BREACHING VULNERABILITY OF COASTAL BARRIERS UNDER
EFFECTS OF TROPICAL CYCLONES:A MODEL STUDY ON THE
HUE LAGOON-VIETNAM
Thieu Quang Tuan
Water Resources University, Hanoi, Vietnam. E-mail: Tuan.T.Q@wru.edu.vn, T.ThieuQuang@tudelft.nl
Marcel J.F. Stive and Henk Jan Verhagen
Delft University of Technology, the Netherlands
Abstract: Under effects of tropical cyclones, the coast is subjected to attack both by surge and wave from
the sea and by flooding from the bay. These forces pose a serious breaching threat to natural sea-defence
works such as barrier spits, barrier islands, lagoon barriers, etc. on the coast. Unintended breaching can be
catastrophic, causing losses of human life and damages to infrastructures and to natural habitats.
Understanding of coastal breaching and breaching potential during tropical cyclone events is therefore of
great importance to the management of coastal flooding hazards as well as to the design of sea-defence
works. In this paper, a model study on breaching vulnerability of the coastal lagoon barrier of HueVietnam during the historical event of the tropical cyclone in 1999 is presented. In spite of scarceness of
available data, attempts were successfully made to simulate the barrier profile response and the barrier
breaching process under this extreme condition. It is learnt that seaward erosion and severe wave overwash
on coastal barriers during storms can considerably narrow and lower the barrier profile and thus exaggerate
significantly the barrier breaching potential. The model predictions for the barrier response to wave
overwash and the time-dependent breach growth seem realistic and in agreement with available
observation data.
Keywords: Barrier breaching, breaching potential, breach growth, lagoon barriers, wave overwash, river
flooding, storm surge, tropical cyclones.
1. CYCLONE-INDUCED BREACHING OF THE HUE BARRIER ISLAND
The coastal lagoon of Hue stretches over 68 km along the central coast of Vietnam (see
Fig.1). Its water surface of 22,000 ha, protected from the sea by a long natural sand barrier, is
a vast aquacultural sanctuary providing major income for over 300,000 local inhabitants. The
lagoon habitat is rich and unique in terms of biodiversity. The barrier island is also the
dwelling place for thousands of local households. Unfortunately, this beautiful coastal area is
highly vulnerable to flooding hazards due to its complex geographical conditions.
Hue shares a common topographic feature of the central region is that the coastal strip is
very narrow as it is formed by the coast-parallel mountain range on the west and the sea on
the east (see left map in Fig.1). The area is therefore subjected to impacts by both the sea and
rivers. In particular, it has a very short response time with regard to river flooding (6 to 7
hours).
The lagoon is highly dynamic with the entrance discharges from six rivers and with two
main outlets to the South China Sea. Because of its location within the cyclone basin in the
South China Sea, this coastal area is prone to attack by tropical cyclones (05 cyclones a year),
annually from June to November. Tropical cyclones are the major cause of flooding hazards
in the area. Statistical data indicate tropical cyclones were responsible for over 80% of
flooding events occurred in the region. The annual precipitation observed in the region is
exceptionally high, which is as a result of high activity of tropical cyclones and sometimes in
combination with one of the local climatologic features such as a cold front or monsoon.
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Because of steep terrain to the sea, heavy rainfall in the upper river catchment quickly results
in high flood level near the coast.
Prior to river flooding, on the sea side, the coast is under attack by high wave and surge
brought in by tropical cyclones. This flooding from the sea is immediate and acute, which
often leads to more catastrophic consequences compared to those caused by river flooding.
Severe flooding is an annual threat to the lagoon area during storm season, especially near
the coast. The barrier, which provides protection for the lagoon water, is exposed to frequent
attack both by flooding from river and by surge and wave from the sea. Therefore, the barrier
is under a serious threat of breaching. It is learnt from historical incidents that there are two
possible scenarios that cause the barrier to breach, in which these two forces can be
simultaneous or one promotes the other. In chronological sequence, the attack by wave and
surge is always in advance to that by river flooding.
In the first scenario, the barrier is pierced through from the sea side. The attack by surge
and wave on the barrier can significantly narrow and lower the barrier cross-section, creating
vulnerable spots. In severe storms, high surge and wave can even overtop and dig through the
barrier at depressed portions.
Breaching due to overtopping the barrier from river side is the second scenario. Due to the
quick-flood-response character, river flooding can peak at the lagoon side in a matter of days.
The discharge capacity of the main outlets is usually insufficient because of earlier
sedimentation processes. Also, the withdrawal of river flood to the sea is hampered by the
alongshore barrier and sometimes by possible high sea water level (surge). As a consequence,
the lagoon water is quickly elevated and overflows the barrier crest at vulnerable sections,
opening new outlets.
In both scenarios, earlier attack by surge and wave (seaward erosion and overwash) can
significantly increase the barrier breaching potential. This is a major promoting force of
coastal barrier breaching.
Fig. 1 Vietnam’ coast with high activity of tropical cyclones (right: 1998’s cyclone tracks).
Hue lagoon (left): a typical vulnerable area in the central coast.
In the past, breaching of the Hue lagoon barrier due to effects of tropical cyclones was
occurred several times, causing catastrophic consequences. The most recent serious incident
was by the tropical cyclone EVE in November 1999. The cyclone EVE made landfall over the
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central coast region in late October 1999. It was in fact a rather moderate storm by its wind
strength but its secondary impacts were fatal. The cyclone induced weeks of heavy rainfall in
a large affected inland area. The region experienced the worst flooding ever in 50 years. The
precipitation measured in Hue was 2288 mm, a new historical record. In the first days of
November 1999 most of the rivers in the central coast were above their highest alarm levels.
The water level of the Perfume River (main river of the lagoon) measured in 01/11/1999 in
Hue was 5.6 m above the mean sea level. Because of high inundation level, the lagoon barrier
was broken at six different sections. Among those the breach at Hoa Duan commune was the
largest with the final width of about 700 m and depth of 5-9 m (see Fig.2). In this serious
flooding event, 352 people were drowned and 25,056 houses were washed away.
Fig. 2 Satellite image of the Hoa Duan breach in 3/11/1999.
2. MODEL APPLICATION
2.1 Objectives
Break-out of the Hue lagoon is almost unavoidable at least at this moment. Better
understanding of this coastal breaching is therefore of crucial interest for the safety
management of the lagoon. From the historical incident in 1999, the model study should be
able to reconstruct the breaching incident at Hoa Duan commune concerning the following
aspects:
• Aggravating effects of seaward erosion and wave overwash on the barrier breaching
potential.
The model for this purpose is called LOBARM, a model of barrier response to wave
attack (low-crested barrier response model).
• Breaching process of the barrier due to flow overtopping (river flood or surge).
For this purpose, a barrier breach model is used (BARBRE).
For the safety management of coastal lagoons in general, the first objective will help
identify weak spots in the barrier so as to avoid unintended breaching by appropriate
protective measures.
Prediction of the time-dependent breach growth is of importance to the disaster mitigation
strategies such as in making a proper evacuation plan, flood risk analysis, and coastal design.
Also, intended breaching, which requires knowledge of breach growth, can be utilized to
alleviate the inland flood level where and when necessary. In the case of the Hue lagoon, it is
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worth considering this viable solution since the existing lagoon outlets have insufficient
capacity to discharge extreme river flood.
2.2 Description of models
2.2.1 Model of barrier response to wave attack LOBARM
Seaward erosion and wave overwash on coastal barriers during storms can considerably
narrow and lower the barrier profile and thus increase significantly the barrier breaching
potential. In this model (Tuan et al., 2006a), the barrier breaching potential is limited to only
the response caused by those seaward across-shore processes.
Modelling of the barrier response to wave attack follows a “wave-averaged” concept, in
which time-dependent wave-induced cross-shore transport and overwash are interrelated.
Wave-induced seaward erosion is based on the model approach of UNIBEST-TC (see
Bosboom et al., 2000). Wave overwash is modelled given the hydraulic input from the
seaward boundary. New wave overtopping parameters, instead of the conventional average
discharge, have been formulated according to an “event-based” approach to better describe
overwash strength and its discontinuous character.
The model is capable of predicting changes of the barrier cross-section during attack by
wave and surge in storm conditions. Also, the lateral growth of the overwash channel (width
increase) can be computed. The model was calibrated using laboratory experiments on wave
overwash.
2.2.2 Model of sand barrier breaching
The breaching model BARBRE (Tuan et al., 2006b) is a hydrodynamic morphological
breach model of sand barriers. The breach flow is primarily based on the unsteady open
channel flow equations. Also, the hydraulic jump is considered as part of the breach flow
conditions. In order to deal with discontinuity (hydraulic jump), mixed-regime flow as well as
abrupt bed level variations during breaching, a robust upwind numerical approach is adopted
to resolve the flow equations.
Effects of turbulence of hydraulic jump on the formation of the scour hole in the breach
channel are taken into account in the breach morphology. The model is capable of predicting
the time-dependent breach growth in both lateral and vertical directions. The model was
calibrated using laboratory breach data of Caan (Caan, 1996) and verified against data of the
Zwin’94 field experiment (Visser, 1998).
2.3 Situation before breaching and basic data inputs
One of the concerns in breach modelling in general is the availability and reliability of
breach data since the data acquisition is not a priority during disaster events. Also, there was
no real observation on the hydraulic and morphological conditions at this location particularly
on the sea side before and after the breaching incident. The most available data are about river
flooding from dozens of inland meteorological stations. To overcome this difficulty, in the
present study we attempted to make use as much as possible of all available data and
information from different sources such as from hind-cast models, eye-witnesses, photos, and
appropriate assumptions.
To do this, we first need to understand the beach morphological situation before the event.
Hoa Duan breach was a naturally weak section of the barrier island, where used to be the
location of an ephemeral river-mouth. Earlier field investigation reveals the Hoa Duan beach
was suffering from severe erosion due to lee-side effects from a system of groins constructed
in 1997 at the neighbouring beach. It shows that a large part of the seaward barrier profile was
swept away and the shallow foreshore in front was also gone. The deepened foreshore in
return had a further negative influence on the barrier, facilitating more severe wave conditions
in front of it.
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Eye-witnesses claimed that the barrier crest width at the breach section was about 50 m one
day before breaching. It is also evidenced from amateur photos and eye-witnesses that severe
overwash had drastically occurred weeks before breaching during the earlier attack by the
cyclone (see Fig. 3). As a consequence, the barrier at this location was markedly narrow and
lowered, which had become a vulnerable spot provoked breaching later on.
In summary, the Hoa Duan breach was in fact as a consequence of the following sequential
causes:
• Lee-side effects from the groyne field had narrowed part of the barrier and especially
cleared away its shallow beach foreshore.
• Partly because of deepened foreshore, severe wave overwash had occurred during the
cyclone EVE, remarkably narrowing and lowering the barrier cross-section. As a
consequence, a highly vulnerable spot was created.
• Overtopping the barrier at the vulnerable section by the sudden release of extreme river
flood (followed up the cyclone) to the sea.
To study the effects of wave overwash, we first need a beach profile at this location just
before the storm (pre-storm profile). The only available information on this is a bathymetric
survey map in late April 1999, about six months before breaching. At that moment, the
barrier at the breach section was 220 m wide above the mean tide and the averaged crest level
was 3.0m. It is assumed that the shape of the foreshore was remained the same as it was six
months ago, and the seaward barrier position is then shifted landward to fit the remaining
width of 210m (estimated erosion rate of about 20m per year according to visual observation
by the local authority). The pre-storm barrier profile is shown in Fig. 5 (initial profile).
Fig. 3 Situation of the Hoa Duan breach section in 31/10/1999, 03 days before breaching
Regarding the sea conditions, we utilized the results from a hind-cast model, which
anticipated the wave parameters and surge height, using the wind characters of the post storm
EVE. This hind-cast model study pertains to a national research program on rehabilitation of
the Hue lagoon (see NCKH-NN, 2001). The astronomical tide at Hue location is perfectly
semi-diurnal with the amplitude of 0.5m. The following sea conditions (Table 1) have been
estimated for the coast of Hoa Duan:
Table 1 Sea conditions during attack by the tropical cyclone EVE at Hoa Duan section
Parameters
Tidal amplitude (semi-diurnal)
m
0.50
Maximum surge height
m
1.50
Deep water incident wave height Hso
m
5.0
Peak period Tp
sec.
10.0
Effective storm duration
tidal cycle
4.0 (2.0 days)
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The sea water level, composed of surge and astronomical tide, is assumed to vary in a
cosine manner (see also Steetzel, 1993). The storm surge hydrograph is shown in Fig. 4.
π (t − tm )
2π (t − tm )
(1)
+ As cos 2
Z sea (t ) =
MSL + At cos
Ta
Ts
In which the first two terms on the right hand side describe the contribution of the
astronomical tide, the mean sea level MSL = 0.20m, the tidal amplitude (first order) At =
0.50m, and Ta =12.42 hours is the standard period of the semi-diurnal tide. The last term on
the right hand side denotes the surge effect with the magnitude As=1.50m, the effective storm
duration equals to four tidal cycles (about 2 days) Ts = 4Ta ≈ 50 hours. It is also assumed that
the maximum surge coincides with the maximum tide at t = tm = 1/4Ta = 3.105 hours.
Quantitatively, with such hydraulic conditions and the pre-storm barrier geometry, the
barrier would have been under attack by severe wave overwash.
Water level (m)
2.5
tide level
surge level
2
water level
1.5
1
0.5
0
-15
-10
-5
0
5
10
-0.5
15
20
25
30
Time (hours)
Fig. 4 Storm surge hydrograph
The barrier profile after attack by the storm is then used for simulating the barrier breach
erosion process. At the moment when the breach initiated, the lagoon water level was reported
to be at about 2.60 m. The breach development was not observed during and even right after
the event. It is not clear when the breach was considered to be ended but it was claimed that
the breach dimensions were about 600∼700m wide and 5∼7m deep. Several months later, this
gap measured 1,000 m wide in a bathymetric survey to design the gap closure.
2.4 Model results
2.4.1 Breaching potential by overwash
It was not able to determine exactly the starting moment of the storm. Also, as the purpose
is to investigate the barrier breaching potential by severe overwash. Therefore, simulation of
the barrier response to wave overwash was carried out for the most intensive day of the storm
(24-hour period in the middle of the storm surge hydrograph). The deep water wave
parameters were kept the same as shown in Table 1. The sand diameters were according to
samples taken from the breach site, with median d50 = 250µm and d90 = 300µm. Since the
dimensions of the initial overwash channel (initial depressed portion) were not known, it was
assumed in the simulation that this channel is sufficiently wide so only the development of the
barrier profile is considered.
Figure 5 shows the computation result of the barrier profile response to wave attack after 12
and 24 hours. It follows that the barrier was appreciably flattened out after 24 hours of wave
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overwash. The remaining barrier crest level was 2.65m, which is some what in agreement
with the water level when the breach was reported to start.
Fig. 5 Barrier profile response to wave attack during storm
In this case, wave overwash significantly increased the breaching potential of the barrier by
digging through it a channel of about 0.5m deep. Wave overwash had made the barrier break
sooner in the following river flood event.
2.4.2 Barrier breaching
In breaching of coastal lagoon barriers due to river flooding, the breach erosion process will
last until the end of the flooding. In the case of the Hoa Duan breach, the flooding and so the
breaching process lasted for about one week. However, it is believed that the major breach
dimensions were formed within the first day of breaching. After that the breach grew mainly
laterally with a lower speed.
In this paper, we present the simulation result of the breach development in the first 12
hours of breaching. With this breaching time, the full breach depth was almost developed.
In the simulation, the upstream (lagoon) water level was assumed to increase linearly from
2.60m at the starting moment of the breach up to 4.0m in the next 12 hours. These levels were
derived from satellite images in a study on flood modelling for Hue area (Villegas, 2004). The
downstream water level variation (sea level) was according to the tide hydrograph in Fig. 4.
For the computation of the breach lateral development (breach width increase), the
dimensions of the initial breach (depth and width) are required. For this, a channel of 0.5m
deep (based on the computed result of barrier response to wave overwash) and 25m wide was
assumed.
Figures 6 and 7 show the model predictions for the breach profile development at different
time levels. Also, the increase of the breach width in time is shown in Fig. 8.
It follows from the computation that the barrier was completely washed out after about 7
hours of breaching. The breach reached a maximum depth of about 7.50m after 12 hours,
which is also almost the final breach depth. Also at this time the breach widened to 350m.
However, the breach width continued to grow vigorously since the flow through the breach
was still strong.
Overall, the computation results are rather realistic and in agreement with the available
reported data on the breach dimensions.
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Fig. 6 Vertical breach development after 7 hours
Fig. 7 Vertical breach development after 12 hours
Fig. 8 Time-varying breach width
3. CONCLUDING REMARKS
Under high activity of tropical cyclones, the coastal area of Hue is subjected to attack both
by high wave and surge at sea and by flooding from rivers. These two forces are the main
causes to the breaching of the coastal lagoon.
Earlier, seaward erosion and severe wave overwash on the barrier can considerably narrow
and lower the barrier profile and thus exaggerate significantly the barrier breaching potential.
A breach in the barrier can then be initiated by waver overflowing (either surge or river
flooding) through a vulnerable spot, which is in most cases as a result of earlier wave
overwash.
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For better understanding of the mechanism causing the lagoon breakout, a model study on
the breaching vulnerability of the coastal lagoon of Hue under effects of tropical cyclones was
carried out with specific data and conditions of the historical cyclone EVE in 1999. The
situations before and after the storm were reconstructed. In spite of scarceness of available
data, attempts were successfully made to simulate the barrier response and the barrier
breaching process under this extreme condition.
The computation results seem realistic and are in agreement with reported data. The
numerical models of LOBARM and BARBRE can be used as predictive tools in assisting the
lagoon safety management.
For the future use, more breach data are necessary in order to calibrate and verify these
numerical models.
The present work can be considered as an example for studies on the barrier breaching
vulnerability in other coastal lagoon areas under effects of tropical cyclones.
ACKNOWLEDGMENT
This research was carried out under the framework of the coastal engineering cooperation project funded by
the Royal Netherlands Embassy in Hanoi between Delft University of Technology and Hanoi Water Resources
University.
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