7.5.1: Introduction

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Page ID: 16378

Judith Bosboom & Marcel J.F. Stive
Delft University of Technology via TU Delft Open

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The research field of cross-shore hydrodynamics, sediment transport dynamics and resulting bed profile dynamics is highly topical and very complex, and simple, analytical treatments do hardly exist. The reason is that over the whole of the shoreface the constituent processes vary strongly. In the upper shoreface, say the surf zone, under normal conditions we encounter a mix of bed and suspended load transport due to undertow, bound and free long waves, short wave skewness in combination with breaking-induced turbulence (Bailard, 1981; Roelvink & Stive, 1989). In this zone, during extreme conditions the undertow is dominant (Steetzel, 1993) although long wave effects (bound and free) cannot be ignored (Van Thiel de Vries, 2009). On the middle and lower shoreface again a mix of bed and suspended load transport is encountered, but here wave boundary layer streaming, bound long waves and short wave skewness are relevant. Also, upwelling and downwelling due to stratification and Ekman currents may play a role here (but are outside the scope of this book).

For calculating bed profile dynamics of the upper shoreface (under storm surge and non-storm surge conditions), process-based, numerical models have been developed. These models describe sediment transport as a function of the wave evolution in the surf zone, which requires a largely empirical description of the highly variable wave energy dissipation and related flow variations. For the middle and lower shoreface analytical approaches are available due to Bowen (1980), who derived expressions for equilibrium profiles by balancing onshore and offshore transport components.

In Sect. 7.5.2, the cross-shore sediment transport rates are decomposed into contributions due to undertow, short wave skewness and bound and free long waves associated with wave groups. Herewith, we gain insight into the relevance of the various flow components to net sediment transport. In Sect. 7.5.3, we reproduce some of the analytical approaches for the middle and lower shoreface as introduced by Bowen.