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Coastal flooding event definition based on damages: Case study of Biarritz Grande Plage on the French Basque coast

01 Jun 2021-Coastal Engineering (Elsevier)-Vol. 166, pp 103873

Abstract: This paper presents a method to include damage at the initial stage of coastal flooding events definition and in return periods computation. The methodology is illustrated within a local study carried out in Biarritz Grande Plage, a meso-tidal, wave dominated beach located on the french basque coast in the south west of France. The work is based on two datasets covering the period 1949–2015: a first one, consisting of observation and synthetic data on wave characteristics and water level, and a second one, gathering storm dates and related damage intensities obtained through investigations in the press and in archives. A statistical analysis was first carried out to find the best combination of source variables explaining the reported damages for the identified storms. Maximal, mean and accumulated values were calculated over storm duration, considering source and aggregated variables based on the empirical run-up formula or the wave energy flux. Most rules combining a wave parameter and water level are found to provide satisfactory damage prediction as soon as maxima variables are considered. Rules based on mean variables are less accurate and those based on accumulated variable values are not relevant. The ability of the rules to be used as generic event definition rules is then tested by performing a retrospective analysis on the whole dataset, checking their efficiency in detecting historical storms (i.e., with damages) without finding too many false positives. Most of the rules formerly studied, except the ones using wave period only as wave parameter, were able to correctly perform this task. Coastal flood event return periods (RP) were then calculated by applying three of the best rules identified previously. The rule using non simultaneous maxima of wave energy flux and water level gives encouraging results for the RP values. Nevertheless, the discrepancy still observed among the different rules calls for further work in this direction.
Topics: Return period (53%)

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Coastal ooding event denition based on damages:
Case study of Biarritz Grande Plage on the French
Basque coast
Florian Arnoux, Stéphane Abadie, Xavier Bertin, Ivan Kojadinovic
To cite this version:
Florian Arnoux, Stéphane Abadie, Xavier Bertin, Ivan Kojadinovic. Coastal ooding event deni-
tion based on damages: Case study of Biarritz Grande Plage on the French Basque coast. Coastal
Engineering, Elsevier, 2021, 166, pp.103873. �10.1016/j.coastaleng.2021.103873�. �hal-03200009�

Coastal flooding event definition based on damages:
Case study of Biarritz Grande Plage on the French
Basque coast
Florian Arnoux
1,2
*, St
´
ephane Abadie
1
, Xavier Bertin
3
, Ivan Kojadinovic
2
Abstract
This paper presents a method to include damage at the initial stage of coastal flooding events definition and in
return periods computation. The methodology is illustrated within a local study carried out in Biarritz Grande
Plage, a meso-tidal, wave dominated beach located on the french basque coast in the south west of France.
The work is based on two datasets covering the period 1949-2015 : a first one, consisting of observation and
synthetic data on wave characteristics and water level, and a second one, gathering storm dates and related
damage intensities obtained through investigations in the press and in archives. A statistical analysis was first
carried out to find the best combination of source variables explaining the reported damages for the identified
storms. Maximal, mean and accumulated values were calculated over storm duration, considering source and
aggregated variables based on the empirical run-up formula or the wave energy flux. Most rules combining
a wave parameter and water level are found to provide satisfactory damage prediction as soon as maxima
variables are considered. Rules based on mean variables are less accurate and those based on accumulated
variable values are not relevant. The best results are obtained with the wave energy flux and water level maxima
over the storm duration. The ability of the rules to be used as generic event definition rules is then tested by
performing a retrospective analysis on the whole dataset, checking their efficiency in detecting historical storms
(i.e., with damages) without finding too many false positives. Most of the rules formerly studied, except the
ones using wave period only as wave parameter, were able to correctly perform this task. Coastal flood event
return periods (RP) were then calculated by applying three of the best rules identified previously. The rule using
non simultaneous maxima of wave energy flux and water level gives encouraging results for the RP values.
Nevertheless, the discrepancy still observed among the different rules calls for further work in this direction.
Keywords
coastal flooding ; return period ; storm ; event definition ; damage ; historical data ; risk ; Basque coast
1
Universit
´
e de Pau et des Pays de l’Adour, E2S UPPA, SIAME, Anglet, France
2
Universit
´
e de Pau et des Pays de l’Adour, E2S UPPA, LMAP, Pau, France
3
UMR LIENS, La Rochelle, France
*Corresponding author: florian.arnoux@univ-pau.fr
1. Introduction
Mitigating coastal flooding is a common concern of coun-
tries with maritime borders. Whereas the problem is more
and more acute due to the growing coastal population and
associated infrastructures [
1
], climate change also increases
pressure on the coast by sea level rise which allows the ocean
to reach usually protected areas [
2
]. Nations cope with this
problem by developing coastal flood management plans, for
which, one important task is the identification of coastal zones
at flooding risk [
3
]. The objective is usually to delineate a
flooding line associated with a given return period (RP). If the
methodologies developed over the years to build these risk
maps may differ from one nation to the other, the processes
that have to be taken into account are generally common, even
though some process may be predominant in one location
compared to the other. In this framework, common flooding
processes include water level variations due to sea level rise,
tides, surge and wave set-up as well as to higher frequency pro-
cesses such as run-up due to infragravity and incident waves
and/or overtopping.
The definition of coastal flood return period (RP) is in-
timately related to the notion of event as described in [
4
],
[
5
] and [
6
]. As explained in theses papers, RP calculation
should first apply a physical declustering on the initial auto-
correlated time series of observations in order to obtain an
independent and identically distributed (i.i.d.) sample of an
event-describing random variable. The second step consists
of determining the statistical threshold above which the ex-
ceedances of the event-describing variable may be modelled
by a suitable statistical distribution, usually the Generalized

Coastal flooding event definition based on damages: Case study of Biarritz Grande Plage on the French Basque coast 2/22
Pareto Distribution (GPD). The physical declustering step is
crucial as it will totally define the meaning and the value of the
RP calculated. It should therefore be adapted to the studied
phenomenon and to the end-users of the study.
At this stage an important distinction has to be made be-
tween source, response variables and impacts. A given storm
is characterized by source variables such as waves, water level,
wind, atmospheric pressure and induces various responses at
the coast such as flooding, overtopping, wave loading, in turn
resulting into various impacts (erosion, damages on infrastruc-
tures, fatalities). Note that erosion can be seen as a response
variable as well as an impact.
Various declustering methods can be found in the litera-
ture. They differ depending on the impact and the number of
source variables considered and the way the source variables
are processed. For instance, [
7
], studying erosion as impact,
based the definition of the events on a wave height exceedance
criterion, the events thus defined, being then complemented
with other variables such as wave covariates and water level
reduced to the surge component. In this case, the event is
defined through a threshold on a unique source variable. But
source variables can also be combined to form a structural
variable or response variable. For instance, [
5
] applied a
declustering procedure to a response function built by sum-
ming the sea level to the nearshore wave height. Total water
level including run-up and set-up is also often used [
8
]. A mul-
tivariate threshold can also drive the declustering procedure.
[
9
] used a bivariate threshold in which an event is defined
when both the significant wave height and the meteorological
surge exceed given values.
As formerly stressed, the value and meaning of the RP may
be highly sensitive to the event definition procedure which
should, therefore, be related to the impact it is supposed to
characterize. Stakeholders being mostly concerned by the
impacts to the coast and population, RP should reflect this
aspect in applied studies. The literature nevertheless shows
that the link between the event definition procedure and the
impact (i.e., erosion, infrastructure damages, etc.) is often
vague or based on general considerations. Nevertheless, this
question must be in some way addressed in order to better
justify crucial choices usually made in the event definition
procedure as the combination or characteristics (i.e., peak
value, integral value, etc. ) of source variables.
In that context, historical data can help better understand-
ing the complex relationship between source variables and
impact. The interest on historical data in the coastal flood-
ing has been acknowledged mostly in the last ten years even
though few older references exists [
10
]. The multiple very
recent references show the growing interest of the community
for using historical data in coastal risk prevention. As exam-
ples, [
11
] combined historical data with numerical hindcast
to investigate the recurrence of major flooding in the central
part of the Bay of Biscay; [
12
] presents a database on 329
coastal flooding events reported in the period 1915-2016 and
ranked using a multi-level categorisation; [
13
] proposes a
historical analysis of the occurrence of storms and their dam-
age intensity from The Middle Ages to the 1960s at several
study sites along the European coasts; [
14
] gathered and used
extensive historical data at the site of Dunkirk (France) to
quantify extreme sea levels and skew surges, and [
15
] joined
historical, statistical and modeling approaches to characterize
past flooding events at the city of G
ˆ
avres in France. Note
that a french Working Group named ”Tempetes et Submer-
sion Marines” and led by the Institut de Radioprotection et
de S
ˆ
uret
´
e Nucl
´
eaire (IRSN) has recently built a database of
historical storms in France, with a qualification of both the
sources and the damages [16].
This paper is an attempt to include impact in the procedure
of coastal flooding event definition. The idea developed is
to test several rules based on the source variables to explain
damages reported in a historical dataset. Based on the se-
lected optimized rules, RPs are finally computed in order to
illustrate the sensitivity of the rules with respect to the vari-
ables they involve. This methodology is applied to a local site
study, Biarritz on the Basque coast (SW of France), for which
historical and oceanographical datasets have been constructed.
The structure of the article is as follows :
In section 2, the damage, wave and water level databases
are first described. Then, the candidate damage rules,
involving source and structural variables, are presented
as well as the associated processing (i.e., calculation of
maximum value, mean value, etc.).
In section 3, the different rules are tested against the
damage historical data and the best ones are retained. A
validation stage consists then of verifying the ability of
the rules to not generate false positives when blind run-
ning. Subsequently, return periods of historical storm
events are calculated using the damage rules obtained
previously.
Finally, results are discussed in section 4.
2. Material and methods
2.1 Site description
The Basque coast (Fig. 1) is a 200 km rocky coast facing
the Atlantic Ocean and stretching from the north of Spain
to the south west of France. In this paper, we specifically
focused on one site : the so-called Biarritz “Grande Plage”
(Fig. 2), a touristic seaside resort where hotels, casino, infras-
tructures are often damaged by storms. The area is indeed
submitted to a highly energetic wave climate. A wave buoy
installed since 2009 off Biarritz in 50 m depth allows to assess
this climate. Statistics computed from 9 years data avail-
able on the Candhis internet web site (
http://candhis.
cetmef.developpement-durable.gouv.fr
) show
that the wave incidence is almost completely restricted to the
west-north west and north west angular sector. Extreme events
can be illustrated by wave height return periods. The data
shows that the significant wave height
10
-year return period

Coastal flooding event definition based on damages: Case study of Biarritz Grande Plage on the French Basque coast 3/22
at this particular location lies between
8.43
and
9.63
m with a
70%
confidence percentage. For these sea states, the expected
peak period is about
16
s. The Grande Plage beach is a mesoti-
dal beach with
4.5
m spring tidal range around a mean water
level of
2.64
m (Charts datum). Due to the bathymetry in
this area, in which deep water is relatively close to the shore,
storm winds do not generally account significantly to surges,
which are completely driven by atmospheric pressure varia-
tions. The Grande Plage beach is an intermediate-reflective
beach with a mean sediment grain diameter equal to
0.3
mm
and typically exhibiting a steep foreshore slope of
8 9%
and
a gentle nearshore slope of
2 3%
[
17
]. Finally, the water
front is composed of a waterfront boardwalk (Figure 2 right)
located at 7.65m CD.
2.2 Dataset
2.2.1 Damage database
A database reporting flooding events and associated dam-
ages was collected at the French Basque coast scale by a
research in archives for the period 1950-2014 [
18
,
19
,
20
].
The main source of data comes from systematic investiga-
tions in the national and local press, and in particular in
the regional newspaper Sud-Ouest, as well as in the pub-
lic archives kept by the government representatives, public
bodies and local authorities. The methodology, see [
20
] for
a detailed description, was inspired by the French project
COCORISCO (
http://www.risques-cotiers.fr/
fr/projets/cocorisco
) on coastal risks and built on
previous experiences research and mapping of historical archives
on disasters conducted by part of the team. An assessment
of the probability of flooding and associated damage was car-
ried out. As already mentioned, in this paper we focus on
the Grande Plage in Biarritz, for which the most continuous
series of information has been collected. We tried to find
some evidences of flooding or non-flooding, the term flooding
referring to the flooding of the waterfront boardwalk (Fig. 2).
The events that have supposedly caused flooding at the Grande
Plage were marked by an index of 1 and 0 in case of no flood-
ing. A confidence index (1: reliable, 0: low reliability) has
been added to this information for more relevant statistical
processing. The same analysis was carried out for the intensity
of the damage. A three-level scale (2 for significant damage,
1 for moderate damage and 0 for zero damage) was chosen as
the most appropriate, taking into account the accuracy of the
information found. A confidence index on this value has also
been added for further processing.
The difference between level 2 and level 1 damage events
was made by analyzing the historical documents and trying
to find explicit proofs of the damage extent. The level 2
category is characterized by significant damages evidences
(photographs, repair quotations, etc.) to the buildings and
especially to the glass windows of the casino or the hotels
lying along the waterfront. Events were classified as level 1
when light damages are reported, for instance public bench
broken, light damages on the road or the seawall but not on
the buildings. Finally, note that waves and water level data
(see section 2.2.2) were used a posteriori in order to eliminate
storm events from the damage database (Table 1), for which
damages were likely not caused by flooding (i.e., rather by
wind or rain). For this, the values of the empirical cumulative
distribution function (cdf) (see equation 4 for the definition)
for wave height, water level and run-up were computed over
the event period when there was a doubt on the origin of the
damages. This concerns only a few cases as for several others,
the flooding is obvious from photographs. The examination of
the cdf values is efficient in discriminating the event as most of
the events exhibits very high cdf values close to the maximum
(typically around 0.99) except a few ones, for which cdf values
are only average. The latter were removed from the database.
Note that this method may not be totally justified for sites
involving coastal defences. Indeed, in this case, the failure
of a critical structure during an event may lead to damages
even for moderately energetic conditions [
15
]. Nevertheless,
as La Grande Plage faces the main energetic wave directions
without any intermediate protective structures, the procedure
described is relevant.
The Biarritz Grande Plage database is summarized in
Table 1. Information indicating whether or not the event was
classified as a natural disaster (i.e., CATNAT statement) are
also added to complete this database.
The number of storms, for which only Biarritz was men-
tioned is 30 and the number of flood events at the Grande
Plage is 13, which represents one third of the storms observed
over the period 1950-2014. Of these flooding events, only
2 are considered as unreliable. With regard to non-flooding
events, the information is most often considered unreliable.
In 2 events only, flooding is excluded with confidence. With
regard to damages, 5 events generated a level damage 2 with
good reliability and 4 caused moderate damage with only
one assessment considered unreliable in this category. The
remaining storms did not cause any damage with a good level
of confidence. The storms that caused the main damage (level
2) due to flooding are the storms of December 28-29 1951,
November 9-10 2010 and the recent events of the winter of
2013-2014 including the so-called Hercules (January 4-7),
Nadja (February 1-2) and Christine (March 3-5) storms.
In the present paper, only the information on damages was
finally used in the analysis. Indeed, the additional information
on the occurrence of flooding and the associated confidence
level, is rather scarce due to the low confidence level associ-
ated to this information (only 5 new events are associated to
good confidence : 3 flooding and 2 no flooding events). In
particular there are not enough trusted storm events with no
flooding to allow defining a clear separation between flooding
and no flooding events.
2.2.2 Wave and water level data
In addition to the former damage database, a corresponding
hazard database composed of wave and water level data and
covering the same period was also established [
21
]. Water
level data was built from tide gauge observations collected at

Coastal flooding event definition based on damages: Case study of Biarritz Grande Plage on the French Basque coast 4/22
Storm Start End Duration Flooding confidence Damage confidence CATNAT recognition
name date date (days) occurrence intensity
1 12/28/1951 12/30/1951 2 1 1 2 1 NA
2 14/12/1958 15/12/1958 1 0 0 0 1 NA
3 12/11/1961 14/11/1961 2 0 0 0 1 NA
4 17/01/1965 20/01/1965 3 1 1 1 1 NA
5 30/10/1967 05/11/1967 6 1 1 1 0 NA
6 11/12/1968 17/12/1968 6 1 1 0 0 NA
7 20/01/1972 22/01/1972 2 1 0 0 1 NA
8 06/02/1974 08/02/1974 2 1 0 0 1 NA
9 24/01/1984 25/01/1984 1 0 0 0 1 NO
10 30/01/1988 01/02/1988 2 0 0 0 1 NO
11 25/02/1989 27/02/1989 2 0 0 0 1 NO
12 30/01/1990 01/02/1990 2 1 1 1 1 30/1/1990
13 07/02/1996 09/02/1996 2 0 0 0 1 NO
14 28/12/1998 02/01/1999 5 1 1 1 1 NO
Martin 27/12/1999 31/12/1999 4 0 1 0 1 25-29/12/1999
16 06/11/2000 10/11/2000 4 0 0 0 1 NO
17 07/12/2000 09/12/2000 2 0 1 0 1 NO
18 14/08/2008 16/08/2008 2 0 0 0 1 NO
Klaus 23/01/2009 25/01/2009 2 0 0 0 1 24-27/1/2009
20 09/11/2010 11/11/2010 2 1 1 2 1 8-10/11/2010
Quirin 15/02/2011 17/02/2011 2 0 0 0 1 NO
22 18/02/2011 23/02/2011 5 1 1 0 1 NO
23 13/12/2011 18/12/2011 5 0 0 0 1 NO
24 26/01/2013 31/01/2013 5 0 1 0 1 NO
25 09/02/2013 14/02/2013 5 0 0 0 1 NO
Dirk 23/12/2013 26/12/2013 3 0 0 0 1 NO
Hercules 04/01/2014 08/01/2014 4 1 1 2 1 6-7/1/2014 (recognized after trial)
Nadja 01/02/2014 03/02/2014 2 1 1 2 1 2/2/2014
Andrea 26/02/2014 03/03/2014 5 0 0 0 1 NO
Christine 03/03/2014 06/03/2014 3 1 1 2 1 4/3/2014
Table 1. Database of damages related to coastal flooding in the Biarritz Grande Plage. When the events correspond to known
storms, their name is used if not their number. The dates and durations are either extracted or inferred from the press and
archives. The Confidence indices are coded as 1 (confident) or 0 (not confident). Damage is coded in intensity 0 (weak/absent),
1 (moderate) or 2 (strong) and flooding occurrence is coded by 1 (presence) and 0 (absence). The last column indicates the
storms recognized as CATNAT and the corresponding dates. The state of natural disaster (natural catastrophe = CATNAT) is a
situation whose recognition in France by the Ministry of the Interior allows for the systematic compensation of victims of
damage caused by various natural agents. This procedure was established in 1982.

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Abstract: The evaluation of coastal damage caused by storms is not straightforward and different approaches can be applied. In this study, damage caused by extreme storms is evaluated at a regional scale based on news information published in regional newspapers. The data derived from the news are compared with hydrodynamic parameters to check the reliability of this methodology as a preliminary” fast approach” to evaluate storm damage and to identify hotspots along the coast. This methodology was applied to the two most extreme storms ever recorded along the Spanish Mediterranean coast, which occurred in January 2017 and January 2020, severely impacting the coast and causing significant community concerns. The news information from different media sources was processed and weighted to describe the resulting erosion, inundation, sand accumulation, and destruction of infrastructures. Moreover, an accuracy index for scoring the quality of the information was proposed. In spite of some limitations of the method, the resulting regional coastal hazard landscape of damage provides a rapid overview of the intensity and distribution of the damage and enables one to identify the location of potential hotspots for the analyzed extreme storm events. The results show that estimated damage intensity is better related to maximum wave energy than cumulative wave energy during a storm, and that beach characteristics should also be included for understanding the distribution of coastal damage.

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