dynamics as reference for evaluating

future changes and addressing management issues.

Tarragoni et al.

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352

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777-792.

Bellotti P., Caputo C., Davoli L., Evangelista S.,

Garzanti E., Moretti Foggia F. & Valeri P. (1999a) - La

piana deltizia del Fiume Ombrone (Toscana meridionale):

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Historical and Recent Environmental Changes of the Ombrone Delta (Central Italy)

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