National-scale geodatabase of catchment characteristics in the Philippines for river management applications.

Quantitative descriptions of stream network and river catchment characteristics provide valuable context for enabling geomorphologically-informed sustainable river management. For countries where high-quality topographic data are available, there are opportunities to enable open access availability of baseline products from systematic assessment of morphometric and topographic characteristics. In this study, we present a national-scale assessment of fundamental topographic characteristics of Philippine river systems. We applied a consistent workflow using TopoToolbox V2 to delineate stream networks and river catchments using a nationwide digital elevation model (DEM) acquired in 2013 and generated through airborne Interferometric Synthetic Aperture Radar (IfSAR). We assessed morphometric and topographic characteristics for 128 medium- to large-sized catchments (catchment area > 250 km2) and organised the results in a national-scale geodatabase. The dataset realises the potential of topographic data as part of river management applications, by enabling variations in hydromorphology to be characterised and contextualised. The dataset is used to reveal the diversity of stream networks and river catchments in the Philippines. Catchments have a continuum of shapes (Gravelius compactness coefficient ranges from 1.05 to 3.29) with drainage densities that range from 0.65 to 1.23 km/km2. Average catchment slope ranges from 3.1 to 28.1° and average stream slope varies by more than an order of magnitude from 0.004 to 0.107 m/m. Inter-catchment analyses show the distinctive topographic signatures of adjacent river catchments; examples from NW Luzon highlight topographic similarity between catchments whereas examples from Panay Island shown marked topographic differences. These contrasts underline the importance of using place-based analyses for sustainable river management applications. By designing an interactive ArcGIS web-application to display the national-scale geodatabase, we improve data accessibility and enable users to freely access, explore and download the data (https://glasgow-uni.maps.arcgis.com/apps/webappviewer/index.html?id=a88b9ca0919f4400881eab4a26370cee). The national-scale geodatabase provides a baseline understanding of fundamental topographic characteristics in support of varied geomorphological, hydrological and geohazard susceptibility applications.

National-scale assessment of decadal river migration at critical bridge infrastructure in the Philippines.

River migration represents a geomorphic hazard at sites of critical bridge infrastructure, particularly in rivers where migration rates are high, as in the tropics. In the Philippines, where exposure to flooding and geomorphic risk are considerable, the recent expansion of infrastructural developments warrants quantification of river migration in the vicinity of bridge assets. We analysed publicly available bridge inventory data from the Philippines Department of Public Works and Highways (DPWH) to complete multi-temporal geospatial analysis using three decades worth of Landsat satellite imagery in Google Earth Engine (GEE). For 74 large bridges, we calculated similarity coefficients and quantified changes in width for the active river channel (defined as the wetted channel and unvegetated alluvial deposits) over decadal and engineering (30-year) timescales. Monitoring revealed the diversity of river planform adjustment at bridges in the Philippines (including channel migration, contraction, expansion and avulsion). The mean Jaccard index over decadal (0.65) and engineering (0.50) timescales indicated considerable planform adjustment throughout the national-scale inventory. However, planform adjustment and morphological behaviour varied between bridges. For bridges with substantial planform adjustment, maximum active channel contraction and expansion was equal to 25% of the active channel width over decadal timescales. This magnitude of lateral adjustment is sufficient to imply the need for bridge design to accommodate channel dynamism. For other bridges, the planform remained stable and changes in channel width were limited. Fundamental differences in channel characteristics and morphological behaviours emerged between different valley confinement settings, and between rivers with different channel patterns, indicating the importance of the local geomorphic setting. We recommend satellite remote sensing as a low-cost approach to monitor river planform adjustment with large-scale planimetric changes detectable in Landsat products; these approaches can be applied to other critical infrastructure adjacent to rivers (e.g. road, rail, pipelines) and extended elsewhere to other dynamic riverine settings.