1 Introduction

Estuary plays in an important role in the human life. There are 22 cities within the 32 largest cities in the world are in estuaries. Many estuaries are important centres of transportation and international commerce. Because of their great commercial and recreational importance, estuaries are often utilized excessively by a burgeoning coastal population. Approximately 60% of the world population now resides near the estuaries.

The regulation work and dredge or the combination of these two can be useful in stabilizing the navigation channel position, diverting water flow, blocking sediment, and minimizing the back-siltation. These methods have been widely used in the estuarine navigation channels over the world. In case of high intensity of back siltation, offshore port will be considered. In 1884, France began regulation work in the Seine Estuary. Two jetties are built along the bank having 60 km length. The distance between two jetties ranges from 300 to 500 m. This work continued until 1980 with a 9 km submerged breakwater constructed in the old North jetty and a dredging volume of 2.5 million cubic meters. The 10.6 m channel depth is designed for vessels 35,000 DWT to reach Rouen. In 1863, for the regulation of Rhine River for Rotterdam Port in Netherland, two jetties were constructed in two banks of the Rhine River. Other examples are the Columbia Estuary, southwest channel of Mississippi Estuary in America and Mersey Estuary in United Kingdom [1].

Lacroix et al., used numerical model to simulate investigated the effect of geotextile submerged breakwater on hydrodynamics in La Capte beach, France. The results shown that the presence of the breakwaters has changed the current at La Capte beach in the positive direction, and the significant wave height and current speed have been reduced to acceptable levels [2].

Many estuaries have been investigated by numerical and physical models, as it is very common to calibrate and validate these two methods by each other. It does not save cost or time, but may increase the effect and development speed of the navigation channel research [1]. Nguyen and Zheng proposed four regulation schemes for Dinh An estuary and used numerical model to investigated and analysed with respect to the behaviour of each scheme to improve the water depth in the navigation channel [3].

In this study, the influence of the offshore breakwater in TH port of Cua Lo estuary on hydrodynamics and wave transformation will be investigated by a couple numerical model.

2 Study Area and Methodology

2.1 Description of Study Area

TH port is in offshore of Cua Lo estuary, Nghe An Province, Vietnam. It is the most important estuary in Nghe An province. Cua Lo estuary will become a modern key project of special significance in economic development not only for Nghe An but also for Laos and north-eastern of Thailand. The existing port includes six berths for mooring vessel of 10,000 to 20,000 DWT. Based on the master plan period from 2021 to 2030 and vision 2050, there are three navigation channels to three ports such as ① Cua Lo port, ② DKC port and ③ TH port, and an offshore breakwater will be constructed in the northern of Cua Lo estuary (Fig. 1).

There are two ports of Vissai Nghe An in the north of Cua Lo estuary. It is a specialized mooring area with a general port and a container in which the general ports and containers for vessel of 30,000–50,000 DWT and up to 100,000 DWT, and international passenger vessel of between 3,000 and 5,000 seats, when conditions permit); Specialized wharves for export of cement, clinker, coal for vessel of 70,000- 100,000 DWT of international routes, specialized wharves for coal importation, additives, cement, clinker, vessel of up to 30,000 DWT in domestic and international; Specialized ports specialized in importing liquid petrol, oil, asphalt and petroleum products for vessels of up to 50,000 DWT and wharfs for 5,000 DWT [4, 5].

The TH port (number 3 in Fig. 2) is a priority project for construction for mooring vessel of 100,000 DWT and an offshore breakwater will be built to protect wave impact on the port and ensure the safety operation. The offshore breakwater was proposed by TEDIPORT with 2520 m length and crest level of 6.5 m (Chart Datum).

Fig. 1.
figure 1

Study area

Wind data was collected from Hon Ngu Island station from 1962 to 2020 with interval time of 3 h. The analysis result shows that offshore wind included two major seasons are NE monsoon and SE monsoon. The maximum speed of 56 m/s occurred in NNE direction on October 1st, 1964.Wind rose is plotted in Fig. 2, and indicated that the main wind direction for N direction with frequency of 25.85%, for NE direction with frequency of 7.51% in the winter and changes to be S direction with frequency of 8.75%, SE direction with frequency of 9.25% in the summer [4, 6].

Tidal in coastal of Cua Lo estuary is semi-diurnal with tidal ranges from 1.0 to 3.59 m [7]. Before Nghi Quang Dam was built, Cam River flow has significant in the formation and existence of tidal creeks in front of Cua Lo port, now with the presence of Nghi Quang Dam the effect of river flow can be ignored. The observation data from TEDIPORRT in December 2016 show that nearshore current speed of 0.65 m/s with NW direction in the NE monsoon [4, 6].

Nearshore wave depends on the offshore wind, the domination wave in NE monsoon are NE & N waves with the average signification wave height ranges from 0.7 to 1.0 m. In SE monsoon, wave prevailing in SE and SW directions. During storm NANCY (May 18, 1982) significant wave height was recorded of 6.0 m. The frequency of waves occurs as following NE waves of 18.4%, N wave of 15.42%, SE wave of 7.59% and SW wave of 5.16%, respectively [4]. Wave rose plotted in Fig. 4 is result of offshore wave collected from NOAA at the location of 1900N, 106015’ E from 1997 to 2016. The maximum wave height observer reached to 4.5 m in NE direction [4, 6].

Fig. 2.
figure 2

Wind rose in Hon Ngu Island station

2.2 Methodology

To investigate the influence of offshore breakwater of TH port in the hydrodynamic of Cua Lo estuary based on the two dimensions couple numerical model in a larger coastal area between Cua Lo and Cua Hoi estuaries. The couple model consists of Mike 21HD and Mike 21 SW models, these models were calibrated and verified of by Nguyen et al. (2021).

Mike 21 HD model used a study domain showed in Fig. 3. Study domain with two river boundaries are Cam and Lam rivers. Cam River boundary is far from the Cua Lo estuary about 2 km and Lam River boundary is far from Cua Hoi about 3.5 km. Two river boundaries are near to estuary and subjected to tidal regime. Therefore, tidal levels were used in these boundaries.

Mike 21 SW model used fully spectral formulation with stationary mode. A spectral-form empirical formula was used in initial conditions type. A wave fetch of 160 km and the maximum peak frequency was 0.4 Hz were setup for the JONSWAP fetch growth expression. Other JONSWAP’s parameters used the default values. Offshore boundaries were applied timeseries wave data obtained from NOOA, and storm wave with return period of 50 years collected from TEDIPORT (2016) was used in simulation of storm scenario.

Fig. 3.
figure 3

Study domain

Six scenarios of simulation will be carried out in which three scenarios simulate the natural condition and three scenarios simulate with the presence of offshore breakwater in climates condition such as NE monsoon, SE monsoon and storm with return period of 50 years. The results will be used to analysis the hydrodynamics and wave features in the Cua Lo estuary.

3 Results and Discussion

3.1 Hydrodynamic Features

In NE monsoon, during the flood tidal the current in the nearshore flow from north to south. However, in offshore due to the presence of offshore breakwater two circulations appeared, one occurred in the southwest of the breakwater and one in mid of coastal zone between two estuaries. The current behind the breakwater very small with the maximum speed reached to 0.08 m/s only. In Cua Lo estuary, the maximum flood current speed of 0.15 m/s is smaller than in Cua Hoi estuary of 0.35 m/s. The longshore current in the northern of Cua Lo is larger than in the estuary and Cua Lo beach. The current speed in Cua Hoi estuary reaches to 0.35 m/s (Fig. 4).

During the ebb tidal, the ebb current from Cua Lo estuary flow to the north while in Cua Hoi estuary still existing of the flood current. This phenomena is similarity with the result in the natural condition without offshore breakwater [6]. The current speed in the harbour basin of TH port is a bit smaller than other water areas. Longshore current from Lan Chau rocky headland to the south jetty in Cua Lo estuary and from the north jetty of Cua Lo estuary to the north very small only ranges from 0.02 to 0.05 m/s. The result also indicated that Hon Ngu Island take an important role for distribution of current field in the Cua Lo coastal and also Cua Lo estuary, the presence of Hon Ngu Island induced decreasing of offshore current during ebb tidal (Fig. 5).

Fig. 4.
figure 4

Flood tidal current field in NE monsoon

Fig. 5.
figure 5

Ebb tidal current field in NE monsoon

In SE monsoon during the flood tidal, the currents in the nearshore flow from the south to the north. The maximum ebb current speed in Cua Lo estuary is 0.41 m/s larger than in Cua Hoi estuary of 0.33 m/s. Longshore current along the coastal of Cua Lo beach ranges from 0.05 to 0.1 m/s (Fig. 6).

Fig. 6.
figure 6

Flood tidal current field in SE monsoon

During the ebb tidal, the nearshore currents also flow from the north to the south. A circulation appeared behind the offshore breakwater. Longshore current a bit higher than in flood tidal. The maximum ebb current speed in Cua Lo estuary is 0.25 m/s larger than in Cua Hoi estuary of 0.12 m/s (Fig. 7).

Fig. 7.
figure 7

Ebb tidal current field in SE monsoon

In the storm with return period of 50 years, during the flood tidal, the currents in the nearshore flow from the south to the north. The maximum ebb current speed in Cua Lo estuary is 0.41 m/s larger than in Cua Hoi estuary of 0.33 m/s (Fig. 8). During the ebb tidal, the nearshore currents also flow from the north to the south. The maximum ebb current speed in Cua Lo estuary is 0.25 m/s larger than in Cua Hoi estuary of 0.12 m/s (Fig. 9). In both flood and ebb tidal regime the circulation occurred behind the offshore breakwater, in mid of coastal between two estuaries and in front of Cua Hoi estuary.

The simulation results indicated that the new navigation channel and offshore breakwater induced a complicated coastal current in the study area with many coastal circulations. These circulations will be induced unpredictable sediment transportation and can affect the stability of the navigation channels in the study area.

Fig. 8.
figure 8

Flood tidal current field in the storm with return period of 50 years

Fig. 9.
figure 9

Ebb tidal current field in the storm with return period of 50 years

3.2 Wave Field

During NE monsoon, the NE wave was blocked by offshore breakwater and wave height in front of the gap between two jetties of Cua Lo estuary ranges from 1.2 to 1.4 m and reduced in the south of estuary and Lan Chau rocky headland. The coastal to Cua Lo estuary the wave height ranges from 0.8 to 1.2 m and increasing in the southern part. This result also indicated that the offshore breakwater has not taken significant in wave height distribution in Cua Hoi estuary in NE monsoon (Fig. 10).

Fig. 10.
figure 10

Wave heigh in NE monsoon

During SE monsoon, the NE wave was blocked by both of Hon Ngu Island and offshore breakwater. However, the present of the offshore breakwater has not taken significance in wave height distribution in Cua Lo estuary while Hon Ngu Island plays an important role in reducing wave height in Cua Lo beach and estuary during the SE monsoon. The wave height in front of the gap between two jetties of Cua Lo estuary ranges from 1.2 to 1.4 m and reduced in the south of estuary and Lan Chau rocky headland. The coastal to Cua Lo estuary the wave height ranges from 0.4 to 0.8 m, and in Cua Hoi ranges from 0.8 to 1.0 m (Fig. 11).

Fig. 11.
figure 11

Wave heigh in SE monsoon

In the storm condition, the present of the offshore breakwater take an important part in reducing the significant wave height in Cua Lo estuary and a half part of coastal area between two estuaries. The Hon Ngu Island also plays an important role in reducing wave height in Cua Hoi estuary. The wave height in front of the gap between two jetties of Cua Lo estuary ranges from 2.0 to 2.4 m and reduced in the south of estuary and Lan Chau rocky headland. The coastal to Cua Lo estuary the wave height ranges from 1.0 to 1.3 m, and in Cua Hoi ranges from 1.4 to 1.8 m (Fig. 12).

Fig. 12.
figure 12

Wave heigh in the storm with return period of 50 years

Table 1 presented the comparison wave height before (N-BR) and after (W-BR) construction of the offshore breakwater in four locations as show in Fig. 12. The results showed that in the Cua Lo estuary (location t1) the wave height reduced from 22.46 to 32.47%; the Cua Lo beach (location t2) the wave height decreased from 2.26 to 16.22%; the wave height behind the offshore breakwater reduced from 77.36 to 95.74% and in the Cua Hoi estuary from 4.48 to 10.95%.

Table 1. Comparison wave height before (N-BR) and after (W-BR) construction of the offshore breakwater

4 Conclusions

  1. 1.

    Offshore breakwater plays an important role in protecting the TH port in both of northeast monsoon and storm conditions.

  2. 2.

    Circulation occurs in both of Cua Lo and Cua Hoi estuaries and in the mid coastal area during storm.

  3. 3.

    The presence of offshore breakwater has good protecting of the TH port from wave impact. However, it may induce sedimentation in existing navigation channels to Cua Lo port due to the complex of hydrodynamic regime induce by the breakwater.