Review of literature
2.2 Sediment dynamics of the major fluvial basins
valley, global estimate of river sediments is incomplete without taking into account of high sediment yielding rivers like Brahmaputra.
an open-water area varying between 600 km2 and 2500 km2, it represents ~13% of the total flooded area of Amazon River. Sediment accumulation occurred during the five months of the flood rise, from December to April. The mean average sediment storage calculated varies between 41% and 53% of the annual flux of sediments entering the floodplain through the main channels.
Martinez et al. (2009) attempted to quantify Amazon River sediment budget by looking at data from suspended sediment discharge monitoring network and remote sensing data.
Suspended sediment discharge was found to increase by about 20% during 12 years since 1995. An increase in sediment discharge may be attributed to stronger erosion processes caused either by a global change (rainfall), or regional changes (land cover change resulting from deforestation for example) or both.
Knox (2006) used 14C and 137Cs isotopic dating methods to determine rates of sedimentation and morphologic change for a reach of the upper Mississippi River. The shift from pre- agriculture, natural land cover to landscape dominance by agricultural land use of the last 175–200 years typically increased rates and magnitudes of floodplain sedimentation in the Upper Mississippi Valley. Large floods have frequently provided major increments of sedimentation on floodplains of tributaries and the main valley upper Mississippi River.
Generally the river mouth is the main depocenter of the riverine sediment (Wang et al., 2007). However, in high flood years the picture is different. For example, in the Yangtze River, the amount of sediment discharged to the sea accounts for 68% of the total sediment supplied into the trunk river in the middle and lower reaches. The remaining 32% is deposited in the river channel and linked lakes. Dai and Lu (2010) examined the sediment process in two large flood years, i.e., 1954 and 1998 based on the re-evaluated sediment supply from the tributaries and the data from selected gauge stations on the main channel.
58% and 52% of the total supplied sediment were deposited in the river channel and floodplain in 1954 and 1998 respectively. The floodplains and channels in the middle and lower reaches of Yangtze River played an important role in regulating sediment discharge during extreme flood events.
Modified sediment load due to human intervention
Human activities are the most important factors that affect the variation in the pattern of river sediment load (Syvitski et al., 2005; Walling, 2006). Sediment budget and human &
climatic impacts in a few large rivers are mentioned in Table 2.3. Liu et al (2008) showed that the average annual sediment yield per area has been decreased significantly in Yellow River over the past 10 years mainly due to impacts of human activities, including the operation of hydropower stations/ reservoirs, construction of dams, as well as soil conservation programs. Liu et al. (2008) showed marked decrease in annual sediment yield at the Lijin Station on the Yellow River in 1961, 1980 and 2000. Before 1960, Yellow River was under essentially natural conditions. From 1960 to 1964, the Sanmenxia reservoir was in operation for impounding water and trapping sediment. Thus, annual sediment transport downstream decreased dramatically. From 1965 to 1973, the operating mode of the Sanmenxia reservoir was changed to provide flood detention and sediment flushing. Thus, both annual sediment transport and runoff increased during this period. From 1974 to 1985, the operating mode of the Sanmenxia reservoir was further modified to provide for “storing clear water during low flow seasons and releasing turbid water during flood periods”. Since 1980, due to increased water consumption by agriculture and industry and soil and water conservation projects, both annual sediment transport and runoff on the Lower Reach of the Yellow River decreased significantly (Liu et al., 2008).
During the past 50 years, the sediment loads are in increasing trend in most of the upstream stations of the Yangtze River basin, but in decreasing trend at other stations (Zhang et al., 2006). Studies by Xu and Milliman (2009) revealed that after the impoundment of the Three Gorges Reservoir (TGR), 60% of the sediment entering the TGR was trapped in flood seasons. Downstream of the TGR, substantial channel erosion is significant. However, downstream channel erosion (70 Mt/y) has not yet counteracted TGR trapping (118 Mt/y) and sediment delivered to the Yangtze estuary will probably continue to decrease.
Nile River, Mississippi River and Red River are other examples where reservoirs and dam construction has changed sediment dynamics of the floodplains. Before the High Aswan Review of literature
Dam, Nile carried an average of 124 million ton of sediment to the sea each year, and deposited another 9.5 million ton on the flood plain. But due to construction of that dam, 98% of the sediment goes to the bottom of the Nasser reservoir (Billi, 2010). In Mississippi River, prior to 1930s, the floodplain was the major sediment source. But due to human modifications in terms of dams & reservoirs, artificial levees, dikes, concrete revetments and a series of channel cutoffs, floodplain provides only a minor amount of sediments today.
Major degradation to the channel including the growth of channel bars has occurred as a result of these engineered modifications (Kesel, 2003). Similarly, the impoundment of two large reservoirs in the Da and the Lo watersheds in the 1980s has resulted in a considerable reduction (70%) of the total suspended load carried to the sea by the Red River (Le et al., 2007).
Table 2.3 Sediment budget and human & climatic impacts in a few large rivers River Sediment budget and human & climatic impacts
Amazon (the largest drainage basin)
Sediment accumulation occurred during the five months of the flood rise, from December to April. Mean average sediment storage varies between 41% and 53% of the annual flux of sediments entering the floodplain through the main channels (Maurice-Bourgoin et al., 2007).
Increased suspended sediment discharge by about 20% during 12 years since 1995, which may be attributed to stronger erosion processes caused either by a global change (rainfall), or regional land cover changes or both (Martinez et al., 2009).
Nile
(the longest river)
Prior to the construction of the Aswan Dam, the river annually delivered 60-180 million tones sediment and water to the Mediterranean Sea. Due to construction of the Aswan dam, 98% of the sediment goes to the bottom of the Nasser reservoir. Decreased sediment supply has created major erosion and shifting of sediment along the coast (Billi, 2010).
Mississippi Increased rate and magnitudes of floodplain sedimentation in the Upper Mississippi Valley due to land use change (Knox, 2006).
Yangtze Since 1950, Sediment load in the Yangtze River has been decreased
(5th in terms of discharge and 4th in sediment load)
sharply due to dam construction (Dai and Lu, 2014), causing serious river management problems including channel erosion and delta retreat (Yang et al., 2011; Li et al., 2014)
Since 2003, the Three Gorges Dam has trapped around 75% of the sediment coming from the upper reaches (Hu et al., 2009). Cumulative sedimentation rate in the Yangtze River basin is approximately 691 (±93.7) Mt y−1. The TGR and other reservoirs are the major contributors to the sedimentation rate of ~ 494 Mt y-1 in the upper Yangtze reaches (Yang and Lu, 2014).
Climate change was considered responsible for a slight increase in sediment influx, approximately 3% (Dai and Lu, 2010).
Yellow (once the world’s largest in terms of sediment discharge)
Sediment budget produced for the period from 1855 to 1968 was characterized by a sediment input of 1.837 × 1011 ton and a distribution between the lower Yellow River, the delta, and the deep sea of 64%, 33%, and 3%, respectively (Shi and Zhang, 2005).
During 1970 – 2008, climate change and human activities were believed to respectively contribute 14% to 86% to the reduction in sediment yield in the upper reaches (Miao, 2010).
Sediment reduction of Yellow River was 41 Gt during 1959 – 2007 due to dams and reservoirs (51%), soil and water conservation (25%), increased water consumption (19%), and channel sedimentation (13%) (Chu, 2014).
Mekong (Asia's third largest river in terms of sediment load)
Approximately 80% of sediment has been trapped within the delta area (Xue, 2011).
As of 2002, the construction of major dams on the headwaters in China appears to have had little impact on the sediment load, although as further larger dams are commissioned, the sediment load of the Mekong can be expected to decrease (Walling, 2008)
Red (the 2nd largest river in Vietnam)
70% decrease of the total suspended load since the impoundment of the Hoa Binh and Thac Ba reservoirs in the 1980s. Following the planned construction of two additional reservoirs, a further reduction by 20% of the suspended load of Red River was predicted, which might be compensated by an expected increase in suspended loading due to enhanced rainfall induced by climate change (Le et al., 2007)
Indus 87% of the annual erosion took place in the three summer months with greatest erosion potential. Major sources of eroded sediment were located in the Karakoram. Substantial sediment storage occurs on the flat Tibetan Plateau and Indus River valley reach (Ali, 2009)
Ganges Of the 794 ×106 t/y transported in the rivers of the Ganga catchment, 80
±10% sediment came from the High Himalaya, 20 ±10% from the Lesser Himalaya and the proportions from the Tethyan Himalaya, Siwaliks, Plain, and Peninsular while unknown are likely to be < 10%.
About 8% of the river sediment was deposited on floodplains and delta plains in Bangladesh. The remaining ~ 45% was deposited in the subaqueous delta and the Bengal Fan (Wasson, 2003)