the grand popo beach 2013 experiment, benin, west africa: from
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Journal of Coastal Research, Special Issue No. XX, 2014
The Grand Popo beach 2013 experiment, Benin, West Africa 1
The Grand Popo beach 2013 experiment, Benin, West Africa: from
short timescale processes to their integrated impact over long-term
coastal evolution
Rafael Almar†, Norbert Hounkonnou‡, Edward J. Anthony∞, Bruno Castelle§, Nadia Senechal§, Raoul
Laibi+, Trinity Mensah-Senoo#, Georges Degbe@, Mayol Quenum‡, Matthieu Dorel†@, Remy Chuchla†‡,
Jean-Pierre Lefebvre†, Yves du Penhoat†‡, Wahab Sowah Laryea#, Gilles Zodehougan, Zacharie Sohou@
and Kwasi Appeaning Addo#, Raimundo Ibaceta†* and Elodie Kestenare† †IRD-LEGOS
Université Paul
Sabatier/CNRS/CNES/IRD
Toulouse, France
‡ University of Abomey-Calavi
ICMPA-UNESCO Chair
International Chair in Mathematical
Physics and Applications
Cotonou, Republic of Benin
∞ Aix-Marseille Université, IUF,
CEREGE, UMR 34 Europôle de
l’Arbois, B.P. 80,13545 Aix en Provence
cedex 04, France
§EPOC
Université de Bordeaux/CNRS
Talence, France
+ Université Abomey Calavi
Département des Sciences de la Terre,
Faculté des Sciences et Techniques
Cotonou, Republic of Benin
# University of Ghana
Department of Marine and Fisheries
Sciences,
Accra, Ghana
@ IRHOB
Cotonou, Republic of Benin
*Universidad Técnica Federico Santa
Maria, Valparaíso, Chile
INTRODUCTION Multi-scale coastal evolution is poorly known, mainly due to
lack of observational datasets, and this is even more pertinent on
tropical coasts. Understanding the processes responsible for
coastal evolution at different timescales is crucial for socio-
economic development, especially in the present context of
vulnerability to climate change. 80 % of the economic activity of
the Gulf of Guinea countries is concentrated along the coast,
together with large population densities (main cities: Abidjan,
ABSTRACT
Almar, R., Hounkonnou, N., Anthony, E., Castelle, B., Senechal, N., Laibi, R., Mensah-Senoo, T., Degbe, G., Quenum,
M., Dorel, M., Chuchla, R., Lefebvre, J-P, du Penhoat, Y., Laryea, W.S., Zodehougan, G., Sohou, Z., and Appeaning
Addo, K., and Kestenare, E., 2014. The Grand Popo beach 2013 experiment, Benin, West Africa: from short timescale
processes to their integrated impact over long-term coastal evolution. In: Green, A.N. and Cooper, J.A.G. (eds.),
Proceedings 13th
International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue
No. 66, pp. xxx-xxx, ISSN 0749-0208.
The first large nearshore field experiment in the Gulf of Guinea was conducted at Grand Popo Beach, Benin, in
February 2013, on an open wave-dominated micro- to meso-tidal coast, located mid-way between Cotonou and Lome
harbours. The overall project aims at understanding at multi-scale (from event to interannual) the causes of the dramatic
erosion observed throughout the Bight of Benin, and caused by the interaction of a large littoral drift with human
engineering works. Grand Popo 2013 experiment was designed to measure the processes over the short term and to test
the ability of an installed video system to monitor the evolution of this stretch of coast over the longer term. The beach,
characterized by a low-tide terrace and a high tide reflective part, experiences a long swell (Hs=1.6 m, Tp=16 s, oblique
incidence ~15-20°). Topographic surveys showed a double beach cusp system interaction and repeated surf-zone drifter
runs revealed high flash and swash rip activity driven by wave dissipation over the terrace and energetic swash
dynamics at the upper reflective beach. Swash was measured over a cusp system at two locations using video poles.
Wave reanalyses (ERAInterim) were used to determine the wave climate and its variability, and to quantify sediment
transport. This robust methodology is thought to be replicated elsewhere in different coastal environments in West
Africa, in particular with the objective to monitor various sites within the framework of the new West African Coastal
Observatory.
ADDITIONAL INDEX WORDS: low-tide terrace, long swell, micro-meso tidal environment, beach cusps, energetic
swash, wave reflection, littoral drift, Gulf of Guinea, erosion
www.JCRonline.org
www.cerf-jcr.org
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DOI: 10.2112/SI65-xxx.1 received Day Month 2013; accepted Day
Month 2013.
Journal of Coastal Research, Special Issue No. XX, 2014
2 Almar, et al.
Accra, Lomé, Cotonou, Lagos). Erosion in Cotonou currently
reaches 10 m/year (Dossou and Glehouenou-Dossou, 2007) due to
recent coastal constructions (i.e. harbour jetty, see Addo et al.,
2011, Angnuureng et al., 2013; Laïbii et al., this issue). It is
important to move towards more integrated coastal zone
management, and a first step is to adequately characterize the
natural coastal system, the prevailing processes and their
integrated effects. In this framework, an international project has
been designed to better understand the processes responsible for
the observed erosion and monitor the long term evolution of this
stretch of coast.
We believe that our observations are crucial, not only to
understand the causes of erosion in the Gulf of Guinea (Anthony
and Blivi, 1999; Blivi et al., 2002; Blivi and Adjoussi, 2004), but
to improve our fundamental knowledge on the morphodynamics
of low-tide terrace beaches (Wright and Short, 1984). These
beaches are mostly described in the literature for micro-tidal and
fairly low energetic waves and remain poorly documented in
energetic swell-wave dominated environments. As such, our
project has the following specific objectives:
1) Describe nearshore wave-driven circulation (scale L~10 m,
T~hour). This is conducted through the characterization of both
alongshore currents and more variable cross-shore currents (e.g.
flash and cusp rips), the influence of relative tide, wave
conditions, and interactions between swash and surf zone
processes.
2) Explore the small-scale swash zone hydro-morphodynamics
(L~metre, T~second). How is wave energy transferred to lower
frequencies, and energy dissipated and reflected in the swash zone
(see the current research challenges proposed by Brocchini and
Baldock, 2008)? This includes the study of individual swash and
swash-to-swash interactions and the influence of tide in generating
beach slope changes in the course of the tidal excursion. The
objective is also to be able to monitor bed evolution at the swash
event scale, and wave groups, and link these events to longer-term
beach profile evolution.
3) Characterize beach cups dynamics and beach profile
evolution (L~10 m, T~hour to day). Routine field observations
show that the Gulf of Guinea beaches remain most of the time in
the Low Tide Terrace state, following the beach morphodynamic
classification of Wright and Short (1984) and the dominant
variability is due to the upper beach alternating from rather
alongshore-uniform to a periodically steep morphology. The
conditions for these transitions and the mechanisms involved are
still poorly known, as well as the influence of antecedent
morphology; this includes, for instance mechanisms involved in
the construction and interaction of stacked beach cusp systems
(Vousdoukas, 2013).
4) Establish at a longer timescale the link between wave
forcing, littoral drift and morphological evolution (L~10 km, T~month to decades). The understanding of the integrated effect
of short-scale processes is crucial although linking these scales
remains a challenging task. To address this aspect, we provide a
general description of the Gulf of Guinean morphological system
and its wave forcing. This includes an analysis of the strong
wave's variability at seasonal and interannual scales, as well as
littoral drift.
In this paper we provide an overview of the project, including
objectives, field experiment set-up, long term video monitoring
system, and main achievements up to now. Closely related to the
project are three other papers. The first gives a general overview
Figure 1. The Bight of Benin coast. a) regional map, with red and yellow crosses indicating respectively the study site at Grand Popo,
and the city of Cotonou b) part of the city of Cotonou showing the main harbour breakwater with updrift accretion and downdrift
erosion, c) Grand Popo beach and d) a picture of Cotonou showing beach erosion.
Journal of Coastal Research, Special Issue No. XX, 2013
of the Bight of Benin beach system (Laïbi et al., this issue),
whereas the other two address, respectively, the dynamics of
double beach cusps (Senechal et al., this issue) and the occurrence
of flash and swash rips (Castelle et al., this isssue) at the Grand
Popo beach experimental site, described below.
STUDY SITE The field study was conducted in 2013 at Grand Popo beach
(6.2°N, 1.7°E, Figure 1) in Benin, Gulf of Guinea, midway (80
km) between Cotonou (Benin) and Lomé (Togo). Like much of
the Bight of Benin coast, Grand Popo beach bounds a sand barrier
system extending from Ghana to Nigeria (Anthony and Blivi,
1999; Anthony et al., 2002). Grand Popo is representative of the
beaches of the Bight of Benin. This bight coast is an open wave-
dominated and microtidal environment (from 0.3 m to 1.8 m for
neap and spring tidal ranges, respectively) exposed to long period
swells (ECMWF 1957-2013 annual wave averages: Hs=1.36 m,
Tp=9.4 s, Dir=S-SW) generated at high latitudes in the South
Atlantic. The beach (Figures 2 and 3) presents an alongshore-
uniform low tide terrace (LTT) and a steep and rather alongshore-
uniform lower beachface and persistent upper beachface cuspate
morphology cut into a well-developed berm. The grain size is
medium to coarse (D50 = 0.6 mm). An eastward littoral drift of 0.8
to 1.5 m3/yr has been reported in the literature (Tastet et al., 1985;
Anthony and Blivi, 1999; Blivi et al., 2002), driven by persistent
oblique long swells year-round.
DATA
Grand Popo beach 2013 experiment As a first step in setting up a robust methodology to be
replicated, a field experiment was conducted at Grand Popo beach
from 17 to 28 February 2013. The experiment was designed both
to measure beach changes at the short timescale and to test the
applicability of a low-cost video monitoring system. Incident
waves were measured with an acoustic Doppler current profiler
(ADCP RDI WORKHORSE) moored on the shoreface at a depth
of 8 m (Figure 3). Surf zone currents were measured by means of
repeated human operators acting as drifters (~10 runs per series)
daily, at selected moments, over the entire experiment duration
(see Castelle et al., this issue, for a description of the method).
Swash was measured over a beach cusp system (~30-m
wavelength, Figure 2) along two cross-shore transects, in a cusp
bay and horn. This was done using conventional pressure sensors
(10 NKE SP2T10-SI) and a new technique of dense arrays of
poles deployed on the beach. Measurements were conducted daily
over daylight hours. Water surface and bed vertical levels were
monitored using 2 high frequency video cameras HD (SONY
HDR-CX 250, 30 Hz, 1920x1080 pixels, Figure 4). These levels
are detected through colour band divergence at the transition
between the pole painted in black, the reddish beach sand, and the
white-blue water (Figures 4.b, 4.c). A large-scale bathymetric
survey was also conducted on the last day of the experiment using
an echosounder (GARMIN GPS MAP526S) mounted on a fishing
Figure 2. a) Grand Popo video system and b) a 15-min time-averaged image. Black lines stand for the time stack image locations (2
cross-shore and 1 alongshore), red line is the shoreline automatically detected using method described in Almar et al., 2012a. Pole
arrays in the instrumented cusp (horn and bay) are visible in the left part of the image.
Figure 3. Alongshore-averaged bathymetric cross-shore profile at
Grand Popo. Z = 0 m, the mean water level (solid blue line),
dashed blue lines are spring high and low tide levels (±0.8 m).
Dashed black lines stand for local slopes.
a) b)
Journal of Coastal Research, Special Issue No. XX, 2014
4 Almar, et al.
boat (Figure 3). Elevations were corrected from the tidal
elevations of the ADCP measurements and the Cotonou harbour
gauge. Beach surveys were performed daily using a GPS
(TOPCON, post-processed using PPP - Precise Point Positioning,
see Ebner and Featherstone, 2008). Atmospheric data (wind and
pressure) are provided by the Cotonou Airport weather services
(http://rp5.ru/).
Permanent video system In parallel, a long-term video station was deployed (camera
VIVOTEK IP7361, 1600x1200 pixels, Figure 2.a) on a 15 m-high
semaphore belonging to the navy of the Republic of Benin, 80-m
distant from the shore (i.e. which is the approximate beach width).
This autonomous system is powered by a solar panel. A computer
on site processes the raw images and store data. Three types of
secondary images are stored: instant, 15-min averages (Figure 2.b,
and time stacks (Figure 6). 3 time stacks are generated, 2 across-
shore, to cope with alongshore non-uniformity, and 1 alongshore,
offshore of the low-tide terrace, at the breaker point (Figure 2).
This station gives continuous and long-term information on beach
morphological change as well as on waves, tides and currents.
During the experiment, 2 Hz raw videos were stored.
Rectification of images from pixels into real world coordinates
was accomplished by direct linear transformation using GPS
ground survey points (Holland et al., 1997) after a correction of
the lens radial distortion (Heikkila and Silven, 1997). Although
varying somewhat throughout the field of view, the pixel footprint
was less than 0.1 and 0.05 m in the region of interest (surf-swash
zones of the instrumented cusp) for cross-shore and alongshore
direction, respectively.
Offshore wave data Wave data (Total Swell, wind waves, Hs, Tp and Dir) in deep
water were extracted from the ECMWF ERAinterim reanalyses
(Dee et al., 2011) from the ECMWF data server
(www.ecmwf.int/research/era) which are on a 1x1° grid and based
on the WAM model. Data are available from 1979 to the present,
with a 6-hr temporal resolution.
Figure 4. Illustration of the high frequency video/poles measurements. a) Camera mounted on a tripod on the berm aligned with a
cross-shore transect of poles and results of a 30 Hz vertical time stack (~1 min) on poles located in the lower (b) and upper (c) swash
zone.
Figure 5. Average energy spectrum from the ADCP over the
experiment duration. Red dashed line is the shore normal
direction.
Journal of Coastal Research, Special Issue No. XX, 2013
PRELIMINARY RESULTS Various aspects of the experiment are discussed by Senechal et
al., (this issue) and Castelle et al. (this issue). Here, some of the
results recorded by the ADCP and obtained from the swash poles
are presented.
Figure 7 shows the long swell conditions at Grand Popo beach
from 22 to 26 March. A noteworthy element is that the long
distance between the swell generation zone in the South Atlantic
and Grand Popo beach results in the the maximum in peak wave
period Tp preceding that of the significant wave height Hs for
about 3 days, which is long compared to the lag generally
encountered elsewhere in smaller oceanic basins. Another key
specificity of this site is that the energy spectrum (Figure 5)
reveals a large shore reflection of incoming swell waves energy (8
s<Tp<20 s), the swash playing the role of a low-pass filter (see
Almar et al., 2012b). A time series (not shown) of this reflection
value denotes a large influence of both incoming period and tidal
level; the reflection peaking when the swash occurs on the steep
upper beachface and when breaking in the surf zone is reduced.
The recently developed method of video pole arrays (Figures 4
and 8, see also Lefebvre et al., this issue,for a description) shows
good skills at describing both bed and swash elevation. This set up
enables a description of the wave-by-wave impact of breakers on
the beach morphology, and swash energy frequency-transfer and
reflection. Large variations (of the order of 1 to 10 cm) of the bed
level are observed at periods longer than group periods (typically
5 - 10 min), suggesting a substantial morphological control on
swash-surf zone dynamics. This will be further investigated.
The first results on the wave climatology characterization using
reanalyzes indicate a strong seasonal and interannual variability.
Monthly swell and wind waves are significantly correlated at -0.2
with the South Antarctic Modulation (SAM) and at 0.4 with the
Intertropical Convergence zone (ITCZ), respectively.
CONCLUDING REMARKS The Grand Popo 2013 experiment, the first of such importance
to be conducted in the Gulf of Guinea, took place from 17 to 28
February 2013 and involved 17 scientists from 7 institutions in
Benin, Ghana and France. The experiment was aimed at
describing processes at event scale responsible for observed
dramatic erosion in the Gulf of Guinea and placed emphasis on a
description of the nearshore circulation (pulsations or rips),
morphological interactions between stacked cusps systems, and
swash hydro-and morphodynamics including energy dissipation,
reflection and bed level change. The experiment also involved the
testing of an original video monitoring system. At a larger scale,
empirical and quantitative (from ECMWF reanalyzes) results were
obtained, allowing a global description of the Bight of Benin wave
forcing and coastal behaviour.
It is still too early to draw conclusions on seasonal and
interannual patterns of the evolution of this stretch of coast, but
the quantitative information obtained from such permanently
operating low-cost coastal video systems is key to building long-
term robust systems of observation and analysis of beach
dynamics and long-term coastal stability in the region. Following
the success of this first system, similar video monitoring systems
are currently being deployed along the West Africa coast within
Figure 6. Secondary images from the video system. a) Cross-
shore and b) along-shore time stacks.
Figure 7. Hydrodynamic conditions recorded by the ADCP from
19 to 28 Feb. 2013. a) tidal elevation, b) significant wave height
Hs, c) peak period Tp and d) peak direction Dir. Red dashed line
stands for shore-normal incidence.
a)
b)
Journal of Coastal Research, Special Issue No. XX, 2014
6 Almar, et al.
the framework of the recently set up West African Coastal
Observatory (MOLOA) and ALOC-GG projects. The current
overall project methodology thus provides a basis for replicated
elsewhere in West Africa in the next years.
ACKNOWLEDGEMENT This study was funded by the French INSU LEFE and EC2CO
programs, the IRD (Action Incitative program), and the UNESCO
co-chair CIPMA. We benefited the ECMWF ERA Interim dataset
(www.ECMWF.Int/research/Era), made freely available. We are
greatly indebted to the naval services of Benin at Grand Popo for
their logistic support during the field experiment and for allowing
the installation of the permanent video system on the semaphore.
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