Sedimentology and evolution of modern gravel bars, Pueblo Viejo River, Tucumán, NW-Argentina

MOYANO, M.S., GEORGIEFF, S.M., IBAÑEZ, L.M.

Delft, Holanda

Conferencia; 8th International Conference on Fluvial Sedimentology; 2005

Universidad de Utrecht, Holanda

Pueblo Viejo River (PVR) flows from high mountains (around 5,000 masl) toward gentle plain of 300 masl. Study area has a humid subtropical climate with a mean annual precipitation of 2,000 mm and a subtropical forest. PVR is a permanent river with peak flows during summer. El Tejar Creek (ETC) is an ephemeral channel, only active during exceptional rainy summers or splays either avulsions of PVR. The interest of this zone is related to splays, partial avulsions of PVR and capture of ETC channel, which means floods of downstream inhabited areas. PVR had a central gravel channel-bar in 2001. The bar migrated during March 2002 due to an exceptional peak flow and a partial abandonment of the right channel was produced at the same time. Periodic detailed maps (January and March 2002, December 2004 and February 2005) with a total station (digital theodolite) and GPS map of an area of 7.5 ha were performed in order to monitor splays and avulsion episodes. The mapping considered the bar shape, channel widths (active and abandoned), location of jetties, possible avulsion points, previous splay channels, topographic levels of different channels, location, size and lithology of blocks in 5 cross sections throughout major channel-bar. There were also compared the map with satellite images from 1990, 1996 and 2000. The first map and the cross sections performed showed a regional slope toward ESE. The sequence maps (from 2002 to 2005) show an evolution from a central channel-bar (January 2002) to a lateral bar (March 2005) and the development of two minor bars up- and downstream of the main bar (2004). These small bars produced a flow diversion and an increment of left bank erosion during 2002-2005 period. In addition, the left margin is composed of pebbles, cobbles and blocks with a fine easily erodable matrix (fine to coarse sands). In the other hand, the right bank is more high and hard than the left bank, and it composed by red fine sediments (sandstones and siltstones). Bar topography and distribution of gravel size on the main bar indicate old chute and cross bar channels. The evolution of the main bar shows a progressive erosion of the left bank, increased by the diverging flow produced by upstream small bar, while the other small bar situated in downstream direction diverts the flow to the main bar’s tail, producing less erosion on the left bank at this area. The highest erodability of the river left bank plus to the regional slope and the position of the El Tejar creek, increase the risk of an avulsion and a permanent capture.