Measuring river planform changes from remotely sensed data – a Monte Carlo approach to assessing the impact of spatially variable error

Abstract

Abstract. Remotely sensed data from fluvial systems are extensively used to document historical planform changes. However, geometric and delineation errors inherently associated with these data can result in poor or even misleading interpretation of measured changes, especially rates of channel lateral migration. It is thus imperative to take into account a spatially variable (SV) error affecting the remotely sensed data. In the wake of recent key studies using this SV error as a level of detection, we introduce a new framework to evaluate the significance of measured channel migration. Going beyond linear metrics (i.e. migration vectors between diachronic river centrelines), we assess significance through a channel polygon method yielding a surficial metric (i.e. quantification of eroded, deposited, or eroded-then-deposited surfaces). Our study area is a mid-sized active wandering river: the lower Bruche, a ∼20 m wide tributary of the Rhine in eastern France. Within our four test sub-reaches, the active channel is digitised using diachronic orthophotos (1950 and 1964), and the SV error affecting the data is interpolated with an inverse-distance weighting (IDW) technique. The novelty of our approach arises from then running Monte Carlo (MC) simulations to randomly translate active channels and propagate geometric and delineation errors according to the SV error. This eventually leads to the computation of percentage of uncertainties associated with each of the measured planform changes, which allows us to evaluate the significance of the planform changes. In the lower Bruche, the uncertainty associated with the documented changes ranges from 15.8 % to 52.9 %. Our results show that (i) orthophotos are affected by a significant SV error; (ii) the latter strongly affects the uncertainty of measured changes; and (iii) the significance of changes is dependent on both the magnitude and the shape of the surficial changes. Taking the SV error into account is strongly recommended even in orthorectified aerial photos, especially in the case of mid-sized rivers (<30 m width) and/or low-amplitude river planform changes (<1 m2m-1yr-1). In addition to allowing detection of low-magnitude planform changes, our approach is also transferable as we use well-established tools (IDW and MC): this opens new perspectives in the fluvial context (e.g. multi-thread river channels) for robustly assessing surficial channel changes.

Investigation of the lateral channel migration: A case study of the South Morava River (Serbia)

“…Riverbank location changes in history can be monitored and determined by using a database that includes many projects and studies, aerial and orthophoto images, and cartographic materials from different time periods (Jautzy et al, 2020). The oldest source of data used in this research are maps (1:25,000) from 1924 ("Section for investigating the Morava River" within the General Directorate of Water).…”

“…Briaud et al (2007) су издвојили и објаснили најчешћа три приступа приликом проучавања проблематике латералног померања речног тока. Највећи акценат је стављен на вредновање стопе и магнитуде померања обала кроз примену емпиријског и теренског метода, приступа просторновременске динамике и приступа моделовања у циљу предвиђања будућих тенденција Историјски развој померања речних обала може се пратити и утврдити коришћењем базе података која укључује бројне пројекте и елаборате, аерофото и ортофото снимке и картографске материјале из различитих временских периода (Jautzy et al, 2020). Као најстарији извор података, у раду су коришћене карте (1:25.000) из 1924. године ("Секција за испитивање реке Мораве" у оквиру Генералне дирекције вода).…”

Using transition matrices to assess the spatio‐temporal land cover and ecotone changes in fluvial landscapes from historical planimetric data

“…The approach developed by Jautzy et al. (2020) based on Monte Carlo simulations might be useful to assess the effects of geometrical errors on metrics derived from transition matrices. In addition, future research should be conducted on floodplain areas to produce knowledge on conclusions about the effects of changes in the physical environment on habitat‐specific biocenoses.…”

“…These data can be further discussed in regard to natural drivers (flood events, changes of flow and/or sediment regime) and/or human‐induced morphological changes (Arnaud et al., 2019; Scorpio & Piégay, 2021). Historical sources also allow estimation of the influence of river regulation on the lateral dynamics of channels and their evolution over time (Jautzy et al., 2020; Mandarino et al., 2019). Such sources may also be combined with other geomorphological data (e.g., longitudinal profiles, cross‐sections, water levels LiDAR dataset), even including geochronological approaches to provide knowledge on temporal trajectories of sediment deposition in the fluvial system (Eschbach et al., 2018; Quik & Wallinga, 2018).…”

Quantitative Assessment of Riverbed Planform Adjustments, Channelization, and Associated Land Use/Land Cover Changes: The Ingauna Alluvial-Coastal Plain Case (Liguria, Italy)

“…The active channel was manually mapped as a polygon on each dataset (Table 1), including wetted channels and bare or partially vegetated bars (Figure 3a,b) [9,11,14,135,[142][143][144]. This spatial unit reflects ongoing geomorphic processes independent of flow conditions at the time of the survey [45,138].…”