Catena 122 (2014) 180–195 Contents lists available at ScienceDirect Catena journal homepage: www.elsevier.com/locate/catena Short-term geomorphic analysis in a disturbed fluvial environment by fusion of LiDAR, colour bathymetry and dGPS surveys J. Moretto a,⁎,E.Rigona,L.Maob,F.Delaia,L.Piccoa, M.A. Lenzi a a Department of Land, Environment, Agriculture and Forestry, University of Padova, Padova, Italy b Department of Ecosystems and Environment, Pontificia Universidad Catolica de Chile, Santiago, Chile article info abstract Article history: Objective: Estimating river's underwater bed elevations is a necessary but challenging task. The objective of this Received 11 November 2013 study is to develop a revised approach to generate accurate and detailed Digital Terrain Models (DTMs) of a river Received in revised form 27 June 2014 reach by merging LiDAR data for the dry area, with water depth indirectly derived from aerial imagery for wet Accepted 30 June 2014 areas. Available online xxxx Methods: This approach was applied along three sub-reaches of the Brenta River (Italy) before and after two major flood events. A regression model relating water depth and intensity of the three colour bands derived Keywords: from aerial photos, was implemented. More than 2400 in-channel depth calibration points were taken using a Fluvial processes Gravel-bed river differential Global Positioning System (dGPS) along a wide range of underwater bed forms. LiDAR data Results: The resulting DTMs closely matched the field-surveyed bed surface, and allowed to assess that a 10-year Colour bathymetry recurrence interval flood generated a predominance of erosion processes. Erosion dominated in the upper part of Floods the study segment (−104,082 m3), whereas a near-equilibrium is featured on the lower reach (−45,232 m3). DoD The DTMs allowed the detection of processes such as riffle–pool downstream migration, and the progressive scour of a pool located near a rip-rap. Conclusion: The presented approach provides an adequate topographical description of the river bed to explore channel adjustments due to flood events. Practice: Combining colour bathymetry and dGPS surveys proved to represent a useful tool for many fluvial en- gineering, ecology, and management purposes. Implications: The proposed approach represents a valuable tool for river topography description, river manage- ment, ecology and restoration purposes, when bathymetric data are not available. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Three-dimensional and high-resolution representations of river bed morphology are used in many applications such as hydraulic and cellu- The study of river morphology and dynamics is essential for under- lar modelling (e.g. Rumsby et al., 2008), evaluation of climate change standing the factors determining sediment erosion, transport and depo- impacts on river systems (e.g. Rumsby and Macklin, 1994), flood hazard sition processes. Natural (e.g. climatic and hydrological variations) and management (Fewtrell et al., 2011; Macklin and Rumsby, 2007; anthropic factors (e.g. water captures, grade-control works, gravel min- Sampson et al., 2012), assessment of erosion and deposition areas ing, deforestation) can act at both the reach- and basin-scales changing along the river corridor (Lane et al., 2007; Picco et al., 2013; Stover the magnitude and timing of these processes (Buffington, 2012). Geo- and Montgomery, 2001). Calculating sediment budgets, estimating morphic variations at the reach scale are a direct consequence of sedi- transport rates, and understanding changes in sediment storage are ment erosion and deposition processes, which are in turn influenced also fundamental aspects to quantify geomorphological changes due by the size and volume of sediment supply, transport capacity of the to flood events and changes in flow regime (Ashmore and Church, flow, and local topographic constraints. The actual ability to quantify 1998; Wheaton et al., 2013). the interaction of these processes is limited by the difficulty of collecting The traditional techniques of terrain survey (e.g. total station de- high spatial resolution data in river environments. Traditional ap- vices, differential Global Positioning System - dGPS; Brasington et al., proaches, based on the application of hydraulic formulas at cross- 2000) in the evaluation of morphological changes across large areas sections, fail when aimed at describing non-uniform natural conditions. have so far demonstrated to be expensive, time-consuming and difficult to apply in zones with limited accessibility. Some innovative methods are good alternatives for producing high-resolution Digital Terrain ⁎ Corresponding author at: Agripolis Campus, Viale dell'Università, 16. 35020 - Legnaro Padova, Italy. Fax: +39 0 498272750. Models (DTMs) of fluvial systems. Recent studies on morphological E-mail address: [email protected] (J. Moretto). channel changes have used passive remote sensing techniques such as http://dx.doi.org/10.1016/j.catena.2014.06.023 0341-8162/© 2014 Elsevier B.V. All rights reserved. J. Moretto et al. / Catena 122 (2014) 180–195 181 digital image processing (e.g. Forward Image Model, Legleiter and changes occurring in river systems over time (e.g. Lane et al., 1994). Roberts, 2009), digital photogrammetry (Brasington et al., 2003; An important component to be evaluated in DEMs is uncertainty, Dixon et al., 1998; Heritage et al., 1998; Lane et al., 2010), active sensors which can be influenced by many factors. The most decisive sources of including Laser Imaging Detection and Ranging (LiDAR) (e.g. Brasington error include survey point quality, sampling strategy, surface topo- et al., 2012; Hicks, 2012; Hicks et al., 2002, 2006; Kinzel et al., 2007), and graphic complexity and interpolation methods (Milan et al., 2011; acoustic methods (e.g. Muste et al., 2012; Rennie, 2012). Panissod et al., 2009; Wheaton et al., 2010). Total uncertainty is usually The main difficulty related to the production of precise DTMs with no- derived from the classical statistical theory of errors (Taylor, 1997) bathymetric sensors concerns the absorption of natural (solar) or artificial where an estimation of DEM accuracy based on survey data is used as (LiDAR) electromagnetic radiation in the wetted portion of the river chan- a surrogate for DEM quality (Milan et al., 2007). nels. The capacity of the electromagnetic signal to pass through water, be This paper implements a revised approach to generate accurate and reflected from the bed and reach a sensor depends on water surface tex- detailed Digital Terrain Models (DTMs) of a river reach by merging ture (pleating, reflexes, etc.), water column (depth and turbidity), and na- LiDAR data for dry areas, with water depth estimates for wet areas. ture of channel bed (substrate type and presence of algae; Marcus, 2012; The main objective consists in the evaluation of morphological patterns Marcus and Fonstad, 2008). Even if different electro-magnetic (EM) of change in three sub-reaches of the Brenta River as a consequence of wavelengths are absorbed by water at different degrees (Smith and two consecutive major floods occurred in November and December Vericat, 2013; Smith et al., 2012), LiDAR signal proved to be adequately 2010. Specific aims related with the proposed approach and targeted reliable to assess channel topography “where flow is sufficiently shallow to achieve an adequate topographic description of the river bed are: to that water depth does not distort the laser” (Charlton et al., 2003), as late- determine physical and empirical relations between local channel ly confirmed also by Cavalli and Tarolli (2011). depths and photo colour intensity; to identify and to filter the factors Only a few tools have proved able to provide an accurate and high- which increase uncertainty in the final DTM, in order to obtain Hybrid resolution measure of submerged bed surface. Moreover, the precision DTMs (HDTMs) at high resolution and low uncertainty. of the surveyed data decreases as water depth increases. Bathymetric LiDAR sensors have recently been developed and should enable the sur- 2. Study area vey of underwater bed surfaces. Nevertheless, they feature high costs, relatively low resolutions, and data quality comparable to photogram- The Brenta River is located in the South-Eastern Italian Alps, has a metric techniques (Hilldale and Raff, 2008). Progress in LiDAR acquisi- drainage basin of approximately 1567 km2 and a length of 174 km. tion of topographic information from submerged areas has been The average annual precipitation, mainly concentrated in spring and au- achieved with a new technology called Experimental Advanced Air- tumn, is about 1100 mm. The geology of the area is rather complex and borne Research LiDAR system (EAARL), which records the full wave- includes limestone, dolomite, gneiss, phyllite, granite and volcanic form of returning laser pulse. Even if this system is affected by rocks. environmental conditions (e.g. turbulence in the pool, bubbles in the The study reach is 19.2 km-long and lies between Bassano Del Grap- water column, turbidity, and low-bottom albedo) and by post- pa and Piazzola sul Brenta (Fig. 1). The dominant morphologies are processing algorithms, its accuracy appears comparable to what is ob- wandering and braided, the active channel width varies between 300 m tained using airborne terrestrial near-infrared LiDARs (Kinzel et al., and 800 m, and the average slope is about 0.0036 m/m. Within this 2013; McKean et al., 2009). study reach, three sub-reaches 1.5 km-long and 5 km apart were se- Surveys of wet areas can thus be approached using two photogram- lected as representative of the upper- middle- and down-stream part metric techniques (manual or automatic) which are able to produce a of the study area and named according to the nearby villages: Nove, cloud of elevation points (Fryer, 1983; Rinner, 1969), or with a tech- Friola and Fontaniva (Fig. 1). The upstream sub-reach (Nove) has a nique based on the calibration of a depth–reflectance relationship of im- single straightened channel morphology with an average width of ages, which can be in greyscale (e.g.