Impacts of breaching on a bar-built estuary, example of the Senegal River new mouth monitored with satellite imagery from 2003 to 2010
Deroin, Jean-Paul; Wade, Souleye; Dia, Abdoulaye
published: Sep 1, 2012
ArtNo. ESP022005603005, Price: 29.00 €
This paper presents the results of a survey of an estuary in Senegal since a humaninduced breach was opened in a bar at the mouth of the embayment in October 2003 to reduce the effect of a major flood event. The evolution of the new mouth's width has been analysed using a Landsat time series of 94 scenes, covering the period 2003-2010. This series is complemented by recent high resolution Alos-PRISM data, and very high resolution Ikonos data. The survey indicates a rapid opening of the new mouth in the first days after the breaching (270 m wide after 10 days) and during the first three years (about 1,900 m wide in September 2006). Then, a strong shortening of the mouth appears (689 m wide in April 2007). The next evolution shows a relatively regular widening of the mouth with limited shortening events during highwater periods (autumn 2007, 2008, and 2009). In May 2010, the mouth was 2,400 m wide and then slightly decreased to 2,145 m wide on September 15, 2010. The general trend is the opening of the mouth with an average of about 300 m per year since the breaching. The effect of the high flow period (August to November) seems to shorten the mouth width but with various amplitudes, -1,210 m in 2006, only -215 m in 2008. During this period the northern sandbar is more or less north-south oriented. In contrast, during the dry season when the littoral drift dominates, the mouth width generally increases except in 2007. The main part of the widening is due to the erosion of the southern part of the mouth (about -2,300 m). The northern part is barely affected because it is regularly fed with sand transported by the littoral drift. The erosion of the southern part results in a NNE-SSW oriented coastline showing also the important effect of the littoral drift. The most recent observations show that the evolution of the mouth will probably change in the near future. The opening of the new mouth in 2010 has been stopped for the first time since the breaching. New changes are now visible such as the natural breaching of Babagueye Island formed in February 2010 in front of the new mouth. All these major changes also induced large-scale ecological and human effects. The direct impact of the new breach on flood mitigation remains unknown. If Saint-Louis City seems less subject to recurrent river flooding by overflow, flooding by stagnation of water from precipitation remains worrying.