Quantifying seasonal ecohydrological roughness along river corridors using environmental sensing techniques

Despite the importance of floodplain vegetation (including dead wood) in fluvial geomorphology and its influence on the aquatic-terrestrial transition zone, current research is dominated by an ecological perspective which misses the bi-directional feedbacks between ecological and geomorphological processes in the river corridor. Prior studies have focused attention on the important roles of climate (light availability and temperature) and fluvial disturbance in controlling river corridor vegetation dynamics, but most of these previous studies have focused not only on trees, but also specific attributes such as canopy height and diameter at breast height. In contrast, much less attention has been paid to the role of understorey vegetation within fluvial systems, despite its potential role in modulating overbank flow (roughness), stabilising banks, and sequestering carbon. Here we define ‘understorey’ vegetation to mean all biomass up to a metre above the ground, irrespective of it being under a canopy or not and we also include large wood and leaf litter. 
Within this context, this research aims to quantify how hydraulic roughness and understorey vegetation co-vary seasonally along river corridors representing different disturbance regimes and river types, by quantifying structural aspects of understorey vegetation and its interactions with flow. Here we present work that is focused on Highland Water, a small flashy stream located in the New Forest, UK, which has riparian vegetation comprising predominantly a heavily-grazed deciduous canopy. The stream and its floodplain are also affected by the presence of developed log jams promoting overbank flow with multiple side channels. The study site is being surveyed monthly as well as during high flows to monitor flood extent. The structural complexity of riparian and floodplain understorey vegetation (<1m) is captured from a variety of complementary methods to ensure comprehensive coverage/capture of all relevant components of the above-ground biomass, while hydrological monitoring is being undertaken to evaluate variations in imposed hydraulic forces. The survey methods include Terrestrial Laser Scanning (TLS), alongside Uncrewed Aerial Vehicle (UAV) Light Detection and Ranging (LiDAR), RGB and Multispectral Imagery. Monthly surveying is conducted to capture the transition between leaf-off and leaf-on conditions, enabling links between phenological cycles, light availability, and understorey growth patterns to be explored in the context of variable fluvial disturbance. The interactions between understorey vegetation, fluvial disturbance, and subsequent morphology are being examined to identify the processes occurring within this section of the river corridor and how they vary in both space and time. 

Keywords:
Environmental sensing, Fluvial biogeomorphology, Phenology, Disturbance, River corridors, UAV, Laser Scanning, TLS, Flood interactions