NHESSNatural Hazards and Earth System SciencesNHESSNat. Hazards Earth Syst. Sci.1684-9981Copernicus PublicationsGöttingen, Germany10.5194/nhess-16-209-2016Can an early-warning system help minimize the impacts of coastal storms? A case study of the 2012 Halloween storm, northern ItalyHarleyM. D.mitchell.harley@unife.itValentiniA.https://orcid.org/0000-0001-7827-6267ArmaroliC.PeriniL.https://orcid.org/0000-0002-9094-7825CalabreseL.CiavolaP.https://orcid.org/0000-0002-7107-8185Department of Physics and Earth Sciences, University of Ferrara, Via Saragat 1, 44121, Ferrara (FE), ItalyHydro, Meteo and Climate Service of the Emilia-Romagna Region (ARPA-SIMC), Viale Silvani 6, 40122, Bologna (BO), ItalyGeological, Seismic and Soil Service of the Emilia-Romagna Region (SGSS), Viale della Fiera 8, 40127, Bologna (BO), ItalyM. D. Harley (mitchell.harley@unife.it)21January201616120922228February201522May201522October201523October2015This 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://nhess.copernicus.org/articles/16/209/2016/nhess-16-209-2016.htmlThe full text article is available as a PDF file from https://nhess.copernicus.org/articles/16/209/2016/nhess-16-209-2016.pdf
The Emilia-Romagna
early-warning system (ER-EWS) is a state-of-the-art coastal forecasting
system that comprises a series of numerical models (COSMO, ROMS, SWAN and
XBeach) to obtain a daily 3-day forecast of coastal storm hazard at eight
key sites along the Emilia-Romagna coastline (northern Italy). On the night
of 31 October 2012, a major storm event occurred that resulted in
elevated water levels (equivalent to a 1-in-20- to 1-in-50-year event) and
widespread erosion and flooding. Since this storm happened just 1 month
prior to the roll-out of the ER-EWS, the forecast performance related to this
event is unknown. The aim of this study was to therefore reanalyse the ER-EWS
as if it had been operating a day before the event and determine to what
extent the forecasts may have helped reduce storm impacts. Three different
reanalysis modes were undertaken: (1) a default forecast (DF) mode based on
3-day wave and water-level forecasts and default XBeach parameters; (2) a
measured offshore (MO) forecast mode using wave and water-level measurements
and default XBeach parameters; and (3) a calibrated XBeach (CX) mode using
measured boundary conditions and an optimized parameter set obtained through
an extensive calibration process. The results indicate that, while a “code-red”
alert would have been issued for the DF mode, an underprediction of the
extreme water levels of this event limited high-hazard forecasts to only two
of the eight ER-EWS sites. Forecasts based on measured offshore conditions
(the MO mode) more-accurately indicate high-hazard conditions for all eight
sites. Further considerable improvements are observed using an optimized
XBeach parameter set (the CX mode) compared to default parameters. A series
of what-if scenarios at one of the sites show that artificial dunes, which
are a common management strategy along this coastline, could have
hypothetically been constructed as an emergency procedure to potentially
reduce storm impacts.
Introduction
The last decade has seen some particularly large coastal storms that have
severely tested and, tragically, demonstrated certain limitations of
established disaster risk reduction (DRR) measures. This list includes
Hurricane Katrina, which struck the coastline of Louisiana in 2005
, Cyclone Sidr in the Bay of Bengal in 2007 , the
2009 “Klaus” storm in the Mediterranean Sea , the 2010 Xynthia
storm on the west coast of France , Hurricane Sandy on the east
coast of the USA in 2012 , Typhoon Haiyan in the Philippines in
2013 and the 2013/2014 series of winter storms in the UK
. Whether or not these events have increased in both intensity
and frequency in the long term is the subject of considerable debate
. An increase in exposure to these storm hazards due to
factors such as the increase in people and economic assets in low-lying areas
nevertheless places increasing pressure to review and update
DRR strategies regardless of any storm trends.
A key aspect of DRR measures is that of community preparedness, of which
early-warning systems (EWSs) play a vital role . Early-warning
systems involve the provision of timely (i.e. a sufficient preparation
window prior to the approaching hazard) and effective (i.e. accurate and
clear) information so that the exposed individuals can undertake the
necessary actions in order to avoid or minimize their risk .
With specific regards to coastal storm hazards, the development of EWSs has
until recently focused on hydrodynamic forecasts for vulnerable low-lying
areas. Some examples of these systems around the world include the
acqua alta surge forecast system for the Venice lagoon
, the UK joint Met Office–EA Flood Forecasting
Centre , the US National Hurricane Center forecast system
and the Bangladesh storm surge EWS . When
performing successfully, the early warnings provided by these systems have
been credited with having greatly reduced the impacts and loss of life of
various extreme events e.g..
For areas where sandy barriers provide a degree of protection from coastal
storms, the inclusion of morphological processes in coastal hazard forecasts
is critical . Waves and currents interact with beach and dune
sediments during storms to dissipate wave energy and act as a natural defence
against storm surge. Dune or barrier breaching as a result of erosion and
overwash on the other hand can potentially have devastating effects on the
coastal hinterland by causing a sudden ingression of marine water and
widespread flooding. Recent advancements in coupled
hydrodynamic–morphodynamic models such as XBeach have enabled
these processes to be simulated with enhanced accuracy and numerical
efficiency. XBeach has been shown to successfully model a diverse range of
extreme scenarios, such as hurricane impacts on barrier islands
, coastal inundation on beaches protected by rubble-mound
breakwaters , dune erosion during Australian east coast lows
and maximum wave run-up .
State-of-the-art EWSs for coastal storm hazards that include both
hydrodynamic and morphodynamic processes have begun to recently emerge in
both the USA and Europe. The Coastal Storm Modeling System (CoSMoS) has been
developed for the US west coast to provide detailed deterministic predictions
of storm impacts for both coastal vulnerability assessments and real-time
forecasts . It comprises a numerical model chain from
large-scale atmospheric and wave forecasts down to detailed (i.e.
metre-scale) predictions of coastal hazards at a series of closely spaced
cross-shore transects along the coastline. In Europe, the MICORE project
Morphological Impacts and COastal Risks induced by Extreme storm
events; established a series of prototype EWSs at nine
diverse sites using a similar methodology to that of CoSMos but with a strong
end-user focus through the application of storm impact indicators
SIIs;. These SIIs are based on the “Frame of Reference”
approach of and are used to translate the comprehensive and
sometimes complex model output into a clear format useful for decision making
in emergency scenarios.
This study presents the EWS for coastal storm hazard initiated as part of the
MICORE project for the coastline of Emilia-Romagna, northern Italy. The
Emilia-Romagna coastline is situated on the Adriatic Sea and is particularly
vulnerable to coastal storms due to its low-lying nature (exacerbated by
decades of anthropogenic and natural land subsidence; ), the
large amount of infrastructure concentrated close to the coastline and
the frequency of winter storm events with water levels exceeding those of the
dune crest and building foundations. A common practice undertaken by
beachfront-property owners in Emilia-Romagna in order to manage this storm
risk is the artificial modification of the beach profile by means of beach
scraping . Prior to the upcoming winter, an artificial
dune of approximately 3 m in height above mean sea level is scraped from
available sub-aerial beach sand and used as a soft revetment from storm waves
and elevated water levels. The relatively ad hoc nature of this management
technique however means that these artificial dunes often fail for
particularly large events, causing flooding of properties in their lee. When
storm events subside in the spring months (i.e. April–May), these dunes are
subsequently removed and the profile is reshaped to its pre-modified form.
Artificial dunes are also used as a coastal-management technique across many
parts of the USA e.g., Europe
e.g. and Australia e.g..
The Emilia-Romagna EWS – hereafter referred to as the ER-EWS – has been
operational (in experimental mode) since December 2012 as a partnership
between the University of Ferrara, the Hydro, Meteo and Climate Service of
Emilia-Romagna (ARPA-SIMC) and the Geological, Seismic and Soil Service of
Emilia-Romagna (SGSS). One of the principal goals of the ER-EWS is the
ability to predict with sufficient warning and accuracy both the amount of
dune erosion and the location, timing and extent of marine flooding
along this coastline. To this end, a chain of atmospheric, hydrodynamic and
morphodynamic models are executed daily to obtain a 3-day prediction of
coastal hazard at various strategic locations alongshore. The
state-of-the-art nature of this system however means that a detailed
understanding of the system performance is required.
On the night of 31 October 2012, a large storm occurred in the Adriatic
Sea that resulted in elevated water levels along the entire Emilia-Romagna
coastline and characteristic of a 1-in-20- to 1-in-50-year event
. Referred to in the media as the “Halloween storm”, the
resulting widespread coastal flooding and erosion called into question
whether more effort was needed to enhance the regional preparedness to such
events. Since the Halloween storm occurred just 1 month prior to the
roll-out of the ER-EWS, the extent to which such a warning system could have
helped reduce these impacts is unknown. This study therefore evaluates this
event by reanalysing the ER-EWS predictions as if it had been operational 1 day
prior to the storm. The results of this analysis have the following aims: (1)
to quantify the accuracy of the EWS predictions with respect to measured
data; (2) to assess potential weaknesses in the early-warning system model
chain; and (3) to undertake “what-if” scenarios with regards to the
presence/absence of artificial dunes of various dimensions.
BackgroundStudy area
The Emilia-Romagna coastline is situated on the northern Adriatic Sea and
comprises 130 km (from the Po Delta at its northern boundary to the township
of Cattolica at its southern boundary) of predominantly sandy beaches
(Fig. ). It is a highly modified coastline, with over
half (57 %) of the region protected by coastal structures such as
rubble-mound breakwaters, groynes and sea walls . These
modifications have been built primarily over the last 60 years in conjunction
with the large growth in tourism on this coastline, which now amounts to
approximately 5 million tourists visiting each year .
Tourist areas are concentrated in the southern section of the region,
particularly in the major townships of Rimini, Riccione and Cesenatico.
Stretches of natural areas meanwhile are more common on the central and
northern parts of the coastline and typically have relatively small foredunes
of up to 4 m in height, a flat beach slope (tan β≈ 0.03) and a
dissipative beach state .
Map of the Emilia-Romagna region in northern Italy.
Environmental conditions for the region are characterized by low wave energy
(mean Hsig≈ 0.4 m, Tpeak≈ 4 s)
with a semidiurnal and microtidal regime (neap tidal range
=±0.15 m; spring tidal range =±0.4 m). Storm waves
meanwhile of up to 3.3 m 1-in-1-year return period; and
storm surge anomalies of up to 0.6 m 1-in-2-year return period;
can occur, particularly in the winter months. These storm waves
are mainly from the east to northeast sectors associated with Bora weather
conditions. Surge events meanwhile predominantly occur during south-easterly
(Scirocco) winds, which coincide with the main SE–NE axis of the Adriatic
Sea. Bora storm waves are generally large and steep, whereas Scirocco waves
are smaller but with a longer wave period. This is because the latter are
generated over a longer fetch but with winds of lower intensity. SE waves are
also sheltered somewhat by Conero Headland south of Emilia-Romagna.
The Emilia-Romagna early-warning system
Within the existing civil protection protocol for the Emilia-Romagna
coastline, 3-day wave and water-level forecasts are undertaken daily by
ARPA-SIMC through its meteo-marine operational forecasting system
. Wave forecasts are performed using the shallow-water wave
model SWAN (Simulating WAves Nearshore; ), forced with 10 m wind output from the atmospheric
model COSMO-I7 (7 km resolution, www.cosmo-model.org). COSMO-I7 wind
speed forecasts have been found to have a mean and rms error of less than 1
and 2 m s-1 respectively . A nested computation grid is
used for SWAN model runs, from a 25 km grid resolution of the Mediterranean
Sea to an 8 km intermediate grid of the Italian region, and finally a
high-resolution 800 m grid of the Emilia-Romagna coastline .
Three-day water-level forecasts are undertaken using the ocean model ROMS
(Regional Ocean Modeling System; ) for the entire Adriatic Sea at a regular grid resolution of
2 km. The ROMS model is forced by atmospheric output (10 m wind, sea level
atmospheric pressure etc.) from COSMO-I7, with the main semidiurnal
(M2,S2,N2) and diurnal (K1,O1) tidal components and measured or
monthly-average river discharge values used as forcing conditions. For a
complete description of the meteo-marine forecasting system for
Emilia-Romagna, see .
The ER-EWS extends this existing offshore forecast system into the coastal
zone through the addition of XBeach (Fig. ). XBeach is a
two-dimensional depth-averaged (2DH) model that solves coupled cross-shore
and alongshore equations for wave propagation, flow, sediment transport and
bed-level changes . It has been designed primarily as a dune
and barrier island erosion model based on the four stages of dune impacts
described by . The main innovation of XBeach compared to other
similar models is that surf and swash-zone processes are solved on the
timescale of wave groups, which drive infragravity wave motions that become
increasingly dominant as the surf zone becomes saturated .
Infragravity waves are therefore the most likely cause of wave bore
collisions with the dune face during storm events . An
avalanching algorithm is used to simulate the effects of slope collapse
during these bore collisions . For a complete description of
the XBeach model set-up, see .
The Emilia-Romagna early-warning system model chain: wind and
pressure fields are forecast using the atmospheric model COSMO-I7, which are
then used to force the ocean model ROMS and wave model SWAN. Nearshore
hydro/morphodynamics are then simulated using XBeach.
Two different SIIs (Fig. ) are used within the ER-EWS to translate
XBeach predictions into indicators of storm hazard, as selected by the
regional geological survey and ARPA to monitor storm impacts and compile them
into an impact-oriented event database . The first SII is
applicable to stretches of coastline with natural dunes and is called the
safe corridor width (SCW). The SCW is a measure of the amount of dry beach
available between the dune foot and waterline for safe passage by beach users
and is given by
SCW(t)=Xdf-Xwl(t),
where Xdf is the surveyed cross-shore position of the
dune foot and Xwl is the model-derived position of the waterline,
which varies through time due to tidal variability, storm surge, wave
set-up/run-up and erosion of the beach face. If the SCW becomes too narrow,
then people could be putting their lives at risk by having no means of
escaping the hazardous marine conditions. A threshold SCW of 10 m has been
selected by end users to separate low-hazard (i.e. “code green”) conditions
from medium-hazard (i.e. “code orange”) conditions. A threshold SCW of 5 m
meanwhile has been selected to separate medium-hazard conditions from high-hazard (i.e. “code red”) conditions.
The two storm impact indicators (SIIs) used to translate XBeach
model output into a format useful for decision makers. (a) The safe
corridor width (SCW). (b) The building–waterline distance (BWD).
The second SII used within the ER-EWS is applicable to locations with
beachfront infrastructure and is referred to as the building–waterline
distance (BWD). Similar to the SCW, the BWD is a measure of the amount of dry
beach available between the seaward edge of a building and the model-derived
waterline. It is given by
BWD(t)=Xb-Xwl(t),
where Xb is the surveyed cross-shore position of the
seaward edge of the building. A threshold BWD of 10 m has been selected by
end users to separate low-hazard conditions from medium-hazard conditions. A
threshold BWD of 0 m (i.e. inundation of the building is predicted) has
meanwhile been selected to separate medium-hazard conditions from high-hazard
conditions.
The complete model chain is executed daily at a total of 22 cross-shore
profile lines along the Emilia-Romagna coastline. These profile lines
correspond to eight different coastal sites, including the tourist areas of
Rimini, Cesenatico and Riccione. Table summarizes the various
beach statistics of each site. Dry beach widths, defined as the distance from
the dune foot or building edge to mean sea level, vary from site to site from
a minimum of 21 m at the site of Lido di Classe to a maximum of 160 m at
Lido di Spina. Grain size information for each site was obtained from SGSS
and range between 120 and 230 µm (D50).
Summary of the eight different sites incorporated into the
Emilia-Romagna early-warning system.
∗ denotes presence of artificial
dunes in topographic data.
Given the impracticalities and costs of continuously updating profile lines,
daily simulations are undertaken by using the same initial profile line. For
most ER-EWS sites, the topographic data used to obtain these initial profiles
are derived from a lidar survey conducted along the entire coastline in March
2010. Having been undertaken towards the end of the winter season, these
profile shapes are characteristic of a more-conservative eroded winter beach
profile. A consequence of using this one-off survey of the entire region is
that some profiles contain artificial dunes and others not (see
Table ). This introduces local variability to the hazard
forecasts depending on how the beach profile was modified at the time.
The exception to this pragmatic approach to updating initial profile lines is
the site of Lido di Classe, which is used as a validation site for the ER-EWS
and is subject to regular (approximately every 2 months during winter and
after major storm events) topographic beach surveys using RTK-GPS technology
(Fig. ). Lido di Classe is a double-barred sandy beach of
approximately 30 m in width and has natural vegetated foredunes backed by a
low-lying pine forest . Eleven closely spaced (alongshore
spacing = 250 m) profile lines over a 2.5 km stretch are used for validation,
with dune crests ranging from 2.1 m at the southernmost profile (profile
name: classe11) to 3.9 m at the northernmost profile (profile name:
classe01).
The Lido di Classe validation site. (a) Locations of all
11 profiles that are regularly monitored using RTK-GPS.
(b) Profile line classe02 after the Halloween storm.
(c) Profile line classe11 after the Halloween storm, indicating an
overwash fan (Photos: Mitchell Harley). (d) The relationship between
dune crest elevation and the minimum safe corridor width (SCW) measured after
the storm.
Daily ER-EWS hazard predictions for the regional authorities are displayed on
a WebGIS platform. The website (Fig. a) presents a general
overview of the current state of the region in terms of coastal hazard for
the next 3 days, with pin colours (i.e. green, orange or red)
corresponding to the worst predicted hazard level for each site. Zooming into
individual profile lines (Fig. b) subsequently indicates the
maximum (i.e. most landward) position of the predicted waterline over the
following 3 days as well as the corresponding SII time series
(Fig. c). An automated email service is also active and sends
emails to the relevant authorities when any predictions exceed the designated
threshold levels.
The Emilia-Romagna early-warning system WebGIS. (a) A daily
representation of the current coastal hazard state at all eight sites is
displayed, with pin colours corresponding to the forecast hazard level.
(b) A close-up of the Rimini profile line, with the pin position
indicating the forecast maximum waterline position for the following 3
days. (c) The corresponding time series of the building–waterline
distance for the following 3 days.
The 2012 Halloween storm
Beginning on the afternoon of 31 October 2012, a cyclonic system centred
over the Ligurian Sea resulted in strong SE (i.e. scirocco) winds
blowing over the length of the Adriatic . At Ravenna, wind
speeds began to develop from 15:00 GMT and peaked at a value of 15.5 m s-1
(ca. 60 km h-1) at 21:50 GMT. The intensity and fetch length of this wind
created a strong surge event that resulted in water levels reaching a maximum
at the Ravenna tide gauge of 1.16 m above mean sea level at 23:30 GMT, tied as highest level recorded in the gauge's 15-year history. In Venice
meanwhile, water levels peaked at 1.43 m (according to the Venice reference
level), which is tied as 14th-highest level recorded since 1923 and was
sufficient to flood approximately 60 % of the city .
Wave conditions during the storm were measured by the Cesenatico wave buoy,
located off the coastline of Cesenatico at 10 m water depth
(Fig. ). According to these measurements, significant
wave heights (Hsig) peaked at a value of 2.43 m at 03:00 GMT on
the 1 November, which when considered in isolation to the extreme water
levels represents only a relatively minor storm event for this coastline
. Wave periods during the height of the storm were around
10 s, and the wave direction was from the east. Both wave and water
levels decreased by the following afternoon, such that the entire event
duration was less than 24 h. Damage due to the event however was
extensive across the whole northern Adriatic Sea. In Emilia-Romagna,
inundation and erosion were observed along the entire 130 km coastline,
including the destruction of several beachfront restaurants, flooding of
villages, major road blockages and sunken boats moored at marinas. A lower
estimate of the damage bill for the Emilia-Romagna coastline was placed at
EUR 4.6 million , which does not include the considerable
flood damage to private infrastructure.
At the Lido di Classe validation site, the low dune crest elevations of the
two southernmost profiles (classe10 and classe11) meant
that overwash occurred at these locations (Fig. , b and
c). The other nine profiles, where the dune crests are higher, underwent dune
face erosion but no overwash. Pre- and post-storm RTK-GPS surveys were
performed (both topographic profile-line and maximum waterline measurements)
and used to calculate the degree of sub-aerial beach erosion (ΔV,
defined by the volume above mean sea level) and the minimum SCW according to
Eq. (). A negative minimum SCW was observed at all 11 profile
lines, meaning that the maximum waterline exceeded the initial dune foot
position along the entire site (Fig. d). This was
particularly the case for the two southernmost profiles, where overwash
resulted in strongly negative values of the SCW (measured minimum SCW at
classe11 =-26 m).
MethodsEWS reanalyses
Three different EWS reanalysis modes were undertaken in order to obtain a
detailed understanding of the EWS performance for the Halloween storm. These
reanalyses concentrated first on the 11 cross-shore profile lines at the
Lido di Classe validation site, followed by an overall reanalysis of all
eight ER-EWS sites. The three reanalysis modes are described below.
Default forecast (DF) mode
The default forecast mode (hereafter referred to as the DF mode) consisted of
running the ER-EWS model forecast chain as if it had been operating at 00:00
GMT on 31 October 2012, i.e. approximately 15 h prior to the onset of
the storm. In order to undertake this reanalysis, 3-day wave and
water-level predictions for 31 October were extracted from the SWAN and
ROMS model prediction archive at each of the eight ER-EWS sites. These output
time series were then used as boundary conditions to run XBeach (revision
4242) for the 22 cross-shore profile lines. No XBeach calibration was
undertaken for the DF mode, meaning that all XBeach model parameters were set
to default values.
Measured offshore forecast (MO) mode
In addition to the DF mode, coastal hazard forecasts based on measured
offshore conditions were also calculated using the wave and water-level
measurements between 31 October and 2 November 2012. This measured offshore
forecast mode (hereafter referred to as the MO mode) was undertaken to remove
the influence of wave and water-level prediction uncertainties and focus
solely on the XBeach model prediction accuracy. Wave conditions were assumed
to be constant along the Emilia-Romagna coastline and extracted from the
Cesenatico wave buoy. Water-level measurements were meanwhile taken from the
Ravenna tide gauge (see Fig. for locations). Similar to
the DF mode, default XBeach parameters were used for this mode.
Calibrated XBeach (CX) mode
The final reanalysis mode consisted of adjusting the MO mode to contain
calibrated (as opposed to default) XBeach model parameters. Hereafter
referred to as the CX mode, this reanalysis represents what should be the
most precise forecasts of region-wide coastal hazard for this storm event by
using both actual offshore wave and water-level measurements as well as a
rigorous calibration of the XBeach model to reflect the site- and
event-specific conditions.
Calibration of the XBeach model parameters was undertaken using the beach
profile and maximum waterline measurements at the 11 Lido di Classe
profile lines. The calibration concentrated on a number of XBeach parameters
deemed critical to wave run-up and beach/dune erosion processes for this
particular type of event and coastal setting:
maximum shields value for overwash processes (smax). It has been observed that XBeach tends to overestimate erosion rates during overwash conditions when the sediment concentration is high and sheet flow conditions occur . This parameter therefore limits sediment transport during sheet flow to a linear function of flow discharge. Similar to , a value of 0.8 was tested against the default setting (default: no limiter).
breaker index in dissipation model (gamma). Energy dissipation due to wave breaking is calculated in XBeach using the formulation of . This equation has three calibration parameters: a power term (n, default = 10), a parameter to adjust the dissipation rate (α, default = 1.0) and a parameter to adjust the fraction of wave breaking depending on water depth (γ, default = 0.55). Default settings of n, α and γ are based on a limited number of tests undertaken predominantly in the wave flume. A variation of these settings (γ= 0.42) was tested for the calibration process here, which has been found to result in improved estimation of maximum run-up at Duck, USA, a sandy beach with similar environmental characteristics (i.e. barred profile, microtidal) to those in Emilia-Romagna.
maximum allowed wave height over water depth (gammax). This limiter restricts wave heights in very shallow water. A value of 1.5 (meaning that maximum wave heights are restricted to 1.5 times the water depth) was tested against the default value (gammax = 2.0).
threshold depth between wet/dry points (eps). This parameter determines the critical depth that defines wet grid points from dry grid points and is hence important for shallow-water processes as well as determining Xwl in Eqs. () and (). Similar to , values of 0.01 and 0.1 were tested against the default value (eps = 0.005).
critical avalanching slope under water (wetslp). This parameter determines the maximum bed slope for wet grid points prior to slumping. A maximum slope of 0.5 was tested against the default value (wetslp = 0.3).
wave asymmetry parameter (facua). This parameter, which can vary between 0 and 1, determines the degree to which short-wave asymmetry and skewness control the direction of sediment transport. Similar to , a value of 0.15 was tested against the default value (facua = 0.1).
An iterative procedure involving 32 different model runs for each profile
(total model runs = 352) was undertaken in order to obtain the optimum
parameter settings. Model performance was assessed by comparing model output
to both the measured minimum SCW/maximum waterline (see
Fig. d) and the measured sub-aerial beach volume change
(ΔV). The parameter set that resulted in the most-accurate estimates
of these two performance criteria (based on the mean error between modelled
and measured values) was subsequently chosen as the optimum parameter set to
be used for the CX mode.
What-if scenarios based on alternative artificial dune designs
The second component of the study was to undertake what-if scenarios in order
to assess the degree to which artificial dunes may have helped reduce the
widespread flooding that occurred. Given the relatively short time frame
(i.e. < 6 h) required to construct such temporary coastal defence
measures, suggested a new coastal-management technique
whereby these dunes are used in conjunction with real-time forecasts as an
emergency reinforcement measure prior to storm arrival. A numerical tool
known as DuneMaker was subsequently developed to rapidly test
the impact of different artificial dune configurations for a given forecast
condition. This tool, which has been designed based on real cases in
Emilia-Romagna, simulates the action of a bulldozer in scraping sub-aerial
beach sand in order to form a dune of a certain height (Zcrest),
width (Wcrest), cross-shore position (Xcrest) and
slope (m).
Dune configurations tested for the what-if scenarios at the site of
Rimini. (a) Dune configurations 1 to 4. (b) Dune
configurations 5 to 8. All configurations were generated using the DuneMaker
software .
The profile line at Rimini towards the south of the Emilia-Romagna coastline
was chosen to undertake such an analysis. With a maximum profile elevation of
just 1.5 m and a large amount of tourist infrastructure concentrated on the
beach itself, this site was particular hit by the Halloween storm and
resulted in damage of approximately EUR 1.1 million . Eight
different dune designs were tested and compared to the actual conditions
whereby no artificial dune was present. These dune designs were divided into
two sets of four dune designs: one set where a buffer of 15 m exists between
the artificial dune and the seaward edge of the building
(Fig. a) and another set closer to the shoreline
with a buffer of 55 m (Fig. b). In each set, the
dune crest height varies in 0.5 m increments between 1.5 and 3.0 m, and the
crest width altered accordingly in order to maintain a comparable dune
volume. Seaward and landward dune slopes were fixed in all cases at 0.25 and
0.75 respectively, which are typical values for the Emilia-Romagna coastline
.
Only the CX forecast mode was used to undertake these what-if scenarios,
since this mode represents what should be the most precise of the three
forecast modes. The ability of each dune to retain the elevated water levels
and wave action was assessed using two parameters: (1) the minimum BWD
obtained over the forecast period and (2) the percentage of artificial dune
volume remaining following the storm event. Whereas the minimum BWD
determines whether or not building inundation occurs, the percentage of dune
volume remaining gauges the degree to which each dune is able to resist the
storm.
ResultsSWAN and ROMS forecasts
An important first assessment of the overall ER-EWS performance is the
accuracy of the offshore wave and water-level forecasts derived from the ROMS
and SWAN model respectively. Results of these 3-day forecasts for the 31
October 2012 forecast date are plotted against co-located wave and water-level
measurements in Fig. . The results indicate that
the SWAN model predicted the observed peak Hsig of 2.43 m to a
high degree of accuracy (forecast peak Hsig= 2.35 m) but
underestimated the duration of the storm. In terms of period and wave
direction (not shown), the SWAN model also reasonably forecast the peak
period (measured peak period = 10.0 s; forecast peak period = 10.2 s) as well
as the direction of the storm (measured direction at peak of
storm = 90∘; forecast direction = 103∘).
Co-located measured (red lines) and 3-day forecast (black lines)
wave and water-level conditions for 31 October 2012.
(a) Significant wave height. (b) Water level. Wave and
water-level data are derived from the Cesenatico wave buoy and Ravenna tide
gauge respectively.
Comparisons between water-level forecasts and measured values show that the
forecasts quite significantly underestimated the extreme water levels over
the entire duration of the event. Whereas water levels were observed to peak
at 1.16 m in Ravenna, forecasts derived from the ROMS model only predicted a
peak of 0.87 m. Additionally whereas the measurements indicate that these
extreme levels were maintained for approximately 12 h, the forecasts
indicate a drop in water levels following this peak. Such a significant
underestimation of the water level over the duration of the event has
implications for the three different forecast reanalyses presented below.
Forecast reanalyses: Lido di Classe validation site
Results of the forecast reanalyses in terms of both the minimum SCW and
ΔV predicted at the 11 Lido di Classe profiles are summarized in
Table , with each profile classified according to the
dune impact regime (CO: collision regime; OW: overwash
regime).
Results of minimum safe corridor width and change in sub-aerial
beach volume (ΔV) predictions for the DF, MO and CX forecast modes.
Values in brackets denote deviations from measured values. CO and OW refer to
the collision and overwash regimes, according to the four dune impact regimes
described by .
a Optimum XBeach parameters: smax = 0.8; gamma
= 0.42; eps = 0.1; facua = 0.15.
b Excluding overwash regime profiles classe10 and classe11.
Considering the DF forecast mode, the results indicate that, while predictions
for this mode exceed the high-hazard threshold for all 11 profiles
(average forecast minimum SCW =-3 m), an overall underprediction of the
maximum waterline reached by the Halloween storm is found. This
underprediction is greatest at the southern end of the site where the dune
crest is lowest and overwash occurred. In terms of the sub-aerial beach
volume change, this same mode resulted in a slight overestimation in the
degree of sub-aerial beach erosion.
The MO forecast mode takes into account the measured extreme water levels and
consequently results in significantly different forecasts in comparison to
the DF mode. Whereas the DF mode underpredicts the maximum waterline due to
the Halloween storm, the MO mode significantly overpredicts this position. At
all 11 profile lines, the waterline for this mode is forecast to overtop
the dune crest and continue down into the low-lying pine forest backing the
dunes. Values of the minimum SCW are subsequently in the range of -16 m
(classe08) to -86 m (classe10). A similar overprediction is
found when considering predictions of ΔV for this mode, with the
forecast sub-aerial beach erosion almost an order of magnitude greater than
measured values.
Switching from the default XBeach parameter set to an optimized set results
in further differences to the coastal hazard forecasts. An optimized
parameter set consisting of smax = 0.8, eps = 0.1, gamma = 0.42 and
facua = 0.15 was obtained through the calibration process of 352 different
model runs detailed in Sect. 3.1.3. The use of this optimized parameter set
results in major improvements to the maximum waterline predictions in
comparison to the MO mode. The forecast minimum SCW for all but the two
southernmost profiles (classe10 and classe11) is comparable to observed
values, with an average difference between forecasts and measurements at
these nine profile lines of just 1 m. A similar outcome is observed for the
sub-aerial beach erosion forecasts, where the average ΔV for nine of
the 11 profiles (again excluding the two southernmost profiles) is
12 m3 m-1 and represents only a slight overestimation of measured
values. For the two southernmost profiles, the calibration process makes
little difference to the large overestimation of both the maximum waterline
and ΔV that was observed in the MO forecast mode.
Figure illustrates the differences between the coastal hazard
predictions for the different reanalysis modes at the second-northernmost
profile, classe02. The figure indicates that the DF mode results in a
reasonable prediction of beach profile change but an underestimation of the
minimum SCW. On the other hand the MO mode indicates a major overestimation
of the beach profile change as well as the maximum waterline. Finally the CX
mode displays only a slight overestimation of the beach profile change and a
near-perfect (i.e. within 1 m cross-shore) prediction of the minimum SCW.
Forecast results at profile line classe02 at the Lido di Classe
validation site: (a) profile change forecasts, (b) safe
corridor width forecasts. Grey lines correspond to the 32 different model
runs undertaken during the calibration process.
Forecast reanalyses: regional level
At the regional level, the three reanalysis modes indicate disparate
forecasts of coastal hazard for the Halloween storm (Fig. ).
For the DF mode, the reanalysis shows that only two of the eight sites
(Marina Romea and Lido di Classe) would have issued high-hazard forecasts had
the ER-EWS been operating on 31 October 2012. Both of these sites
correspond to natural areas where the SCW is predicted and the dry-beach
width is particularly narrow. For all other sites (including the site of
Rimini) the DF mode would have mistakenly forecast low-hazard conditions,
implying that no major threat to buildings or dune systems was imminent at
these sites.
Summary of forecast hazard levels at the eight early-warning system
sites for the three different reanalysis modes.
In contrast to the DF mode, all eight ER-EWS sites in Fig.
indicate code-red conditions, meaning that high-hazard conditions would have
been forecast. Since some of these sites include artificial dunes in the
topographic data (see Table ), this suggests that these dunes
would have failed for this particular storm and the waterline would have
subsequently reached the buildings (i.e. BWD = 0 m).
Finally, the CX mode shows that six of the eight sites would have issued a
high-hazard forecast. The two sites where low-hazard conditions would have
been forecast are those of Riccione and Cesenatico. These sites correspond to
profiles where particularly high artificial dunes are present in the
topographic data, and closer inspection (not shown) of the forecast
simulations indicate that these dunes were capable of retaining the elevated
water levels and waves of the Halloween storm. This concept is explored in
greater detail in the results of the what-if scenarios for artificial dunes
below.
What-if scenarios for artificial dunes at Rimini
The reanalysis of the ER-EWS above indicate that the CX mode would have
forecast high-hazard conditions for the low-lying and dune-free Rimini site.
Had this hypothetical forecast been available a day before the Halloween
storm, the what-if scenarios based on the eight different artificial dune
configurations would subsequently have provided a valuable means of testing
and optimizing potential emergency dune constructions prior to the storm
arrival. The results of these scenarios (Table ) indicate
that all dunes, barring those with the lowest crest elevations (dunes 1 and 5,
Zcrest= 1.5 m), would have resulted in significant reductions to
the hazard forecasts, from a high-hazard forecast for building inundation
down to a low-hazard forecast. Furthermore these artificial dunes appear
relatively resistant to erosion for the Halloween storm, since the percentage
of dune volume remaining in all cases except for dunes 1 and 5 is at least
87 %. In the cases of artificial dunes built further away from the shoreline
(i.e. dunes 2–4), these dunes are seen to actually increase in volume, as a
fraction of the sediment eroded close to the shoreline accumulates at the
base of the dune.
Results of the eight different dune configurations tested at Rimini
for the what-if scenario analysis. Xcrest, Zcrest and
Wcrest refer to the cross-shore position, elevation and width of
the artificial dune crest respectively, and Vdune is the dune volume
per alongshore metre.
This case study for the 2012 Halloween storm in northern Italy highlights
not only the current challenges but also the great potential of early-warning
systems as a tool for coastal management in the future. The challenges lie in
the fact that uncertainties in the deterministic forecasts can be introduced
from a large number of sources and subsequently propagate along the forecast
model chain. With regards to the Halloween storm, three main sources of
uncertainty appear to limit the overall forecast accuracy:
Uncertainties due to offshore water-level forecasts
Water-level forecasts derived from the ROMS model were found to underpredict
the extreme water levels over the duration of the Halloween storm. For a
low-lying coastline like Emilia-Romagna, this underprediction made the
difference between low-hazard code-green forecasts along most of the
coastline and high-hazard code-red forecasts for all eight ER-EWS sites.
While discerning the causes of the water-level underpredictions for this
storm is beyond the scope of this study, it is worth noting that a
water-level underprediction of similar magnitude was observed for the same
event by the acqua alta surge forecast system in Venice, located
just to the north of the study region . Following an extensive
review of the possible forecasting errors related to this event,
attributed the underprediction of the surge in Venice
predominantly to the forecast complexity of this particular storm, which
consisted of complex localized wind fields that amplified the degree of surge
at certain locations.
Uncertainties due to XBeach parameterization
Comparisons between forecasts based on default XBeach parameters and an
optimized parameter set obtained through the testing of six different
parameters also resulted in significant differences to the overall coastal
hazard forecasts. In general, the default XBeach settings were found to
significantly overpredict the maximum waterline and degree of sub-aerial
beach erosion for this event. Improvements to the model predictions were
observed as parameters were changed one by one from their default settings.
Specifically, the shields limiter smax resulted in the most notable
model improvements for this event, followed in order of influence by the
parameters gamma, eps and facua. Note that no
significant prediction changes were observed for the parameters
gammax and wetslp based on the values tested. The optimized
parameter set using these four parameters resulted in satisfactory results
from an end-user perspective, with a mean error in the maximum waterline
predictions of only 1 m for 9 out of the 11 profile lines tested. A
significant overprediction of the maximum waterline and sub-aerial beach
volume change for the two southernmost profile lines where overwash occurred
was however still observed, which suggests that further improvements need to
be made to the XBeach model under overwash conditions.
Uncertainties due to coastal topography
With the exception of the Lido di Classe validation site, topographic data
used to perform the forecast reanalyses were derived from a region-wide lidar
flight undertaken in March 2010. While the Emilia-Romagna coastline can be
considered relatively stable in comparison to more exposed coastlines, the
use of these 2-year-old topographic data introduces a degree of uncertainty
into the forecasts. This is particularly the case when considering the
artificial modification of the beach profile, which is undertaken on an ad
hoc basis along the coastline and can vary considerably from year to year.
Addressing the above uncertainties can be achieved in part through a
combination of numerical model improvements, enhanced coastal monitoring and
comparable studies of other major storm events. As a means of reducing
uncertainties in the water-level forecasts for instance, steps are currently
being made to upgrade and improve the offshore forecasting system by
increasing the model resolution, coupling the SWAN and ROMS models as well as
including additional tidal components and river flow inputs .
Coastal monitoring has also been increased through the use of real-time video
imaging in order to provide a more precise snapshot of the current state of
the coastline, including the size and location of artificial dunes
.
While these measures are likely to improve the confidence of coastal hazard
forecasts, uncertainties will always remain. discusses various
methods of estimating confidence intervals of morphological forecasts through
the use of ensemble forecasting (i.e. performing multiple runs by slightly
varying the input conditions) as well as long-term observations of
morphological forecast error. As this study has demonstrated, ensemble
forecasting should ideally require multiple permutations not only of the
offshore forecasts but also of different XBeach parameter sets and
variations in coastal topography (e.g. natural vs. artificially modified and
eroded vs. accreted beach profiles). While this would lead to a significant
increase in forecast computation time, a greater understanding of the
forecast uncertainty would be achieved in order to better inform decision
makers. Long-term observations of morphological forecast error on the other
hand require several years of monitoring data that are currently unavailable.
Despite these challenges, the benefits of an operational forecasting system
for the Emilia-Romagna coastline are clearly apparent. Considering the site
of Rimini, the series of what-if scenarios suggest that an artificial dune
with a crest elevation of at least 2 m and dune volume of 9 m3 m-1 may have
been sufficient to retain the elevated water levels and waves and prevent the
extensive flooding that occurred at that site. Such information could have
prompted authorities to undertake low-cost emergency actions in the form of
artificial dune constructions based on the appropriate dune designs. While
further testing of this approach is needed, the encouraging results
demonstrated here point towards a new coastal-management tool based on
real-time forecasts that could help minimize the impacts of coastal storms in
Emilia-Romagna and ultimately lead to more resilient coastal communities.
Conclusions
Early-warning systems for dunes and sandy barrier coastlines are still in
their infancy and hence require a careful assessment of their forecast
performance. This study has presented one of only several such
state-of-the-art systems currently operational worldwide that has been
developed for the low-lying, vulnerable coastline of Emilia-Romagna on the
Adriatic Sea, northern Italy. The aim of the study was to reanalyse this
system for the 2012 “Halloween” storm that occurred just 1 month prior to
the system's roll-out and to ascertain to what extent the forecasts may have
helped minimize the subsequent widespread flooding that occurred.
The results indicate that, had this system been operational 1 day prior to
this major storm, a high-hazard code-red forecast would have been issued
for only two of the eight forecast sites, with the remaining six sites
mistakenly issuing low-hazard forecasts. Careful inspection of these results
indicate that the main reason for these low-hazard forecasts was the
significant underestimation of the extreme water levels observed for this
particular event. Had the model chain forecast water levels in line with
measured values, a high-hazard forecast would have been issued for all eight
forecast sites. The results also indicate the importance of undertaking an
extensive calibration of the XBeach model parameters, since considerable
improvements were observed when using an optimized parameter set compared to
default values.
Despite the limitations of the early-warning system for this particular
event, the study highlights the overall benefits of an early-warning system
for a vulnerable coastline such as Emilia-Romagna. A series of what-if
scenarios with regards to the emergency construction of artificial dunes
illustrates that, had accurate forecasts been available at the time, the rapid
construction of these artificial dunes could potentially have withstood the
elevated waves and water levels and significantly reduced storm damage.
Current development efforts are focused on reducing the forecast
uncertainties of this operational system through continued coastal
monitoring, numerical model improvements and further performance assessments.
Acknowledgements
This study has been supported by the Geological, Seismic and Soil Service of
Emilia-Romagna (SGSS) and the EU FP7 collaborative project RISC-KIT
(Resilience-Increasing Strategies for Coasts – toolKIT, EU Contract 603458).
M. D. Harley would like to thank Francesco Droghetti for his assistance with the Lido
di Classe field surveys. The authors would like to thank Francesco Marucci of
SGSS for designing the ER-EWS WebGIS platform and the two reviewers of the
manuscript for their insightful comments.Edited
by: J. Brown
Reviewed by: three anonymous referees
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