Flood Mitigating Effects of Open-Pit Quarrying in Rizal, Philippines

The occurrence of flooding is already a common phenomenon in the Philippines where numerous tropical cyclones pass annually. The study is focused on flood hazards in Angono River Watershed in Rizal, Philippines which has historically experienced significant flooding events. Local residents believe that the open-pit quarrying activities in the area aggravates flooding in the town of Angono. To confirm this assumption, two-dimensional modelling of historically reconstructed as well as the present topography of the quarry area was done using FLO-2D software. The reconstructed topography modelling of the quarry area was digitally reconstructed using a 1:50 000 topographic map dating back prior to the start of quarrying. A digital elevation model (DEM) with 10x10 meters resolution, soil infiltration and saturation parameters, land use maps, and rainfall data were are the primary input of the model. The rainfall data were represented by Rainfall-Intensity-Duration-Frequency (RIDF) values sorted into 5-year, 25-year, and 100-year rainfall return rates. The delineated flood hazard maps produced will help in the interpretation and recognition of the effects of the presence of the quarry in the areas to rainfall intensity-specific flood hazards. In the simulated flood hazard map of the current topography, the total average affected area is 0.4742 square kilometers and 0.2541 square kilometers for the low and moderate flood hazard respectively. The reconstructed topography hazard map has a total average affected area of 0.4755 square kilometers and 0.3186 square kilometers for the low and moderate flood hazard respectively. This is a reduction of 2.3%-3.9% in the flood affected area is calculated with the presence of the quarry. This is because the quarried area serves as an artificial catchment basin which detains approximately 578,868 cubic meters of floodwaters from flowing directly to the lower watershed. The results of these models show that quarrying activities reduce flood hazards by creating catchments basins which detain and delay floodwaters during extreme rainfall. As opposed to public belief, the simulated maps show that quarrying does not enhance flood hazards and in fact mitigates its effects.