The spatial variability of lightning over the railway network of Sri Lanka is presented in this section. Moreover, the variability of lightning activities over the nine railway lines were considered separately for a period of 17 years from 1998 to 2014.
3.1. Spatial Variability of Annual Lightning Flash Density over the Existed Railway Network
Figure 3 presents the spatial variation trend of the lightning flash density along with the railway network and the disparity of lightning flash density along the railway line at each point of 1 km distance from 1998 to 2014.
Table 1 shows the extracted information of recorded maximum flash density on each railway line. According to the spatial variability of lightning flash density over the railway network, Kelani Valley is more vulnerable to lightning activities than the other railway lines. It extends from Colombo to Maradana East to Avissawella, which extends up to 30 km from Colombo. According to
Figure 3 and
Table 1, along this railway line, the maximum lightning flash density of 24.13 flashes/(km
2⋅year) was near the Avissawella railway station (6°56′41″ N/80°12′00″ E). Operations at Seethawaka Export Processing Zone and power stations installed within the region may be significantly influenced by a rise in lightning activities over the region.
Moreover, significantly, more lightning activities had been recorded over the Coastal line, Puttalam line, and Mainline. Starting from Colombo-Fort station, the coastal line runs towards the Southern end. It extends by connecting the regional centres of Moratuwa, Panadura, Kalutara South, Aluthgama, Ambalangoda, and Hikkaduwa towards Galle before the termination at Beliatta. Along this railway line, the maximum lightning flash density of 12.19 flashes/(km
2⋅year) was recorded near the Colombo-Fort railway station (8°56′01″ N/79°51′01″ E), and Minimum flash density values are recorded from close proximity to the Matara district. In the Puttalam line, which extends from Ragama to Periyangavillu, the highest lightning activities had been recorded at close proximity to Ragama railway station (7°02′09″ N/79°54′43″ E) in the Gampaha district with a maximum lightning flash density of 22.82 flashes/(km
2⋅year). Results suggest that with the increase in the latitude values, there is a decreasing trend in the lightning flash density in a specific region. On the other hand, Mainline runs from Colombo to Badulla, and it reaches its summit at Pattipola, which is 6226 feet (1898 m) above sea level, before continuing its descending to Badulla through Bandarawela. As stated in
Figure 3 and
Table 1, along the railway line, the maximum lightning flash density of 22.27 flashes/(km
2⋅year) was recorded near the Ragama railway station (7°02′06″ N/79°55′12″ E).
High aerosol emission and variation of surface temperature in a particular area may be the causative influential factors to the incremental trends in lightning activities in the study area. The above-indicated paths are bound to relatively high populated districts (Gampaha, Colombo, Kalutara, and Galle). Therefore, the highest population density may rigorously influence to increase in the traffic on public road networks, which will eventually pave the path to make a direct influence to enhance the atmospheric pollution by releasing aerosols generating air pollutants.
On the other hand, low lightning flash density is shown over the Mannar, Trincomalee, and Matale lines. Lightning activities over the railway network show a clear decrement in lightning activities near the coastal line of the country. Observed results suggest that the lightning activities are low over the Northern line at higher latitudes relative to the lower latitudes. Furthermore, over the Trincomalee line, the lightning flash density is decreasing with the rise of latitude.
Railway lines in the wet zone such as the Kelani Valley line, part of the Mainline (Colombo-Fort to Peradeniya junction), part of the Coastal line (Colombo-Fort to Matara), and part of the Puttalam line are highly susceptible to lightning activates relative to the other climate zones. Furthermore, lower lightning activities account over the railway lines in the intermediate zone and dry zone. The wet zone accounts for the highest annual rainfall and the dry zone and semi-arid zone receive the lower annual rainfall. Variations in the climate and inclined geographical features towards lightning hits featured, manmade structures prone to lightning in the wet zone can assume to be accounted for this variation.
Figure 4a presents the spatial variability of the annual lightning flash density, which was obtained by LIS satellite data in the period of 1998 to 2014 over the railway network of Sri Lanka. Satellites have observed about 3316 lightning flashes from the total lightning flash counts over the railway network from 1998 to 2014; as a percentage, it is 18.29%. According to
Figure 4a, the Kelani Valley line is highly susceptible to lightning in the considered duration. Furthermore, one of the main controlling centers at the railway network–the Maradana–and its surrounded area account for higher lightning flash density values. Moreover, the lightning flash density and lightning activities over the railway network in the southern part of the country are comparatively higher than that of the north part of the country. In particular, railways running across the high populated areas were recorded with more lightning activities. Established industrial zones, implemented major thermal power plants, elevated buildings and towers, and other manmade climbing structures may be the influential factors to raise the lightning activities in those highly populated areas. Furthermore, aerosol emissions have been increasing over this area during the considered period due to excessive burning of huge amounts of carbon by diesel locomotive engines. This eventually makes a critical impact on aerosol pollution, which harshly affects cloud formation processes and precipitations. The statistical representation of the annual lightning flash density distribution and population density over the considered buffer area of each railway line from 1998 to 2014 is indicated in
Table 2. The maximum average of the 16.58 flashes/(km
2⋅year) is shown over the Kelani Valley railway line (maximum = 28.05 flashes/(km
2⋅year), minimum = 10.13 flashes/(km
2⋅year), and standard deviation = 4.34 flashes/(km
2⋅year)). The minimum average of the 5.08 flashes/(km
2⋅year) is shown over the Mannar line (maximum = 8.27 flashes/(km
2⋅year), minimum = 1.37 flashes/(km
2⋅year), and standard deviation = 2.29 flashes/(km
2⋅year)).
Figure 4b depicts the variability of the population density within the 5 km buffer zone in 2020 by centering each railway line. The maximum population density had been recorded in the vicinity of Colombo-Fort (32,657 people km
−2), Colombo-Maradana (30,240 people km
−2), and Ragama (8241 people km
−2) railway stations. On the other hand, lightning flash density is higher over the aforementioned area. Furthermore, both population density and lightning flash density over the vicinity of the coastal line are greater than over the other railway lines in the network. Although the population density accounts for a higher value in areas such as Kankasanturai, Batticaloa, and Peradeniya junction relative to the other mentioned areas in the railway network, recorded lightning flash density values are relatively low. According to
Table 2, the maximum population density (32,657 people km
−2) is associated with the Mainline. In general, throughout the globe, the use of rail transportation as a mode of public transportation is more popular among the public in areas with a high population density. Therefore, high attention should be drawn towards the aforementioned areas to mitigate the hazardous influences due to lightning.
As described in the previous study, the average annual lightning flash density of Sri Lanka from 1998 to 2014 was 8.26 flashes/(km
2⋅year) [
21]. Lightning flash density disparity along with the railway network of Sri Lanka in the aforementioned period at each point of 1 km distance and the percentage deviation of the lightning flash density along with the railway network relative to the average annual lightning flash density of Sri Lanka is illustrated in
Figure 5.
Figure 5 emphasizes that the maximum lightning flash density has been recorded in the vicinity of the Avissawella railway station in the Kelani Valley line, and relatively, a broad area of the Mainline is vulnerable to lightning activities. On the other hand, low lightning flash density is shown over the Mannar and Matale lines. Analysis of lightning activity disparity over the railway network of Sri Lanka is important for the electrifying process of the Sri Lanka railway network.
Moreover, over the Kelani Valley railway line, a maximum percentage deviation of 192.10% has been reported near the Avissawella railway station and it is gradually decreasing by reporting a 42.04% of percentage deviation as a minimum, near the Colombo-Fort railway station. The maximum percentage deviation of 176.22% has been reported at the Ragama railway station in the Puttalam line, and 27 km from the Ragama railway station, the recorded lightning flash density over the Puttalam line is lower than the average annual lightning flash density of Sri Lanka. Moreover, the maximum percentage deviation of 169.72% has been reported 16 km from Colombo-Fort railway station in Mainline, and the lightning flash density over this railway line is higher than 50% of the annual average flash density of Sri Lanka– in between 0 km (Colombo-Fort railway station) and 100 km. On the other hand, all the recorded values of the flash density over the Mannar line are lower than the annual average flash density of Sri Lanka (maximum percentage deviation = −4.98%, minimum percentage deviation = −81.62%). However, lightning flash density over the Coastal line from 0 km (Colombo-Fort railway station) to 120 km (near the Galle railway station) is higher than the annual average flash density of Sri Lanka; after more than 120 km, lightning flash density values are relatively lower than the annual average flash density of Sri Lanka (maximum percentage deviation = 47.58%, minimum percentage deviation = −77.96%). Furthermore, over the Northern line, lightning flash density values have fluctuated around the value of the annual average flash density of Sri Lanka (maximum percentage deviation = 109.74%, minimum percentage deviation = −80.96%). Lightning flash density over the Batticaloa line, Trincomalee line, and Matale line express minor variations relative to the annual average flash density of Sri Lanka.
Table 3 shows the percentage distribution of lightning flash density within five different ranges, relative to the annual average flash density of Sri Lanka over the railway network, from 1998 to 2014. According to the reported details of
Table 3, all recorded lightning flash density values along the Mannar line are lower than the average annual flash density of Sri Lanka, and 39.05% of lightning flash density points are below 50% of the average annual flash density of Sri Lanka. As per the reported details of the study, over the area of the Kelani Valley line accounts for the maximum average lightning flash density. Along the full length of the line, 14.55% of lightning flash density values are in between the annual average flash density and 150% of the annual average flash density of Sri Lanka, 41.82% of lightning flash density values are in between 150% of the annual average flash density and 200% of the annual average flash density of Sri Lanka, and 43.64% of lightning flash density values are greater than 200% of the annual average flash density of Sri Lanka. Furthermore, 43.67% of lightning flash density values over the Kelani Valley line, 29.69% of lightning flash density values over the Mainline, 6.76% of lightning flash density values over the Puttalam line, and 0.90% of lightning flash density values over the Northern line are more than 200% of the annual average flash density of Sri Lanka. This information implies that the aforementioned fragments of the railway line are more vulnerable to lightning accidents. Therefore, responsible authorities of the Sri Lankan railway should be attentive in enhancing the precautionary measures by conforming to the safety of passengers, the working crew of the railway, and the properties of the railway.
3.2. Spatial Variability of Lightning Flash Density over the Railway Network in Climate Seasons
The distribution of lightning activities in all climatic seasons over the country varies in different geographical regions. Moreover, an average flash density of 22.25 flashes/(km
2⋅year) was reported in the FIM in the country whereas, in the SWM it was 5.67 flashes/(km
2⋅year) [
21]. As mentioned in [
21], an average lightning flash density of 9.29 flashes/(km
2⋅year) was recorded in the SIM in the country; for the NEM, it was 2.99 flashes/(km
2⋅year).
The spatial variation pattern of the lightning flash density over the railway network of Sri Lanka in FIM from 1998 to 2014 is depicted in
Figure 6a. According to
Figure 6a, in this monsoon season, the railway lines of Colombo-Fort to Peradeniya junction and Colombo-Maradana to Avissawella (Kelani Valley line) have been highly susceptible to lightning activities. Furthermore, the vicinity of the railway stations of Talai Mannar, Kankesanturai, Trincomalee, and Batticaloa have shown lower lightning activities. Previous studies show that the FIM is the highest lightning active monsoon season in Sri Lanka [
18,
20,
21], and
Figure 6a also depicts the same behavior for the railway network of Sri Lanka.
Figure 6b depicts the spatial variation of the lightning flash density over the railway network of Sri Lanka in the SWM from 1998 to 2014. During this seasonal period, the railway road between Galoya junction and Batticaloa is more vulnerable to lightning activities than other railway roads in the network.
Figure 6c illustrates the spatial disparity of the flash density over the railway network of Sri Lanka in the SIM from 1998 to 2014. In this seasonal period, the railway road of Colombo-Fort to Moho junction and Kelani Valley line is more vulnerable to lightning activities than other railway roads in the network. The vicinity of Polgahawela junction has recorded more lightning activities in the SIM.
Figure 6d presents the spatial variability of the lightning flash density over the railway network of Sri Lanka in the NEM from 1998 to 2014. The coastal line is more highly vulnerable to lightning activities than other railway lines in the network during this monsoon season. Moreover, previous studies show that the NEM is the active monsoon season in Sri Lanka with the lowest amount of lightning [
18,
20,
21], which shows similar behavior for the railway network of Sri Lanka.
Table 4 presents the statistical description of the annual and seasonal lightning flash density variation over the considered buffer area of the railway network from 1998 to 2014.
The FIM season accounts for high lightning activities in the railway network of Sri Lanka covering all the climatic zones, relative to the other seasons. Moreover, the Kelani Valley line and part of the Mainline (Colombo-Fort to Peradeniya junction) running across the wet zone are extremely vulnerable to lightning activities during this season. Thunderstorms with dense convections are pronounced in the afternoon hours over the southern part of the country in the FIM season. In the SIM season, more lightning activities was recorded over the railway lines in the dry zone. Within this seasonal period, thunder clouds are drawn towards the northwest of the country by southwest winds. This leads to an intense occurrence of lightning activities over the dry zone. In the SIM season, the Kelani Valley line and part of the Mainline (Colombo-Fort to Polgahawela junction) in the wet zone have more lightning activities. During this seasonal period, in the afternoon hours, thunderstorms are occurring over the wet zone of the country. Significant lightning activities have been observed over the proximity of the Aluthgama and Ambalangada railway stations in the Coastal line and Avissawella railway station in the Kelani Valley line in the wet zone during the NEM. It seems that the above-mentioned railway lines in the wet zone are highly susceptible to lightning during the FIM, SIM, and NEM seasons, but the vulnerability towards lightning is considerably low in the SWM.
3.3. Spatial Variability of Lightning Flash Density over the Proposed Railway Electrification System
As a future masterly proceeding of the Sri Lankan railway, they plan to electrify a 102 km distance of the existing railway network by including the 15 electric multiple units and a signaling system [
26]. Direct lightning flashes and the induced surges due to cloud–cloud and cloud–air lightning can harshly influence the overhead transmission line of the electrified railway network or compromise the telecommunication system and interrupt the transmission of the railway signal via temporary malfunction, degradation, or permanent damages [
27,
28,
29,
30,
31,
32,
33]. Therefore, a study on the spatial variability of lightning activities over the existing railway system is an essential requirement for the Sri Lanka railway as a part of the feasibility study on the proposed railway electrification network.
Figure 7 shows the spatial disparity of lightning flash density over the railway lines that are expected to be electrified. Railway lines of Colombo-Maradana to Veyangoda (36 km), which is part of the Mainline, Ragama to Negombo (23 km), which is part of the Puttalam line, and Colombo-Maradana to Kalutara (43 km), which is part of the Coastal line, are expected to be electrified in the near future by the railway department of Sri Lanka. According to the results of the study, lightning flash density over the expected electrified railway lines is higher than the other lines of the railway network.
Table 5 shows the descriptive statistics of the variability of the annual lightning flash density over the railway lines expected for electrification. The maximum annual flash density of 22.85 flashes/(km
2⋅year) was recorded at the Ragama railway station. Accordingly, for the study period from 1998 to 2014, the maximum average annual lightning flash density of 16.53 flashes/(km
2⋅year) was recorded over the Colombo-Maradana to Veyangoda railway line (Maximum = 22.85 flashes/(km
2⋅year), Minimum = flashes/(km
2⋅year), and Standard deviation = 2.94 flashes/(km
2⋅year)). Considering the information in
Table 5, a high population density can be detected in the selected area, which is proposed to launch the electrification. This implies the inevitability of implementing a strong lightning protection system for the proposed electrified railway network by the Sri Lanka Railway Department in order to achieve sustainable development in the railway network in Sri Lanka, through proper minimization of lightning hazards. Otherwise, lightning accidents may act as a booster to reduce the operational efficiency of the proposed electrified railway network.