Field observations of instantaneous water slopes and horizontal pressure gradients in the swash-zone

Baldock, T. E. and Hughes, M. G. (2006) Field observations of instantaneous water slopes and horizontal pressure gradients in the swash-zone. Continental Shelf Research, 26 5: 574-588. doi:10.1016/j.csr.2006.02.003

Author Baldock, T. E.
Hughes, M. G.
Title Field observations of instantaneous water slopes and horizontal pressure gradients in the swash-zone
Journal name Continental Shelf Research   Check publisher's open access policy
ISSN 0278-4343
Publication date 2006-04
Sub-type Article (original research)
DOI 10.1016/j.csr.2006.02.003
Volume 26
Issue 5
Start page 574
End page 588
Total pages 15
Place of publication Oxford, England
Publisher Pergamon-Elsevier Science
Collection year 2006
Language eng
Subject C1
290899 Civil Engineering not elsewhere classified
770400 Coastal and Estuarine Environment
0905 Civil Engineering
Abstract Field observations of instantaneous water surface slopes in the swash zone are presented. For free-surface flows with a hydrostatic pressure distribution the surface slope is equivalent to the horizontal pressure gradient. Observations were made using a novel technique which in its simplest form consists of a horizontal stringline extending seaward from the beach face. Visual observation, still photography or video photography is then sufficient to determine the surface slope where the free-surface cuts the line or between reference points in the image. The method resolves the mean surface gradient over a cross-shore distance of 5 m or more to within +/- 0.001, or 1/20th -1/100th of typical beach gradients. In addition, at selected points and at any instant in time during the swash cycle, the water surface slope can be determined exactly to be dipping either seaward or landward. Close to the location of bore collapse landward dipping water surface slopes of order 0.05-0.1 occur over a very small region (order 0.5 m) at the blunt or convex leading edge of the swash. In the middle and upper swash the water surface slope at this leading edge is usually very close to horizontal or slightly seaward. Behind the leading edge, the water surface slope was observed to be very close to horizontal or dipping seaward at all times throughout the swash uprush. During the backwash the water surface slope was observed to be always dipping seaward, approaching the beach slope, and remained seaward until a new uprush edge or incident bore passed any particular cross-shore location of interest. The observations strongly Suggest that the swash boundary layer is subject to an adverse pressure gradient during uprush and a favourable pressure gradient during the backwash. Furthermore, assuming Euler's equations are a good approximation in the swash, the observations also show that the total fluid acceleration is negative (offshore) for almost the whole of the uprush and for the entire backwash. The observations are contrary to recent work suggesting significant shoreward directed accelerations and pressure gradients occur in the swash (i.e., delta u/delta t > 0 similar to delta p/delta x < 0), but consistent with analytical and numerical solutions for swash uprush and backwash. The results have important implications for sediment transport modelling in the swash zone.
Keyword Oceanography
Long wave runup
Pressure gradients
Flow acceleration
Boundary layers
Sediment transport
Dam break
Q-Index Code C1

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Created: Wed, 15 Aug 2007, 08:45:48 EST