Post-STS Vinta Assessment of Affected Areas in Zamboanga del Norte

By Lawrence R. Zamoras, PhD.


The Mines and Geosciences Bureau – Regional Office IX conducted a post-disaster assessment of the heavily affected municipalities in Zamboanga Peninsula during the onslaught of the Severe Tropical Storm Vinta (STS Vinta), which entered eastern Mindanao on December 21 and exited western Mindanao through Siocon before midnight of December 22, 2017. The Zamboanga Peninsula Region has long been outside the paths of Typhoons or Tropical storms. The path of STS Vinta on December 22, 2017 appears to be the first in recent period in Zamboanga Peninsula. It entered from Aurora-Tukuran area, the easternmost tip of the Region, passing through Pagadian, Buug, Kabasalan, Ipil, Baliguian and exited Mindanao mainland through the Siocon coast. (Figure 1)

Figure 1. [A] Path of STS Vinta across Mindanao from December 21 to 23, 2017; [B] Location of the Zamboanga Peninsula

This assessment was conducted in STS Vinta affected areas in order to understand the nature of the flooding and landslides which brought about by this meteorological event. The assessment team composed of Dr. Lawrence R. Zamoras (Supervising Geiologist) and Benjamin Sanaani (Cartographer) conducted the quick assessment in four days. It is intended to determine the different factors that caused the flooding and to assess whether it is due to man-made activities such as effects of logging and/or mining activities, or due to natural factors such as high rainfall or storm surge. Understanding the nature of the disaster will be helpful in our future preparations to prevent or mitigate the effects of such weather events.

Data collection was taken from LDRRMC as well as from the short field visit in STS Vinta-affected municipalities and interviews with local residents. Rainfall data were taken from PAGASA and other local rain gauge stations such as in TVI-Siocon and SODACO-Sirawai. These data are used in determining the factors that caused the STS Vinta flooding.

BACKGROUND

STS Vinta traversed across Mindanao leaving so much havoc and casualties (Table 1). For the people of Mindanao particularly in Zamboanga Peninsula, the severe flooding brought by STS Vinta with its devastation was a lifetime experience. Victims often blame their fate on logging or mining activities, forgetting the fact that typhoons or tropical storms naturally and commonly generate widespread flooding in Luzon, the Visayas and Mindanao, particularly Cagayan de Oro City. Flooding naturally follows when the rainfall of a typhoon or tropical storm is very high, and the STS Vinta traversing Mindanao is not an exception.

After the aerial inspection by the Department of Agriculture Secretary Manny Piñol around Siocon, Sirawai and Sibuco areas, he made pronouncements blaming the logging operations for the massive flooding and recommended to President Rodrigo Duterte to halt alleged logging operations of the Consunji family in the Zamboanga Peninsula (http://news.abs-cbn.com/news/01/07/18/piol-blames-logging-for-zamboanga-floods-after-vinta). Consequently, the Department of Environment and Natural Resources expedited its investigations on the logging activities over Siocon, Sirawai and Sibuco. Various aspects of assessments had been undertaken by its several agencies including the Mines and Geosciences Bureau.

Table 1. Varying impacts of STS Vinta in selected affected towns of Zamboanga del Norte (data taken from the OCD-Region IX)

FINDINGS

A. STS VINTA & RAINFALL

The center of STS Vinta passes through affected areas between 5pm (in Buug) and 11 pm in the Sulu Sea near Siocon. Its path is practically latitudinal, traversing Pagadian City, Buug, Kabasalan, Ipil, Baliguian to Siocon. However, the estimated diameter of STS Vinta exceeds 200 km based on satellite images thus having very wide area affected. The areas traversed experienced heavy rainfall hours or even a day before the passing of the center of STS Vinta (Figure 2).

Figure 2. The passing of STS Vinta across the Zamboanga Peninsula and the different periods of flooding in the region.

 The actual measure of rainfall however is somewhat very limited because only few rain gauges have relayed readings. Table 2 shows the list of municipalities with recorded rainfalls. Different rainfall intensities and durations have been noted through anecdotal accounts. The highest rainfall reported was that in Sirawai with 299 mm followed by that in Labason with 219 mm. Due to limited rainfall data however, especially the lack of rainfall data along mountainous inner section of the Zamboanga Peninsula, we do not have measured basis to systematically understand where and when floods are bound to occur. The distribution of rainfall data could have depicted the concentration of rainwater as to which catchment basin they concentrate, which could have explained the magnitude of flooding. However, based on the Himawari-Floater loop satellite image of NOAA, it appears that around 5 pm of December 22, there is a distinct concentration or thickening of clouds near the center of STS Vinta around 5pm onwards covering the municipalities Salug, Liloy, Labason, Gutalac, Baliguian, Siocon, Sirawai and Sibuco of Zamboanga del Norte; and Kabasalan, Ipil, Titay, RT Lim and Tungawan of Zamboanga Sibugay (Figure 3). This was likely the re-intensification of the STS Vinta as it regained more moisture while approaching the Sulu Sea. From that point, STS Vinta formed a well-defined circulation which expanded as it moved westward through Sulu Sea.

Table 2. Amounts of rainfall STS Vinta-affected towns in Zamboanga del Norte where rainfall data were available
Figure 3. Satellite image of STS Vinta showing the bulge of clouds near the center of STS Vinta at about 5 pm Dec 22, 2017, when it was directly above the flood-affected municipalities of the Zamboanga Peninsula. (NOAA, Himawari-Floater)

B. CATCHMENT BASINS

The size of a catchment basin of a particular river system will determine the volume of rainwater that will gather to cause floods. Larger catchment basins tend to generate higher floods and with longer durations. When a high-rainfall typhoon passes and pours rain over a region, the larger catchment basins naturally gather more volumes of water such as Kipit River/ Labason River, and Siocon River/ Lituban River (Table 3; Figure 4). Those catchment basins in Sirawai and Sibuco areas are relatively smaller than those in Labason or Siocon. However, for Labason and Siocon despite the bigger volumes of floodwater, their floodplains are very wide, thus, allowing the floodwater to spread over the plains and lower the flood height. For Gutalac and Sibuco terrains that are mountainous, floodwaters are confined along narrow valleys, thus, resulting in extremely high flood heights.

Table 3. List of catchment basins of western Zamboanga del Norte
Figure 4. Map of the western portion of Zamboanga del Norte showing the river systems and their inland extents.

C. LANDSLIDE AND FLOODING

1.Salug Municipality
The Geohazard Map of Salug Municipality shows that majority of its terrain falls under Moderate Susceptibility to Landslide. Areas with High Susceptibility to landslide are the narrow steep terrains near river channels. The floodplains along the downstream section or rivers mostly near the coast are under Low to High Susceptibility to flooding. Experiencing a high-rainfall with wind gustiness, the portions of the map shaded red are likely to generate landslides, while the portions shaded lavender to purple are likely to undergo flooding.

The Geohazard Map of Salug Municipality shows that majority of its terrain falls under Moderate Susceptibility to Landslide. Areas with High Susceptibility to landslide are the narrow steep terrains near river channels. The floodplains along the downstream section or rivers mostly near the coast are under Low to High Susceptibility to flooding. Experiencing a high-rainfall with wind gustiness, the portions of the map shaded red are likely to generate landslides, while the portions shaded lavender to purple are likely to undergo flooding.

Figure 5. Salug Geohazard Map 1:10,000 Scale shows an area with widely varying landslide susceptibility ratings from Low, moderate to High, very limited areas that are landslide-prone. (MGB, 2014)

As per anecdotal accounts a resident in Poblacion Salug received a phone call informing her that torrential rains started upstream earlier around 2pm. This rainfall corresponds the area of Godod Municipality where the reported rainfall was 166 mm. In the lowland Poblacion Salug, the torrential rains poured later around 4 to 6 pm. Such timing of heavy downpours creates a scenario wherein the arrival of surging floodwaters from the upland coincides with the flooding in the lowland due to later heavy downpour. The combined larger volume of floodwaters from the lowland and surging floodwater from the upland results in flashflooding. The LGU mentioned of collapse of an ‘undocumented’ NIA dam upstream, which may have caused the flashflood, but such dam could not be found in the Google Satellite Image, thus nonexistent and unlikely to cause the flashflood.

Photo 1. [A] Uprooted coconut trees carried by the flashflood dumped along the flood path showing alignment that indicates flow direction; [B] height of the flashflood reaching 2.5 meters in Sitio Campo in Poblacion Salug; [C] View of Sitio Campo showing only few houses that survived the flashflood [D] debris materials of uprooted trees and house materials (8°06’ 20.46”, 122°45’ 05.6”).

According to residents the flashflood was short-lived that lasted only around 30 minutes from 5:30 pm to 6:00 pm of December 22. In Sitio Campo, Poblacion Salug where the total casualty was 17 persons, the height of the flood from their ground base was 2.5 to 3 meters, but the total height of the flood from the normal level of the river is estimated 6-meter high. The surge was very strong carrying uprooted coconut trees and other trees as well as corrugated roofs and other house materials. The flood surpassed the elevated national highway and bridge by about 1 meter.

As per anecdotal accounts the residents of Poblacion Salug particularly near the river channel of Salug River had been ordered to evacuate to safer grounds but some residents particularly in Sitio Campo refused to follow as they are used to floods in their area and believed that they can manage to save themselves. When the raging waters of flash flood quickly engulfed their place, it was too late to escape.

Figure 6. Google Satellite Image of Salug showing the locations of [A] Poblacion Salug and its two parallel rivers namely Mukas River and Salug River; [B] devastated Sitio Campo (encircled).

2. Labason Municipality
The Geohazard Map of Labason Municipality shows three main characteristics: the flood-prone coastal plains in the north, the moderately susceptible to landslide western portion, and the high to very highly susceptible to landslide eastern portion. For the STS Vinta with such intense rainfall, these areas identified as highly to very highly susceptible to landslide will generate landslides in sporadic locations and in varied extents. The debris materials will then be carried by floodwaters to cause thick mud deposits in the floodplains.

Figure 7. Labason Geohazard Map 1:10,000 Scale (MGB, 2014) showing that most of the southeast portion is under High to Very High Susceptibility to landslide, while the most of the southwestern is under Moderate to Low Susceptibility to landslide; the northern coastal portion mostly flood prone at Moderate to High Susceptibility ratings.

The catchment basin of Kipit River that drains in Labason town is very extensive and extends far south from Labason to Gutalac, and Kalawit ZDN and even farther south to Titay ZSP. Its total catchment area is 70,750 hectares. Such huge catchment basin will gather larger volume of floodwater and can cause bigger flood downstream. During the STS Vinta, the lowland Labason area experienced peak rains from 4 pm to 10 pm, with recorded 219 mm rainfall. It generated flashflood around 6 pm to 8 pm with flood height of 1.5 meters in the Poblacion area. The vast coastal plains of Labason significantly lowered the flood height as the extensive flat grounds allowed the water to spread out, thus having relatively lower flood height. Furthermore, the successful Forced Evacuation ordered by the LGU averted local residents from the flood hazard resulting in zero casualty.

3. Gutalac Municipality
The topography of Gutalac Municipality is generally hilly to mountainous. Its geohazard map shows that the northeastern portion comprising about a third of the municipality is generally gently sloping making landslide susceptibility low. The larger southwestern portion is categorized as moderately to highly susceptible to landslide for its hilly to mountainous topography. Its geohazard map also shows that the Gutalac Municipality has very minimal flood prone areas, which are confined along river banks and along few coastal areas. The rivers in the Gutalac area are V-shaped, characterized by very narrow floodplains, extending upstream about 10 kilometers long. Its two river channels are Panganuran River and Sibalik River.

Figure 8. Gutalac Geohazard Map 1:10,000 Scale showing generally safer grounds (Low Landslide Susceptibility) in the northeast portion and Moderate to High Landslide Susceptibility in the southwest. (MGB, 2014)

During the STS Vinta period, the rain started in the morning, but the peak rainfall occurred between 3 pm to 6 pm. In the more populated northeastern area characterized by gently rolling terrain no serious flooding was reported. The mountainous southwestern portion with narrow river channels were filled with floodwater, which surpassed the Panganuran bridge level by 1.5 meters in Brgy Mamawan. The impact of the flashflood around 7 to 8 pm in Brgy Panganuran along the downstream section of the Panganuran River brought destruction of properties of local residents. The total number of casualties is 28 persons which took place along the Panganuran River.

Photo 2. Panganuran River Bridge in Brgy Mamawan, Gutalac Municipality: The bridge was completely submerged during the flood on Dec. 22, 2017 resulting in the cutting-off of the bridge approach section. It resulted in the isolation of Sirawai, Siocon and Baliguian municipalities from Dec. 22 to 27, 2018 hindering transport of relief goods to these municipalities; [A] Looking south, [B] Looking north (7°57’ 06.47”, 122°18’ 25.77”).

4. Baliguian Municipality
The Baliguian Municipality topography is mostly mountainous with steep slopes. In the Geohazard Map of MGB, about 95% of its land area is landslide prone with susceptibility rating of mostly High to Moderate. It features a topographic divide, splitting the northwest coastal side from the southeast inner side. The northwest forms a narrow watershed with shorter river channels draining directly to the sea, while the vast southeast forms a watershed that drains to the southwest toward Siocon Municipality. When it comes to intense rainfall such as STS Vinta, landslides are potential problems anywhere in the 95% of the municipal area, but for the huge watershed in the southeast flooding is big hazard since its river channels will carry huge volumes of floodwaters, which will flow toward Siocon River. The coastal northwest side has smaller river channels that independently drain to the sea without converging, thus, not prone to generate high flood levels except with coastal flooding due to storm surge.

STS Vinta generated flooding in southeastern watershed of Baliguian particularly in Barangays Linay, Diangas, San Jose and Guimotan. From these barangays the total number of casualty is 10 persons. A collapsed NIA dam accordingly had saved the residents of these barangays since flood waters began diverting into these barangays prior to the collapse of the said dam.

Figure 9. Baliguian Geohazard Map 1:10,000 Scale (MGB, 2014) showing that most of the Baliguian area is under High to Moderate Susceptibility to landslide due to the mountainous terrain. Only very narrow areas are flood prone such as coastal flat areas and the upstream section of Siocon River which runs across the mid-section of the municipality.

5. Siocon Municipality
The Geohazard Map for Siocon Municipality shows that the Siocon plain is generally highly susceptible to flooding while the surrounding mountainous areas are highly susceptible to landslide. The effects of STS Vinta with its high rainfall would trigger rampant landslides in these areas shaded red (High Susceptibility to Landslide). Debris materials including rock fragments, soil and vegetation (trees) would then be washed out by flood to be transported downstream.

Figure 10. Siocon Geohazard Map 1:10,000 Scale (MGB, 2014) showing an extensive flood-prone coastal plain that is surrounded by mountainous terrain categorized as High Susceptibility to landslide. Two river systems drain across the coastal plain where Poblacion Siocon is located.

The Municipality of Siocon developed on a flat area which serves as the floodplain of the Siocon River. This river system has a very huge catchment basin with a total area of more than 50,500 hectares covering the municipalities of Siocon and Baliguian. The main Siocon River extends northeast through the Baliguian. Lituban River, draining the eastern portion of Siocon town, merges with Siocon River in the New Lituban area. The confluence of Siocon River and Lituban River can generate flashfloods to the Poblacion during heavy downpours. During the STS Vinta there was heavier rainfall in the southeast side of Baliguian which is the upstream of Siocon River, while relatively lighter rains prevailed over the eastern Siocon Town as per TVI Canatuan, as supported by its 142 mm rainfall for December 22 (24 hrs). This was further supported by anecdotal accounts that the surge of floodwaters from Siocon River caused backflow along the Lituban River starting from their point of confluence.

Figure 11. STS Vinta centered within the Sulu Sea may have caused Storm Surge raised the sea level in the Siocon Bay, which prevented the floodwater from draining into the sea causing 7 to 10 hours of flooding in Siocon and Sibuco (Brgy Anongan). (NOAA, Himawari-Floater)

The rain in Poblacion Siocon began 6:30 am to 9:00 am, and then heavy rainfall poured from 12:00 nn up to 6:30 pm. Flooding in the upland Siocon River was reported 6:30 pm. At 7:30 pm the head of the flashflood was reported at the spillway section of the Siocon River. Before 8:30 pm, the flash flood had already arrived in the Poblacion area. Local residents believed that the flashflood was caused by the collapse of their NIA dam and the breaching of their river control structure.

Flooding in Siocon lasted for more than 10 hours from 8:30 pm of December 22 up to the day time of December 23, but the peak flooding was around 9 pm December 22 to 1 am December 23. The flood height in the market area was roughly 2 meters high, other areas it is 1.5 meters. Since Poblacion Siocon is a flat coastal town, it is hard to imagine that floodwaters can remain on land for more than 10 hours when the coast is just about 500 meters away. Thus, the factor of Storm Surge is taken into consideration. By 10 pm and onwards the center of STS Vinta had already entered the Sulu Sea and had re-intensified, it may have caused rising of the sea level affecting the Siocon Bay. The high tide for Siocon area (Figure 12) is further around 10 pm December 22 up to 2am December 23, with maximum level of 1.3 meters at 0:25 am December 23 (http://www.tides4fishing.com/). These combined factors of Storm Surge and the High Tide may have hindered the draining of huge volume floodwaters over the plains of Siocon. Even during the low tide at 7:08 am December 23, the floodwaters still remained at about 0.5 meter high, the storm surge may still have some effects on the sea level of Siocon Bay.

Figure 12. High tide for Siocon occurring at 0:25 am Dec 23, but the tide is generally high between 10 pm Dec 22 and 2 am Dec 23; this further added the height of the sea level, which prevented the floodwater from draining into the sea (http://www.tides4fishing.com/).

Siocon reported 505 partially or totally damaged residential houses and a total number of 10 casualties. Accordingly, the LGU implemented forced evacuation earlier but some residents still returned to their houses to monitor their belongings, only to be taken by the flashflood. Having such tremendous volume of floodwater, it is advantageous that the Siocon plain spans about 5 km long and about 4 km wide. The floodwater had enough space to spread out that significantly lowered the height of the flood.

6. Sirawai Municipality
The Geohazard Map for Sirawai shows a vast area characterized by High (red) to Moderate (green) Susceptibility to landslide. The topography of such landslide-prone area is mountainous with steep slopes. The SODACO area in Sirawai occupies the eastern upland portion which is within generally moderate susceptibility to landslide, but there are also extensive narrow zones under high susceptibility to landslide. The potential effects of STS Vinta with high rainfall would include landslides particularly steeper areas, and soil erosion in bare areas that are currently being planted.

Figure 13. Sirawai Geohazard Map 1:10,000 Scale showing that most of its areas are categorized under Moderate to High Susceptibility to Landslide. (MGB, 2014)

There are two main river systems within Sirawai Municipality, such as Siragwai-Panabutan River and the Piacan River. Their catchment basins are confined within the Sirawai area. The Siraguay & Panabutan Catchment basin spans 9,875 hectares, while that of the Piacan River has 7,108 hectares. These river systems have estimated lengths of around 5 to 7 km, which are relatively shorter, thus, its floods may not be as massive as those in Siocon and Labason. However, since STS Vinta rainfall was abnormally high, flooding was inevitable.

Rains occurred for about 8 hours from1 pm to 9 pm on December 22, 2017, but the torrential rains poured around 5 pm to 6 pm. The rainfall data from SODACO-Sirawai rain gauge registered 299 mm rainfall for December 22, the highest among the available rainfall data. Flooding however occurred much later between 9 pm to 11 pm giving flood height of 0.5 meter in the Poblacion area. 

By anecdotal accounts from coastal residents described the 1-meter height of the floodwater in the coastal residential area or about 3-meter high from the mean sea level. This indicates occurrence of coastal flooding, which is likely caused by a storm surge event. Storm surges can be generated in the sea area that is directly below a typhoon or cyclone. It is during this time about 10 pm when STS Vinta had entered the Sulu Sea. Furthermore, it was also between 10 pm December 22 to 2 am December 23 when the high tide occurred that peaked at 12:25 am December 23 with 1.3 m height above the mean sea level. The combined effects of the storm surge and the high tide significantly raised the local sea level of Sirawai Bay, which resulted in obstructing the flow of floodwater coming from the catchment basin into the sea. The rise of the sea level is estimated to be 3 to 4 meters above the mean sea level. The obedience of the residents or their willingness to cooperate when ordered to evacuate resulted in zero casualty for Sirawai.

7. Sibuco Municipality
The coastal municipality of Sibuco has an elongated shape and has an extensive land area of roughly 782 sq km. But access or contact to most of its barangays are hampered by poor road infrastructure, poor telecommunication and serious security issues. Thus, the implementation of pre-disaster preparations of the LGU such as forced evacuation or monitoring for the safety of their constituents may have been challenging. As the torrential rains of STS Vinta and the succeeding flashflood in Sibuco occurred at night, most residents were already inside their respective homes. In inhabited areas that are highly susceptible to flooding, the presence of residents in their houses at night time during an onset of flashflood is very dangerous. Most of the casualties in Sibuco occurred in Brgy Anongan, the farthest coastal barangay from the Poblacion, particularly along Anongan River and vicinity wherein most residential houses were swept by the flashflood. This barangay had the highest number of casualties in the Zamboanga Peninsula during STS Vinta reaching 36 persons.

Figure 14. Sibuco Geohazard Map 1:10,000 Scale indicating that most of its areas are highly susceptible to landslide (MGB, 2014)

Sibuco Municipality has 4 river systems namely Anongan River, Panganuran River, Sibuco River and Lintangan-Malayal River with respective catchment basin areas of 10,279 hectares, 13,298 hectares, 14,122 hectares, and 9,401 hectares. During the STS Vinta, flashfloods occurred in the northern two rivers such as Anongan River and Panganuran River. Heavy rainfall poured from 7 pm to 12 mn of December 22, 2017, relatively at a later time compared with the other towns of Zamboanga del Norte. The intensity of rainfall in the area is 299 mm as measured by SODACO rainfall gauge. Flashflood occurred at 8 pm carrying uprooted tree trunks that smashed against the light-material houses located in the delta area of Anongan River. The flood in the coastal area of Brgy Anongan lasted from 8 pm of December 22 up to 3 am of December 23, 2017. Since the populated coastal ground has elevation of only about 1 to 2 meters, the general flood height of 5 meters with uprooted tree trunks definitely devastated the area. The LDRRMC of Sibuco reported 80% household damage for Brgy Anongan. Furthermore, the extremely high amount of rainfall, 299 mm, recorded at the upland area of SODACO, caused much higher flood level of 15 meters in the upstream section of Anongan River based on the markings on coconut trees, as per Sibuco LDRMM.

Figure 15. Densely inhabited strip of coastal grounds adjacent to Anongan River delta, which is categorized under High Susceptibility to flooding by the MGB Geohazard Map (Google Satellite Image 2018)

The roughly 7-hour duration of the flood in coastal Brgy Anongan, Sibuco may also be partly caused by a storm surge generated by STS Vinta, which also occurred in Siocon and Sirawai. The 5-meter high flood on a coastal area of Brgy Anongan may be mainly caused by the extremely high rainfall, but its 7-hour flood duration definitely implies a certain sea level rise connecting the sea with the floodwater from STS Vinta. This was further compounded by the high tide period from 10 pm December 22 to 2 am December 23, 2017. The overall effect was blocking of the floodwater from draining into the sea, which resulted in widespread and prolonged flooding around the delta and along the floodplain of Anongan River.

D. FOREST COVER FACTOR

Based on the 2010 data of the Forest Management Bureau of the DENR, the total Forest Cover for Zamboanga del Norte is 82,757 hectares out of 730,100 hectares, or 11.34%; this is the largest in Zamboanga Peninsula. The areas with thicker forest cover are particularly found in vast portions of Baliguian, Siocon, Sirawai and Sibuco based on the 2016 Google Satellite Image (Figure 16). This forest cover can be observed along the newly opened Zamboanga del Norte Highway across Baliguian Municipality. Such thick forest cover cannot be found in Salug, Liloy and Labason Zamboanga del Norte, as these areas had already undergone deforestation and converted to agricultural land. Although there are patches of denuded sections, the forest covers of Baliguian, Siocon, Sirawai and Sibuco are still more extensive and much thicker than the surrounding municipalities of Zamboanga del Norte (Gutalac, Labason and Salug), as well as those of Zamboanga Sibugay.

Figure 16. Thick forest cover is generally extensive in the municipalities of Baliguian, Siocon, Sirawai and Sibuco as encircled (Google Satellite Image 2016).

During the onslaught of STS Vinta, flooding appeared to have no preference in occurrence. Flashfloods took place in the municipalities of Zamboanga del Norte regardless of the presence of forest covers. Salug, Labason and Gutalac experienced severe flooding as much as the extensively forested Baliguian, Siocon, Sirawai and Sibuco. It is therefore difficult to prove that the severe flooding during STS Vinta Siocon, Sirawai and Sibuco towns were particularly due to the logging activities.

CONCLUSIONS

  1. Widespread flooding: Severe flooding due to STS Vinta occurred on December 22, 2017 in the Zamboanga del Norte municipalities of Salug, Labason, Gutalac, Baluguian, Siocon, Sirawai and Sibuco. It also occurred in Zamboanga Sibugay such as the municipalities of Imelda and Kabasalan. The timing of flooding occurred in sequence; earlier in eastern towns about 4 pm of December 22, and about 7 pm onwards in the western towns coinciding with the movement of STS Vinta.
  2. Rainfall intensity: Although the rainfall data were very limited, across affected areas, the rainfall intensity was indicated by the bulge of cloud near the center of STS Vinta about 5 pm of December 22, thus, indicating the high magnitude of rainfall on the municipalities directly below including the heavily affected areas of Salug, Labason, Gutalac, Baliguian, Siocon, Sirawai and Sibuco. Such rainfall intensity coincides with anecdotal accounts of local residents who survived the flashfloods.
  3. Landslide occurrences: Most areas traversed by STS Vinta, particularly the inner section of the Zamboanga Peninsula, are mountainous, which have been categorized mostly under High Susceptibility to Landslide. It is natural for these steep terrains to generate landslides during Typhoons, Tropical Storms or even just monsoonal rains, depending on the intensity and duration of rainfall. Thus, regardless of the presence or type of vegetation whether forested or grass-covered, such steep slopes under the prolonged torrential rainfall of STS Vinta, occurrences of landslides or slope failures are inevitable. Several landslide occurrences were encountered during the field visit, fortunately no residential houses were directly affected. The floodwater then transported the landslide debris materials and dumped as thick deposits of mud on the floodplains and coastal grounds of Siocon and Labason after floodwaters subsided.
  4. Storm surge: Around 9 to 10 pm of December 22, STS Vinta had already re-entered the body of water, thus possibly causing a storm surge along the coastal plains of Siocon, Sirawai and Sibuco. The effects of rising sea level due to storm surge is further aggravated by the high tide period which lasted 4 hours from 10pm December 22 up to 2am December 23, with peak height of 1.3 meters at 12:25 am of December 23. Without the storm surge factor, the sea level could have only risen by 1.3 m, but what happened during the STS Vinta is that coastal grounds had much higher flood heights 2 to 5 meters with longer durations of up to 10 hours. Therefore, the occurrence of storm surge has likely caused the severe flooding in Siocon Sirawai and Sibuco, which was estimated to be 2-meter high in the Siocon market area, and up to 5-meter high in Brgy Anongan, Sibuco. The volume of floodwater coming from their generally huge catchment basin had not drained effectively into the sea due to rising sea level, causing the floods which lasted for 7 to 10 hrs.
  5. Casualties. The areas with highest number of casualties such as Brgy Anongan (Sibuco) with 36, and in Brgy Panganuran (Gutalac) with 28, are generally remote areas whose respective accessibilities are difficult, and where the implementation of forced evacuation is rather challenging. Our interviews with the LDRMM of the concerned municipalities of Zamboanga del Norte found out that those municipalities where forced evacuation was successfully implemented had zero casualty while those that failed had numbers of casualties. The casualties in the barangays of Siocon and Baliguian, were reportedly due to the collapse of a headwater NIA dam, which submerged low lying communities along the floodplain. For Salug Municipality the number of casualties as per anecdotal accounts resulted from refusal of residents to obey preemptive evacuation order.
  6. SODACO Logging Activity Issue: Flooding occurred along all river systems from Salug, Labason, Gutalac, Baliguian, Siocon, Sirawai to Sibuco of Zamboanga del Norte, many of which are outside the influence of the SODACO area. These floods are mainly associated with being along the path of the STS Vinta, in similar manner that flooding in Luzon or Visayas normally happen when being traversed by a high-rainfall typhoon or LPA. The major factors causing the widespread flooding under STS Vinta include (1) the extremely high rainfall and, (2) the compounding effects of the high tide and the storm surge for the western municipalities of Siocon, Sirawai and Sibuco. Thus, there is no apparent correlation whether or not the flooding was caused by logging activities particularly in the SODACO IFMA in the Baliguian-Sirawai area.
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