Chapters authored
Bedload and Suspended Sediment of a Watershed Impacted by Dams
Sediment transport relates to suspended sediment and bedload. The suspended sediment plays the most important role on the land-ocen sediment flux. On the other hand, the bedload should be considered in order to assess the impacts of dams on sediment transport and sediment yield. Recent effects of dam construction have been widely reported. The sediment load has been reduced by more than 75% for major rives, such as Nilo, Orange, Volta, Indus, Ebro, Kizil Irmark, Colorado, and Rio Grande and more than 40% of its lux is trapped within large dams. In addition, the multiple trapping through sequential dams has impacted the sediment transfer from terrestrial to coastal zone, triggering the coastal erosion. In terms of sediment retention and transport, China stands out the most impacted country by dams, followed by United States, and continents such as Europe, Africa, and South America. Based on the foregoing, the impact of dams on sediment transport and yield of an important Brazilian watershed with multiple dams will be the focus of this chapter. Thus, a three years field sampling (2009-2011) was carried out to measure the sediment yield of Capibaribe Watershed, and also its contribution to coastal erosion. The ratio between QB and SSQ ranged from 0.12% to 27.3% with 76% of all values lower than 5%. Usually, the bedload transport rate of a river is about 5–25% of the suspended sediment transport. This ranging sheds light on the lack of bedload reaching the coastal zone and it is likely one of the reasons to yield coastal erosion. The low rates can be attributed to the presence of dams which have been admitted to have a strong effect on sediment transport. The sediment yield was equal to 3.69, 4.36, and 6.7 t km-2 ano-1 in 2009, 2010, and 2011, respectively. In comparison with bedload yield, the suspended sediment yield was higher than 95% for all studied years. Therefore, the limited bedload supply – mainly responsible for construction of coastal landform – is likely contributing to the coastal erosion along part of the northeast region, Brazil. The multiple dams along the Capibaribe River watershed produce a deficit in sediment flux to coastal zone of Pernambuco State, Brazil, which relies on the low ratios between bedload and suspended sediment. As a result, it generates energy to coastal erosion of the Brazilian northeast.
Part of the book: Effects of Sediment Transport on Hydraulic Structures