A Short Geological Discussion on The Proposed Dumaguete 174 Reclamation Project: Spatial and Temporal Implications

by Ralph MT

"Civilization exists by geological consent, subject to change without notice” ~ Will Durant

Through a Private Sector Partnership, the local government of Dumaguete City is in view to enter into a Memorandum of Understanding with E.M Cuerpo, Inc., a construction company based on Quezon City, for the proposed “Smart City”. The projected “Smart City” is a 174-hectares artificial, off-shore island, situated 30 meters from the mainland, which would stretch the entire coast of Dumaguete City. The project is said to amalgamate 5G ready connections, coastal waste water treatment facility, shoreline slope and water protection esplanades, modern ferry boat, marina and open area for sports facility, residential, condominiums, malls and other business hubs. With a grandeur project such as this, sustainability, geological factors, and mitigation of geohazards is of utmost significant that needs to be considered and evaluated to check the feasibility of the project. This paper will look into the spatial and temporal geological standpoints to demonstrate that the development of the Dumaguete 174 “Smart City” is indeed a disaster waiting to happen.

Spatial Standpoint – Negros Island is part of the Philippine Mobile belt, located at the the western block of the Philippine Fault Zone. It accommodates the East-Negros Fault System which is an NNE oriented fault system comprised of almost linear segments that run along the northeastern section towards the southern portion of the island parallel with the volcanic arc from Mt. Cuernos de Negros to Mt. Kanlaon in the north. Dumaguete City is situated at the southeastern area of Negros Island. 

The Mines and Geosciences Bureau (2004) and Ramirez (2016) identified that the local surface geology of southeastern Negros Island is dominated by Quaternary Alluvial Plain Deposits derived as products of erosion from the Okoy River which is deposited into the low topographic relief of Sibulan-Dumaguete-Bacong that could extent further from the current shoreline of the island. This is also correlated to the presence of San Manuel Loam and San Manuel Taal Complex soil series in the said areas, this soil series commonly occurs along river course or in plains by formed by streams. As the principle of lateral continuity dictates, the extent of the alluvial deposits could go further into the margins of the shoreline. Steady transgression of sea level and the direction of littoral drifts in the past decades could also contributed to the aggradation of sediments towards the shoreline. Presently, the active deposition of sediments such as clays and silts by Banica River in Dumaguete and Okoy River in Sibulan is controlled by the artificial levees and dikes but accretion of the deposits is still observed beyond the mouths of the downstream rivers. Thus, we can infer that the shoreline and offshore area of Dumaguete City is dominated by thick unconsolidated, soft sediments from the accumulation of fluvial, alluvial and beach deposits, which will serve as the foundation for the proposed 174 hectares development of the “Smart City”.

All objects have their own natural period or resonance, which is the number of seconds it takes for the particles to naturally vibrate back and forth (IRIS, 2020; Rial, 1992). When a period of ground motion is stimulated to match the object’s resonant frequency, the amplitude of the vibration is greatly increased. Thus, when an earthquake produces a ground period motion that ties with the natural resonance of the building or an object, it will undertake the largest oscillations possible and experience the greatest damage.

Several studies recognized that soft sediments have lower frequencies compared to hard bedrock (Rial, 1992; IRIS, 2020; Kumar, 2020). Therefore, low frequency earthquakes passing through soft sediments will generate a maximum sustained wave, and high-stories buildings constructed over it will suffer catastrophically.

The soft sediments in the offshore of Dumaguete City is at high risk to these low frequency earthquakes. High-rise buildings for condominiums and malls that will be constructed on the proposed artificial islands for the “Smart City” will be subjected to these oscillations when facing the low frequency earthquake and could eventually cause devastating damages, casualties and risks to the neighboring community.

Temporal Standpoint – The persistence of global warming and its associated sea level rise and coastal inundation is a big threat to all the coastal communities, the proposed “Smart City” is not an exception. It has been projected that by 2030 sea level would have risen by 12 centimeters. Flood map simulation with 12 cm rise of sea level shows a 20–40-meter inundation into the coast Dumaguete City. Storm surges is also a crucial factor that needs to be evaluated because it can concur damages and significant risk especially for Philippines that experiences an average of 20 tropical storms per year, however, due to lack of data and studies and the restriction posed by the COVID-19 pandemic for collection of anecdotal accounts, this remains inconclusive for now.

Since 1911 to 2015, 1,866 earthquakes were recorded in the Visayas region with average magnitude that ranges from 4.4 to 4.7 and given the geotectonic setting of Negros Island with the active subduction zone at the southwestern portion, the generation of low-frequency earthquakes in the future are very high (Torres, 2017).

The construction of the proposed “Smart City” in the offshores of Dumaguete City offers a chance for socioeconomic development, however, the outcome is overweighed by the geological risks it is subjected to.

Both spatial and temporal geological factor in the construction for the 174 hectares “Smart City” points to a number of geohazards such as the low frequency earthquakes and storm surges that might be triggered in the near future and could cause unnecessary damages and risks, along with the continuous threat of the sea level rise. The project is feasible but without the consideration of the implications stated above will only lead to a disaster. It is thus my conclusion that the development of the 174 hectares “Smart City” offshore of Dumaguete City is very costly both for its reclamation and foundation strengthening and is prone to seismic risk and hydrological risk due to the constant threat of the sea level rise, storm surges and earthquakes.

References:

Articles

Kumar, P., Mahajan, A.K. New empirical relationship between resonance frequency and thickness of sediment using ambient noise measurements and joint-fit-inversion of the Rayleigh wave description curve for Kangra Valley (NW Himalaya), India. Environ Earth Sci 79, 256 (2020). https://doi.org/10.1007/s12665-020-09000-8

MGB, 2004. Stratigraphy of Central Visayas. Retrieved from: http://r7.mgb.gov.ph/files

Ramirez, A. 2016. A Short Discussion on the Stratigraphy of Southeastern Negros: In Time and Space. Requirement submitted to Negros Oriental State University. Published at academia.edu

Rial, J., Saltzman, N., & Ling, H. (1992). Earthquake-Induced Resonance in Sedimentary Basins, American Scientist, 80(6), 566-578. Retrieved July 29, 2021, from https://www.jstor.org/stable/29774781

Torres, C.J.A. 2017. Geology, hydrothermal alteration, geochemistry, mineral geochemistry, and alteration textures of the lithocap at Bantug, Negros-Philippines. Thesis presented to the University of Tasmania

Links

Building Resonance: Structural stability during earthquakes. Retrieved from https://iris.edu/hq/inclass/animation/building_resonance_the_resonant_frequency_of_different_sesmic_waves

Pal, I.F. (July 2021). Dumaguete Mayor seeks public support for reclamation project. Inquirer.net. http://newsinfo.inquirer.net/1460636/Dumaguete-mayor-seeks-public-support-for￾reclamation-proj