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Impact of a single dam on sediment transport continuity in large lowland rivers
Published in Silke Wieprecht, Stefan Haun, Karolin Weber, Markus Noack, Kristina Terheiden, River Sedimentation, 2016
Due to damming a reservoir is formed, in the backwater of which water movement slows down to the value critical for bed load transport and, consequently, the material is discharged in the deltaic form. In the lower part of the reservoir conditions favour at least partial decantation of the transported suspension (Babiński, 1992). Such deficiency in clastic-load transport is replenished below the dam due to bed and lateral erosion (among others: Babiński, 1992; Williams & Wolman, 1984; Andrews, 1986; Brandt, 2000; Juracek, 2002; Kondolf, 2004). Right below the front of the erosion zone, an accumulation braided-type reach is formed (Babiński, 1992). Bed erosion below a dam is particularly noticeable in the direct vicinity of the structure. The zone of intensive bed erosion below dams occurs within certain reach of the river and relocates in time, at various rate. The relocation speed of the zone front is directly related to the dynamics of water flowing through the reservoir, topography and geological structure of the channel bed (Kondolf, 1997), as well as time function of the occurrence (Babiński, 1992). When considering the movement rate of the erosion zone front, erosion proceeds until the base level is reached (sea, lake, etc., tributary of a major river) or erosion-resistant bed is encountered.
River Flood Erosion and Land Development and Management
Published in Saeid Eslamian, Faezeh Eslamian, Flood Handbook, 2022
Giovanni Barrocu, Saeid Eslamian
Slope stability may be simply improved by building a weir or a sill only downstream of the unstable slope to favor alluvium deposition. A weir, or low head dam, is a barrier across the horizontal width of a river that alters the flow characteristics of water and usually results in a change in the height of the river level. There are many weir designs, but commonly water flows freely over the top of the weir crest before cascading down to a lower level. The raising of the base level decreases the shear forces acting upstream along the slope of the riverbed and banks. Thus, it prevents the slope failures and debris formation of various masses and sizes, which would increase the kinetic energy of the flood and its destroying capacity downstream.
Geomorphology and Flooding
Published in Saeid Eslamian, Faezeh Eslamian, Flood Handbook, 2022
Giovanni Barrocu, Saeid Eslamian
One may assess the erosion activity of a river by analyzing its long profile, given by the line obtained by plotting the elevations against the respective distances measured along the Thalweg, the deepest part of the streambed from the source to the mouth. The mouth represents the river base level, which is the lowest altitude, reached at the moment by erosion. The general base level is the medium sea level of the sea, where all continental waters outflow.
Origin of the Baltic Sea basin by Pleistocene glacial erosion
Published in GFF, 2020
Adrian Hall, Mikis van Boeckel
Superimposition of the long profile of the Orinoco, a river of equivalent length to the former Baltic River, indicates a plausible datum for Pleistocene glacial erosion (Fig. 3). Correction for residual isostatic rebound indicates that the present shoreline of the BSB will be tilted southward and warped at 60 ka in the future, when all rebound is complete. The rebound surface and the Orinoco profile lie at similar relative elevations, supporting the proposition that fluvial erosion had lowered rocks around the BSB close to the erosional level of the present shoreline prior to the onset of major glacial erosion. As rivers do not erode below base level, the presence of the BSB requires overdeepening by erosion from the FIS. This pattern and depth of erosion is the same for the present and +60 ka scenarios as the form of the BSB remains the same when it is tilted and warped.
Case study on debris-flow hazard mitigation at a world natural heritage site, Jiuzhaigou Valley, Western China
Published in Geomatics, Natural Hazards and Risk, 2020
Wanyu Zhao, Yong You, Xiaoqing Chen, Jingfeng Liu, Jiangang Chen
Based on the characteristics of the debris-flow gully catchments in the Jiuzhaigou Valley, a debris-flow prevention and control system consisting mainly of stabilization, blocking, and deposition structures is proposed. The stabilization structure in this context consists mainly of check dam groups and is applied to stabilize the gully bed and slump sources on the gully banks. These structures reduce the material sources for debris flows in the downstream area. The blocking structure is composed mainly of debris, beam, silt and filter dams, and it intercepts and stores debris flows, regulates the debris-flow discharge and velocity, raises the base level of erosion, stabilizes the gully bed, and prevents the downward gully erosion. The deposition structure in this study consists primarily of a deposition field to deposit debris flows. Therefore, the total amount of debris-flow material escaping the outlet of the gully should be allocated along its course into several parts (Equation 1). where Wc is the total amount of debris-flow material escaping the outlet of the gully; Wstable is the amount of stabilized material; Wdrainage is the amount of drained material; Wblock is the amount of blocked material; Wdeposit is the amount of deposited material.
Sequence stratigraphy and Quaternary depositional systems in the Sanhu area of the Qaidam Basin, northwest China
Published in Australian Journal of Earth Sciences, 2019
Z. Liu, S. Fu, H. C. Jia, J. X. Tian, Y. H. Chen, C. Y. Dong
With increasing lake level (representing base level in lacustrine setting, controlled by tectonic and climate), in which the stratigraphic sequence develops TST, the association of the vertical facies can change from the base coarse-grained delta front subfacies to pre-delta subfacies and to upper fine-grained semi-deep lake and deep-lake subfacies (showing retrograding of delta deposits); the lateral extent of facies also increases. As the relative lake level (representing base level) decreases (i.e. formation of RST), vertical facies associations change from the base fine-grained deep-lake subfacies to pro-delta subfacies and relatively coarse-grained delta front subfacies that suggest progradation of delta deposits. Figure 10 shows the lake-level fluctuation of the Qigequan Formation in the Sanhu area, and its corresponding sequences, system tracts and subfacies and lithofacies associations. The RST mostly develops a prograding-type sand body extending into the basin centre; TST mainly develops retrograding-type sequence sand bodies extending landward. The sand bodies of the Sanhu area are vertically superimposed delta sand and lacustrine sand bodies.