Photographic Techniques for Characterizing Streambed Particle Sizes

Abstract

We developed photographic techniques to characterize coarse (>2‐mm) and fine (≤2‐mm) streambed particle sizes in 12 streams in Anchorage, Alaska. Results were compared with current sampling techniques to assess which provided greater sampling efficiency and accuracy. The streams sampled were wadeable and contained gravel—cobble streambeds. Gradients ranged from about 5% at the upstream sites to about 0.25% at the downstream sites. Mean particle sizes and size‐frequency distributions resulting from digitized photographs differed significantly from those resulting from Wolman pebble counts for five sites in the analysis. Wolman counts were biased toward selecting larger particles. Photographic analysis also yielded a greater number of measured particles (mean = 989) than did the Wolman counts (mean = 328). Stream embeddedness ratings assigned from field and photographic observations were significantly different at 5 of the 12 sites, although both types of ratings showed a positive relationship with digitized surface fines. Visual estimates of embeddedness and digitized surface fines may both be useful indicators of benthic conditions, but digitizing surface fines produces quantitative rather than qualitative data. Benefits of the photographic techniques include reduced field time, minimal streambed disturbance, convenience of postfield processing, easy sample archiving, and improved accuracy and replication potential.

Catchment‐scale mapping of surface grain size in gravel bed rivers using airborne digital imagery

“…These methods typically rely on photographs of the bed. Also called photosieving, they involve taking plan view images of gravels either in exposed areas [Adams, 1979;Ibbeken and Schleyer, 1986;Butler et al, 2001a] or shallow submerged areas [Whitman et al, 2003]. Generally, images of the riverbed are collected from standing position with some form of scale information in each image.…”

Comment: Photographic Techniques for Characterizing Streambed Particle Sizes

“…The use of a photographic approach is particularly appropriate for fisheries and monitoring applications because, unlike conventional measurement techniques, it is non-invasive, preserving the substrate that is being studied. Whitman et al (2003) found a significant difference between the grain-size distributions and mean particle sizes determined by their photographic technique and Wolman sampling at five sites (as determined by Kolmogorov-Smirnov and t-tests). These results are confirmed by their Figure 3a, which shows a clear difference between the grain-size distributions obtained by the two techniques for a single site.…”

“…In contrast, all of the grains recorded in the Wolman samples are larger than 8 mm (their Figure 3a). It is therefore unsurprising that the grain-size distributions and mean particle sizes, reported by Whitman et al (2003), are different. Whitman et al (2003) may have retained different lower size limits in their analysis to highlight the bias against fine material that is inherent in Wolman sampling (Fripp and Diplas 1993).…”

“…They concluded that these differences may result from an error in one of the techniques, but were unable to fully account for them or determine which of the techniques gave the most reliable results. In fact the disparity is, at least partly, accounted for by reanalyzing their data in light of two principles of grain-size sampling that Whitman et al (2003) overlooked. This re-analysis illuminates their comparison and, along with a suite of results from the sedimentological literature, suggests that Wolman and photographic sampling can be of comparable quality.…”

Interactions between a cushion plant (Azorella selago) and surface sediment transport on sub-Antarctic Marion Island

“…As an alternative, photogrammetric techniques have been applied for the quantification of grain size parameters since at least the 1970s, especially in river channel hydraulics (Adams, 1979;Ibbeken and Schleyer, 1986;Whitman et al, 2003;Graham et al, 2005a). Many previous photogrammetric studies of sediment grain sizes have involved manually outlining sediment grains on the digital image (Ibbeken and Schleyer, 1986;Ibbeken et al, 1998;Whitman et al, 2003), although, more recently, the focus of photogrammetric studies has been on automating grain detection and data generation (Butler et al, 2001;Sime and Ferguson, 2003;Graham et al, 2005a). In protected, relatively undisturbed areas, such as Marion Island which has Special Nature Reserve conservation status, it is desirable to use techniques such as these photogrammetric methods as they minimise disturbance to the environment.…”