Wave distribution and proposed seawall design around Tanjung Emas Port, Semarang

. On May 23 rd , 2022, an extreme coastal event occurred around the northern coast of Java. This event had caused the collapsed of several seawall segments around Tanjung Emas Port, Semarang. Combined with wave overtopping, it led to severe flooding and economic damages to the port operation. Responding to the event, this research aimed to characterize the hydrodynamic condition around the port and proposed a seawall design. The research was done through field observation, wave climate characterization, and numerical simulation. The field observation confirmed the extent of the damage and wave condition around the harbour. The wave climate study, based on a global wave reanalysis data, confirmed that the offshore wave height could reach 3 m during high wave season, with wave period of 4 – 6 seconds coming from the northwest. The tidal range is up to 1 m with highest astronomical tide (HAT) is at +0.65 MSL. The extreme value analysis gave a 3.7 meter of offshore design wave height for 100-year return period or up to 1.7 meters wave height nearshore. The proposed seawall crest elevation is at least at +3.40 meters above the mean sea level to avoid overtopping, which is 1.1 m higher than the existing seawall.


Introduction
Port or harbour is important for both social and economic activities in Indonesia, as an archipelagic or maritime country.Port primarily serves the function of marine transportation activities [1], where ships park, load, and unload passenger or cargo.Therefore, a port should be protected from high waves or tidal floods in order to function properly [2].One of the important ports in Indonesia is Tanjung Emas Port, Semarang.Fig. 1 shows the port location (Fig. 1a), and photos of some parts of the port (Fig. 1b and Fig. 1c).
Tanjung Emas Port is in Semarang, the northern part of Java Island where tidal flood is commonly observed.Tidal floods could be naturally occurred during high tide.Around the northern part of Java, land subsidence and sea level rise have been exacerbating the flood [3].The soil around Semarang coast experiences natural compaction leads to land subsidence [4].Based on previous research, the rate of land subsidence around Semarang is 9 to 13 cm/year [5].In addition to that, sea level rise was predicted to be about 0.6 -0.9 m from 1990 to 2100 due to global warming [6].
Coastal floods are influenced by several natural phenomena, namely tides, wave set up, wind set up, and sea level rise.Tidal floods routinely occur during high tides [7].It occurs when high tides inundate land that has a lower elevation [8].The term tidal flood is used to describe a rise in sea level which is high enough during full moon tide to inundate coastal areas [9].The construction of a seawall was carried out as an effort to anticipate the impact of tidal floods.On May 23, 2022, an extreme tidal flood had occurred around the northern coast of Java.This tidal flood had impacted several areas along the northern coast, but the damage around Semarang was quite significant due to the disturbance to the port operation.Moreover, the wave overtopping on the seawall had also occurred because both the wave height and tidal surge exceeded the existing seawall height.Thus, several seawall segments around the Tanjung Emas port were recently broken due to the extreme coastal events as shown in Fig. 2. The resulting damages had caused severe economic loss to the port operation [10].Overtopping events were also reported at several locations, even though the northern coast of Java is assumed to have mild wave height.
Therefore, questions were raised regarding the real characteristics of wave climate around the location.Moreover, with the increasing evidence of intensifying wave condition due to climate change around the globe [11][12][13], understanding wave climate is important when designing vital public infrastructure.Therefore, this study is aimed at understanding the hydrodynamic characteristics around Tanjung Emas Port and providing the seawall design based on tidal observation data and wave reanalysis data from the European Centre for Medium-Range Weather Forecast (ECMWF) Reanalysis 5-th Generation (ERA5) using Coastal Modeling System (CMS) model.

Data and method
This research was conducted through two main steps: field observations of flooded locations around the harbour, and numerical simulation using CMS Model.

Field observation
Field observations were conducted on January 7 th , 2023 around Tanjung Emas Port to gather evidence of the flood impact following the extreme coastal event that occurred on May 23 rd , 2022.Despite the visit which taken place several months after the event, the evidence was still visible in the field, as illustrated in Fig. 3.One of the impacts of tidal flood on May 23 rd , 2022 was the wave overtopping over the seawall which damaged the structure (Fig. 3).However, during the visit, several seawall segments were updated with new seawall, as shown in Fig. 4.
Based on the field observations around the harbour, it was found that the waves in the harbour basin were calm and there were no breaking waves, as shown in Fig. 1.This is attributed to the presence of breakwater, which protects the harbour basin from the incoming waves.According to previous research [15], land subsidence has occurred around the port.This observation aims to identify evidence related to the land subsidence that has taken place.As shown in Fig. 5, several locations of land subsidence can be seen.Some buildings are sinking from street level.However, the land subsidence around the port did not occur uniformly in all areas.It was limited to only in a few parts.

Hydro climatology
The ocean around Tanjung Emas Port is characterized by two wave seasons.The high wave season, which usually starts around November to March, dominantly coming from the northwest (Fig. 6).The wave height during high wave seasons could reach 2.5 -3 m (Fig. 7a).The calm season occurs from April to October.The waves are coming from the northeast (Fig. 6), with the wave height ranging from 0.5 to 1 m (Fig. 7a).The extreme coastal event discussed in this study was occurred in the end of May 2022 which was usually a calm season.
The average wave period is ranging from 4 to 6 seconds (Fig. 7b).The wave rose might shows that the most wave are coming from the northeast.However, the magnitude of northeast waves is relatively small.Moreover, a headland exists in the east side of the study area, protecting the harbour from the east waves.Therefore, it is reasonable for structure design around Tanjung Emas Port to consider waves coming from the northwest in the design criteria.This characterization was based on ERA5, which represents the offshore wave.It is then be distributed by using CMS model to understand the wave propagation to nearshore.
The wave data was then further analysed to calculate the design wave, a wave height with certain return period as a design criterion for coastal structures.In this study, 100-year return period was selected as the design criterion considering the high economic value of the port.Extreme values were selected based on annual maxima of a 20-year data series and then calculated using Gumbel and Weibull method which gave 3.4 m and 3.7 m of wave height for each method respectively.This result is align with previous research which compared various wave reanalysis data, including SEAMOS-South Fine Grid Hindcast (SEAFINE) data [16], where they pointed out that the 100-years return period of wave could ranging from 3 to 4.5 m [17].In this study, Weibull analysis was chosen as the wave design (3.7 m) with wave period of 6 seconds coming from the northwest.Water level variation data were obtained from both a 30-days of tidal survey [18] and measurement data from Tanjung Emas Maritime Station, Semarang [19].Both data are presented in Fig. 8 showing that the tidal range is around 0.6 and 1 meters during neap tide and spring tide respectively.
The tidal type at that location is mixed semi-diurnal with a Formzhal number of 1.24.Tidal component used in the simulation are presented in Table 1.

CMS model setup
The wave propagation was simulated using CMS model, both CMS-Wave and CMS-Flow model.The CMS-Wave was used for wave distribution, while CMS-Flow was used to accommodate water level fluctuation and current.
The domain for both CMS-Wave and CMS-Flow model are shown in Fig. 9.In CMS-Wave we used 50×50 m grid and 8 detailing points with the minimum grid of 15 m.In CMS-Flow we used varying grid sizes: 50×50 m in area 1, a maximum of 35 m in area 2, and a maximum of 15 m in area 3. We used the manning roughness value of 0.025 uniformly across the model domain.Bathymetric data from the Pushidrosal sea chart was used with some adjustment around Tanjung Emas Port as shown in Fig. 10.
Tanjung Emas Port is equipped with breakwater which protects shipping lanes and harbour pools from certain wave height.The breakwater helps to maintain calm wave condition inside the pool.Therefore, the breakwater structures were also installed in our model domain as shown in Fig. 11.
The CMS-Wave model utilizes spectral wave at one location as model input.We arranged the model domain to coincide with the location of our data point in ERA5 (110°, -6.5°) as offshore wave input (Fig. 9a).The calculated 100-year design wave (Hs = 3.7 m, Tp = 6s) was then be distributed across the domain using CMS Wave.Fig. 10 shows the boundary line locations where the WSE-forcing is applied.The simulation run was set for 30-days covering with 2-hours of ramp duration and 600 s time step.The CMS-Flow read the wave distribution result from CMS-Wave in hourly steering interval.The simulation result was then verified using the tide observation data.

Model verification
The simulation result was verified by comparing water level fluctuation between model result and field observation data.Fig. 12 shows comparison between model result from CMS Flow and field survey measurement, while Fig. 13

Tidal flow
The CMS Flow model show the direction of tidal flow and current.The current at high tide flows from the sea towards the land is shown in Fig. 14.At high tide, the sea level is quite high so area that is inundated by sea water is quite large.At low tide, area that is inundated by sea water is less than during high tide.Current at low tide flows from land to sea is shown in Fig. 15.

Wave distribution
Fig. 16 shows wave propagation from offshore to onshore from the CMS Wave model.Changes occurs in wave height and direction due to refraction and shoaling.From the extreme value analysis, a 3.7 meter of wave height is assigned in the offshore.The wave propagated and transformed nearshore into about 1.7 meters around the harbour area (Fig. 17).It is seen from Fig. 17 that the breakwater arrangement successfully protects the harbour area from high wave.The models also showed that the wave breaks around the breakwater, so there are no breaking waves in the harbour turning basin.Wave height around the turning basin is about 0.1 -0.3 meters.However, during the extreme rise of sea water level, overtopping still occurs in several locations as observed from the site visit.Thus, the coastal flooding event which occurred on May 23 rd , 2023 was mainly due to an extreme water level surge instead of high wave.The hydrodynamic characteristics from the model simulation were then be used to design the suitable seawall around the study area.

Seawall design
A concrete vertical seawall was design after analysing the hydrodynamic characteristic around the harbour.Crest elevation is calculated using Equation 1.

𝐷𝑊𝐿 = 𝐻𝐴𝑇 + 𝑆𝑆 𝑜𝑟 𝑊𝑆 + 𝑆𝐿𝑅
(1) where HAT represents Highest Astronomical Tide, SS represents Storm Surge, WS represents Wind Set Up and SLR represents Sea Level Rise.The tidal constituent in Table 1 gives HAT of 0.65m.The sea level rise was assumed as 0.9 meters following the IPCC prediction, while the wind set up was estimated as 0.3 meters based on 9.30 km/hour of wind speed and 538 km of average fetch.The calculation concluded a +1.85 meters of design water level (DWL).
Seawall crest elevation is estimated by adding wave runup (Ru) and freeboard (Fb) to the previously calculated design water level as stated in Equation 2.
Since there are no breaking waves around the harbour, wave runup can be omitted from the calculation.Seawall crest elevation was designed only from the DWL and freeboard.In this case, we considered 1.5 m as the freeboard.Thus, the crest elevation would be at +3.40 m above the mean sea level (Fig. 18).
The design of seawall at each location will vary, especially if there is land subsidence at the seawall location.Thus, land subsidence that occurs at site should be considered when determining the seawall height.In this study, a land subsidence of 1.3 meters (Fig. 18) was assumed based on a land subsidence rate of 13 cm/year or 0.13 m/year around the port for the past 10 years, as reported in previous studies [5].The seawall crest elevation needs to take the land subsidence into account.Thus, the crest elevation was estimated at +4.70 m above the mean sea level.If the wharf floor is at an elevation of +1.25 m, then seawall height would be about 2 meters without considering land subsidence and 3.50 meters when accounting for land subsidence.The width of a seawall depends on its function and location.The width of designed seawall was based on the measured field observation at several seawall segments at the port.The observed seawall had a width of 0.4 meters and hence, the proposed seawall in this study was designed to be 0.5 meters in width to support the construction method.
In addition, a drainage system needs to be incorporated to control the water in the seawall and adjust it to the needs of the seawall location.The drainage channels discharge water into the sea with a specific construction comprising water pumps (Fig. 19a) and drainage channels (Fig. 19b).The drainage system was designed based on the field visit on January 7 th , 2023.A drainage channel has been constructed around the back side of the seawall to collect water from overtopping.This water then flows through the drainage channel and is pumped back into the sea using the existing water pumps.

Conclusions
This work described the hydrodynamic characteristics around Tanjung Emas Port using wave reanalysis data ERA5.The analysis showed that waves are generally calm, however it might up to 3 m of wave height during the high wave seasons.Based on the wave's climate characterization, the last extreme coastal event occurred during calm season, whose wave heights are usually ranging from 0.5 to 1 m.According to ERA-5 wave reanalysis, no extreme wave height was observed at the end of May 2022.Therefore, we concluded that the last extreme events could be caused by other meteorological events such as tidal surge or ocean storm event.
Hydrodynamic model in this study has shown a good agreement with the field observations in terms of tidal fluctuation with RMSE value of around 0.1 with respect to tide data from both field observation and data from the Maritime station.The model showed that waves around the harbour is generally calm, as it is damped by the breakwater.With an incoming offshore wave of 3 m coming from the northwest, wave height near the harbour is about 1.7 m, while the wave height around the turning basin is only about 0.1 -0.3 m.
The proposed seawall is designed with a DWL of +1.85 m MSL.Considering a freeboard of 1.5 m, the crest elevation is designed at +3.34 meters to avoid overtopping.

Fig. 19 .
Fig. 19.Illustration of the existing (a) water pump station and (b) drainage channel.

Table 1 .
Tidal component used in the simulation.