Samar, Philippines

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Catbalogan PAGASA Station

H. Climate and Rainfall

The province falls under the 2nd and the 4th type of climate. Type II is characterized by having no dry season with a very pronounced maximum rain period from December to February. Minimum monthly rainfall occurs during the period from March to May. Areas characterized by this climate are generally along or very near the eastern coast, thus, open to the northeast monsoon. Municipalities at the southeastern part of the province experience this type of climate.

Type IV climate has no maximum rain and no dry season (rainfall is more or less distributed throughout the year). This is an intermediate between the first and second types of climate, although it resembles the second type more closely, hence it has no dry season. Those areas located at the northwestern part of the province have this type of climate that includes the municipalities of Gandara, San Jorge, Pagsanghan, Tarangnan, Sta. Margarita, Sto. Niño, Almagro, Tagapul-an, Calbayog City and portions of Matuguinao and San Jose de Buan.

Monthly and Average Rainfall in Millimeters (2008-2012)

The average rainfall in the past five (5) years (2008-2012) was 302.2 millimeters. The highest rainfall was registered at 1,111 millimeters in February, 2008 due to continued heavy rains during the northeast monsoon. The lowest rainfall for the past five years was recorded in February, 2010 (7.90 mm.) because of the occurrence of "El Niño" which lasted for five months (February-June). Fair weather was noted in 2009 and 2011 except for the months of January and May at the latter year which recorded quite high rainfall at 991.6 millimeters and 669.9 millimeters respectively. See Table above and Figure below.

Annual rainfall in 2011 (5,227.2 mm.) was the highest in the past 5 years (2008-2012 mm.) which is nearly doubled from 2010 and likewise its average. High rainfall was noted in the months of January (991.6 mm.), March (823.8 mm.) and May (669.9 mm.).

Rainfall Pattern in Millimeter as of 2008-2012

Climate scientists have declared that climate change is upon us. And this global phenomenon that is happening now has been observed to cause extreme weather events- droughts and intense tropical cyclones/typhoons- that impact adversely on man and environment. What is more appalling is the increasing frequency and intensity of these events that is foreseen to make disastrous impacts on the climate sensitive sectors, the agriculture, health infrastructure, coastal water and forestry resources.

The government is doing considerable efforts necessary to prepare for these impacts- primary of which is by incorporating disaster preparedness and adaptation in development planning and step up investment in these areas.

Planning for disaster risk reduction and climate change adaptation was initiated by the National Economic and Development Authority (NEDA) with funding from the United Nations Development Programme (UNDP) and Australian Agency for International Development (AUSAID) and was implemented nationwide.

Initially, a disaster risk and vulnerability assessment study was conducted to identify the multiple natural hazards that affect the province and the effect of the occurrence of these hazards on the vulnerable sectors.

The province's disaster risk and vulnerability assessment (DRVA): The province is prone to three (3) hydrometeorologic hazards namely flood and flashflood, rain induced landslides, and storm surge, and three (3) geologic hazards or earthquake related hazards namely ground shaking, liquefaction and earthquake-induced landslide.

Tropical Cyclones that affected Samar Province

Hydrometeorologic Hazards:

FLOODS. A flood is defined as a "rise, generally brief, in the water levels in a stream to a peak from which the water level receded at a slower rate" (UNESCO/WMO, 1992). Some floods overflow the normal confines of a stream or other body of water and cause flooding over areas which are not normally submerged. Floods are part of the natural hydrological cycle and are generally an outcome of a complex interaction between natural random processes in the form of precipitation with the basin or watershed characteristics (World Meteorological Organization).

Floods in the province are classified into three (3) as follows: (1) River flood – which is caused by the overflowing of a river when run-off water exceeds the holding capacity of the channels or depressions and covers adjacent to low-lying areas called the "floodplain". (2) Flash flood – which is caused by a very short period of unusually heavy rainfall in a mountainous or hilly area this usually occur in the Catbalogan area; and (3) Coastal flood – which occurs when strong onshore winds push the water inland and caused a rise in sea level and floods the low-lying coastal areas.

At least 20 typhoons pass through or enter the Philippine Area of Responsibility (PAR) every year and about three (3) of these directly hit the province before moving away from the PAR to another country or dissipate into the sea.

Typhoon that pass through, and others that may not directly hit but the influence area are wide enough to affect the province, bringing heavy and continuous rains would provoke flooding on deltas or floodplains adjacent to the river systems provincewide. This is aggravated by the lesser catchment capacity of the watershed areas, clogged waterways, heavily silted rivers, and the restricted flow of floodwaters into the sea.

Of the 951 barangays provincewide, 262 barangays are highly susceptible to flooding because the settlements are situated adjacent to or within the floodplains, 165 barangays are moderately susceptible and the rest are least susceptible to flooding.

FLASHFLOODS. Catbalogan City, the capital of the province is highly susceptible to flash flooding because of its proximity to a mountainous or hilly section of the land that serves as the receiving portion of a bigger volume of rainwater that is released into the long and winding Antiao River that passes through the city proper with its tributaries within the city, before it discharges into the Maqueda Bay/sea. This is aggravated by poor drainage system of the city, improper disposal of garbage/waste materials that cause clogging of waterways and high tide occurrence during heavy and continuous downpour.

RAIN-INDUCED LANDSLIDE. The rainfall threshold that could trigger landslide is 100 millimeters of rain per day. Steep slopes, thick soil or fractured rocks or tension cracks are contributory factors. There are 229 barangays in the province that are highly susceptible to rain-induced landslides because of their proximity to the identified ground or environmental defects that is aggravated by the lack of vegetative covers of these areas.

STORM SURGE. Storm surge refers to the onshore rush of water into the coastal or low lying areas due to abnormal water level caused by high wind and low pressure during typhoon occurrence. The islands and coastal municipalities of the province are prone to storm surge.

From 2008-2011, the local disaster council have recorded about 50,000 families affected by hydrometeorologic hazards with 6 fatalities and 5 injuries and around 95,000 of the province' population are vulnerable to flooding. Damage to properties, infrastructures, agriculture and the temporary stoppage of traffic along the Maharlika Highway that cost the government millions of pesos have been reported.

Coastal erosion or shoreline retreat is currently affecting many coastal areas in the Philippines. Natural factors such as wind and waves, long shore currents and tectonic activities, as well as anthropogenic factors such as dam construction, sand mining, coral reef destruction, groundwater extraction, wetlands conversion, dredging of inlets for navigation, and boat traffic have been identified as the factors contributing to the hazard.

The impact of this coastal hazard is expected to become more widespread due to climate change and sea level rise as well as with the continuing urbanization and development of coastal communities in the country. The hazard can inflict serious adverse impacts on society. Land, properties, infrastructure, and natural resources, such as sandy beaches, can be destroyed.

Landslide are described as downward movement of a slope and materials under the force of gravity which includes a wide range of ground movement, such as rock falls, deep failure of slopes, and shallow debris flows. Landslides are influenced by human activity (mining and construction of buildings, railroads and highways) and natural forces (geology, precipitation, and topography).

What causes a landslide? Landslides occur when masses of rock, earth or debris move down a slope. Gravity acting on an overly steep slope is the primary cause of a landslide. They are activated by storm, fires and by human modification to the land. New landslides occur as a result of rainstorms, earthquakes, volcanic eruptions, and various human activities.

The measures of landslides are mudflows (or debris flows), which are flows of rock, earth, and other debris saturated with water. They develop when water rapidly accumulates in the ground, such as during heavy rainfall, changing the earth into a flowing river of mud or "slurry". Slurry can flow rapidly down slopes or through the channels and can strike with little or no warning at avalanche speeds. Slurry can travel several miles from its source, growing in size as it picks up trees, cars and other materials along the way (US FEMA. Understanding Your Risk: Identifying Hazard and Estimating losses).

Rainfall thresholds for landslides, based on the worldwide comparisons and trends, reveal that about 100mm of rainfall per day could trigger a landslide. Based on this observation, independent studies by PHILVOLCS and their experts reveal that the amount of rainfall that trigger landslides in Southern Leyte (2003) and in the Northern Quezon and Aurora (2004) was more than three times the worldwide threshold.

Geologic Hazards:

EARTHQUAKE. An earthquake is a weak to violent shaking of the ground produced by the sudden movement of rock materials below the earth's surface (L. Bautista, 2008). Earthquakes are caused either by the sudden movement along faults and trenches (tectonic), or by the movement of magma beneath volcanoes (volcanic). Faults are fractures in the earth's surface where rock movement has taken place and earthquakes produced. Earthquakes can trigger hazards that can cause destruction to the lives and properties such as ground shaking, ground rupture, earthquake induced landslides, and tsunamis.

Samar is prone to earthquake related hazards because of the presence of three faults/lineaments within the provincial boundary. The longest fault line called the Southern Samar Lineament 1 spanning about 48 kilometers is located at the southern portion of the province near Marabut and Basey. The second longest fault is called the Southern Samar Lineament 2 that runs 30.5 kilometers long is also located near Basey and Marabut. And the third fault called the Central Samar Lineament and about 29.5 kilometers long runs from Darahuway islands to Catbalogan city and San Jorge.

Several strong earthquakes, ranging from intensity 6 to 7.8 in the Richter scale, have been recorded to have struck near Samar from 1608 to 2012 but no death nor heavy damage to infrastructure and properties have been reported.

Ground shaking is the main hazard created by seismic earth movements. This term is used to describe the vibration of the ground during an earthquake. During an earthquake seismic waves travel rapidly away from the source and through the earth's crust. Upon reaching the ground, they produce shaking that may last from seconds to minutes (Kramer, 1996).

Earthquakes strength is measured in terms of either its magnitude or intensity. Magnitude measures the total energy released at the earthquake's point of origin (below the earth's surface) based on information derived from a seismograph typically reported in Arabic numerals (e.g., 6.3, 7.2). Intensity is the perceived strength of an earthquake based on relative effects to people and structures of the earth's surface. The Philippines uses the PHIVOLCS Earthquake Intensity Scale (PEIS), which helps explain the intensity assigned to a specified location based on observations made on the consequences.

Based on the six "worst case" scenarios simulated, Samar Province has a total 1,142 barangays and sitios out of 951 barangays, sitios and islands with a very high level of earthquake induced landslide susceptibility. The highest number of barangay which are highly susceptible to ground shaking are in Basey, Paranas, Calbiga, Gandara, Daram, Catbalogan and Calbayog.

Provincewide, a total of 1,142 barangays and sitios are ground shaking prone. It is worth mentioning however, that out of the 26 municipalities/cities only the municipality of Tagapul-an is not highly susceptible to ground shaking. About 98.19% of the total land area of the province is exposed to ground shaking.

Within the province are three earthquake faults/lineaments. The longest fault line is the Southern Lineament1 which spans about 48 kilometers, starting from the northwest portion of Giporlos and running northwestward through the northeast of Balangiga, the western portion of Llorente and Balangkayan, Eastern Samar to the northeastern tip of Marabut and the western section of Basey. The second longest fault is the Sothern Samar Lineament2 which is approximately 30.49 kilometers runs from the southwestern tip of Marabut northwestward to the southwestern part of Basey. The third fault is the Central Samar Lineament has an estimated length of 29.43 kilometers starts from Darahuway Islands runs along the central portion of Catbalogan City northwestward to the central portion of San Jorge.

Liquefaction is a process where particles of loosely-consolidated and water saturated deposits of sand are rearranged into a more compact state. This results in the squeezing of water and sediments towards the surface in the form of "sand fountain" and creating a condition resembling "quicksand". In this phenomenon, the strength of the soil is reduced to a point where it is unable to support structures Kramer, 1996).

Liquefaction commonly occurs in areas that are water saturated (shallow water table), low lying and situated in typically loosed (unconsolidated) foundation or in sandy or silty deposits. Typical examples of these areas are river banks, abandoned rivers, flood plains, coastlines and swamps.

The liquefaction encompasses several related phenomena, among which are lateral spreads, subsidence and sand boils. Lateral spread involves the horizontal displacement of surface blocks as a result of liquefaction in a subsurface layer. Characterized by incremental displacements during earthquake shaking, it can produce damage in the abutments, foundations and superstructures of bridges, pipelines, bridge piers and other structures with shallow foundations, especially those located near river channels or canal banks on floodplains (Kramer, 1996).

The municipalities which are moderately susceptible to liquefaction include all the towns' except Almagro, Matuguinao, Sto. Nino, Tagapul-an, and Talalora. It is however worth mentioning that the municipality of Tagapul-an is the only municipality in the province that is not susceptible to any level of liquefaction. The high and moderately susceptible liquefaction areas in the province are located near the river bank, downstream and coastal area. About 40.16 percent (224,552.17 hectares) of the total land area of the province is prone to liquefaction.

Summary Matrix for all Hazards by Municipality

Note:    GS - Ground Shaking    LIQ - Liquefaction    EIL - Earthquake-Induced Landslide    RIL - Rain-Induced Landslide

Source: DRVA 2012

Liquefaction. Liquefaction commonly occurs in areas that are water saturated (shallow water table), low lying and situated in typically loosed (unconsolidated) foundation or in sandy or silty deposits. Typical examples of these areas are river banks, abandoned rivers, flood plains, coastlines and swamps.

The liquefaction encompasses several related phenomena, among which are lateral spreads, subsidence and sand boils. Lateral spread involves the horizontal displacement of surface blocks as a result of liquefaction in a subsurface layer. Characterized by incremental displacements during earthquake shaking, it can produce damage in the abutments, foundations and superstructures of bridges, pipelines, bridge piers and other structures with shallow foundations, especially those located near river channels or canal banks on floodplains (Kramer, 1996).

A total of about 224,550 hectares of the province is susceptible to liquefaction. This covers 16 municipalities which are considered highly susceptible and 5 municipalities which are considered moderately susceptible.

Earthquake-Induced Landslide (EIL). Earthquake-induced landslide is the slope failure resulting from severe ground shaking. This can be in the form of rapidly moving detached masses and bits of pieces of solid and rock materials. In general, steep slopes with weathered bedrock or soft rock units, hilly to mountainous areas exposed to high precipitation rate, slope modified by roads cuts, quarrying operations, river erosion and denuded forests are areas highly prone to landslide occurrences (Punongbayan, 1998).

Almost all of the municipalities and cities in the province, except Almagro, Tagapul-an and Sto. Nino, are highly susceptible to EIL.

Earthquake-induced landslide, which is an earthquake associated hazard, is the slope failure resulting from severe ground shaking. This can be in the form of rapidly moving detached masses and bits of pieces of solid and rock materials. In general, steep slopes with weathered bedrock or soft rock units, hilly to mountainous areas exposed to high precipitation rate, slope modified by roads cuts, quarrying operations, river erosion and denuded forests are areas highly prone to landslide occurrences (Punongbayan, 1998).