Flood Variability Determines the Location of Lobe‐Scale Avulsions on Deltas: Madagascar

Abstract

River deltas grow through repeated lobe‐scale avulsions, which often occur at a location that correlates with the backwater lengthscale. Competing hypotheses attribute the avulsion node origin to either the morphodynamic feedbacks caused by natural flood discharge variability (backwater hypothesis) or to the prograding delta lobe geometry (geometric hypothesis). Here, using theory, historical flood records, and remotely sensed elevation data, we analyzed five lobe‐scale delta avulsions in Madagascar, captured by Landsat imagery. Avulsion lengths were 5–55 km, distances significantly longer than the backwater lengthscale and inconsistent with the geometric hypothesis. We show that the steep, silt‐bedded rivers of Madagascar have flood‐induced bed scour, driven by backwater hydrodynamics, that propagates farther upstream than the backwater lengthscale. The avulsion lengths are 3.1 ± 1.5 times the predicted flood scour lengths, similar to low‐gradient deltas, and consistent with backwater hypothesis. Results demonstrate that erosion initiated by nonuniform flows in the backwater zone is a primary control on delta avulsion locations.

Additional Information

© 2020 American Geophysical Union. Issue Online: 20 October 2020; Version of Record online: 20 October 2020; Accepted manuscript online: 21 September 2020; Manuscript accepted: 16 September 2020; Manuscript revised: 12 September 2020; Manuscript received: 14 May 2020. We thank R. C. Mahon for constructive comments on the manuscript. This work was supported by the National Science Foundation EAR 1935669 grant to Ganti and EAR 1427262 to Lamb. Data Availability Statement: Data sets used in this research are publicly available, with USGS/NASA Landsat satellite imagery and SRTM 1 arcsec elevation data downloaded from the USGS Earth Explorer (https://earthexplorer.usgs.gov/) and Sentinel 2 imagery downloaded from the Copernicus Open Access Hub (https://scihub.copernicus.eu/), river discharge data from the RivDIS and GRDC global discharge data sets (GRDC, 2015; Vorosmarty et al., 1998) were downloaded from or requested from https://doi.org/10.3334/ORNLDAAC/199 and https://www.bafg.de/GRDC, respectively, with surface water masks provided by the JRC Global Surface Water 1984–2018 data sets (Pekel et al., 2016) downloaded from https://global‐surface‐water.appspot.com/download. Representative global compilation of low gradient avulsions were compiled from previously published studies (Chatanantavet et al., 2012; Ganti et al., 2014, 2019; Jerolmack & Mohrig, 2007; Jerolmack & Swenson, 2007; Moodie et al., 2019). Tabular data for this study have been provided in the supporting information. All code and data used in this study can be downloaded from these sites (https://doi.org/10.6084/m9.figshare.12948671 and https://gitlab.com/sambrooke/avulsion‐flood‐scour‐paper/‐/releases/v1.2).

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Published - 2020GL088797.pdf

Supplemental Material - grl61264-sup-0001-2020gl088797-ts01.csv

Supplemental Material - grl61264-sup-0002-2020gl088797-si.pdf

Supplemental Material - grl61264-sup-0003-2020gl088797-ts02.csv

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