Manila Observatory analyzes LPA that affected Mindanao

Background

A low-pressure area (LPA) has been dumping heavy rains over Mindanao and parts of the Visayas since Jan. 10. The National Disaster Risk Reduction and Management Council (NDRRMC), local DRRM councils and the media have reported numerous landslides and flashfloods resulting from the continuous rainfall.

As of the NDRRMC report on the night of Jan. 16, the effects of the LPA had claimed over 31 lives, while seven people were missing and 36 had been injured.

The heavy, continuous rainfall has affected around 376,000 people (approximately 79,000 families). Thousands of families from Regions 10, 11 and Caraga have been evacuated and are being served by relief operations. Several local government units (LGUs) have declared a state of calamity. However, according to the DPWH, many roads are still impassable due to the damages caused by landslide debris and floods, which washed out and completely destroyed some bridges along the Surigao del Sur-Davao Oriental Coastal Road, among many other roads. Classes and flights were suspended or cancelled due to inclement weather conditions. The Libuganon River and Saug River swelled from the immense amount of runoff. Communities and households near the rising Magpayang River were evacuated.

Southern Leyte, recently devastated by Typhoon Yolanda (Haiyan) two months ago was also affected by the LPA, with reported landslides in Sogod municipality. In addition, some of these strong floods and rainfall-induced landslides occurred in the provinces of Davao Oriental and Compostela Valley, which were areas affected by Typhoon Pablo (Bopha) in December 2012. Typhoon Pablo was a major disaster that caused massive losses and damages. Many communities currently affected by the LPA have not yet made a full recovery.

Landslide and flood hazards

Numerous landslides and floods resulted from the LPA. Figure 1 shows the location of several reported areas where there were landslides and floods on a topographic map, with a 10-meter inundation. Most reported floods and landslides were from the eastern side of Mindanao, although there were notable landslides in Lanao del Norte, ARMM and Zamboanga del Norte as well, based on recent NDRRMC data.

The provinces of Dinagat Islands and Southern Leyte also experienced landslides as a result of the heavy rainfall while island province Camiguin reported flooding. Based on the topographic map and the location of the LGUs, the reported floods and landslides happened in low-lying areas, at the foot of areas of higher elevation, or close to rivers. For example, much of the area near Tagum City (including New Corella, Asuncion) lies within the low elevation coastal zone (LECZ) that would be completely flooded by 10-m inundation. Also within the LECZ is Butuan City in the northern part of Mindanao, where the LGU reported a landslide event. The towns of Carrascal and Lanuza are also close to the LECZ.

Exposed population

Based on population statistics from 2010, the flooded areas of Tagum City and its environs are heavily populated (Figure 2), and this is where some landslides occurred as well. The Dinagat Islands are also heavily populated. Based on the data, most coastal towns have higher population than inland municipalities. A zoom-in on this area (Figure 3) shows that the relatively larger populations of Tagum, Asuncion, Nabunturan and other Compostela Valley cities and municipalities had to evacuate due to floods and the occurrence of landslides in their area, as reported by the NDRRMC.

Satellite-based rainfall estimates

An LPA is an area characterized by low atmospheric pressure relative to other surrounding areas. The difference in pressure drives the formation of clouds as warm air cools, which leads to precipitation.

Data from the Tropical Rainfall Measuring Mission (TRMM) show the amount of daily total rainfall in millimeters for the period of Jan. 11 (Saturday) to Jan. 15 (Wednesday). The highest rainfall values were recorded, between 350-400 mm on Jan. 12 (Sunday) over the northeastern regions of Mindanao and toward Southern Leyte (Figure 4).

The TRMM data show that rainfall was consistent over the period from Saturday to Wednesday as the LPA moved northward, with the Lanao region receiving a peak of 150-200 mm. This continuous rainfall would have saturated surface water basins, groundwater and accumulated as runoff, causing conditions conducive to flashfloods and landslides.

Historical comparison

Compared to historical rainfall data for these areas, how heavy was the rainfall compared to the normal monthly averages? Figure 5 shows areas of interest where the NDRRMC received reports of landslides and flashfloods (refer to Figure 1).

The province of Dinagat Islands, located north of mainland Mindanao, has an average daily rainfall total of 11.97 mm and a monthly average rainfall of 370.99 mm in January. Data from the TRMM show that daily rainfall was well over the historical daily average, and reached over 200 mm on Jan. 12 (Sunday) and Jan. 13 (Monday), with an accumulated rainfall total of around 725 mm (Figure 6).

The province of Lanao del Norte similarly received more than its daily rainfall average of 3.78 mm, with the highest amount of rainfall of 70.9292 mm recorded on Jan. 11 (Saturday) and an accumulated rainfall total of around 163 mm, which is higher than its total monthly rainfall average of 117.08 mm (Figure 6).

The province of Davao Oriental received more rainfall on Jan. 11 than its average monthly rainfall, receiving 246.616 mm in one day. Normally, it only receives 6.46 mm per day and has around 200.26 mm for its usual January average rainfall (Figure 6). Davao Oriental received a total of 559.2726 mm from Saturday to Wednesday.

Compostela Valley received most of its rainfall on Jan. 12, with 239.115 mm of rain compared to its January mean daily rainfall of 6.66 mm and monthly average of 206.41 mm. A total of 594.7321 mm accumulated in Compostela Valley for this period because of the LPA (Figure 6). Similar to Compostela Valley, Davao del Norte also received its peak rainfall on Jan. 12, around 198 mm. Rains continued throughout Wednesday to accumulate 511.6468 mm for the five-day period, which is significantly higher than its average monthly rainfall of 146.54 mm (Figure 6). 

In an area of particularly high rainfall (see inset in Figure 5), in the area between Surigao and Agusan del Norte, rainfall was measured to be the highest on Jan. 12 with 293.107 mm, the highest in a day among the selected areas. The accumulated total rainfall was 592.2803 mm, close to 150 mm above the mean monthly rainfall of 455.54 mm (Figure 6). This is an area that receives an average of January daily rainfall total of 14.69 mm. Key cities near this area have previously experienced extreme rainfall. In February 1964, a total of 123.78 mm was recorded in Butuan City, while Surigao City experienced 221.48 mm of rain in January 1978. These two cities received rainfall of more than 250 mm during the Jan. 12 event.

Among the four provinces discussed here, the Dinagat Islands received the highest accumulated rainfall. This was followed by Compostela Valley, the area between Surigao/Agusan del Norte and the Pablo-affected province of Davao Oriental.

Future conditions and conclusion

The rain from the previous days has caused a large amount of rainfall to accumulate in most regions of Mindanao and some parts of Visayas. This rainfall has already resulted in numerous flashflood events and rainfall-induced landslides and rock/debris fall.

Floodwaters have damaged many roads and bridges, which may make relief operations difficult in some areas due to access. Recovery in areas affected by Pablo in the past may have been particularly vulnerable to the effects of this LPA. Rivers and other water bodies may have higher levels and stronger currents, and high amounts of sedimentation due to the large amount of runoff. It is recommended that residents of affected communities take precautions especially in landslide-sensitive areas as the soil is already saturated with water from nearly a week of continuous rain and as such is prone to landslide events.

As of 2 p.m. of Jan. 16, JTWC reported an area of convection located near 11.3 N, 128.3 E (east of Samar island). Based on satellite imagery, this convective area is “highly disorganized,” especially on the low levels. However, there is a “medium” possibility to develop as a tropical cyclone in the next 24 hours due to current moist conditions and sea surface temperatures.  This has now developed into a tropical depression and has been named Agaton by PAGASA. According to the weather bulletin of PAGASA issued at 5 p.m. of Jan. 17, Agaton had been stationary but was moving toward the Surigao provinces. Areas in Southern Leyte and Northern Mindanao were also assigned with signal no. 1 warnings. LGUs and communities should be prepared should the development of this tropical cyclone continue, so monitoring key weather information is important.