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Geomorphology and Flooding
Published in Saeid Eslamian, Faezeh Eslamian, Flood Handbook, 2022
Giovanni Barrocu, Saeid Eslamian
Among various factors involved, there is the downslope gravity component tangent to the riverbed, the attrition due to the roughness of the bed, and the water turbulence caused by the irregular paths of the load elements of a different type, so that flow lines continuously deviate from one side of the channel to the other. In the flood plain, the bends, rapidly modified in any direction, tend to assume the shape of loops similar to the capital Ω, typical of the classic river Meander, the present-day Menderes in southwestern Turkey, characterized by a very convoluted and winding path along the lower reach, for which reason they are named meanders. The lateral flood erosion of two opposite banks may eventually cut off the narrow neck of a meander, which so becomes a crescent-shaped lake, named oxbow lake, separated from the river (Figure 2.12B). The stream short-circuits its course with increased momentum so that the flood enhances its erosion power.
Rivers: Restoration
Published in Brian D. Fath, Sven E. Jørgensen, Megan Cole, Managing Water Resources and Hydrological Systems, 2020
Yet, if riverbed regulation is needed, the solutions should be based on the following principles[36,38,40–49]: Natural riverbed protection: rechanneling is done through initializing the meander pool by imposing a triangular section.Smoothing natural riverbed meanders by means of linear structures with cross-beams: dams are replaced with river walls that only reinforce concave banks and are pitched against the existing banks (apart from cases when this solution cannot be used).Meandering, including riverbank walls or short structures resembling groyne heads (only for riparian protection, low impact on riverbed flows and the riverbed itself).Transverse structures that stabilize the riverbed bottom: made as gentle-sloped erratic riffles (1:10 is the optimum gradient, tested in practice), thus preserving the biological passability of the watercourse; if possible, the location of the riffle is selected so that it can also act as a cataract and not only as bottom gradient reducer and stabilizer.Stone backfill with appropriate scarp inclination from 1:2.5, the recommended ratio being 1:4 and lower (wherever possible): more capacious watercourse bed, accessible to people and animals; regulative structures are more durable.Preservation of the riverbed’s and the river bottom’s morphological form: construction of stone backfill from the meander pool bottom (elimination of the necessity to disturb the natural gravel bottom shield during the formation of partitions from these alluviums; the structures are made without partitions).Stone backfill durability and visual appeal enhancement: boulder placement and turfing initialization by soil filling of interboulder space and grass seeding according to pertinent formulas.Application of defenses made of stone netting baskets: only for protection of building structures situated in the vicinity of the watercourse bank.River bar or island leveling or correction—riverbank reinforcement.
Flow through side-combined structure in a channel bend under subcritical flow regime
Published in ISH Journal of Hydraulic Engineering, 2022
Masoud Ghodsian, Farshid Feyzollahi, Mohammad Ghodsian
Generally, a natural river consists of alternate meanders separated by short straight reaches. The knowledge of flow characteristics in a channel bend is important for river diversion plans because a bend is often the best site for an intake. The streamlines in a channel bend are not parallel to the channel boundaries. The surface currents are directed to the outer bank of bend, the bottom currents to the inner bank of the bend; and the entire flow moves as a spiral flow. Hence, the flow structure is three-dimensional, leading to the helical flow structure in curved channels (Razvan 1989).
An automated approach in estimation and prediction of riverbank shifting for flood-prone middle-lower course of the Subarnarekha river, India
Published in International Journal of River Basin Management, 2021
River naturally erodes their banks and changes their inherent morphology to achieve an equilibrium condition with the ever-changing effects imposed on the channel. The selected alluvial (middle-lower) course accommodates changes with the imposed discharge by eroding the banks and subsequently deposits the eroded material in the lower reaches. Riverbank erosion is strongly associated with the expansion and migration of meander bends. In the studied river course, zones A, C and D are very common where the riverbed and bank materials are erodible and cohesive, which further insists to channel meandering. In these zones, thalweg impinges on the outer bank (concave) of the meander bend. The materials of the outer bank are eroded by the hydraulic action of the high-velocity current, which transfers the eroded material toward the downstream section. Meanwhile, the low-velocity current with less energy acts on the inner bend (convex) of a meander, which leads to sediment deposition. A secondary circulation is fashioned due to an uneven distribution of shear stress along the cross-sectional and longitudinal profile of the meandering bend. This process further intensifies the sediment deposition at the inner meander bend. Therefore, the outer meander bends are gradually eroded and sediment deposited at the inner bends. The result of bank shifting at the meandering stretch (Figure 6 and Table 6) shows that most of the outer bends (except L1, L2, R2, L4, R5, L10 and L11) are gradually eroded. Among these meander bends, the outer bend of L1, R2, L4, L10 and L11 remains stable as hydraulic action is protected by the strong embankment structures (Figure 12). The other meander bends (L2 and R5) are shifted in the reverse direction (e.g. left bank shifts leftward) as the hydraulic head is reflected by the erosion protection embankment structure. As a result, the hydraulic action and uneven spatial distribution of shear stress influence the spatial pattern of erosion and accretion along the middle-lower course. The spatio-temporal pattern of riverbank shifting leads to reform the river course through narrowing or widening of channel and bank shifting toward left or right in different stretches.