Multiband (X, C, L) radar amplitude analysis for a mixed sand- and gravel-bed river in the eastern central Andes

Abstract

<p>Synthetic Aperture Radar (SAR) amplitude measurements from spaceborne sensors are sensitive to surface roughness conditions near their radar wavelength. These data can be exploited to measure gravel-to-sand transitions and downstream gradients in grain size related to geomorphic setting in tectonically active high mountain environments at large spatial scales. The bedload of mixed sand- and gravel-bed rivers can be considered mixed smooth (compacted sand) and rough (gravel) surfaces. Here, we assess backscatter gradients over a large high-mountain alluvial river with aerially exposed sand and gravel bedload using X-band TerraSAR-X/TanDEM-X, C-band Sentinel-1, and L-band ALOS-2 PALSAR-2 radar scenes. In a first step, we compare backscatter response over vegetation-free endmember surfaces within the dry channel bed to assess expected responses and limitations of SAR roughness measurements. We then develop methods to extract smoothed backscatter gradients downstream along the channel using kernel density estimates. In a final step, the presence of sand and gravel bars is analyzed using Fourier frequency analysis, by fitting stretched exponential and power-law models to the power spectrum. We find a large range in backscatter depending on the heterogeneity of contiguous smooth- and rough-patches of bedload material. The SAR signal responds primarily to the fraction of smooth-sand bedload, but is further modified by gravel elements. The sensitivity to gravel is more apparent in longer wavelength (L-band) radar. Because the spatial extent of smooth sand bars is typically < 50 m, only higher resolution sensors (e.g., TerraSAR-X/TanDEM-X) are useful for power spectrum analysis. Our results show the potential for mapping sand-gravel transitions and local geomorphic complexity using SAR amplitude at the scale of large high mountain catchments with aerially exposed bedload.</p>

Identification of Debris‐Flow Channels Using High‐Resolution Topographic Data: A Case Study in the Quebrada del Toro, NW Argentina

“…These include steep topographic gradients, pervasively fractured bedrock in the steep hanging walls of basin‐bounding faults, frequent rainstorm events during summer, and ongoing tectonic activity. In addition, the strongly deformed and cataclasized Proterozoic phyllites of the Puncoviscana Formation predispose this region to debris flows (e.g., Garcia et al., 2019; Olen & Bookhagen, 2018; Purinton & Bookhagen, 2020; Tofelde et al., 2017). Sudden rupture events along basin‐bounding faults in this environment can also trigger rockfalls and landslides (Strecker & Marrett, 1999) or generate pervasive rock fractures that supply large volumes of unconsolidated material to debris flows, similar to tectonically active high‐relief environments elsewhere (e.g., Tang et al., 2009).…”

“…Previous regional studies have revealed high erosion rates based on 10 Be cosmogenic nuclide dating of sands from the Río Capilla subcatchments (Bookhagen & Strecker, 2012; Tofelde et al., 2017). In addition, grain‐size measurements indicate a significant increase in pebble sizes downstream of the confluence of the Río Capilla with the Río Toro mainstem (Purinton & Bookhagen, 2020, 2021). These observations are consistent with the distribution of channels with high debris‐flow similarity, which are widespread within the Río Capilla catchment (Figure 14).…”

“…Farther south, the valley narrows and is dominated by Precambrian through Paleozoic basement rocks (Escayola et al., 2011; Jezek et al., 1985). In this lower sector, the incision of the Toro river has created a steep, narrow gorge that is frequently affected by debris‐flow activity (e.g., Cencetti & Rivelli, 2011; Olen & Bookhagen, 2020; Purinton & Bookhagen, 2020; Tofelde et al., 2017; Wayne, 2011) (Figure 4). Repeated mass wasting from the steep hillslopes and contributing catchments has led to riverbed aggradation, which is associated with the risk of flooding, loss of agricultural land, and temporary blockage of the main valley.…”

Tracking Downstream Variability in Large Grain‐Size Distributions in the South‐Central Andes

“…Contemporary hydro‐meteorologic conditions are characterized by a wet and dry season (coinciding with the South American Summer Monsoon) with frequent, high‐intensity rainfall events and recent trends of increasing rainfall and river discharge related to climate change (Castino et al., 2016a, 2016b, 2017, 2020). Finally, high rates of bedload transport have been observed along the Río Toro (Purinton & Bookhagen, 2018), and associated spaceborne radar measurements of this bedload material indicate the wide range and variability of grain size (Purinton & Bookhagen, 2020), along with the seasonal motion of this material (Olen & Bookhagen, 2020).…”

Mapping riverbed sediment size from Sentinel‐2 satellite data

“…Satellite data, despite their low spatial resolution (metres per pixel), have been used in sub‐pixel methods to study landscape units with characteristic dimensions below 100 m, or even below 10 m (Busetto et al, 2008; Verhoeye & De Wulf, 2002). A recent study by Purinton & Bookhagen (2020) showed that radar amplitudes collected by several satellites, with different wavelength and resolution, have the potential to identify sediment size in mixed sand‐ and gravel‐bed rivers.…”