Morphodynamic study of an artificial inlet in Langue de Barbarie (Senegal) from 2003 to 2014 using Landsat images

Abstract

The Senegal River is the second largest river in West Africa. It has a length of 1800 km and a watershed of 337000 km2. Born in Mali resulting from the confluence of two rivers (the Bafing River and the Bakoye River), it crosses through Guinea and finally along the border between Mauritania and Senegal, before flowing into the Atlantic Ocean. All along this coast the prevailing NW swell generates a southward longshore drift that has contributed to the formation of a sandy spit with an average width of 300 m, which runs parallel to the coast along more than 45 km, starting in South Mauritania and ending in North Senegal. This spit covers an area of approximately 15 km2 and it is characterized by low topography vegetated dunes, around 5 m in the higher areas. The soil is infertile and consists on medium and fine quartz sands. The vegetation is Sahelian, haracterized by Ipomoea pescaprae, Sea Alternanthera, Sporobolus spicatus and Sesuvium portulacastrum. In 1976 the southern part of this sand barrier was declared as Langue de Barbarie National Park by ecological reasons. The weather is marked by two distinct seasons: The wet season takes place from July to October, and the dry season the rest of year. This is reflected in the river discharge, so that during the wet season average discharge is 1500-2000 m3/s, while during the dry season it is around 200 m3/s. In 1986 the Diama dam was built, which changed the hydrological conditions in the river mouth. In October 2003 heavy rains originated the flood in the estuary of the Senegal River, endangering the surrounding population and the city of Saint Louis (Capital of Senegal from 1872 to 1957 and World Heritage Site by UNESCO). Due to this situation the authorities decided to open a channel through the spit to facilitate river discharge. The channel dimensions was 4 m wide, 200 m long and 1.5 m deep, and it was located approximately 7 km south of Saint Louis. The pressure of the river water flowing at very high speed originated the quick channel widening, so that two days after opening it was 80 m wide and three weeks later it was about 330 m. Since then the morphodynamic change in this area has been very fast. Few months after the opening, the artificial channel evolved to form the new river mouth, and the ancient embouchure was naturally closed due to the weakness of river flow and the strong longshore drift. Nowadays the inner zones of the lagoon that are opposite the new mouth are completely exposed to wave action, and therefore they are suffering very strong coastal retreat. This strong erosion has forced people to leave their properties, as in the case of Doun Baba Die`ye, where the whole village has completely disappeared and people were forced to move inland (Sy., 2010). This paper aims to assess how it has been the evolution of this highly dynamic system since the situation previous to the new opening to nowadays. To cope with this task a small area 12 km long was selected, covering the whole new mouth and adjacent areas. 17 Landsat images with a spatial resolution of 30 m were used, covering most of the study period. These images were treated with ArcGis 9.3. Since the opening of the drainage channel, the spit was divided into two fragments, located north and south relative to the new mouth, which have evolved in different ways. To quantify the changes three parameters were measured: shoreline length and area in each of the two spit sectors, and amplitude of the river mouth. The first image analyzed corresponds to April 5, 2003 previous to the channel, so there is a single polygon with 4050000 m2 area and perimeter of 26.4 km. During the early days from the channel formation, as mentioned above, the change in the gap was very fast, reaching 330 m in three weeks. In November 15, 2006 the river mouth was about 1750 m wide and the loss of total area was 760000 m2 (19% of the initial area). In February 2014 the area loss is 1310000 m2, approximately 33% less than the initial study area, since the opening has reached 5250 m length. Nevertheless changes at both sides of the gap shows opposite pattern. The southern coast suffers constant erosion on its outer face, due to the waves attack and longshore drift. Therefore the spit becomes narrow and overwash processes may take place, with the subsequent spit fracture and temporal formation of small islands that finally disappear. Coastal retreat is approximately 7 km from the opening and the average erosion rate is 580 m2/day. On the contrary, the northern coast shows a net progradation due to the sedimentation of particles pulled by littoral drift due to the hydraulic barrier produced by the river flow on its way out to the ocean. This effect produces sand accumulation upstream of the gap in the form of sandy hooks. Average accretion is 290 m2/day, due to a net southward displacement of the river mouth by 1750 m. To summarize, the Senegal River mouth migrates southward as a result of the slow progradation north of the gap and the rapid erosion at the south.