PIAHSProceedings of the International Association of Hydrological SciencesPIAHSProc. IAHS2199-899XCopernicus PublicationsGöttingen, Germany10.5194/piahs-373-161-2016Effects of anthropogenic land-subsidence on inundation dynamics:
the case study of Ravenna, ItalyCarisiFrancescafrancesca.carisi@unibo.ithttps://orcid.org/0000-0003-3745-2513DomeneghettiAlessiohttps://orcid.org/0000-0003-4726-5316CastellarinAttiliohttps://orcid.org/0000-0002-6111-0612School of Civil, Chemical, Environmental and Materials Engineering,
DICAM, University of Bologna, Bologna, ItalyFrancesca Carisi (francesca.carisi@unibo.it)12May2016373161166This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://piahs.copernicus.org/articles/373/161/2016/piahs-373-161-2016.htmlThe full text article is available as a PDF file from https://piahs.copernicus.org/articles/373/161/2016/piahs-373-161-2016.pdf
Can differential land-subsidence significantly alter
river flooding dynamics, and thus flood risk in flood prone areas? Many
studies show how the lowering of the coastal areas is closely related to an
increase in the flood-hazard due to more important tidal flooding and see
level rise. The literature on the relationship between differential
land-subsidence and possible alterations to riverine flood-hazard of inland
areas is still sparse, although several geographical areas characterized by
significant land-subsidence rates during the last 50 years experienced
intensification in both inundation magnitude and frequency. We investigate
the possible impact of a significant differential ground lowering on flood
hazard over a 77 km2 area around the city of Ravenna, in Italy. The
rate of land-subsidence in the study area, naturally in the order of a few
mm year-1, dramatically increased up to 110 mm year-1 after World War II,
primarily due to groundwater pumping and gas production platforms. The
result was a cumulative drop that locally exceeds 1.5 m. Using a recent
digital elevation model (res. 5 m) and literature data on land-subsidence,
we constructed a ground elevation model over the study area in 1897 and we
characterized either the current and the historical DEM with or without road
embankments and land-reclamation channels in their current configuration. We
then considered these four different topographic models and a
two-dimensional hydrodynamic model to simulate and compare the inundation
dynamics associated with a levee failure scenario along embankment system of
the river Montone, which flows eastward in the southern portion of the study
area. For each topographic model, we quantified the flood hazard in terms of
maximum water depth (h) and we compared the actual effects on flood-hazard
dynamics of differential land-subsidence relative to those associated with
other man-made topographic alterations, which resulted to be much more
significant.
Study area and union of flooded area in scenarios A (current DEM
without infrastructures) and C (1897's DEM without infrastructures): blue
indicates areas flooded in both scenarios, red and green areas are flooded
exclusively in Scenario A and C, respectively.
Introduction
As clearly highlighted in the recent literature, the study of hydrological
processes cannot neglect the effect of anthropogenic impacts on the territory
(e.g. Montanari et al., 2014 and many writings in the context of the new
science of socio-hydrology, proposed by Sivapalan et al., 2012; Di
Baldassarre et al., 2013 and references therein). Human and water systems are
in fact closely related, so that various authors demonstrated how flood-risk
evolution and the increasing of potential damages during extreme flood events
are often linked to the strong land-anthropization, rather than to climate
change (see e.g. Domeneghetti et al., 2015; Bouwer et al., 2010). Our study
considers the human-induced land-subsidence due to the pumping of underground
fluids in densely populated areas. This phenomenon has been documented,
especially in the last half of the XX century, in different parts of the
world, such as in Japan (see e.g. Daito and Galloway, 2015), Mexico (Toscana
and Campos, 2010), Thailand (Phien-wej et al., 2006) and Bangladesh (Brown
and Nicholls, 2015; Howladar and Hasan, 2014). Literature on the effects of
land-subsidence in coastal areas is rich, see e.g. the effects of salt-water
intrusion (see e.g. Schmidt, 2015) and the decrease of the coastal floods
return period (see e.g. Yin et al., 2013), while the dynamics of hydraulic
risk in rivers flood-prone areas is still poorly investigated and understood.
The aim of our study is to understand whether and in to what extent the
human-induced land-subsidence can change the riverine potential flooding. To
investigate these aspects we focus on the most resounding case of
anthropogenic land-subsidence in Italy, which is the area near the city of
Ravenna (Northern Italy; see Fig. 1). Here the monitoring of ground elevation
by traditional as well as more advanced techniques (see e.g. Bitelli et al.,
2000) showed that the land-subsidence rate experienced a sudden acceleration
in the aftermath of World War II due to an intense water and gas extraction
from underground (Gambolati et al., 1991; Carminati et al., 2002). The latter
produced more than 1.5 m cumulative lowering near the historical city centre
of Ravenna (see Fig. 1). Ravenna is surrounded and crossed by natural streams
that are characterized by artificial embankment systems protecting the city
from frequent flooding. In case of extreme events or levee failures (i.e.
what is usually identified as “residual flood risk”; see e.g. Castellarin
et al., 2011; Di Baldassarre et al., 2009b) it would lead to higher damages,
than those which would occur if rivers could expand freely in the surrounding
plain. This paradox is called “levee effect” and describes the frequent
phenomenon in which the flood control systems encourages urbanization in
areas that are even closer to rivers (Tobin, 1995). We selected this study
area because of its location (only a few kilometers from the coast) and all
factors described above. We investigate the effects of land-subsidence and
man-made infrastructures in the study area on the flood dynamics that is
expected in case of a levee-failure in the proximity of the urban area of
Ravenna. The analyses are performed by adopting fully-2-D models which
consider different topographic scenarios.
Study area
The study area consists of a 77 km2 area around the city of Ravenna, in
the Emilia-Romagna region (Northern Italy; Fig. 1). With a municipal area of
653 km2 and a population of 160 000 inhabitants, the city is located
few kilometers away from the Adriatic coast. Ravenna is one of the oldest
Italian towns, presumably founded in the eight century BC.
Although an inland city, Ravenna is directly connected to the Adriatic Sea
by the Candiano Canal and is crossed by the Montone River (United Rivers
after the confluence of the Ronco River). High population density, as well
as a complex network of road infrastructures characterize the study area.
Like many other coastal lowlands and deltaic plains, the eastern Po plain
and in particular the district of Ravenna lye on a subsiding sedimentary
basin, where extremely significant changes in terms of ground elevation
occurred over centuries relative to the Adriatic mean sea level. The
land-subsidence rate in the area, naturally in the order of few mm year-1,
increased enormously after World War II. The main driver is believed to be
the increase in the extraction of deep non-rechargeable groundwater, related
to the growth of the economic activities in the Po Basin. The close
relationship between groundwater pumping and land-subsidence was confirmed,
among the other studies, by the lowering of subsidence rate experienced
after a strong reduction of groundwater withdrawal (see Carminati et al.,
2002 and references therein). Other studies identified the exploitation of
several on-shore and off-shore deep gas reservoirs in the Ravenna area as an
additional factor that contributed to the growing of land-subsidence rate up
to some centimeters per year (see e.g. Gambolati et al., 1991). In 2005,
Teatini et al. (2005) provided a detailed georeferenced map of land-subsidence in
the eastern Po River plain over the period 1897–2002, based on the main
levelling surveys databases available in the Ravenna area for the last
century (IGM, Ravenna Reclamation Authority, Geological Service of the
Ravenna Municipality, ARPA and ENI-E&P).
Although land-subsidence rate before the 1950s could be assumed to be almost
constant (Teatini et al., 2005), we chose this map as the starting point to
perform our study, therefore investigating the possible role of
land-subsidence on flood-hazard evolution in the period 1897–2002. As shown
in Fig. 1, land-subsidence drops in the last fifty years are larger than 1 m
over more than one third of the study area, with peaks higher than 1.5 m
over an area of 10 km2 between the historical center and the coastline.
Apart from the significant ground lowering, one must also consider the
potential negative effects of differential subsidence occurred in the study
area: as an example, in the northeast part of the study area the topographic
lowering passes from 1.55 to 1.25 m, with a ≈0.30 ‰ horizontal gradient.
Topography of the study area: current and reconstructed conditions
The current topography of the study area is described by a 5 m Digital
Elevation Model (DEM), available as a GIS Service and provided by the
cartographic offices of the Emilia-Romagna for the entire region (Fig. 1).
With the aim of comparing inundation dynamics under current and historical
topographic conditions, we reconstructed the ground elevations before the
land-subsidence occurred during last decades. In particular, the cumulative
land-subsidence contour lines between 1897 and 2002 described in Teatini et
al. (2005) were used for back-warping the current 5 m DEM obtaining a
historical DEM describing ground elevations in 1897. This procedure is based
on the assumption that between 2002 and today no significant change in
ground elevations occurred.
The area to the Northeast of the center of Ravenna suffered the greatest
drop (ca. 155 cm) and for this reason it is raised the most in the
back-warped DEM. The area located approximately 4 km from the city of
Ravenna to the South-West experienced the opposite situation, so the ground
elevation was raised about 80 cm only, when reconstructing the historical
morphology.
The influence of main infrastructures on the flooding dynamics are considered
by modifying the discontinuities elevation according to the real topographic
characteristics of the elements: we lift the railways elevation by 1 m and
we lower the greater channel elevation by 1.5 m.
2-D numerical model
We perform our study by means of the fully-2-D hydrodynamic model TELEMAC-2-D,
which solves the 2-D shallow water Saint-Venant equations using the
finite-element method within a computational mesh of triangular elements
(see Galland et al., 1991; Hervouet and Van Haren, 1996). TELEMAC-2-D was
adopted in previous studies and similar geographical context, proving to
accurately reproduce the real flooding dynamics in a complex floodplain
topography (e.g. Di Baldassarre et al., 2009a). One of the advantages of
TELEMAC-2-D as finite elements model is the possibility to use structured or
non-structured computational meshes. These last, in particular, provide a
densification of the triangular elements at certain critical points and
allow to better describe the topographical discontinuity that influences the
inundation process, such as levees, road and railway embankments
(Domeneghetti, 2014; Di Baldassarre et al., 2009b). For this preliminary
study, we refer to a non-structured triangular mesh densified at the major
discontinuities that can influence our process of flooding, such as
boundaries, channels, roads and railways.
As far as what the Manning's coefficient is concerned, we rely on land-use
maps available for the Emilia-Romagna region and retrieved from aerial
imagery available for 2008 (AGEA-2008), classified on the base of the
standardized classes aggregation adopted by the CORINE (COoRdinated
INformation on the Environment) project (EEA, 2009). For each land-use class
in the study area we use a different value of the Manning coefficient
according to the indications provided in the literature (see e.g.
Vorogushyn, 2008; Domeneghetti et al., 2013).
TELEMAC-2-D is used to simulate the inundation dynamics in the area of
interest, assuming the formation of a single breach in the left embankment
of the Montone River, near the confluence with the Ronco River (see Fig. 1).
The breach, which is about 120 m wide and 4.5 m deep (from the embankment
crest to the elevation of the ground), is assumed to develop
instantaneously, triggered by a hypothetical embankment overtopping. The
dimension of the breach is typical for embankment systems similar to the one
of the River Montone (see e.g. breaches in the Serchio River, December
2009).
Cumulative distribution function of water depth differences between
scenarios, Δh (i.e. differences between water depths simulated for
two Scenarios X and Y); “Scenario X – Scenario Y” indicates Δh
values computed as water depths simulated for Scenario X minus water depths
simulated for Scenario Y; grey dashed areas highlight non significant Δh values (i.e. absolute values lower than 10 cm).
The overflowing discharge at the breach has been calculated by referring to
a quasi-2-D model of the Montone-Ronco river system (see Castellarin et al.,
2011 for an analogous modelling scheme) adopted for the simulation of a
30-years return period flood wave (AdB-RR, 2011). The overflowing discharges
simulated at the breach by the quasi-2-D model are used in our study as
boundary conditions for the 2-D model in order to simulate the inundation
dynamics associated with 1897's and the current topographic conditions.
In order to assess the role of land-subsidence compared with man-made
topographic alterations in river flood-hazard, independently by all other
factors, the simulations are performed by considering the bathymetry
evolution in the period of interest (1897–2002). We consider four resulting
topographic conditions:
Scenario A: current morphology without infrastructures;
Scenario B: current morphology with main infrastructures (i.e. minor
channels, railways, roads, etc.)
Scenario C: 1897 reconstructed morphology (i.e. back-warped DEM) without
infrastructures
Scenario D: 1897 reconstructed morphology with main infrastructures.
Results and discussion
In order to describe the flooding dynamics in the area of interest, we focus
on the maximum water depth (h) resulting from the simulations for all time
steps and for each scenario (A, B, C and D).
Figure 1 shows an example of the results in terms of significantly (water
depths h≥10 cm) flooded areas in two different scenarios: A (current
DEM) and C (1897 reconstructed DEM). The blue areas indicate the portion
flooded in both topographic scenarios, while areas that are either flooded
for 1897 or current are reported in green and red, respectively. Although
the extent of the blue area is much larger than the green and red ones, it
is evident that in the present scenario the flood-risk affects mainly the
urban area of the city of Ravenna, while in 1897 the rural areas in the
Eastern side were mostly impacted by inundation. The cause, as expected, is
the ground lowering due to the land-subsidence, which had its peak in the
historical city center.
Using as reference area the union of the significantly flooded areas in all
four scenarios (representing the areas with simulated water depths h≥10 cm at least in one scenario), we computed the differences of the water
depths (Δh) for all scenarios pairs. The comparison of different
scenarios are shown in terms of the exceedance probability (F) of a certain
difference of water depth (Δh) (see Fig. 2, left and right panels).
Very flat lines around Δh=0 in Fig. 2 indicate that the two
compared scenarios are very similar in terms of maximum simulated h. Lines
that deviate from 0 indicate scenarios whose simulations provided
significantly different simulated maximum h.
A first comparison, shown in the left panel of Fig. 2, is performed by
considering Scenario B as reference scenario, as it represents the situation
closer to reality (current DEM and schematization of major infrastructures).
Water depths in any other scenarios (A, C and D) are therefore subtracted
from water depths in Scenario B, in order to understand which scenario is
more deviated from the real one. The black line in Fig. 2 (left panel)
represents the differences between h in Scenarios B and D (1897's DEM with
major infrastructures); the green line shows the comparison between
Scenarios B and A (current DEM without major infrastructures); the orange
line represents the differences between Scenarios B and C, the latter
considering the 1897's topography and the absence of major infrastructures.
The results demonstrate that taking into account the land-subsidence in the
study area leads to maximum water depths that are quite similar to those
that result from the simulation with the current DEM (black line). Only
11 % of the reference area experiences significant Δh, i.e. larger
than ±10 cm (in 4 % of the flooded extent, maximum water depths in
Scenario B are lower than in Scenario D and in 7 % of the area, the
opposite occurs), while in the remaining 89 % the differences can be
neglected (i.e. Δh lower than ±10 cm, dashed grey areas in
Fig. 2, left panel). As far as the comparison between Scenarios B and A is
concerned (green line), the percentage of flooded area with negligible
Δh (lower than ±10 cm) is equal to 39 %, while in the
remaining 61 % of the extent the differences are more significant. On the
basis of these results, it is rather evident that the effects of
differential land-subsidence on flood risk in the study area are negligible
if compared to the impacts of major infrastructures. The comparison between
Scenarios B and C (orange line) shows 18 % of the flooded areas with
negligible Δh and 82 % with significant differences in terms of
maximum water depths. These values show that Scenarios B and C are the most
different ones, as expected, but the cumulative distribution function of
Δh in this comparison has a very similar trend to that given by the
comparison between Scenarios B and A.
A second graph, shown in the right panel of Fig. 2, compares differences in
terms of maximum h due to the ground drop caused by land-subsidence (starting
either from a simple configuration, Scenarios A–C, and by configurations in
which the infrastructure are considered: Scenarios B–D, respectively) and
the differences due to the modification of major discontinuities (starting
either from the current configuration, Scenarios B–A, and from the 1897's
configuration – Scenarios D–C).
The results in terms of percentages of the reference area with significant
Δh are presented in Table 1 and confirm that the change in elevation
associated with major infrastructures is more important than land-subsidence
when simulating the flooding dynamics.
Comparison of different scenarios, percentage of flooded areas with
significant (i.e. absolute values larger than 10 cm) Δh (i.e.
differences between water depths simulated for two Scenarios X and Y); “
X - Y” indicates Δh values computed as water depths simulated
for Scenario X minus water depths simulated for Scenario Y.
Our study assesses the effects of anthropogenic land-subsidence on river
flood hazard in the geographical area close to the city of Ravenna, which
was affected by an important ground drop in the last century (i.e. more than
1.5 m in the historical city center). The analysis shows that large and
rapid differential land-subsidence does not seem to lead to significant
alterations to the flooding hazard (if we only consider the maximum water
depth as local indicator).
Comparing differences arising from the comparison between simulation in the
current configuration and in presence of ground lowering with those caused
by the effects of major infrastructures, we can see that human-induced
drivers, like construction of canals and road embankments, has an higher
impact on flood-hazard than anthropogenic land-subsidence.
In addition, the study further shows the importance of an accurate
identification of specific topographic data that have to be considered in the
modelling exercise, which should represent the best compromise between
precision, maximum expected accuracy and computational efficiency (Dottori et
al., 2013).
Acknowledgements
The study is part of the research activities carried out by the working
group: Anthropogenic and Climatic Controls on WateR AvailabilitY (ACCuRAcY)
of Panta Rhei – Everything Flows: Change in Hydrology and Society (IAHS
Scientific Decade 2013–2022).
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