A protection method for the coast of Phan Thiet City, Vietnam

Vietnam is located in monsoon tropical region. That is received a lot of typhoons and tropical depresion annual. Coastal area has been erosion seriously from north to south of Vietnam recently. For developing social economic in the area, some coastal protection solutions have been applied away from natural and hurman influence. Phan Thiet belonging to Binh Thuan province is one of active cities in the South Central Coast. There has strengths in developing aquaculture, fishing and tourism. With a long coastline and beautiful beachs, Phan Thiet attracts a large number of domestic and foreign tourists any time of the year. Some coastal areas were constructed concrete wall or revetment to protect from erosion, however, that are damaged, depleaced or sinked by wave and natural impact. At present, government is supporting in recovering or rebuilt the new protection construction for creating lanscape at the coastal area. The research shows a solution revetment to protect the coastline that could be suiltable for local requirement of stability construction, serving tourism and community activities. That could reduce wave run up and foundation erosion, adapt to working condition of construction.


Introduction
The coast of Phan Thiet City is along 42km with sand dune, villages and coastal streets.The topography of the coastal seabed is steeply, the 2m deepth isoline close to the coast.In particular, the area has two large estuaries: Ca Ty River and Phu Hai River, as shown in figure 1 (Google map 2023).The coastline in the construction area has the main direction of northeast -southwest, almost parallel to the northeast monsoon direction.At the northeast area, there are jetties to protect Phan Thiet fishing port and Phan Thiet seafood industrial zone.These jetties act as a barrier against the northeast waves, but also reduce sediment movement along the coast.Before the jetties were built at the estuaries, in the dry season, the left bank of the estuaries appeared sand spit extending from the bank to the south, obstructing the access to the estuary.In the rainy season, large river flows can erode these spits and reopen the rivermouth.This evolution of estuary morphology is a characteristic of the estuaries in Binh Thuan province with effects of longshore current mainly from the Northeast to the Southwest.Actual developments also show that this area is strongly eroded in the dry season, with East or Northeast winds.The study area was a fairly flat topography, dense residential areas of Thanh Hai Ward, located along the coast.Previously, there was a widely sandy beach with the fisherman activities.In recent years, the coast has been carved deep into the residential area, creating an almost vertical slope, lot of houses have been destroyed, losing residential land.The purpose of study is choosing an appropriate solution to build a work that can be stable, protect the coast from erosion, contribute to the development of economic, tourism and community.

Natural condition
According to the survey results, the topography of the region is in the ranges from 0.0 m ÷ -1.0m elevation, the seabed slope is about 0.2%.The geological structure along the coast in order from top to bottom includes the following geological layers: Layer 1: Fine sand to medium sand, at the deepth from 2.5m to 5.5m, the thickness around of 4m.Layer 2: Gravel sand with seashell, the medium thickness of 2.25m.Layer 3: Mix clay, hard plastic, the thickness around 1.75m.The highest temprature is 37.6 0 c, the lowest temprature is 12.2 0 c.Rainy season is from May to December.The highest rainfall is 467mm.Phan Thiet area in particular and Binh Thuan province in general are influenced by the monsoon regime.
In the dry season, from November to April of the following year there is a northeast monsoon with prevailing wind directions being the East and Northeast, with a frequency of 20-40%, the speed of average wind 6 -9m/s.The maximum wind speed could be 31m/s.[1] In the rainy season, there is a southwesterly wind with the prevailing wind direction being West and Southwest, with a frequency of 30-40%, with an average wind speed of 4-6m/s.The tides in Phan Thiet bay belong to the mixed tidal regime because it is located in the transition zone between the uniform diurnal tide regime in the north (Qui Nhon) and the irregular semi-diurnal tide in the south (Vung Tau), but natural about the irregular diurnal regime.The high tidal range is 1.5m to 2.0m, usually occurs in the months of October, November and December.According to monitoring data and reference documents, some basic characteristics of water level and tidal amplitude in this area are shown as follows: -Average tide level: -0.11m -Maximum high tide level: + 1.12m -Minimum low tide level: -1.42m -Average tide range: 122cm -Maximum tidal amplitude: 222cm

Revetment design
The coast in the center of a tourism city as Phan Thiet is one of the places that citizen and managers pay the most attention to.A construction being built to protect the coast should be adapted a combinative requirement suitable with the community activities.The concrete block revetment was choosen for the coast at this area.The natural topography is low, the altitude of 2.2m to 2.5m.It is necessary a high crest of the revetment to avoid wave run-up or overtopping on the coast.The construction could break the lanscape and enviromental of coastal zone in the city.The revetment with wave overtoping on the crest is a suitable solution for protecting coast in the area.The discharge of wave overtopping allowed to overflow on the embankment depends on the quality of the structure protecting the crest, protecting the slope on the field side and the area that allows flooding on the residential side.This embankment is combined with a pedestrian traffic road with a self-inserting brick structure placed on a cement-sand base and a compacted buffer layer.Thus, the top of the embankment is considered as a solid protection with drainage ditches.Therefore, it is possible to choose the allowable overflow flow as q=20-50 (l/s/m), by the National Standard of Vietnam TCVN 9901-2014.The project area is characterized as a beach resort, behind the embankment are resorts and entertainment activities.It is recommended to choose the allowable overflow discharge of 30 l/s/m to limit the amount of water overflowing the crest to facilitate tourism exploitation.
So, [q]= 30l/s/m=0.03m3/s/m.The height of requirement on the sea water level is determined as equation 3.

Embankment crest elevation
2.0 <b.o 7.0 : The embankment crest elevation is defined of +3.0m, it is ensured with the wave overtopping discharge of q = 30l/s/m.That is suitable to the characteristics, nature and objectives of the area.project; therefore, it is recommended to be used as the design elevation for the embankment crest.

Concrete block calculation
The thickness of concrete block for revetment is defined as following [4].
In which: u indicates stability coefficient of the embankment, Concrete block Tsc178 => u = 2.5  is critical coefficient of the block being movement.That is determined as following: In there: Smax is the equilibrium scour depth.Ho is the deep-water wave height.Lo is the deep water wave length.h is the depth of water in front of the structure.
Using two rows of concrete pipe at the revetment toe, with 2m depth, that could protect the toe from erosion.
The weight of the embankment foot stone must be stable under the effection of currents generated by waves at the foot.The stable weight Gd of the rock at the foot of embankment is not less than the value specified in The National Standard of Vietnam, defined by local velocity at the foot.The maximum velocity of the flow at the embankment foot is determined by the formula:

Stability calculation for revertment toe
The critical erosion depth of the revetment toe depends on wave energy and the geotechnical conditions of the embankment site, which is determined by formula 6.
Where, Vmax is the maximum velocity of current at the embankment foot.Lsp is the design wave length, Lsp = 52m; Hsp is the design wave height, Hs = 1.86 m; h is the water depth in front of the dike, h=2.9m; g is the gravity acceleration, 9.81 m/s2.Thus, the velocity Vmax is detemined as Vmax = 1,78 m/s, and the weight of rock should be more than 40kg.

Stability calculation for the revetment
The embankment cross-sections at the locations and the geological condition along the construction area are relatively similar, that a representative cross-section is calculated.The revetment stability is evaluated the slip failures.The method for calculating slope stability is Slope/W in GeoStudio software program.Geostudio Office is a geotechnical software from GEO-SLOPE International Canada that is used to analyze the stability of proven rock-soil slope.Some key features of Slope/W software are: + Slope/W is a software that applies limitation balance theory to determine the safety factor of earth and rock slope.In Slope/W includes many different calculation methods to calculate the factor of safety such as: Bishop method, Janbu, Spencer, Mogor-Price, Crop of Engineers, GLE method and finite element method.Therefore, the operator is free to choose the method of calculating the factor of safety.+ Slope/W has options to calculate the sliding block including many types of soil and rock, submerged in water or not and according to different sliding blocks such as cylindrical slide, broken form in case of soft soil layer, with rock foundation, sliding along the assumed cross-sections like the slope of the foundation pit.+ Slope/W allows integration with Seep/W so that results from Seep/W can be used.Calculation case: Revetment cross section with slope of m=4.0.
Case 1: Loading on crest of revetment, lowest sea water level at the toe.Case 2: Revetment working with highest sea water level.Case 3: Revetment working with wave rundown from highest level to the toe.  3 shows the stability coefficient of Case 1 is Kmin = 2.346 larger than the requirement [K] = 1.25 by National Standard of Vietnam.This is a dangerous case in loading on embankment, the structure is stable of slipping.The situation in case 2, revetment working with highest sea water level, the struction submerges under water.The core of embankment could be seepaged leading to failure of construction stability.However, the stability coefficient in slipping slope Kmin = 2.959 is much higher than requirement from standard of 1.25.The embankment is stable in high rank of this case, as results shown in figure 4. Result of stability calculation of Case 3, revetment working with wave rundown from highest level to the toe is presented in figure 5.In this case, the seepage water draining to surface of the slope combinate with wave rundown, the slope could be slipped.The calculation result shows the stability coefficient in slipping of the slope is 1.817, larger than standard requirement [K]= 1.25.Thus, the embankment is completely stable.

Conclusion
Recently, effecting of sea water level rising and climate change, the coastal area is severely affected.The coastline that is undergoing eroded due to sea wave is likely more gravely erosion in coming decades, especialy in the South Central Coast of Vietnam, include of Binh Thuan province [5].The coast in Phan Thiet City, Binh Thuan, Vietnam is damaged seriously.Some coastal areas have been protected by embankments but also destroyed after period.To save the community activities and residential areas in the central coast of the city, a protection method from wave erosion was introduced.The solution could be stable in adverse situations.Calculation results have shown that the structure is able to stable with a high safety coefficient.
5)Pb is coefficient effecting of drainage of revetment and foundation.Pb = 0.1 Sb is the Initial failure parameter.Using from 0.5 to 2.0.N indicates number of incident wave in a storm.m is the rate of block material and structural style.∆  = With b = 0.5; m = 1.33; p = 1.36;Hs = 1.75mThus, the thickness of the blocks is determined larger than 27.6cm.Choosen D = 28 cm.1298 (2024

Figure 2 .Figure 3 .Figure 4 .Figure 5 .
Figure 2. Calculation model for revetment stability [1] Pham Trung and Nguyen Duc Vuong 2016 Computaion of wind field and pressure, which creates by storm in the sea "Dong" and establish research scenarios in Binh Thuan province (Proceeding of science and technology results 2015, Southern Institute of Water Resources, Vietnam) [2] National Standard of Vietnam TCVN 9901:2014 [3] Sawaragi T 1995 Coastal Engineering -Waves, Beaches, Wave-Structure Interactions (Elsevier) [4] Pilarczyk K 1998 Dikes and Revetments: Design, Maintenance and Safety Assessment (A.A. Balkema) [5] Pham Trung 2019 Mophology change of coastal estuaries in the South Central region under the impact of sea level rise (Journal of Water Resources Science and Technology, Vietnam Academy for Water Resources)

Table 1 .
Prevailing wind dierection at Phan Thiet Zc is the crest altitude.ZSWL is sea water level with designing frequency.Rcp is the height on the sea water level basing on wave overtopping.Sea water level is determined basing on the location of contruction and designing frequency.With the type of embankment, the designing frequency is choosen 3.33%.As shown in table B.4 of National Standard of Vietnam [2], the sea water level is interpolated ZSWL = +1.36m.The height on sea water level Rcp is calculated aproximately.The surf similarity parameter 0 is defined as [3]:

Table 2 .
Results of crest altitude calculation