Characterisation of hydromorphological attributes is crucial for effective river management. Such information is often overlooked in tropical regions such as the Philippines where river management strategies mainly focus on issues around water quality and quantity. We address this knowledge gap using the River Styles Framework as a template to identify the diversity of river morphodynamics. We identify eight distinct River Styles (river types) in the Bislak catchment (586 km2) in the Philippines, showing considerable geomorphic diversity within a relatively small catchment area. Three River Styles in a Confined valley setting occupy 57% of the catchment area, another three in a partly confined valley setting occupy 37%, and two in the remaining 6% are found in a laterally unconfined valley setting. Five characteristic downstream patterns of River Styles were identified across the catchment. We observe that variation in channel slope for a given catchment area (i.e., total stream power) is insufficient to differentiate between river types. Hence, topographic analyses should be complemented with broader framed, catchment-specific approaches to river characterisation. The outputs and understandings from the geomorphic analysis of rivers undertaken in this study can support river management applications by explicitly incorporating understandings of river diversity and dynamics. This has the potential to reshape how river management is undertaken, to shift from reactive, engineering-based approaches that dominate in the Philippines, to more sustainable, ecosystem-based approaches to management.
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.
<p>The design and implementation of water management strategies in the Philippines, where&#160; precipitation is abundant and groundwater reserves are substantial, are compromised by extreme hydrometeorological events that create hazards such as flooding, bank erosion and landslides. Additionally, structural and institutional factors, such as responsibility for land and water management being divided among 38 agencies, inhibit integrated land and water management. Such a fragmented context threatens the sustainability of water resources and provide challenges for risk management. Integrated river basin management and master plans have been formulated to address catchment-related concerns which include water resources, disaster risk, biodiversity, mineral resources, and socio-economic development. These plans typically include assessment on physical variables such as hydrology and geology. One critical aspect that is missing is baseline understanding of dynamic river geomorphology. Such understanding of river character, behaviour and pattern is required to underpin scientific guidance from a rational evidence base that informs management applications. The Bislak Catchment (593 km<sup>2</sup>), north-western Luzon Island, is underlain by interbedded clastic sedimentary and volcanic rocks. It has a Type I climate which is described as having distinct dry and wet seasons. Early this year, the region suffered a prolonged drought which resulted to huge agricultural damage. In 2018, two severe tropical storms hit the area that caused destructive flooding to communities and infrastructure. In response, flooding and erosion are currently being mitigated by new and repaired defences such as gabion walls and concrete dikes. Satellite images from 1970 to 2019 show spatially variable channel change, in response to channel network and valley geometry. Here, the morphodynamic units throughout the catchment are described using the River Styles Framework which provides a geomorphic template to assess management trajectories. This approach is demonstrated for the Bislak Catchment, and is proposed as a template wider use in the Philippines.</p><p>&#160;</p><p>&#160;</p>
<p>In-stream large wood (LW) can have significant effects on channel hydraulics and thus water and sediment connectivity. The relationship between LW structures and their hydraulic function is generally quantified through drag force. Drag analyses, however, are often not straightforward, especially in complex debris jam settings where LW accumulations often consist of wood pieces of variable sizes. Here, we introduce simple LW (dis-)connectivity and sediment storage potential indices, especially developed for river management assessments. The LW (dis-)connectivity index (ID<sub>LW</sub>) is calculated based on visually estimated, field-derived parameters such as the degree of channel blockage. The LW sediment storage potential index (IS<sub>LW</sub>) is based on a classification scheme differentiating between different types of LW accumulation. Both indices were calculated and tested in two medium-sized mixed-load streams in Austria, further assessing fine sediment retention volumes behind LW structures. In both systems a variety of different types of LW accumulation with different degrees of blockage and storage potential have been detected. The larger system (river length = 5.7 km) had IDLW and ISLW values of 0,75 and 0,027, the smaller system (river length = 1.3 km) of 1,76 and 0,057. In the larger system in total 88.7 m&#179; fine sediment have been found to be retained by LW, while 4.7 m&#179; have been accumulated behind LW structures in the smaller river system. The application of the newly developed indices has shown to be a straightforward and valuable method to assess the effects of LW on water and sediment (dis-)connectivity, especially in a river management context.</p>
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