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How charred detritus dispersed from Galeta Beach after the 2018 Montecito, California debris flow — ScienceDaily


The catastrophic debris flow that hit Montecito, California in early January 2018 was the result of a rare confluence of major events. The Thomas Fire has been raging for weeks in Ventura and Santa Barbara counties, and an unusually heavy winter storm dumped half an inch of rain in five minutes on the freshly charred hills above the suburban enclave. Due to the heavy vegetation supporting the hillsides burned by the fire, tons of water, mud, burnt plant material and rocks came crashing down the slopes and engulfed the population below, causing massive damage and the death of 23 residents.

As the community dug itself out of the mud after the disaster, Santa Barbara County officials faced a major challenge: what to do with the mud and other debris that flooded streams, clogged watersheds and covered homes. One solution was to take it to Goleta Beach for disposal, which they did later that month for several weeks.

“They were trying to work with Mother Nature to disperse the debris,” recalls Haley Lowman, then a graduate student in the Santa Barbara Long-Term Coastal Environmental Research Program led by UC Santa Barbara ecologist John Melak. . She explained that since the winter, spring is the area’s heaviest rainfall, when waves and storms hit the coast and rain fills streams that flow into the sea, which can increase the spread of material offshore.

For Lowman and her colleagues, who were able to watch the development of Galeta Beach County from the Institute of Marine Sciences on campus, the situation amounted to a study to find out how far the trash from the emergency disposal would actually travel. Will it wash up somewhere else along the coast or out into the open ocean? Will it accumulate in the marine habitat where it could cause environmental consequences? They conducted the research in collaboration with scientists from the University of Florida and the University of Quebec in Montreal, with support from the Coastal Foundation and the National Science Foundation (through the SBC LTER). The results of their research are published in the journal Science of the general environment.

Tracking of ground debris

“The high biodiversity of the Santa Barbara coast is due to a rich and productive mosaic of coastal marine habitats that includes kelp forests, sandy beaches, surf areas, rocky reefs, surf and eelgrass beds and soft benthos, all closely connected in space,” said Coastal UCSB marine ecologist Jennifer Dugan. “This means that even on a small stretch of coast, the impact of litter can potentially affect many marine habitats and their biodiversity. In light of this, increasing our understanding of the fate of this type of material and its disposal is a critical step in conserving these marine habitats and their biodiversity as we respond to climate change and the likelihood of future severe events here and elsewhere.”

To understand where the detritus from the trash went after it was dumped on Galeta Beach, the researchers collected samples from the beach and Galeta Bay. They also sampled the seabed in the coastal area south of Goleta Slough and along the transect that runs west. In order to determine whether the sediment was from a terrestrial source, such as a debris flow, they looked in particular for charcoal and compounds that indicate burnt material and terrestrial plant matter. Using samples collected near a creek-draining swamp that was not affected by the Thomas Fire, the scientists were able to compare the sediments for a precise “charcoal signal,” which was a sure sign that material had been burned in a fire.

“The good news is that we found that the debris appears to be largely removed from the beach,” said Lowman, who is currently completing his graduate studies at the University of Nevada, Reno. “And we really haven’t found that in other tidal locations that we’ve studied over a long period of time.

“Although the debris was not detected in the shallow core samples at Galeta Beach, it may have been buried by the large amount of sand moving from west to east along the region’s beaches,” Dugan added.

In the nearshore cores, however, the charcoal signal was strong, indicating that the debris did not travel very far from the beach.

“We can say with a high degree of confidence that the charcoal remained mostly in the marine sediments,” Lowman said. Riparian areas in the Santa Barbara Channel are also home to diverse kelp forest communities that are home to fish, crustaceans, and occasionally marine mammals and birds. Debris found in shallow water showed a fair amount of degradation due to wave action, but some material in deeper water was more recent. This is what scientists would expect from organic debris that has not been acted upon by microbes or degraded by normal downstream movement, but rather has been carried from the mountains, then scooped up and deposited directly into the ocean.

“This means that there was a huge influx of organic matter from the terrestrial environment into the marine environment in one big pulse,” Lowman said. “We saw evidence of fresh terrestrial material at a water depth of about 20 meters.”

They didn’t estimate the impact of this debris on the coastal marine environment, Lowman added; this study was mainly to find out if and how far the debris traveled.

“Goleta Bay has shaly beds that are very sensitive to sedimentation and a rich community of subtidal benthic infauna,” Dugan said. “Some of the sandy beaches that line the bay are some of the richest in the world, and the rocky parts of the shoreline can thrive in surf grass.” The bay historically supported a large kelp forest that stretched from Campus Point to Galetta Pier. This kelp forest had an unusual growth form that allowed it to thrive on the bay’s sandy benthos, Dugan added.

Given the increased likelihood of severe weather — the Thomas fire was the largest in California history at the time, but dwarfed by seven wildfires since then — this may not be the last time burned organic material from the mountains is transported to the ocean. According to the researchers, it is important to know what effect these pulses of organic matter have on the coastal community.

“The purpose of this study was to determine whether or not the debris material was trapped and to motivate further research into the effects of this influx of material from the terrestrial to the marine environment,” Lowman said. “Now that we really know it’s here, we need to better study its effects because it’s not dissipating as far as we thought.”

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