Preliminary Investigation and Assessment of Landslide Dam Occurring in Blumenau, Southern Brazil

As a direct result of continuous rainfall, following a small landslide occurring at about 18 : 30 on October 22, 2015, a large landslide occurred at about 2 : 00 am local time on October 23, 2015, on the left slope of the Garcia River, a branch of the Itajaí-Açu River, southern Blumenau, State of Santa Catarina-Brazil. The landslide occurred immediately after a month period that was marked by unusually high levels of rainfall locally (354.6 mm/month). The landslide initiated within an approximately 300-m-high steep slope and about 200 m of altimetry gradient at the base of the landslide in the geological context area of sedimentary rocks of the Neo-Proterozoic Gaspar Formation. The formation is composed mainly of sandstone and conglomerates and intercalated with fine -grained volcanic tuff bed. The landslide debris struck the community of Willy’s Rancho at the base of the landslide slope, seriously damaging 4 houses and temporarily damming the Garcia River. The landslide debris travelled downslope and partially into the Garcia River, and completely blocked the original course of the Garcia River, leading to changed flow course around the base of the landslide slope and formed landslide dam with a crest height of about 6.0 m above the riverbed. Shortly after the formation of the landslide dam it was immediately impounded to a depth of about 6.0 m above the valley and a length of about 250 m behind the dam, and then partially failed due to overflow and erosion. By the time of our site visit the Garcia River has been partially blocked at an estimated water level of about 4.5 m above the riverbed in the area of the natural dam. The landslide debris still accumulates mostly on the Disaster Report