MGB Central Office – Coastal geohazards assessment and shoreline mapping conducted by the Mines and Geosciences Bureau in several cities and municipalities in Negros Occidental reveals that the southern coast of the region had been losing approximately 9.5 hectares to coastal erosion annually in the past 58 years.
Post-survey data processing and interpretation of shoreline mapping data using ArcGIS software revealed that coastal erosion is more widespread than sediment accretion in the south-central coast of Negros Occidental. The survey is based on the coastal geohazards assessment and shoreline mapping carried out in December 2014 in Bago City and the in municipalities of Pulupandan, Valladolid, San Enrique, Pontevedra, Hinigaran, and Binalbagan.
Analysis of coastlines between 1956 and 2014 in southern Negros Occidental suggested that the corridors eroded away by waves and currents summed up to 550 hectares while the accreted zones summed up to about 226 hectares. Thus, it is apparent that the emergent landmass along the southern coast of Negros Occidental lost 324 hectare (550 has eroded minus 226 has accreted) to coastal erosion.
The widths of eroded corridors within the study area range approximately from 9 to 334 meters with a median value of 144 meters, the widest being in Barangay Enclaro (334 meters) in the town of Balibagan. Barangay Tapong, Pulupandan (272 meters), Barangay San Juan, Binalbagan (266 meters), Barangay Barangay Calumangan, Bago City (231 meters), and Barangay Poblacion, Valladolid (195 meters) follow the list of widest eroded corridors in terms of width respectively.
Barangay Miranda, Municipality of Hinigaran (77 has.) has the most extensive zones of eroded corridors in terms of hectarage. The areas of eroded corridors ranges more or less from 0.01 to 77 hectares. Barangay Tapong, Municipality of Pulupandan (75 has.); and Barangay Enclaro, Municipality of Binalbagan (71 has.) follows the list of most eroded corridors in terms of hectarage. The survey concludes that Hinigaran (133 hectares) has the largest distribution of eroded corridors per municipality/ per city. Binalbagan (108 hectares), Pulupandan (100 hectares), Bago City (88 hectares), Valladolid (78hectares), Pontevedra (42 hectares), and San Enrique (2 hectares) follows the list from largest to least distribution of eroded corridors.
For the same period, the total area gained thru sediment accretion is estimated to be 226 hectares. Assuming a uniform rate, the area formed thru sediment accretion is roughly 3.90 hectares per annum. This magnitude corresponds to the Moderate susceptibility rating for
sediment accretion in accordance with the yardsticks adopted in this report.
To abate the high rate of coastal erosion, a mix of soft engineering measures or bioengineering, and well-engineered hard structures should be launched by the concerned LGUs. Planting of tree species adaptable to coastal environments at the upper beach slope and on emergent accreted zones, as well as massive planting of mangroves, mangrove reforestation, and aforestation complemented with adequate coastal setbacks could be an option to mitigate the serious effects of coastal erosion in settlement centers behind high energy areas.
Furthermore, the report recommends that land titles should not be issued to individuals who may apply or claim for occupancy rights on the emergent accreted lands to reduce the risk of exposure to coastal geohazards. The zones are situated in fragile environments.
The study area is also highly prone to coastal flooding. This coastal hazard could be a direct consequence of either storm surges or tsunami wave trains, which could be aggravated by extreme high tide regimes and ground subsidence. The potential tsunami generators are the Negros Trench and the Sulu Trench. The areas rated to have Very High to High susceptibility potentials to coastal flooding are those with ground surface elevations of 0 to 3 and 3 to 6 meters above mean sea level, respectively, particularly those areas facing the open sea and without wide and dense mangrove forests as buffers or wave energy dissipaters.
By: Marine Geological Survey Division