INCIDENT REPORT: MAY 14, 2021 ZAMBOANGA DEL SUR AND SIBUGAY FLOODING INCIDENT CAUSED BY POST-TROPICAL DEPRESSION CRISING

By Abbygail B. Soco

INCIDENT ACCOUNT AND ACTIONS TAKEN

The Philippine Atmospheric, Geophysical, and Astronomical Services Administration (PAGASA) declared Tropical Depression Crising reduced to a low-pressure area at 8 am on May 14, 2021. However, the remnant low still brought prolonged light to heavy rain over Zamboanga del Sur and Sibugay in the noon of the same day. The extensive downpour caused flooding along major rivers in the said provinces that affected several barangays in Bayog, Diplahan, Imelda, Siay, and Kabasalan. In response to the situation, the Hydrogeology and Environmental Geology Team of the Mines and Geosciences Bureau, Regional Office-IX Geosciences Division stationed in Buug for the Groundwater Resource Availability and Vulnerability Assessment of the municipality interviewed the staff of Local Disaster Risk Reduction Management Office of the Municipalities of Diplahan and Bayog, and surveyed and documented the event’s aftermath in some of the affected barangays on the evening of May 14, 2021, and in the morning of May 15, 2021.

According to the Local Disaster Risk Reduction Management Office of Bayog, Zamboanga del Sur, an hour of light to moderate downpour, which at times became heavy, caused a flash flood in the Dipili River, a tributary of the Sibuguey River, and affected Barangays Dimalinao, Pulang Bato, Dipili, Kahayagan, and Salawagan. This flash flood washed out ten houses in Barangay Dimalinao, two in Barangay Kahayagan, and damaged a hanging bridge and other structures in Barangay Salawagan. In addition, as floodwater from the Dipili River drained to the Sibuguey River, flooding also affected residents in Barangays Lamare, Depore, and Damit. The water level of the Dipili and Sibuguey River rose about three to six meters but immediately dissipated to almost normal in less than an hour. Furthermore, the oversaturation of the ground from the extensive downpour caused landslides along the barangay road in Dimalinao and the Samico Road in Purok 5 of Barangay Depase.

The downpour in the Municipality of Diplahan also started at noon. The staff of the municipality’s Local Risk Reduction Management Office recounted that the sheet flooding started at about three in the afternoon as water overflowed from the unprotected sections of the Sibuguey River channel. This event continued to progress and caused the backflow of water in the Luop River, which aggravated the current condition and affected residents in Barangays Paradise, Pilar, Luop, Sampoli A, Sampoli-B, Balangao, Gaulan, and Lobing. Residents in some affected barangays were rescued and brought to safety in motorboats as floodwater rose to over two meters. Later in the evening, the flood water level already started to drop. However, inundation is still present in some portions of Pilar and Luop the following day. According to interviewed residents, it will take five to seven days for the floodwater to dissipate.

Due to time constraints, not all of the affected municipalities were included in the post-incident inspection. However, the team reached out to their LDRRM units for accounts of the event and lists of flooded barangays.

Post-Incident Surveyed Areas

MunicipalityBarangayPurokGeographic CoordinatesFlood Hazard RatingLandslide Hazard Rating
Diplahan, ZSPilarPurok 17°44’26.7” N 122°57’42.6” EHigh Flood Susceptibility 
  Purok 37°44’36.1” N 122°57’46.7” EHigh Flood Susceptibility 
  Purok 47°44’47.6” N 122°57’51.7” EHigh Flood Susceptibility 
  Purok 67°45’13.9” N 122°58’6.5” EHigh Flood Susceptibility 
  Purok 77°44’52.4” N 122°57’53.8” EHigh Flood Susceptibility 
 ParadiseAquino7°46’5.9” N 122°58’15.7” EHigh Flood Susceptibility 
  Magsaysay7°46’ 17.9” N 122°58’15.3” EHigh Flood Susceptibility 
  Marcus7°45’24.6” N 122°58’18.2” EHigh Flood Susceptibility 
  Quezon7°45’51.4” N 122°58’17.4” EHigh Flood Susceptibility 
  Roxas7°46’26.6” N 122°58’12.9” EHigh Flood Susceptibility 
 Sampoli-APurok Mabini7°47’8” N 122°57’52.4” EHigh Flood Susceptibility 
 Sampoli-BPurok 17°47’8.9” N 122°57’50.4” EHigh Flood Susceptibility 
Bayog, ZDSDamitPurok 47°49’1.4” N 123°0’11.6” ELow to High Flood Susceptibility 
 DepasePurok 57°49’33.3” N 123°3’37” E High Landslide Susceptibility
 DeporePurok 17°50’52.3” N 123°0’52” ELow Flood Susceptibility 
 SalawaganPurok 27°50’0.8” N 123°1’14.1” ELow to Moderate Flood Susceptibility  
Figure 1. A map showing the flood and landslide susceptibility ratings of the areas visited after the Post-Tropical Depression Crising flood and landslide incident on May 14, 2021.
Figure 2. Location Map of the municipalities and verified barangays affected by flooding and rain-induced landslides during the prolonged downpour on May 14, 2021, caused by Post-Tropical Depression Crising.

RECOMMENDATIONS

The following usual recommendations based on the Geohazard Report of Flood and Rain-induced Landslide prone areas and some site-specific recommendations are included in this incident report for reiteration and guidance on future actions to mitigate the effects of flood and landslide hazards in the affected municipalities:

  1. Utilize the 1:10,000 scale Flood and Landslide Hazard Maps for the consideration of geologic hazards in land-use planning and classification and land development in the municipality, especially those within the floodplains of large river channels such as the Sibuguey River, Kabasalan River, and Dipili River, and smaller tributaries of the Siay River.
  2. Observe proper legal easement from bodies of water, in this case, from significant river systems.
  3. Install or maintain Flood and Landslide Hazard Warning Signages in critical areas.
  4. Develop or maintain flood hazard monitoring and early warning structures such as water level markers in the major river systems of the affected municipalities.
  5. Constantly monitor the situation of rivers during times of heavy or prolonged rainfall. Observe rapid increase or decrease in water level, possibly accompanied by an increase in turbidity.
  6. Maintain constant communication of updates between the barangay officials and the Municipal Disaster Risk Reduction Management Office in dealing with geohazard situations.
  7.  Increase individual awareness on flood and landslide hazards in critical areas to improve personal disaster preparedness and response.
  8. Empower and increase the Barangay Disaster Risk Reduction and Management Council (BDRRMC) capacity in attending to geohazard situations.
  9. Drainage maintenance, such as clearing of clogged river channels and mouth and proper dredging of the river channel to maintain adequate depth and volume capacity, is encouraged.
  10. Improve, properly extend, and repair damages on the flood mitigating structure established along the Sibuguey River.
  11. The teams observed that most of the severely affected houses and structures in Barangays Paradise and Pilar of the Municipality of Diplahan are those confined between the unprotected sections of the Sibuguey River and the elevated provincial road. Therefore, to ensure the safety of these barangays’ residents and properties, relocation is recommended.
Advertisement

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.