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Published in G.J.C.M. Hoffmans, H.J. Verheij, Scour Manual, 2021
G.J.C.M. Hoffmans, H.J. Verheij
When conditions to which a river has been adjusted change, overall degradation or aggradation occurs. Examples of changes in conditions are increasing flow or increasing sediment transport in a river. Some examples of features, resulting either from human interference or from natural changes, to which the flow regime could respond, are as follows: Flood embankments, flood detention basins and weirs;Channel improvement schemes involving dredging or weed clearance;Mining of sand and gravel;Changes in water patterns (confluence or bifurcation of river channels);Schemes for transfer of water between river basins;Meander cut-offs.
Basic concepts of soil erosion and sediment transport
Published in Arved J. Raudkivi, Sedimentation, 2020
Aggradation means the building up of the stream bed elevation. However, the accumulation of sediment in a reservoir, which is also a form of aggradation, is usually called siltation. Aggradation of stream beds is usually associated with excessive sediment supply, for example, from a badly eroding catchment. Although it is basically the same problem as degradation, it is frequently made more difficult through spatial variation of the sediment supply, i.e. sediment may be supplied from both banks overland and by tributaries over a considerable distance of the stream, not just an input boundary condition at the upstream end. However, a tributary, for example, may also bring in an excess amount of sediment at a section x1 of a river. The flow in the river will then continue to change until a new sediment transport equilibrium is established over that section of the river. The upstream reach will slowly aggrade to the new slope imposed by the equilibrium at x1 and its hydraulic conditions change to carry the incoming sediment at the new slope. The bed elevation changes logarithmically with time. From the input station downstream, the aggradation proceeds as a propagating wedge over which equilibrium conditions prevail and downstream of the front of this sediment wedge flow remains essentially unaffected, i.e. at the initial conditions.
Stratigraphy and Sedimentation
Published in Supriya Sengupta, Introduction to Sedimentology, 2017
Sediments accumulate within basins by aggradation, progradation and lateral accretion. Aggradation is the process of vertical filling, as in the flood basin of a river. Progradation is the filling from the margin, as in the case of an advancing delta front. Lateral accretion is caused by the process of accumulation against the margin of a channel, as in a river point bar (see Fig. 8.1). The vertical relationship between succeeding sedimentary units produced in this way may be either conformable or non-conformable. A conformable relationship may be either sharp or gradational. Two types of gradations are possible: a continuous gradation or an intercalated contact. A body of sand grading vertically up into a shale section through mixtures of sand and shale in various proportions, is an example of a continuous gradation. A sand body passing vertically up into a body of shale through an intermediate layer of closely spaced lenticles of sand and shale is an example of intercalation.
Evaluation of the equilibrium of the River Nile morphological changes throughout “Assuit-Delta Barrages” reach
Published in Water Science, 2018
It is worth mentioning that the morphological unbalance can result in unfavorable conditions. For example, when the riverbed aggrades due to the sediment deposition resulting from the flow failure to carry the sediment load, bars and islands form. These islands sometimes develop and merge with the channel banks causing part of the active river channel to be blocked and decommissioned. Also, aggradation causes what so-called “navigational bottlenecks”. These necks always hinder the inland navigation movement and in most cases bring it to a complete stop (Sadek and Hekal, 2008). Moreover, bed aggradation causes flooding of unexpected floodplain areas. This event could cause severe harm and losses to human activities existing close to the river channel particularly during high and emergency flows. Additionally, sediment deposition around water intakes affects the water abstraction and supply process by blocking the inlets of the supply pipes. This requires continuous maintenance and dredging works which is considered costly and uneconomic.