Land Classification and Flood Characteristics of the Pampanga River Basin, Central Luzon, Philippines

Abstract

The Pampanga River basin, which is the second largest drainage basin on Luzon Island (Republic of the Philippines) , frequently suffers from severe flood events, caused by monsoon rainfall and typhoon strikes. Therefore, the aim of this study is to determine local flood characteristics and potential flood vulnerability, based on the basin's geography (e.g., distribution of topography, land use and past flood records) . Our land classification shows that the basin consists of three major topographic regions: mountain and hill, volcano, and alluvial plain. The mountain and hill region is further divided into topographic units of mountain and hill, and volcano region is subdivided into volcanic slope, volcanic piedmont gentle slope, and volcanic fan. On the other hand, the alluvial plain is divided into fan, terrace, back marsh, swamp, delta, valley plain, natural levee, meander scroll, and former channel. In the upper alluvial plain, the supply of sediments, triggered by the Philippine Fault activities, contributes to a southwestern fan I, fan II and terrace II development on the western side of the Pampanga River. Terrace I and terrace II on the other hand, develop in western direction on the eastern side of the Pampanga River. Owing to the mountains, hills, and volcanoes that surround the alluvial plain, its width is reduced to 20 km at Arayat. As a consequence, floodwaters easily concentrate and stagnate here, creating the two swamps of San Antonio and Candaba. In the lower portion of the alluvial plain, low gradient bed slopes, land subsidence, and tidal intrusion of sea water enhance poor drainage situations. As a result, floodwaters from Candaba Swamp cannot drain efficiently, causing severe floods. The alluvial plain was divided into four zones (I-IV) , based on the flood patterns that were identified by the geographical conditions of the basin. Floods in zone I and zone II located on the western and eastern side of the Pampanga River are smoothly drained. On the other hand, people in the most downstream flood-prone area (zone III and zone IV) receive much benefit from the cyclic floods by stimulating the agricultural and fishery production, but the deep inundation also damages their houses. Nonetheless, some coexist with the floods seemingly without fear, and did not even evacuate during the largest recent flood event of 2011. The population of the basin is still increasing, and flood risks should be reduced through government-initiated actions. Furthermore, in order for the region's inhabitants to take effective measures to fight flooding and sustain flood-adaptive lifestyles, they should be required to understand the local geographical characteristics and regional flood vulnerability.

Shrinking Rice Bowls: Tracing the Decline of Philippine Rice Lands

“…Land subsidence is a signi cant problem affecting rice lands in the Philippines, especially in areas surrounding Metro Manila, such as Bulacan, Pampanga, and Nueva Ecija (Espiritu et al, 2022;Rodolfo & Siringan, 2006). One major reason for this subsidence is groundwater over-extraction (Nagumo & Sawano, 2016). This leads to a quick fall of the water table and compaction of the aquifer systems, with annual subsidence average of about 2.5 cm and maximum 4 cm.…”

“…This land subsidence affects rice production in several key ways. First, it reduces the drainage capacity of rivers and increases ooding in low-lying agricultural areas (Nagumo & Sawano, 2016). Prolonged ooding damages rice crops and makes it di cult to plant and harvest rice.…”

Improving the Estimation of Rice Crop Damage from Flooding Events Using Open-Source Satellite Data and UAV Image Data

“…The region is susceptible to recurrent flooding, particularly during the rainy seasons, owing to its classification as an alluvial plain, with a portion of it transformed into a swamp. These floods primarily result from the overflow of the Pampanga River, causing water to accumulate and impede efficient drainage, leading to severe floods [31]. In an inter-view with the Candaba Municipal Agricultural Officer, it was revealed that a damage report on agriculture reaching 20% and above should be initially reported to the local officials of the Disaster Risk Reduction and Management Office (DRRMO).…”