What are the most significant factors affecting the Coastal landscapes on pulau ubin and sentosa?

Fieldwork Question and links to the syllabus This investigation aims to answer the following fieldwork question - What are the most significant factors affecting the coastal landscapes on Pulau Ubin and Sentosa?

 

This investigation links to Option B: Oceans and Coastal Margins, specifically “Physical influences on coastal landscapes” (physical factors) and “Management strategies” and “Land-use pressures on coastlines” (human factors).

A coastal landscape is a length of coastline molded by depositional and erosional processes (Britannica). Figure 2 shows the main factors that affect coastal landscapes. These are classified into four groups: terrestrial, marine, atmospheric and human.

The fieldwork question was investigated on two of Singapore’s 64 islands (VisitSingapore): Pulau Ubin and Sentosa (Figure 4). These islands were chosen as their coastal landscapes are similar in some ways (both offshore islands in Singapore), but they differ in terms of physical factors (e.g. fetch and lithology/geology) and human factors (e.g. land-use pressures and management strategies).

The following hypotheses were formed to help investigate and answer the fieldwork question -

Human factors are more significant than physical factors in affecting coastal landscapes on Sentosa, but physical (specifically atmospheric) factors are more significant than human factors in affecting coastal landscapes on Ubin.

 

Since most of Sentosa’s tourist resources are secondary, there is much more coastal management. Physical factors affecting Sentosa’s coastline are mostly redundant due to the artificial deposition of sand and the hard engineering used to protect the coastline. Whereas Pulau Ubin’s coastal landscape has less coastal protection and management and therefore is likely to be more influenced by physical factors, in particular atmospheric factors.

 

Two sub-hypotheses were also created to help investigate the main hypothesis.

There is a positive relationship between wind speed and wave height.

 

Higher wind speeds provide a greater force onto waves, thus causing them to have greater heights. This will help to investigate the interaction between atmospheric and marine factors.

 

The percentage of fine sediment will be greater in the backshore and lesser in the foreshore.

 

Aeolian transportation would cause a greater amount of fine sediment to be transported to the back-shore. This will help to investigate the influence of wind as a key atmospheric factor.

Site selection

 

In order to answer the fieldwork question, two coastal landscape sites were chosen on each island: Sensory Garden Beach, and Police Post Beach (Figure 5) and Siloso Beach, and Rimau Beach (Figure 6). These sites were chosen using stratified sampling as random sampling may have resulted in inaccessible locations being selected. Furthermore, although random sampling reduces bias, it may have misrepresented coastal landscapes on the two islands as some key features could have been missed. Three transects were chosen on each beach, left, centre, and right (looking out to sea), using systematic sampling to ensure representative data was collected at each beach.

Beach profiles (steepness) are influenced by marine, terrestrial, human, and atmospheric factors. The beach profile of the sites can give an indication of the most significant coastal factors acting on them. The beach profile was found using the method described in Figure 7.

Littoral drift, explained in Figure 9, is important as it determines the formation of coastal landforms like spits and tombolos and influences beach morphology (shape). Thus it may be a significant factor in affecting coastal landscapes as it is a process that involves the interaction of atmospheric (wind) and marine factors. Figure 9 describes the method used to measure littoral drift.

The sediment size at different points on the beach is important as it indicates the erosional power of marine processes. It also indicates aeolian (wind) transportation factors like saltation, a key atmospheric factor. A sample of sediment was taken from the fore-shore, mid-shore, and back-shore at each transect. The samples were later dried and weighed to determine the percentage of coarse, medium, and fine sediment.

In order to assess non-quantitative factors like coastal management (and other human influences) and coastal landforms, qualitative data was collected. This included photographs and observations as written notes.

The Spearman’s Rank Correlation Coefficient test was conducted between the wind speed and wave height data. This statistical test demonstrates the significance of the correlation between these two variables, this assesses whether wind speed is the cause of wave height, and thus erosional power.

The statistical test is done using the formula below -

 

\(P=1-\frac{6∑d6}{n(n^2-1)}\)

 

Where ⍴ is the Spearman’s Rank Correlation Coefficient, d is the difference between the ranks of a pair of data, and n is the number of pairs of data.The outcome is then tested for significance using the graph in Figure 12.

The rocks’ shape and texture indicates the significance of marine factors, particularly erosion. Interestingly, the observed waves were very small. Since the beach is only 70 metres from the jetty, it is reasonable to assume much of the wave energy came from boats.

Mangroves (a key terrestrial factor) provide coastal protection (see Figure 14), reducing the rate of erosion of the Sensory Trail Beach.

Due to 400 metres of rip-rap to the West of Rimau beach, sediment will continue to be transported Eastwards. As it approaches Rimau Beach, sediment is deposited in the extruding area. This aids in the formation of the long, flat, beach. Furthermore, due to the short fetch (400m), approaching waves have low energy and thus deposit sediment. This combination of human and terrestrial factors explains the shape and presence of the beach.

Due to the many ships passing by, artificial wave energy is likely to have an effect on Siloso beach. However, due to the man-made off-shore breakwaters, much of the wave energy is dissipated, allowing Siloso to maintain its shape. They also lead to the formation of salients and eventually tombolos through littoral drift (see Figure 17).

The high percentage of fine sediment on Siloso (53%) and the flattened beach profiles after 14 metres indicate the presence of human influence (Figure 19), specifically beach nourishment. This is contrasted by the higher percentage of medium sediment (54%) and the gentle sloping beach on Rimau (Figure 20). On both beaches, a generally positive trend between distance from the shore and percentage of fine sediment is observed, alluding to the significance of aeolian factors described in Figure 23. This same trend implies that marine factors may not be significant (see Figure 21). The anomaly in Figure 18 is discussed in Figure x.

The high percentages of fine sediment on both Police Post Beach and Sensory Trail Beach (49% and 47% respectively) indicate the significance of erosional factors, specifically attrition (see Figure 24). Similar to the beaches on Sentosa, a more significant percentage of fine sediment was found on the backshore, indicating the significance of aeolian transportation, however, an overall greater amount of sediment was medium-sized, indicating less human influence. Attrition and hydraulic action occurring at a large rock outcrop may lead to larger rock fragments nearby, explaining the anomaly in Figure x.

Due to artificial offshore breakwaters at Siloso (Figure 16), wave energy has been dissipated. Although the wind direction may determine the direction of littoral drift in the West and Centre, it is more likely that sediment is being transported East and West due to wave refraction at the breakwaters. In the centre transect, the wind direction and speed likely had a larger impact on the direction of littoral drift than the direction of wave approach, due to the relative strength of the waves and wind.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 

Similar to Rimau Beach, the South transect has an anomalous high wave energy (149 joules), this is again likely due to a passing vehicle. In the Centre and North transects, wave energy is extremely low. This is likely due to the Northeasterly wind direction (opposing the wave approach), causing dissipation of energy, this also reduces the littoral drift rate.

Because the angle of wave approach is almost perpendicular to the coastline, there is a very low rate of littoral drift (1.58 metres/minute max). The opposite direction of littoral drift to wind direction for two out of three transects indicates little significance of aeolian factors. Interestingly, wind direction and the angle of wave approach are in somewhat the opposite direction, these factors likely work against each other, reducing the overall energy of the coastline, explaining the low littoral drift rate.

Figure 28 demonstrates the large influence of aeolian factors on the coastline. There is a strong correlation between the wind direction, the angle of wave approach, and the direction of littoral drift. In the Centre transect, the rate of littoral drift is 0.95 metres/minute, this is relatively low compared to the West and East transects (2.43 and 2.31 metres/minute respectively), this can be explained by the wind direction being perpendicular to the shoreline, reducing its effect on the rate of littoral drift.

Figure 30 demonstrates little to no relationship between wind speed and wave height, implying other factors such as swash from boats have a greater effect on wave height than wind, indicating the significance of human factors. Furthermore, due to an extremely short fetch (>1km on all beaches) wind has less time to cause an increase in wave height (see Figure 31).

The first sub-hypothesis (pg6) can be fully rejected. As shown in the Spearman’s Rank test (Figure 30), the weak correlation between wind speed and wave height has occurred by chance as it is below the 95% significance level. This implies other factors play a greater role than wind speed in determining wave height, such as extremely short fetches and swash from boats. The second sub-hypothesis (pg6) can be fully accepted as all beaches show a greater percentage of fine sediment in the backshore.

 

The main hypothesis can be partially accepted, as discussed below, human factors have some significance in Sentosa, however, it is not the most significant factor. Atmospheric factors (particularly Aeolian) are the most significant factors affecting Pulau Ubin’s coastline. The inverse relationship between sediment size and distance from the shoreline on both sensory trail and police post beach demonstrate aeolian transportation of fine sediment. Analysis shows that strong Northeasterly winds on the sensory trail beach reduce wave energy, thereby reducing the significance of marine factors. On police post beach, Northwesterly winds in the West and East transects lead to a greater rate of littoral drift (2.43 and 2.31 metres/minute). Although evidence of marine factors can be seen in high percentages of fine sediment on both police post (49%) and sensory trail beaches (47%), the significance of atmospheric factors outweighs the significance of marine factors. Smooth beach profiles on sensory trail and police post beaches indicate a lack of human factors. Although there was evidence of a passing vehicle (wave energy was 149J), this was the only one of six measurements above 36J, suggesting it was an anomaly. Since aeolian factors determine the areas that are eroded based on wind direction and speed (indirectly affecting energy), terrestrial factors do not play a large role in affecting the coastal landscape.

 

On Rimau beach, the most significant factors were atmospheric (particularly Aeolian) and marine factors. While the inverse correlation between sediment size and distance from the shoreline indicated the presence of aeolian factors, the overall higher proportion of medium (51%) and fine (38%) sediment indicated the presence of erosion. On the East transect, the direction of littoral drift was concurrent with the direction of the wind. In the center transect, wind direction and the angle of wave approach was opposing, this caused a low rate of littoral drift (0.33metres/minute), suggesting they worked against each other. On Siloso beach, atmospheric (particularly Aeolian), human, and marine factors played a significant role. Similar to Rimau, a negative correlation between sediment size and distance from the shoreline indicated the presence of aeolian factors. Human factors were indicated by flattening beach profiles, an extremely high proportion of fine sediment (53%), and the presence of an offshore breakwater causing the formation of artificial bays, salients, and tombolos. Marine factors were indicated by wave refraction, leading to high rates of littoral drift to the East (2.31metres/minute) and West (4.86metres/minute) of the bay.

Overall, atmospheric (particularly Aeolian) and marine factors are the most significant factors affecting coastal landscapes on both Pulau Ubin and Sentosa.

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