SEDIMENTARY PROCESSES ASSOCIATED TO ACTIVATION OF EPHEMERAL STREAMS INDUCED BY EXTREME RAINFALL EVENTS: AN EXAMPLE FROM COMODORO RIVADAVIA, CENTRAL PATAGONIA

JOSÉ M. PAREDES; S. MARIELA OCAMPO; NICOLÁS FOIX; OLAZABAL, SABRINA XIMENA; MAURO N. VALLE; MONTES, A; JOSÉ O. ALLARD

Puerto Madryn

Congreso; Séptimo Congreso Argentino de Cuaternario y Geomorfología; 2018

Universidad Nacional de la Patagonia San Juan Bosco

Starting on March 29 and during the 3irst week of April 2017, the Comodoro Rivadavia city(Chubut province, central Patagonia) had experienced the worst 3loods because of unprecedented rains.Massive damages to property and infrastructure took place owing to extensive 3looding of sevenephemeral streams that currently debouch in the Atlantic Ocean crossing lowland areas into the city.This catastrophic event occurred due to persistent rains with high runoff production. Although themean annual rainfall of the city in the period 1956-2017 is 240 mm, the extreme rainfall event reached399,4 mm, and on March 31 amounted to 232 mm.We present the results of detailed 3ield surveys carried out after the extreme rainfall event. Theobjectives are (i) to analyze the spatial variation of sedimentological processes along the catchments,(ii) to investigate the in3luence of anthropogenic processes of land use on 3low characteristics anderosion, and (iii) to evaluate mitigation actions to reduce the impact of future rains.The study area is located in the southeast of the Chubut province between latitudes 45°36´ - 45°58´S and longitudes 67°22´ - 68°09´W. The geomorphology of the area shows typical attributes of 3luvialprocesses and allows to identify seven ephemeral river catchments which 3low to the Atlantic Ocean,but it lacks permanent rivers. The seven catchments cover approximately 1546 km2. As the area is inthe rain shadow of the Patagonian Andes, climatic conditions are semi-arid to arid. The altitude raisedfrom 0 m a.s.l. at the catchment outlet to ca. 751 m a.s.l. at the drainage divide, the Pampa del Castillo.The mean air temperature for the period 1956-2017 is 12,9°C at Comodoro Rivadavia airport.The catchments are characterized by high slopes, with ranges for their longest stream between0,0027 to 0,0371. Steep slopes favored high runoff velocity and gully initiation during the storm, anddue to the high intensity of the rain, some gullies reach up to 15 m depth and 870 m long (Paredes etal. 2017). Gullies excavated loose sediments that cover the Chenque and Santa Cruz formations (lowerand middle Miocene, respectively), have V-shaped cross sections, and their side slopes degradedrapidly, providing a huge amount of sand-sized sediments to the streams. Moreover, heavy rains duringthe 3irst days of the storm exceeded the in3iltration capacity, generated intense rainsplash erosion andoverland 3low, providing high-concentration of suspended particles to the streams. Furtherincorporation of cobble to pebble-size clastic components associated to roads, oil locations and locallyparticles obtained from aggregate quarries favored the downstream increase of the kinetic energy anddischarge of the 3low. Although some proportion of the sediment was deposited along the channelnetwork by overbank 3loods, a large proportion of the suspended sediment arrived to higher-orderstreams and entered the city either throughout their natural courses (Arroyo Belgrano, Arroyo La Mata,Arroyo de la Quinta) or following new routes after blocking the manmade waterways. Theanthropically-induced avulsion of the ephemeral streams was followed by massive 3looding oflowlands areas of the city, where reduction of 3low velocity and further deposition of suspendedsediments occurred. The remaining 3low and sediments arrived at the coast and formed transient,medium to 3ine sand-sized deltas of up to 500 m of diameter. Due to exceptional discharges, wideningand deepening of the channels occurred in most of the higher-order streams; as an example, the RestingaAlí stream involved up to 70 m of channel-margin displacement near the outlet