Post-Analysis of the Zamboanga and Dumaguete flooding

The massive flash flood and coastal inundation that struck Zamboanga City, Zamboanga Del Norte, and Dumaguete City was particularly controversial, where forecast models and observation data dispute facts on the ground. Official reports from the National Disaster Risk Reduction and Management Council (NDRRMC) issued at 6AM-PhT of October 16, 2017, states that a flash flood incident occurred at around 3PM-PhT of October 15, due to heavy rainfall brought about by the Intertropical Convergence Zone (ITCZ). NDRRMC also reported that the flooding affected seven (7) barangays to the West, and another two (2) to the East of Zamboanga City. Furthermore, it was initially reported that 2,101 families or 8,404 persons were affected with a total of 151 houses damaged or destroyed. Assessments were made by DRRMO and DSWD Region IX as they conducted inspection in the hard-hit zones.

It can be recalled that Severe Tropical Storm KHANUN (ODETTE) has already left the the Philippine Area of Responsibility (PAR) for Southern China the night before the Zamboanga disaster. WeatherPhilippines has issued its Final Update on the system on October 14, 2017 (See Figure 1).

figure-1

Fig. 1. Final Update on Severe Tropical Storm KHANUN (ODETTE) issued 5PM-PhT, October 14, 2017. Credits: WeatherPhilippines Foundation, Inc.

Storm Surge

First, we must set things in the right perspective using the National Oceanic Atmospheric Administration (NOAA) definition of what a storm surge is (See Figure 2). According to NOAA, it refers to the abnormal rise in sea water level during a storm, measured as the height of the water above the normal predicted astronomical tide and caused primarily by a storm’s winds pushing water onshore. “The amplitude of the storm surge at any given location depends on the orientation of the coast line with the storm track, intensity, size, and speed of the storm, and the local bathymetry,” as defined by NOAA.

figure-2

Fig. 2. A diagram showing the different characteristic of tide behavior during an astronomical tide as opposed to a storm-driven wave that is a Storm surge. Credits: NOAA https://oceanservice.noaa.gov/facts/stormsurge-stormtide.html

A storm surge can only be generated by a Tropical Cyclone such as a Severe Tropical Storm or Typhoon at the very least. Since ODETTE was too far to directly impact any part of the country, this theory cannot be proven correct. During an active Westerly to Southwesterly moisture inflow, in the absence of the Southwest Monsoon (Habagat) this time of the year, a sustained onrush of squally conditions and rough seas can cause coastal inundation as it approaches a land mass. Zamboanga City is no exception to this threat from the sea. Rising sea level is no longer a myth, and it is happening in many locations around the globe and the Philippines face the same threat as any other coastal archipelago. The topography of the Zamboanga Peninsula plays a part to this. Apparently, a locally-occurring weather system such as a thunderstorm can give rise to catastrophic conditions on a local scale (<1 km) or more, but is not far-reaching and intense to generate a storm surge.

What do we know so far?

As we try to piece together what may have caused this unfortunate incident, our post-assessment suggests that the flooding may have been caused by a rapidly developing line of thunderstorms headed Southward as it reached Sibuco Bay and Pangian Point. This is to the Northwest of Zamboanga City just before 2PM-PhT on Sunday, October 15, 2017. It took about 30 minutes to 1 hour for the storm to reach its mature stage as opposed to Zamboanga Del Norte coast. Interaction with land mass may have built up momentum of high wind, drawing more moisture across Siocon and Zamboanga City to the South of it (See Figure 3). 

figure-3

Fig. 3. An infrared satellite imagery showing a mass of thunderstorm at the time of impact over Zamboanga Peninsula at around 2PM-PhT, October 15, 2017. Credits: WeatherPhilippines Foundation, Inc.

Without warning, heavy winds blew hard from West to West-Southwesterly direction as it hurtled towards the Western sections of Zamboanga Peninsula. Our nearest Automated Weather Station (AWS) 980874 to Zamboanga City was estimated to be around 85 km South of Siocon, Zamboanga Del Norte. Just before 2PM-PhT, a thick band of cloud started to approach the Northwestern shores of Zamboanga Del Norte, with Siocon getting some reading of wind gust of 50 km/hr within the hour (See Figure 4). 

figure-4

Fig. 4. High winds bring rough sea waves onshore destroying homes along the coast of Zamboanga City. Credits: ABS-CBN News and Current Affairs.

Other meteorological data gathered by WeatherPhilippines include barometric reading that time dropped to 1007 hPa based on PAGASA Synoptic station in Zamboanga City. Our AWS in Siocon measured 112.8mm of rainfall in a day. Significant rainfall readings occurred at 2PM-PhT, 8PM-PhT, and 2AM-PhT of October 15-16, 2017.

figure-5

Fig. 5 Highest wind gust as of 2pm-PhT, October 15, 2017. Credits: WeatherPhilippines Foundation, Inc.

State of calamity declared

In the wake of the massive destruction and the loss of lives, the City government of Zamboanga has declared a State of Calamity. According to media reports, the cause of the widespread flooding, high winds, and following storm surge along the coast was the ITCZ.

figure-6

Fig. 6. Total 24-hour rainfall as of 8AM-PhT, October 16, 2017. Credits: WeatherPhilippines Foundation, Inc.

Zamboanga City is strategically located North of the Basilan Strait, flanked by nearby Titauan Island and Basilan Island. The narrow strait squeezes the strong currents from the large expanse of the Sulu Sea before entering the Moro Gulf. This, also have a relative impact on its local weather dynamics.

Negros Oriental also hit hard

A combination of factors resulted to tragic flood and landslide in Negros Oriental, with Dumaguete City and neighboring towns submerged with raging torrents as rivers burst their banks. Classes and businesses were disrupted as torrential rains reaching 177.2 mm overnight fell. Bad weather was attributed to the strengthening Westerly to Southwesterly windflow pulled in by a developing tropical cyclone to the East of Mindanao, which later on intensified into a full-fledged typhoon with an international name LAN. It was locally named PAOLO by state weather bureau PAGASA.

LAN’s massive circulation have successfully induced the Southwest Monsoon (Habagat), in conditions where on and off squally rains and gusty winds in excess of 45-60 km/hr at times batter the Southern coast of Negros Island. The neighboring regions in the Visayas and Mindanao were also impacted by the severe weather condition for days.

According to local media source Sunstar, torrential rains have fallen several hours after midnight of Wednesday, October 18, which caused the Banica and Ocoy Rivers to overflow, rendering spillways and roads impassable in Dumaguete. It also triggered landslides in the nearby town of Valencia.

Dumaguete City Mayor Felipe Antonio Remollo ordered the cancellation of classes early Thursday, October 19, while Adrian Sedillo, Executive Officer of the Provincial Disaster Risk Reduction and Management Council (PDRRMC) announced the same for other parts of the province that were also inundated by heavy rains.

figure-7

Fig. 7. Banica River burst its banks in Dumaguete City, Negros Oriental. Credits: Dumaguete Awareness Page.

There were also incidents involving landslides, particularly in Barangay Palinpinon, where 20-30 families were initially evacuated to safer grounds. It affected the operations of Energy Development Corporation’s Negros Island Geothermal Business Unit as roads became impassable to vehicles due to flooding, and a series of landslides took place in Barangays Puhagan, Pulangbato, Malaunay, and Caidiocan.

figure-8

Fig. 8 Raging torrent. Dumaguetenos woke up to a grim day with torrential rains swamping the river systems across the mountainside of Talinis. Credits: Dumaguete Awareness Page.

Counting the cost

As of posting, a total of 51 barangays were affected across the Zamboanga Peninsula (Region IX), with 7,646 families or 34,705 persons affected by the flood and driven from their homes. Local authorities have temporarily housed them in five (5) evacuation sites. Latest report reflects that an estimated Php 49.3 million worth of crops and property was lost as the result of the incident attributing to agriculture and aquaculture industry and infrastructure damage.

In Dumaguete City alone, six (6) barangays were affected by the flash flood with another one in Siaton, where a total of 243 families or 1,215 persons were affected and temporarily relocated to three evacuation sites. All these bring misery to the afflicted communities, but relief works are well underway while rehabilitation of the badly hit areas are still being drawn by authorities.

Local events like thunderstorms are quick to develop and dissipate, possibly in about 30 minutes to an hour, and it can impact nearby areas (<1km). Long after the thunderstorm activity have come to past, the numerical models have yet to capture the variables involved in the system several hours late. This continues to become a challenge to forecasting agencies around the world, all the more reason why we must be prepared at all times.


By: Adonis S. Manzan

References:

https://oceanservice.noaa.gov/facts/stormsurge-stormtide.html

http://ndrrmc.gov.ph/attachments/article/3219/Progress_Report_re_Flashflood_Incident_in_Zamboanga_City_as_of_16OCT2017_0600H.pdf

http://news.abs-cbn.com/news/10/15/17/big-waves-sink-boat-wash-away-60-houses-in-zamboanga-city

https://www.rappler.com/nation/185748-zamboanga-city-council-state-of-calamity-typhoon-paolo

http://www.sunstar.com.ph/dumaguete/local-news/2017/10/20/heavy-rains-cause-massive-flooding-dumaguete-570450

https://www.facebook.com/pg/Dumaguete-Awareness-132338650170772/photos/

https://www.facebook.com/132338650170772/photos/pcb.1699219346816020/1699219040149384/?type=3&theater

http://www.ndrrmc.gov.ph/attachments/article/3224/Update_SitRep_No_5_re_Preparedness_Measures_and%20Effects_of_Weather_Disturbances_in_the_Country_Enhanced_by_Typhoon_Paolo_IN_Lan_and_LPA.pdf

 

©2019 WeatherPhilippines Foundation

or

Log in with your credentials

or    

Forgot your details?

or

Create Account