Airborne LiDAR as a new tool for fluvial geomorphology

Detailed topographic data are important for both quantitative and qualitative fluvial studies. Topographic data available from traditional topographic maps are often not sufficiently accurate to detect, and map all relevant landform features. Moreover, traditional ground surveys may exert a high demand on operator time and cost. The airborne laser altimetry technology known as airborne LiDAR (Light Detection And Ranging) provides very accurate topographical data (vertical accuracy of few centimeters), which can significantly contribute to a better representation of land surface. A valuable characteristic of this technology is the capability to derive high-resolution Digital Terrain Models (DTMs) (1-2 m) from ground points generated after removal of vegetation and man-made features. LiDAR data have been widely applied in all geomorphological fields, especially for studying landslides and soil erosion. In fluvial geomorphology, the first studies used LiDAR data mainly to improve numerical flood models. Recently, applications in the same field concern studies estimating mapping gravel-bed river geomorphology (Charlton, 2003), rapid geomorphological mapping of river valley environments (Jones et al., 2007), evaluating the river long-term changes created by fluvial and debris flow activity (Magirl et al., 2005), tidal channel geomorphology (Mason et al., 2006), and objectively mapping of channel network (Tarolli and Dalla Fontana, 2009). Volumetric assessments of geomorphic change made by differencing sequential LiDAR-derived DTMs can be used to obtain distributed patterns of erosion and deposition, to compute sediment budget and thereby quantify rates of active fluvial processes, such as river bank erosion (Thoma et al., 2005). Nevertheless, significant errors in airborne LiDAR-derived DTMs can be due to the filtering process required to derive points belonging to the ground surface, and be associated to the presence of deep water. An assessment of the quality of the LiDAR data is therefore necessary before utilization. It is worth noting that in the last few years significant advances have been made with the airborne LiDAR bathymetry (ALB) technology, which allows the representation of river channel bed even under deep water. In the headwater environment, airborne LiDAR technique has been proven useful in identifying and mapping gullies, and measuring small ephemeral channels, even under forest cover (James et al., 2007). Where water depth is not exceeding few tens of centimeters, LiDAR data can be used for characterizing channel bed morphology by differentiating step pools from riffle pool reaches through measure of bed surface roughness (Cavalli et al., 2008). These applications highlight the clear potential of airborne LiDAR technology offering new opportunities but also challenges in fluvial geomorphology research.