Abstract
Abstract. Forested, gravel-bed streams possess complex channel morphologies which are difficult to objectively characterize. The spatial scale necessary to adequately capture variability in these streams is often unclear, as channels are governed by irregularly spaced features and episodic processes. This issue is compounded by the high cost and time-consuming nature of field surveys in these complex fluvial environments. In larger streams, remotely piloted aircraft (RPA) have proven to be effective tools for characterizing channels at high resolutions over large spatial extents, but to date their use in small, forested streams with closed forest canopies has been limited. This paper seeks to demonstrate an effective method for classifying channel morphological units in small, forested streams and for providing information on the spatial scale necessary to capture the dominant spatial morphological variability of these channels. This goal was achieved using easily extractable data from close-range RPA imagery collected under the forest canopy (flying height of 5–15 m above ground level; ma.g.l.) in a small (width of 10–15 m) stream along its 3 km of salmon-bearing channel. First, the accuracy and coverage of RPA for extracting channel data were investigated through a subcanopy survey. From these survey data, relevant cross-sectional variables (hydraulic radius, sediment texture, and channel slope) were extracted from high-resolution point clouds and digital elevation models (DEMs) of the channel and used to characterize channel unit morphology using a principal component analysis-clustering (PCA-clustering) technique. Finally, the length scale required to capture dominant morphological variability was investigated from an analysis of morphological diversity along the channel. The results demonstrate that subcanopy RPA surveys provide a viable alternative to traditional ground-based survey approaches for mapping morphological units, with 87 % coverage of the main channel stream bed achieved. The PCA-clustering analysis provided a comparatively objective means of classifying channel unit morphology with a correct classification rate of 85 %. An analysis of the morphological diversity along the surveyed channel indicates that reaches of at least 15 bankfull width equivalents are required to capture the channel's dominant morphological heterogeneity. Altogether, the results provide a precedent for using RPA to characterize the morphology and diversity of forested streams under dense canopies.
Highlights
Channel morphological units such as pools and riffles constitute the building blocks of reach-scale channel morphologies (Buffington and Montgomery, 2013), with spatial variability in these units providing critical habitat diversity
The channel-averaged vertical survey error was estimated by calculating the root-mean-square error (RMSE) and the mean error (ME) of differences between the elevations of check points collected with the total station survey and those estimated from the digital elevation models (DEMs)
The results of this study provide a precedent for using remotely piloted aircraft (RPA) to characterize morphological units in small, forested streams below the forest canopy
Summary
Channel morphological units such as pools and riffles constitute the building blocks of reach-scale channel morphologies (Buffington and Montgomery, 2013), with spatial variability in these units providing critical habitat diversity. Morphological unit classification may be important in forested, gravel-bed streams, where episodic and transient geomorphological processes (Pryor et al, 2011; Wohl and Brian, 2015; Hassan et al, 2019) can lead to a high degree of channel complexity even within a relatively homogeneous channel type (Madej, 1999; Nelson et al, 2010; Gartner et al, 2015) Within these streams, classification schemes can serve an important role in facilitating discussions on stream management (Buffington and Montgomery, 2013). A common challenge of these classification approaches, is their descriptive nature (Buffington and Montgomery, 2013; Hassan et al, 2017) and that their implementation can be subjective, differing between classifiers
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