Aspects of find sediment dispersion along the zambales shelf, North Western Luzon,...

Aspects of find sediment dispersion along the zambales shelf, North Western Luzon, Philippines after Pinatubo

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James Paul H. Esguerra
Environmental Science Program, Ateneo de Manila University
Loyola Heights, Quezon City, Philippines
Tel : (63-2) 426-4321 Fax : (63-2) 426-6088
E-mail: [email protected]

Fernando P. Siringan
National Institute of Geological Sciences, University of the Philippines
Diliman, Quezon City, Philippines
Tel : (63-2) 920-5301 loc 7111 or 7120 Fax : (63-2) 929-6046/47
E-mail: [email protected]

Abstract
The eruption of Mt. Pinatubo presents the possibility of studying run off from its major river tributaries into the South China Sea (SCS) and dispersion using the pyroclastic material laden discharge as tracer. This uses remotely sensed images and ground truth data to validate observations on the shelfal area of the coast. Aspects of sediment dispersion in relation to coastal circulation and river run-off are investigated and some questions are raised on the eruption's effect on the coral communities of the shelf.

Introduction
In the Mt. Pinatubo eruption of 1991, which climaxed on June 14 to 16, the volcano expelled an estimated 7 to 12 km3 of volcanic material; 5 to 7 km3 of pumiceous pyroclastic flow and about 0.2 km3 of tephra-fall wee deposited on its slopes; as much as 5 km3 of hot, massive, pumice-rich non-eelded ignimbrite (Remotigue, 1996) was emplaced on the upper slopes of the volcano, mostly within pre-existing river valleys. Heavy ashfall was observed within a 40 km.radius and to a lesser degree in the nearby provinces (Pierson et al., 1992). Remotigue (1996) estimated the volume flow in the October 1995 lahar episode at the Bucao River junction at 6,400 m3.

Lahar is a rapidly flowing mixture of rock debris and water (other than normal stream flow) from a volcano. A lahar is an event which can refer to one or more discrete processes (such as debris flow and dilute lahar or hyperconcentrated flow) but does not refer to a deposit. Lhars are major contributors to siltation (Rodolfo, 1995).

The area of highest productivity along the coast would be the shelf. Heavy siltation of rivers and coastal waters has endangered aquatic organisms (Evangelista, et al., 1993). This sudden elevation of sediment supply in the fluid, coastal, and shelfal regimes, is reflected in the diminished fish yield in the area. Fish biomas declined from 23 to 69% along the coast of Zambales Province (Ochavillo, et al., 1992). Ashfall deposit, on the shelf, ranges from 0 cm to 15 cm deep (Table 1), depending on the distance of the sampling site from the source of the ashfall plume (the Mt. Pinatubo crater), and some other parameters (Ochavillo, et al. 1992).
 

Table 1. Depth of ashfall deposit along the shelf
Site Depth of ashfall deposit
Capones Island 15 cm.
La Paz, San Narciso 10 cm.
Laoag, North Cabangan 5 cm.
Palauig 0-1 cm.
San Salvador, Masinlic 0 cm.

It is possible to use radar data to observe volcanic eruptions and associated events. Atrigenio, et al. (1992) was able to detect land cover change surrounding the eruption, using SPOT and MOS data. The area covered by each type of material on-shore and offshore was measured. Evangelista, et al. (1993), studied the impact area of ashfall. This provided data on the extent of the impact area of the eruption and its recovery. Lahar hazards were also studied using the SIRC Radar data. Growth of lahars that were emplaced on the volcano, between February and August 1994, were illustrated by Mouginis-Mark (1995). ERS data was successfully used to study the impact of Mt. Pinatubo. These impacts were in the form of flood due to typhoon, and lahar flow. This provided keys to analyze active lahars (Chorowicz et al., 1997). One-third of the material is on the eastern side, while the other 2/3 is on the west (Rodolfo, 1996)

Mt. Pinatubo's west flank tributaries empty into the SCS. The SCS is frequented by northeasterly monsoons, while in summer it is dominated by southwesterly monsoon. The dominant waves are southwestward in winter, and northeastward in summer Sui (1994). More over, there exists a gradual change from north to south, and a slight difference in time range Wyrtki (1961) early on, described the region as ideal for monsoons, and described the pressure distribution as very stationary; the winds have a high constancy, especially over the sea. Pohlmann (1987) modeled the SCS circulation for winter and summer using 12 layers of thickness ranging from 10 m to 3000 m, increasing with dept. This provided insight into new features like deep-reaching up and downwelling phenomena, and was validated against the observational data of Wyrtki (1961),

Liu et al. (undated), using acoustic Doppler current porfiler (ADCP) measurements and geostrophic calculations, described how and why a counterclockwise gyre exists in the SCS north of 15 N latitude. The existence of a flow split at 15 N from the open sea into a northwards and southward current is also described in some detail. This is generally consistent with the description of Wyrtki (1961) : the general circulation of the water off the coast of North Western Luzon (NWL) is described to be persistently northward throughout the year.

Materials and Methods
Surface sediment samples from the shelf of Zambales running roughtly 65 km were collected using a Van Veen grab sampler (English et al., ..1994) from 45 sites. These were labeled (sample point Location and depth) in the field, and analyzed for grain size in laboratory (Lewis and Mc Conchie. 1994). The samples were dried, weighted and then separated by wet sieving with a 4 phi (64 microns) sieve.

Results and Discussion
Based on the rainfall data over 4 decades (1960-1990) provided by PAGASA, the supply of water into the river peaks during the rainy season in the months of July to October. At this time the winds come from northeast of Zambales, causing the river discharge to be pushed by the oncoming waves due to the wind and further pushed by the refracted waves as they leave the shoreline.

The contribution of the Sto. Tomas River is significantly less than Bucao's; that of Pamatawan River is even less, especially sometime after the eruption. Prior to the eruption, the Pamatawan River contributed a significant amount of material (water and sediment) into the SCS relative to the other feeder rivers. It is possible that this decrease is due to dike construction, or to sediment deposition along the river channedls as in what happened to the Mapanuepe Lake, and the general effect of deposition on the Pamatawan fan. This may affect the supply of sediment in the shore area near the Pamatawan River, which may contribute to shoe line movement/changes (Siringan and Ringor, 1996).

The material flowing out of the river mouth and into the coast can be carried on thesurface for a number of kilometers, both seaward and along shore (i.e. the case of Bucao River during the monsoon season). There is a turbidity plume which separates into two layers: a surface layer and a bottom layer. The surface layer appears to be dispersed at a greater distance by surface waves, while the bottom layer tends to go down and is possibly deposited and moved by bottom shearing forces along the bottom substrate by advection.

In general, the plot of the coarse-fine components of the sediments is consistent with the dominant flow of material that is coming from the Bucao and the Sto. Tomas Rivers.

Material from Subic Bay and Manila Bay also moved northward, hugging the Zambales coast at certain times of the year. The existence of gyres in the northern and southern end of the study area may be due to the long shore current, a rip current system, geostrophic currents, the river discharges, or an interaction of these. The phenomena of rip currents and near shore circulation are described by Leeder (9182) and Komar (1975).

Leckie and Krystinik (1990), in clarifying questions about their work, pointed to the importance of differentiating between shore-parallel paleoflow indicators generated by geostrophic flow and those generated by semi-permanent oceanic currents. These gyres are critically important to the Zambales shelf circulation. It is possible that such a condition may lead to the retention of a significant amount sediments of the shelf. This may explain the presence of mud in a significant portion of this shelf, especially in the central area. In what way will this determine the settlement of life forms? How then would life forms differ between these two zones?

Conclusions and Recommendations The evidence gathered indicated the existence of gyres in the northern and southern ends of the Zambales coast. Further studies need to be done on the nature of these gyres. In the open ocean, the SCS, monsoon-related processes basically control the fluxes of particulate matter (Weisner et al., 1996). A similar regime exists in this shelfal area of the SCS.

From Baban (1995), it is possible to correlate the suspended sediment (SS) concentration with the pixel digital number (DIN) in the future. This will require a synaptic shot and SS concentration measurement. Spectrophotometry can be used to determine the absorbance, transmittance or reflectivity characteristics of the pyroclastic material for substrate classification purposes (Nadaoka 1997, Nadaoka et al., 1997).

A more systematic approach to the use of RS data can be done. This may include tapping to databases for archived images before and after the eruption. Using other approaches such as automated image processing, UV or IR images to explore other aspects of the flow may provide other insights. Studying river discharge during storm surges or similar episodes to understand aspects of flow dynamics in more details can provide a better understanding of the system.

On the aspect of sediment properties, tests can be performed on the buoyancy of sediment-laden river discharge to verify or understand their behavior in salt-water environment. Fundamental to describing the transport of these fine-grained sediments is a quantitative analysis of their deposition and resuspension at the sediment-water interface. Properties and mix complicate this aggregation and disaggregation of fine-grained sediments in seawater have already been done (Burban et al., 1991; Xu, 1988).

Our study suggests that a strategy can already be developed for core sampling in the shelfal area sediment wave surfaces can also be studied for localized effects of local and regional circulation. This will provide the ground truth data for satellite imaging. For all cases, various forms of numerical modeling can be used to complement and validate the results of other methods.

Acknowledgemnt
This research was supported by the Philippine Council for Aquatic and Marine Research and Development (PCAMRD), Philippine council for Advanced Science and Technology Research and Development (PCASTRD) with additional support from the University of the Philippines : National Institute of Geological Sciences (UP-NIGS) and the Office of Research Coordination (UP-ORC)

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