Fire (plus) flood (equals) beach: coastal response to an exceptional river sediment discharge event

Abstract

Wildfire and post-fire rainfall have resounding effects on hillslope processes and sediment yields of mountainous landscapes. Yet, it remains unclear how fire–flood sequences influence downstream coastal littoral systems. It is timely to examine terrestrial–coastal connections because climate change is increasing the frequency, size, and intensity of wildfires, altering precipitation rates, and accelerating sea-level rise; and these factors can be understood as contrasting accretionary and erosive agents for coastal systems. Here we provide new satellite-derived shoreline measurements of Big Sur, California and show how river sediment discharge significantly influenced shoreline positions during the past several decades. A 2016 wildfire followed by record precipitation increased sediment discharge in the Big Sur River and resulted in almost half of the total river sediment load of the past 50 years (~ 2.2 of ~ 4.8 Mt). Roughly 30% of this river sediment was inferred to be littoral-grade sand and was incorporated into the littoral cell, causing the widest beaches in the 37-year satellite record and spreading downcoast over timescales of years. Hence, the impact of fire–flood events on coastal sediment budgets may be substantial, and these impacts may increase with time considering projected intensification of wildfires and extreme rain events under global warming.

Fires, floods and other extreme events – How watershed processes under climate change will shape our coastlines

“…Similar patterns of coastal progradation followed by recession and sediment spreading are observed at river mouths around the world as they respond to time-varying sediment inputs (Inman and Nordstrom, 1971;Hicks and Inman, 1987;Cooper, 1993;Anthony and Blivi, 1999;Barnard and Warrick, 2010;Casalbore et al, 2011;Giosan et al, 2012;Milliman and Farnsworth, 2013;Anthony et al, 2014;Bendixen et al, 2017;Besset et al, 2017;Luo et al, 2017;East et al, 2018;Warrick et al, 2019). Satellite imagery can provide important observations of the spatial and temporal response of shorelines to new contributions of river sediment (Besset et al, 2016(Besset et al, , 2019Guzman et al, 2020;Warrick et al, 2022), and example satellite records of the multitude of coastal systems with active river sediment supplies are provided in Fig.…”

“…Additionally, the records also highlight the effects of perturbations, such as wildfires, floods, earthquakes, typhoons, and human impacts, on year-to-year variations in sediment discharge (Fig. 5), which are described more fully in original investigations of these and other rivers (Dadson et al, 2004;Gran and Montgomery, 2005;Wang et al, 2007;Hovius et al, 2011;Lee et al, 2015;Gray, 2018;Montanher et al, 2018;Collins et al, 2020;Warrick et al, 2022).…”

“…Sediment discharge is especially elevated when a landscape perturbing event, such as wildfire or an earthquake, is followed by heavy precipitation, such as shown in the https://doi.org/10.1017/cft.2022.1 Published online by Cambridge University Press records from the Big Sur River and Choshoi River watersheds (Fig. 5a,b) (Dadson et al, 2004;Hovius et al, 2011;Warrick et al, 2022). Over larger watershed scales of 100,000s to millions of square kilometers, events such as earthquakes or wildfires impact relatively small areas compared to the river's total drainage basin area and thus contribute only marginally to the overall temporal variations in sediment fluxes (Milliman and Farnsworth, 2013;Francis et al, 2022).…”

“…The high temporal variability in the Big Sur River is largely related to the combined effects of two wildfire and heavy precipitation events (labeled "Fire + Flood"; Fig. 5a) (Warrick et al, 2022), although sediment discharge from this river is still strongly variable in time if these events are not considered in the records (Fig. 5e).…”

“…The shoreline accretion from the eruption of Santa Maria, Guatemala continues to extend more than 1 km offshore of the pre-eruption shoreline even though the volcanic event occurred over 120 years ago, providing evidence for the longevity of coastal impacts from rare, but massive, sediment discharge events (Kuenzi et al, 1979;Fig. 3b Detailed accounting of shoreline responses to river inputs can be obtained where physical monitoring or remote sensing records are adequately frequent (Hicks and Inman, 1987;Barnard and Warrick, 2010;Besset et al, 2016Besset et al, , 2019Vos et al, 2019a;Guzman et al, 2020;Warrick et al, 2022). For example, the combination of satellitederived shoreline positions from Landsat and Sentinel-2 imagery using the CoastSat technique (Vos et al, 2019b) and estimates of littoral-grade sediment discharge (Barnard and Warrick, 2010) for the Santa Clara River, California shows how the shoreline responds differently in space and time to elevated river sediment discharge (Fig.…”