Why Pampanga floods, and what sinking ground does to a floor height

Why Pampanga floods

• A large basin and a river that cannot carry a common flood. The Pampanga River Basin is the fourth largest in the country, about 10,540 km², and PAGASA says it “experiences, on an average, at least one flooding in a year” (PAGASA). A 2011 JICA study, drawing on earlier assessments, found the river from Masantol to Cabiao could “hardly cope with even a 5-year return period flood” (JICA and NWRB, 2011).
• The Candaba swamp. About 250 km² of swamp takes in most of the floodwater from the east of the basin (PAGASA). It works as a natural flood basin in the rainy season, so the land around it floods deep and often.
• The tide. PAGASA lists high tides coinciding with heavy rain among the causes of floods, and a 2018 river bulletin names Macabebe, Masantol, Sasmuan, Guagua, Lubao, Bacolor and Apalit for coastal flooding where the tide reaches. A high tide in Manila Bay holds back the water trying to leave.
• Pinatubo’s lahar. The 1991 eruption left more than 5 km³ of ash and rock on the volcano, and lahars carried about half of it down in the next four rainy seasons (USGS). Riverbeds filled, drainage from the fields backed up, and parts of Bacolor flooded from behind (USGS, 1996). JICA found in 2011 that San Fernando’s replacement channel could carry less than a 2-year flood.
• Sinking ground. Pumping groundwater faster than it refills compacts the soil underneath. Geologists Kelvin Rodolfo and Fernando Siringan wrote in 2006 that around northern Manila Bay it lowers the land “by several centimetres to more than a decimetre per year”, against sea-level rise of one to three millimetres (Disasters, 2006). JICA found the ground floors of several old buildings in Guagua and Sasmuan had sunk 2 to 3 m over several decades, and that sunken land “is hardly recovered once it occurs”.

How fast the ground is sinking, town by town

The European Union’s Copernicus emergency service mapped ground movement across the lower Pampanga basin from satellite radar, November 2014 to March 2021 (EMSN091). The radar tracks fixed points such as buildings and roads, so the rate is the rate at which built things go down. We took its layer and worked out each town’s median and spread. In Bulacan, ground surveys of nine control points in Hagonoy and Calumpit gave a median close to the radar’s, though point by point they differed and the authors call their rates still inconclusive (Reyes and colleagues, 2026).

“Range” runs from the slowest tenth of points in the town to the fastest. “Tide” marks the towns PAGASA lists for coastal flooding where the tide reaches. The years assume the 2014 to 2021 rate holds and the flood level stays where it is, which fits tidal flooding best; an asterisk marks a town not on that list, where the years show only what the sinking alone would do. At one of the method’s own checks against a ground GNSS station the radar was about 7 mm a year off, so towns with a median under 15 mm a year are left unstated. Sasmuan’s figure rests on few radar points.

Older measurements point the same way. JICA’s 2002 comparison of benchmarks over about 50 years gave 0.5 cm a year near Mexico and 2.5 cm a year in Santo Tomas (JICA, Vol. III). The radar gives Santo Tomas 84.8 mm a year for 2014 to 2021, so the inland rates look several times their long-run average, though JICA called its figures preliminary and the two methods differ.

How long a floor’s margin lasts

Set a floor a margin above the highest flood you know of. If the ground and the house sink while the flood line stays tied to the sea, the margin shrinks every year by the sinking rate. In Macabebe, at the median 82.3 mm a year, a 2-foot (0.61 m) margin is gone in about 7.4 years. Move the controls to try any town, height and rate.

The arithmetic is years = margin ÷ (sinking rate + sea-level rise, if added). It is a what-if under fixed assumptions, and the real rate under one lot can sit anywhere in the town’s range. Global sea-level rise is the IPCC’s 3.7 mm a year for 2006 to 2018 (IPCC AR6).

The last three rainy seasons

• July 2024, Carina and the monsoon. 671,706 people in 333 barangays across 21 Pampanga towns and cities were affected, and the province was declared in a state of calamity from 26 July. In Sta. Maria, Macabebe, water 1 to 6 feet deep came on 23 July and was reported gone by 9 August (NDRRMC).
• July 2025, the monsoon with Crising, Dante and Emong. The province declared a state of calamity on 24 July after flooding in 224 barangays across 18 towns and cities (PIA).
• August to September 2026, the enhanced monsoon and four storms. 1,216,016 people in 437 barangays were affected. On 20 September, 24 of Macabebe’s 25 barangays were still listed as flooded, most of them since 4 to 9 August, with water up to 5.5 feet (NDRRMC); on 28 September PIA reported Macabebe and Masantol still “continue to experience prolonged flooding” (PIA).

On a visit to Macabebe in May 2025, the UP Resilience Institute’s team saw a house “where a once-elevated window now rests at ground level” (UPRI NOAH).

What the rules say about building on a lot that floods

• No national floor height. The National Building Code asks that sites for homes be a safe distance from streams and bodies of water, as competent authorities decide, but sets no floor height above a flood (PD 1096). Ask the building official whether your town requires one.
• The 2-foot guidance. HLURB’s 2014 guidance recommends that towns require the lowest floor 2 feet above the 100-year flood level. It binds you only where a town has written it into its zoning ordinance, and it does not allow for sinking ground.
• The riverbank easement. Along rivers, streams and shores, a strip of 3 m in urban areas, 20 m in farmland and 40 m in forest is kept for public use, and no structure may be built in it (Water Code, Art. 51). The Water Code guide covers how it is measured.
• Subdivisions. The rules for socialized and economic projects tell developers to avoid flood-prone areas (BP 220 IRR), and PD 957 says the same for subdivisions. A project that follows these rules can still flood.
• The permit. A raised house needs a building permit, with drawings signed and sealed by the professional responsible for each trade, the architect, the civil engineer and the others, as the building permit guide explains.

Before you buy a lot or set a floor height

• Ask how high the water came. Neighbours and the barangay remember, and marks on old walls often show it. The municipal disaster office is required by law to keep a local risk map (RA 10121).
• Read hazard maps as dated. HazardHunterPH and the LiPAD flood maps are a good start, but the LiPAD maps were published in 2017 on older elevation data, and in the delta towns the ground has gone down since.
• In the tide towns, plan for the sinking. A margin set from today’s flood mark shrinks every year, so where the lot is matters as much as how high the floor goes.
• Leave the structure to an engineer. Raising an existing house, adding a floor or building on a lot that floods needs a civil or structural engineer, and so do cracks, tilting or sinking floors after a flood. Our house design service covers houses for flood-prone lots.

During and after a flood

• Leave early. PAGASA advises moving to a safe area before floodwater cuts off access (PAGASA). Under RA 10121 the local disaster council can recommend a pre-emptive or forced evacuation; when one is called, go.
• Switch off the power at the main switch before you leave, and do not switch it back on until a competent electrician has checked it (PAGASA).
• Keep out of floodwater. Do not cross flowing water above the knee, and keep children out of it (PAGASA).
• Get checked after wading. DOH says leptospirosis spreads through mud or water contaminated by the urine of infected animals and can show 2 days to 4 weeks later (DOH), and DOH officials have asked anyone who waded to get checked at a health centre within 24 to 48 hours, even without symptoms (Manila Bulletin, 2026). DOH lists pregnant and breastfeeding women and children aged 8 and under among those who should not take the preventive medicine it describes, so ask a health worker first.

What we make of this

Our reading of the evidence above. The rest of the page reports the sources and the data.

In Pampanga the same flood means different things for a floor, depending on where the lot is. In Candaba, at the swamp, the water comes often and deep: in the worst flood of 2019 to 2024, most households in its usually flooded barangays saw 4 feet or more (Nagumo and colleagues, 2026). But the ground there sank only about 1.2 cm a year, a rate too small for the radar to pin down, so the floor height chosen against the known flood is the main decision and the margin changes slowly.

In five of the seven tide towns, Macabebe, Masantol, Sasmuan, Bacolor and Apalit, the ground under houses and roads sank about 5 to 8 cm a year over the same years, and in the other two, Guagua and Lubao, about 2.5 to 4.0 cm. At the five towns’ rates a 2-foot margin is used up in roughly 7 to 12 years if nothing else changes. There, a floor height is a decision that runs out, and the choice of lot matters as much as the height. Lasting relief for those towns depends on cutting the groundwater pumping that drives most of the sinking, which no single house can do.

Sources

• Copernicus Emergency Management Service, EMSN091, subsidence in the lower Pampanga river basin; town figures are our statistics from its vertical ground-motion layer. Read 1 November 2026.
• Rodolfo K. S. and Siringan F. P., Disasters 30(1), 2006. JICA and NWRB, integrated water resources study, 2011, Volumes II and III. USGS Fact Sheet 114-97, and Major, Janda and Daag, 1996. Reyes and colleagues, ISPRS Archives, 2026.
• PAGASA, Pampanga River Basin flood forecasting page and its floods learning page. IPCC AR6 Working Group I, Summary for Policymakers, A.1.7.
• NDRRMC Situational Reports for Carina (2024) and the August to September 2026 monsoon; PIA, 24 July 2025 and 28 September 2026; UP Resilience Institute, Macabebe fieldwork, 2025. Nagumo, Ohara and Jaranilla-Sanchez, Journal of Disaster Research, 2026.
• Water Code, PD 1067; National Building Code, PD 1096; Revised IRR of BP 220 (2008); RA 10121; HLURB supplemental guidelines on climate and disaster risk (2014). DOH, Leptospirosis; Manila Bulletin, 19 August 2026.