Scour protection

Scour Analysis Scour Calculation Pressure Flow Scour from Model Tests Bridge Scour Investigation and Prevention Introduction to Bridge Scour Inspection Real-Time Monitoring Scour Protection... 

This chapter presents basic concepts, methods, and procedures in bridge scour design and protection, including hydrology study, hydraulic analysis, scour evaluation, and scour protection. 

Foot protection blocks are placed to prevent through-wash of the rubble foundation and the sand below, while shape-designed concrete blocks are installed to act as the armor layer of the rubble foundation. The water depth d above the rubble mound including the armor layer is 10 m at LWL, and the berm width Bm of the rubble mound is 12.8 m. For scour protection, additional gravel is placed, being called a gravel mat. A vinyl sheet is also used to prevent scouring of the sand under the rubble foundation. 

Scour is usually inevitable for vertical breakwaters built on a sandy sea bottom. However, scour is not a fatal problem due to the protection features provided by the rubble mound. Nevertheless, scour protection should be included in breakwaters in which severe scouring is expected. There are several scour protection methods, e.g., the use of gravel, geotextile, or asphalt mats. These methods can prevent scouring to some extent, though no fully sufficient method has yet been realized. 

These advances, together with numerous advantages of GSCs as soft rock, have contributed to extend the use of GSCs to permanent coastal defenses, including a wide range of types of structures such as seawalls, revetments, groins, artificial reefs, offshore breakwaters, perched beaches, dune reinforcement, core of rubble mound structures, scour protection, etc. (Fig. 21.1). 

In particular, they are more versatile in application and are used for different class of structmes, including dune reinforcement, seawalls and revetments, detached breakwaters and artificial reefs, groins, etc. (see Fig. 21.5). Comprehensive large-scale model investigations on GSCs used for the scour protection of monopile foundation of offshore wind turbines have also been conducted by Griine et al. The results are described in more detail in a final report by Oumeraci et... 

Moreover, the force coefficients Cd, Cl, and Cm are also given as a function of the Reynolds number for different locations and boundary conditions which may represent different practical applications (scour protection on the sea bed, artificial reef, slope containers, and crest container of a surface piercing structure such as revetments, seawalls, groins, etc.). The proposed values of Cd, Cm, and Cl have been determined on the basis of systematic laboratory experiments. ... 

Geotextile sand containers (GSCs) represent nowadays a soft and low cost alternative to conventional hard structmes made of rock and concrete. Moreover, GSC-made structures are environmentally more appropriate and more easily reversible as they need essentially sand as construction material which is generally available at any coastal site. As soft rock GSCs can be manufactured at any size and used to build any type of shore protection structure, including scour protection, dune reinforcement, and repair of undermined structures. 

J. Griine, U. Spaxboom, R. Schmidt-Koppenhagen, Z. Wang and H. Oumeraci, Sta-bihty test of geotextile sand containers for monopile scour protection, ASCE Proc. Int. Conf. Coastal Eng., San Diego, USA (2006), pp. 5093-5105. 

H. Oumeraci, J. Griine, H. Sparboom, R. Schmidt-Koppenhagen and Z. Wang, Investigations on scour and scour protection for monopile foundation of wind offshore turbines, Forschungszentrum Kiiste (FZK), Res. Report (2007) 79 and Annexes (in German). 

The above equation (22.15) is based on experiments where the breakwater slope was 1 1.5. Therefore, for slopes milder than 1 1.5, the width necessary for protection might be reduced (and for steeper slopes increased). Furthermore, Eq. (22.15) is for scour protection against the local scour caused by the combined effect of steady streaming and stirring up of sediment by waves. Due considerations must be given to global scour caused by the far-field flow circulations around the breakwater. Finally, the recommended width is for protection at the offshore side of the head. 

Design and analysis of inlet structures, scour protection around bridge foundations, channel revetments, etc. 

Stilling basins and downstream scour protections will be designed for the following conditions [ER 1110-2-1458, Hydraulic Design of Shallow Draft Navigation Projects (USAGE 1998)] ... 

Typical applications under unsteady cyclic (or reversing) flow conditions are bank revetments, separation and reinforcing layers in submerged or submersible foundations, offshore foundations, scour protections, and pipeline protections. The range of boundary conditions, in terms of vertical effective stress and hydraulic gradient applied, characteristic of these applications is shown in Fig. 8.15. 

Geotextiles are used for many different types of offshore engineering applications. Four applications are discussed here underwater dykes, scour protection for offshore wind turbines and support and protection for offshore pipelines. 

Offshore wind turbines normally have a large monopile foundation (typically 6 m in diameter) that has been driven into the seabed to a stable depth. To prevent scour around these monopUe foundations (which can lead to monopile instability), scour protection measures are employed. Generally, two different procedures are used to prevent foundation scour. First, a scour protection system is placed on the seabed around the base of the monopUes during the installation process (Fig. 20.39). Second, some scour is allowed to occur around the monopile after installation the resulting scour hole is then filled with erosion-resistant materials. 

In applying a scour protection system as part of the monopile installation (Fig. 20.39), a geotextile filter and stone protection layer are first placed on the seabed to the required horizontal extent to prevent future scour. Then the monopile is driven through the stone layer (and geotextile) and into the seabed to the necessary depth. Finally, an armour protection layer is placed on top of the stone layer around the base of the monopUe. 

Figure 20.39 Scour protection around monopile foundations for offshore wind turbines. 

Figure 20.40 Use of geotextile bags for scour protection around monopile foundations. 

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