River Engineering

RIVER ENGINEERING WITH REFERENCE TO THE VAR RIVER AND ITS LOWER VALLEY RIVER CONTROL • Engineers have developed means to control rivers: 1. Vertically 2. Horizontally • In the XXth century, engineers considered that a river could (or had to be) “tamed” (see the Mississippi, Rhine and other). HydroEurope 2 March 2004 RIVER MANAGEMENT • Today, engineers recognise the need to “manage” a river, instead of taming it. • This trend has also to do with the growing recognition of the ecological values of rivers. • Managing the river requires a proper understanding of the functioning of the river systems. HydroEurope 2 March 2004 RIVER MANAGEMENT • This short presentation will be mainly focussing on the management issues in rivers like the Var Lower Valley. • Management will be seen as a combination of channel control, riverbed management and flood management. • The “measures” must preferably be reversible, to account for unforeseeable river responses. HydroEurope 2 March 2004 VERTICAL CONTROL • When there is a pronounced unbalance (natural or man-made) in sediment transport and sediment supply, usually in the upper reaches (see “LANE’s” balance), a riverbed will tend to degrade (to incise, to lower). • Fixed weirs may help stopping the incision (see the case of the sills n° 2 to 16 in the Var’s lower valley). HydroEurope 2 March 2004 VERTICAL CONTROL • The weirs (or sills) are called “grade-control structures” (structures to control the slope). • By diminishing the slope, the power of the flow is reduced (remember that the power - i.e. the work that can be done per unit time - of the flow per unit area is the product of the flow velocity times the shear stress). HydroEurope 2 March 2004 VERTICAL CONTROL • When the river is incised, there is usually no need to control the river course laterally. • However, because of the energy dissipation at the sill, it is recommended to have the weir with its lateral parts higher, so as to concentrate more flow in the central part. • Sills can be build in a variety of materials, like concrete, gabions, masonry, and other. HydroEurope 2 March 2004 VERTICAL CONTROL • Vertical control structures may be studied with: 1. Scale models for the structure itself (such as a broad-crested weir of the Var). 2. One-dimensional numerical models to predict the impact on flow and sedimentation (see presentation of Lauro Rossi) HydroEurope 2 March 2004 LATERAL CONTROL • Lateral control of the river course is needed to protect something: the valley walls, river banks, a floodplain, levees, bridges, etc. • Sometimes, lateral control structures are built to change the shape of the river course, for navigation or to ease the evacuation of the flood waves. HydroEurope 2 March 2004 LATERAL CONTROL • There are several ways to control the plan- form of the river course: 1. River bank revetments 2. Retard structures 3. Groynes, spur dykes 4. Guide bunds (also called guide banks) 5. Vanes HydroEurope 2 March 2004 LATERAL CONTROL • In this presentation, only bank revetments will be discussed in more detail. • Retard structures are open constructions through which the flow can pass and that induce sedimentation of the solids in suspension (fine material). They may be built in simple material, and even vegetation may be used (such as Catkin grass). HydroEurope 2 March 2004 LATERAL CONTROL • Groynes are structures that are protruding in the river, to concentrate the flow in only part of the river, so that eddy flow is created in between them, where (suspended) sediment may settle. • Groynes are always in fields (several ones) and require most often to protect the opposite riverbank. HydroEurope 2 March 2004 LATERAL CONTROL • Guide bunds are heavy structures, usually at bridges or headworks (mobile dams). • Guide bunds are required to avoid “outflanking” of the channel (i.e. the tendency of the river to erode the bank behind a structure such as a bridge abutment). • Guide bunds may help convey the flow correctly towards a weir (sill). HydroEurope 2 March 2004 LATERAL CONTROL • Vanes are special structures that induce secondary (helical) flow, which move the sediment (bed load) away from the channel and concentrate the flow in it (protecting thus the riverbank where the sediment settles). • Use of vanes requires experience, to know where and how to put them in place (see the use of “bandalls” in India and Bangladesh) HydroEurope 2 March 2004 LATERAL CONTROL • Lateral control structures are studied with: 1. Scale models, for complex geometries. 2. Numerical models for simple geometries. • The presently available numerical tools have not yet been able to include correctly some processes, such as the sediment transport (bed load) and the way it moves through the channels. HydroEurope 2 March 2004 LATERAL CONTROL • Bank protection has been constructed since ages, not always understanding the mechanisms responsible for the bank failures. • The principle is to isolate the bank material from the flowing water. • A traditional way was to place over the lined bank an inverted filter (coarse material at the water-side, finer at the land-side) HydroEurope 2 March 2004 LATERAL CONTROL • A major problem with bank protection is the scouring of the riverbed at the toe of the protected slope. • The traditional solution is to extend the bank revetment with an “apron” on the riverbed. • The extend of the apron is established by computing the maximum scour depth. HydroEurope 2 March 2004 LATERAL CONTROL • In rivers like the Var, the scour depth has been usually calculated in the hypothesis that the flow remains parallel to the bank. • Experience show that this hypothesis is not correct, and that the flow attacks the bank obliquely, creating deeper scour holes. HydroEurope 2 March 2004 LATERAL CONTROL • Locals have discovered this since long and have developed their own, original system that allows the bank protection to glide deeper in the local scour hole without threatening the whole structure. • Concrete blocks are placed over a lined concrete sloping wall; they glide down as soon as a local scour appears. HydroEurope 2 March 2004 LATERAL CONTROL • Engineers responsible for designing the new bank protections have ignored the local experience and use large blocks placed loose over a geotextile. • These large blocks produce additional turbulence, which increase the scour depth and make the bank protection collapsing more rapidly. HydroEurope 2 March 2004 RIVERBED MANAGEMENT • All structures will create a “response” of the river, an example being the bank erosion opposite to a groyne field. • We have seen, during the visit in the Lower Var Valley, how the remains of sills n° 2 and 3 control the development of the gravel bars and the (chute) channels, producing strong bank erosion. HydroEurope 2 March 2004 RIVERBED MANAGEMENT • Dredging and excavations may be required to correct the negative river response. • Most important is to understand the mechanisms producing the unwanted river response, such as a bank erosion. • The heavy protection put in place along the left bank between sills n° 4 and 3 could be avoided by dredging a channel in the bar. HydroEurope 2 March 2004 CONCLUSIONS • River works may be needed to benefit from the positive aspects offered by a river, as for navigation. • River works are sometimes needed to protect valuable areas or infrastructures, such as cities or bridges. • There is a need to manage the rivers taking into account all its functions. HydroEurope 2 March 2004 CONCLUSIONS • River engineering can rely on several tools for studying possible solutions: models (scale and numerical) and the field (a model at scale 1/1). • Tools are not sufficient, and expertise is required. HydroEurope 2 March 2004 CONCLUSIONS • A challenge for our education system is to make students aware of this and to generate their interest for modelling while keeping in mind the physics and the mechanisms that are at the basis of the river’s behaviour. • The Var’s lower valley is a case in which for sure expertise is required besides tools such as models. HydroEurope 2 March 2004 .

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