Interactions between river mobility and land use in the Philippines: implications for space to move policies

Rivers need space to sustain key ecological and geomorphological functions, and to convey floodwater. However, river management efforts have often used structural engineering approaches to mitigate erosion hazards to land that has been developed for agricultural, industrial and urban land uses. In the Philippines, current easement regulations require a minimum 3 metre buffer along the bank of a river for urban areas and 20 metre buffer for agricultural lands. We use a four-decade long archive of satellite imagery, processed in Google Earth Engine, to investigate river mobility across the Philippines, enabling us to quantify how mobile rivers are and the land covers that are eroded due to river migration. In more detail, our study assesses Land Use and Land Cover (LULC) changes and river mobility across ten catchments using national-scale LULC datasets (2003, 2010, 2015, and 2020) and satellite imagery from 1988 to 2021. We standardised LULC classifications and analysed transitions within catchments, identifying key changes in dominant land cover types. Intersections between active channel edges and LULC maps revealed the types of land cover rivers interacted with over time, highlighting areas of encroachment and potential risk. Using Digital Shoreline Analysis Software, we quantified river migration rates between 2000 and 2020, along each river, identifying spatial patterns of river movement and areas where rivers migrated into new LULC types. The analysis of LULC distributions at varying distances from the maximum active channel extent provides insights into how easement regulations could be informed by observations of actual river mobility. Our findings are a demonstration of a nature-based solution to defining how much space rivers need, informed by big data. The findings have direct implications for Philippine easement laws, which mandate buffer zones along waterways to protect against flood and erosion risks, and environmental degradation; there is potential to re-evaluate static buffer zones and consider adaptive, risk-based approaches to easement enforcement.