Abstract
Geophysical changes in river outlet, river upstream and coastlines near the rivers of Bucao and Santo Tomas in Zambales, Philippines were analyzed using the Google Earth’s historical satellite imageries from 2004 to 2013. Data in 2015 were gathered from in situ field measurements ground validation. The study aimed to measure and determine changes in the width of river outlet, width of river bank upstream and shifting of coastline. <br><br> Results revealed that there was a decrease and increase in the width size of the Bucao and Santo Tomas river outlets, respectively during the study period. Geophysical changes occurred in the two rivers due to the continuous supply of lahar as an after effect of the Mount Pinatubo eruption in 1991. Coastline positions near the two rivers also changed. The highest rate of erosion along the coastal area was prevalently observed near the river outlet of both rivers. Moreover, accretion was observed in the coastline of Santo Tomas and erosion phenomenon was observed in the North and South coastlines of Bucao River. The shifting was caused by natural processes such as erosion, sedimentation and natural calamities as well as anthropogenic processes such as reclamation/quarrying. Occurrence of erosion and sedimentation played active roles in the changes of coastlines during the study period. <br><br> Furthermore, the upstream of the Bucao river changed physically due to deposits of lahar present in the upstream which are being discharged directly and continuously going down to the river. Generally, the width of the Bucao River upstream decreased its size because of the accumulated sediment in the riverbank. On the other hand, the observed erosion is caused by high velocity of river during heavy rains and typhoons. The width of the Santo Tomas river bank upstream did not change due to the construction of concrete dikes which prevent the lahar-filled river from breaching the embankment and flooding the agricultural, residential and commercial areas near the river.
Highlights
Remote sensing (RS) based approach is one of the most popular and promising methods for land use/land cover changes detection and can be adapted for mapping dynamic coastal activities and fluvial processes taking place in coastal areas such as coastal flooding, erosion/accretion, sedimentation, land subsidence, salt water intrusion, water pollution and cyclone hazards etc. (Mahmoud, 2011)
Bucao Watershed is considered as the largest watershed in Zambales and one of the most important watersheds in the municipality of Botolan, Zambales as it is the source of water for Bucao River supporting the major irrigation system of the municipality
The highest rate of erosion along the coastal area of Bucao River was prevalently observed near the outlet
Summary
Remote sensing (RS) based approach is one of the most popular and promising methods for land use/land cover changes detection and can be adapted for mapping dynamic coastal activities and fluvial processes taking place in coastal areas such as coastal flooding, erosion/accretion, sedimentation, land subsidence, salt water intrusion, water pollution and cyclone hazards etc. (Mahmoud, 2011). Image analysis or extraction of meaningful information from Google Earth which displays satellite images of the exact same location is one way of detecting or analyzing numerous changes occurring in rivers and coastline environments. These changes are results of either natural or anthropogenic processes. Coastal change is driven by processes that vary significantly in both space and time. Shoreline and river evolution occur due to seasonal changes in summer/winter wave environments, earthquake, extreme storm events, changes in natural sand supply and transport, alongshore variations in coastal geomorphology and elevated water levels caused by long-term sea-level rise (Long and Plant, 2012)
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