Experimental Analysis of Erosive Cohesive Coastline Morphology

Experimental Analysis of Erosive Cohesive Coastline Morphology

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ABSTRACT
Laboratory experiments have been performed in a wave flume to investigate the coastal cliff recession under regular waves forcing. The different processes of the cliff erosion cycle are described and we focus on bottom evolution, which seem mostly depend on the surf similarity parameter ξ. We observed steep planar (ξ > 0.7), gentle planar (0.5
< ξ < 0.7) and bared (ξ < 0.5) profiles. We noticed different sandbar dynamics including either steady or unsteady self-sustained oscillating states. Then we estimate the role of the self-organized material on the cliff recession rate. We show that the cliff erosion increases with the wave energy flux and is stronger for a gentle planar profile than for
a bared profile of bottom morphology. However, the cliff recession rate as a function of the cliff height is not monotonic due to a different dynamics of bottom morphologies.
 
EXPERIMENTAL SETUP / METHOD
The experiments were performed in a wave flume about 5 m-long, 14 cm-wide and 25 cm-high during about 20 hours. The flume is equipped with a flap wave maker producing monochromatic waves with a height up to 5 cm and periods between 0.5 s and 2 s. The offshore water depth is d = 15 cm. A nearshore slope of tan(β) = 1/10 is used to model the shore and a cliff is built on the shore with wet sand. The cliff front is cut to obtain the cliff depth of 40 cm. The initial conditions, in terms of cliff length and water content in material, are identical for every experiment. The cliff height h is measured from the free surface at rest to the cliff top. The sand used is calcite with a median grain diameter of d50 = 0.41 mm and a density of ρs = 2.76 g/cm³ (corresponding to a fall velocity in water of ws = 6.5 cm/s).
Wave parameters are measured by capacitive probes (acquisition at 100 Hz). 3 are located offshore and 2 close to the cliff. The offshore incident wave height (H) is estimated with a least square method and incident and reflected wave height are distinguished (Mansard and Funke, 1980)Two PCO 2000 cameras have been positioned, on the side of the flume (with a field of view 1 m x 25 cm) to detect the water free surface and the bottom and cliff positions, and above the flume (with a field of view 40 cm x 15 cm) to detect the cliff position. During about 4 hours, the video sampling rate is close to the wave frequency and then is decreased to ~1/10 of this frequency until the end of the experiment.
Cliff erosion rate and sediment transport on the bottom are studied in function of incident wave energy, wave shape and cliff height. The monochromatic wave climate is characterized by two parameters, the surf similarity parameter ξ and the incident wave energy flux F which are written ξ = tan(β)/sqrt(H/L) and F = EcG.
 
CONCLUSIONS
Coastal cliff erosion by regular waves has been investigated in an experimental wave flume. Our results show that the type of bottom morphology depends strongly on the surf similarity parameter:
  • ξ > 0.7 – steep planar profile.
  • 0.5 < ξ < 0.7 – gentle planar profile.
  • ξ < 0.5 – bared profile.
For bared profiles, we either observed steady or unsteady self-sustained oscillating states for a constant wave climate. Unsteady states show self-sustained sandbars oscillations, reflecting an organization of the system governed by the hydrodynamics/morphology coupling. The influence of cliff height on the cliff recession rate depends mostly on the self-organised material, previously mentioned; we can therefore correlate the influence of the parameters chosen with the cliff recession rate for the same bottom morphology dynamics. Thus, the cliff erosion increases with the wave energy flux and is stronger for a gentle planar profiles than for a bared profile. We have shown that the sediment supply plays an important role on cliff recession. A periodical bar removal yields an approximately constant cliff recession rate. Our experiments show that average
volume of a collapse event increases and number of collapse events decreases with cliff height.
Attribution: 
Bastien Caplain, Dominique Astruc, Vincent Regard, and Frederic Y. Moulin (2011) Experimental Analysis of Erosive Cohesive Coastline Morphology. Costal Engineering Proceedings 1.32.
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