Sedimentary processes associated to activation of ephemeral streams induced by extreme rainfall events: An example from Comodoro Rivadavia, Central Patagonia.

PAREDES, JOSÉ MATILDO; OLAZÁBAL, SABRINA XIMENA; ALLARD, JOSÉ OSCAR; FOIX, NICOLÁS; MONTES, ALEJANDRO; OCAMPO, SILVINA MARIELA; VALLE MAURO NICOLÁS

Puerto Madryn

Congreso; Séptimo Congreso Argentino de Cuaternario y Geomorfología; 2018

Asociación Argentina de Cuaternario y Geomorfología

Starting March 29 and during the first week of April 2017, the Comodoro Rivadavia city (Chubut province, central Patagonia) had experienced the worst floods because of unprecedented rains. Massive damages to property and infrastructure took place owing to extensive flooding of seven ephemeral 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 the mean annual rainfall of the city in the period 1956-2017 is 240 mm, the extreme rainfall event reached 399,4 mm, and on March 31 amounted to 232 mm. We present the results of detailed field surveys carried out after the extreme rainfall event. The objectives are (i) to analyze the spatial variation of sedimentological processes along the catchments, (ii) to investigate the influence of anthropogenic processes of land use on flow characteristics and erosion, 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 fluvial processes and allows to identify seven ephemeral river catchments which flow to the Atlantic Ocean, but it lack of permanent rivers. The seven catchments cover approximately 1546 km2. As the area is in the rain shadow of the Patagonian Andes, climatic conditions are semi-arid to arid. The altitude raised from 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 between 0,0027 to 0,0371. Steep slopes favored high runoff velocity and gully initiation during the storm, and due to the high intensity of the rain some gullies reach up to 15 m depth and 870 m long (Paredes et al., 2017). Gullies excavated loose sediments that cover the Chenque and Santa Cruz formations (lower and middle Miocene, respectively), have V-shaped cross sections, and their side slopes degraded rapidly, providing huge amount of sand-sized sediments to the streams. Moreover, heavy rains during the first days of the storm exceeded the infiltration capacity, generated intense rainsplash erosion and overland flow, providing high-concentration of suspended particles to the streams. Further incorporation of cobble to pebble-size clastic components associated to roads, oil locations and locally particles obtained from aggregate quarries favored the downstream increase of the kinetic energy and discharge of the flow. Although some proportion of the sediment was deposited along the channel network by overbank, a large proportion of the suspended sediment arrived to higher-order streams and entered to 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. The anthropically-induced avulsion of the ephemeral streams was followed by massive flooding of lowlands areas of the city, where reduction of flow velocity and further deposition of suspended sediments occurred. The remaining flow and sediments arrived to the coast and formed transient, medium to fine sand-sized deltas of up to 500 m of diameter. Due to exceptional discharges, widening and deepening of the channels occurred in most of the higher-order streams; as an example, the Restinga Alí stream involved up to 70 m of channel-margin displacement near the outlet.The extreme rainfall event has leaved huge damages to urban and industrial infrastructure and revealed lack of planning at catchment scale. To reduce the risk of inhabitants and land degradation, several issues must be addressed: 1) Most of the ephemeral streams currently contain large volumes of in-channel loose sediments, making necessary to build up sediment-reservoirs along the more unstable catchments to reduce both the transit of particles during heavy rains and the flow velocity, 2) The manmade waterways into the city and emplaced over former streams resulted under-sized and require a review of their dimensions, and 3) Urban planning and industrial use of the land within the catchments should respect the inherited geomorphology, minimizing soil erosion and gully initiation.