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Scour Manual

In February 1953, many dikes in the southwestern provinces of the Netherlands failed during a heavy storm flood. As a result of this disaster, various hydraulic and geotechnical problems had to be solved to determine suitable solutions to the breaches in the flood defences. To study the effects of closures, small-scale tests were conducted at Delft Hydraulics to obtain information on the critical flow rate for the stone and concrete block stability, the overlap of mattresses, and the scour effects downstream of the hydraulic structures.

It is crucial to design hydraulic structures that are reliable and safe during their life cycle. To ensure safe long-term functioning of hydraulic structures, it is necessary to consider failure mechanisms, boundary conditions, and design criteria. After addressing the boundary conditions, fault tree analysis and risk assessment are discussed in the new Scour Manual, which is an update to the version published in 1997. Two methods are treated: one based on safety factors and one based on failure probability.

Alternative Shields diagram; load due to turbulent flow versus strength
Floating peat in Gouwe channel due to thrusters

The flow close to hydraulic structures is usually expressed by either a turbulence intensity or a dominating flow velocity or both. The combination is the most important phenomenon determining erosion. Relations for the turbulence intensity and the critical flow velocity are presented in the Scour Manual for various situations. The Shields design graphs are also presented, not only for non-cohesive bed materials, such as sand and rock, but also for cohesive soils, such as clay and peat.

Scour is the result of the near-bed load and the soil characteristics. Based on a systematic investigation of the time scale for two- and three-dimensional scour in loose sediments, Breusers derived relations for predicting the maximum scour depth as a function of time. In the 1990s, these scour relations were modified and used in the design of the storm surge barrier in the New Waterway near Rotterdam and for the prediction of the scour process downstream of the barrier in the Eastern Scheldt.

Storm surge barrier, Eastern Scheldt (courtesy Rijkswaterstaat)

The scour depth as a function of time can be predicted by the so-called Breusers equilibrium method. Basically, this method can be applied to all situations where local scour is expected. The new Scour Manual discusses not only the experiences of the Dutch Delta Works but also the innovative scour formulas for predicting the equilibrium scour depth. Evaluating a balance of forces for a control volume, Hoffmans Advice deduced scour equations for different types of flow fields such as jets, propellers, broken pipelines, and various hydraulic structures such as abutments and bridge piers.

Scour hole due to plunging jet (application of Newton’s Law)
Local scour due to broken pipeline (Courtesy NRC / Sam de Voogt)

The new Scour Manual presents mathematical soil and erosion models, risk assessment, and the erosion of clay and grass revetments. Some of these experiences have been described in case studies. The scour process has not yet been explained in a generally accepted manner, and it would therefore be appropriate to discuss its mechanism and formulations of soil and erosion models for the simplest cases. However, with assistance from Hoffmans Advice, steps can be taken toward designing a safe hydraulic structure or assessing existing scour-related structures.

Local scour in the Dutch tidal river Spui (Courtesy Deltares / Kees Sloff)
Scour depth around bridge pier (turbulence approach)

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Piping

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Grass revetments

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