14 research outputs found
Cliff retreat and sea bed morphology under monochromatic wave forcing: Experimental study
Wave flume experiments have been performed to investigate a sandy cliff recession under monochromatic wave forcing. We varied the wave climate through the wave energy flux F and the surf similarity parameter j. The various processes of the cliff erosion cycle are depicted. The sea bed evolution mostly depends on the surf similarity parameter j. Steep planar (j > 0.7), gentle planar (0.5 < j < 0.7) and bared (j < 0.5) profiles are observed. We observed different bar dynamics, including steady and unsteady self-sustained oscillating states. Then we analyze the role of the eroded material on the cliff recession rate. We show that the cliff recession rate increases with the wave energy flux. Moreover, for a given wave energy flux, it is larger for a gentle planar profile than for a bared profile. However it is similar for both a bared profile and a steep planar profile. The cliff recession rate is not a monotonic function of the cliff height as the type of bottom profile influences the wave energy at the cliff
Etude expérimentale de l'érosion d'un massif de sable cohésif par une houle monochromatique
La plupart des cĂŽtes de la Terre reculent et 80% sont rocheuses. La prĂ©vision du recul des falaises littorales est primordiale afin dâanticiper les risques futurs pour les amĂ©nagements littoraux. Cependant, la comprĂ©hension de ce recul est difficile car de nombreux paramĂštres le contrĂŽlent. Des expĂ©riences en canal Ă houle de petite Ă©chelle ont Ă©tĂ© effectuĂ©es oĂč nous avons mis en place un massif de sable humide soumis Ă lâattaque des vagues par sapement. Le but est de comprendre comment lâeffet des vagues contrĂŽle lâĂ©rosion des falaises. La technique de mesure par ombroscopie a Ă©tĂ© employĂ©e et nous a permis de dĂ©tecter la surface du sable et la surface libre en fonction du temps. Nous avons ainsi analysĂ© lâinfluence du forçage des vagues (F, Ο) (oĂč F est le flux dâĂ©nergie des vagues incidentes au large et Ο est le paramĂštre de similitude de âsurfâ) sur la vitesse de recul de la falaise et sur la profondeur des Ă©vĂšnements dâeffondrement. La vitesse de recul de la falaise augmente linĂ©airement avec le flux dâĂ©nergie F. Les dĂ©bris de falaise Ă©rodĂ©s changent la morphologie du fond, les types de morphologie du fond dĂ©pendent fortement du paramĂštre de similitude de âsurfâ au dĂ©ferlement, ou encore du paramĂštre de Dean Ω. Des profils du fond instationnaires prĂ©sentant une oscillation auto-entretenue de la barre sĂ©dimentaire ont Ă©tĂ© observĂ©s. Nous avons de plus Ă©tudiĂ© lâeffet de la granulomĂ©trie du sable utilisĂ© : pour un sable plus fin, la falaise est plus cohĂ©sive et sâeffondre au cours dâĂ©vĂšnements de plus grande ampleur. Etonnamment, le recul de la falaise est plus important pour du sable fin. Ceci est probablement dĂ» Ă une modification de la morphologie du fond conduisant Ă une dissipation de lâĂ©nergie des vagues moins importante. Le volume de sable injectĂ© dans le systĂšme a finalement Ă©tĂ© quantifiĂ©, la barre sĂ©dimentaire a dâabord Ă©tĂ© prĂ©levĂ©e pĂ©riodiquement et il a Ă©tĂ© observĂ© que la vitesse de recul de la falaise vr est constante. Puis, la hauteur de falaise a Ă©tĂ© modifiĂ©e, le recul des falaises est plus important pour des petites falaises. Il semblerait que lâinstationnaritĂ© dâun profil du fond se dĂ©clenche Ă partir dâun volume seuil de sable Ă©rodĂ©. ABSTRACT : Most of the Earth coasts recedes and 80 % are rocky. Prediction of sea-cliff recession is essential to anticipate future risks for coastal development. However, it is difficult to understand this recession because many parameters control it. In addition, both the space and time scales are too big for the different mechanisms of cliff erosion to be fully analysed. Experiments in a small-scale wave flume were conducted in which a massif made of wet sand is submitted to wave attack. The aim is to understand how cliff erosion is wave-controlled. The technique of shadow graph measurements was used to detect the time evolution of sand and water surfaces. We have analyzed the influence of wave forcing (F, Ο) (where F is the incident offshore wave energy flux and Ο is the surf similarity parameter) on the cliff recession rate and on collapse event size. The cliff recession rate increases linearly with the wave energy flux F. The eroded cliff materials change the bottom morphology ; the types of bottom morphology strongly depend on the surf similarity parameter at the breaker point, or the Dean parameter Ω. Bottom profiles characterized by unsteady self-sustained sandbar oscillation were observed. In addition, we studied how sand granulometry change the system evolution. Finer the sand is, more cohesive is the cliff and bigger are cliff collapses. Contrary to what was expected, cliff recession is more important for a finer sand : this could be due to a more dissipative bottom morphology built by fine sands. The sand volume within the system changes following cliff collapses and a sandbar removal during particular experiments. The cliff recession rate is constant when the sandbar is removed and decreases with cliff height. It seems that the unsteadiness of the bottom profile is activated when the volume of eroded sand exceeds a threshold value
Sediment processes and ïŹow reversal in the undular tidal bore of the Garonne River (France)
A tidal bore is a series of waves propagating upstream as the tidal flow turns to rising, and the bore front corresponds to the leading edge of the tidal wave in a funnel shaped estuarine zone with macro-tidal conditions. Some field observations were conducted in the tidal bore of the Garonne River on 7 June 2012 in the Arcins channel, a few weeks after a major flood. The tidal bore was a flat undular bore with a Froude number close to unity: Fr1 = 1.02 and 1.19 (morning and afternoon respectively). A key feature of the study was the simultaneous recording of the water elevation, instantaneous velocity components and suspended sediment concentration (SSC) estimates, together with a detailed characterisation of the sediment bed materials. The sediment was some silty material (d50 â 13 ÎŒm) which exhibited some non-Newtonion thixotropic behaviour. The velocity and SSC estimate were recorded simultaneously at high frequency, enabling a quantitative estimate of the suspended sediment flux at the end of the ebb tide and during the early flood tide. The net sediment flux per unit area was directed upstream after the bore, and its magnitude was much larger than that at end of ebb tide. The field observations highlighted a number of unusual features on the morning of 7 June 2012. These included (a) a slight rise in water elevation starting about 70 s prior to the front, (b) a delayed flow reversal about 50 s after the bore front, (c) some large fluctuations in suspended sediment concentration (SSC) about 100 s after the bore front and (d) a transient water elevation lowering about 10 min after the bore front passage. The measurements of water temperature and salinity showed nearly identical results before and after the tidal bore, with no evidence of saline and thermal front during the study
Field Measurements in the tidal bore of the Garonne River after a recent flood
A tidal bore is a natural phenomenon associated with the rising flood tide. Composed of surface waves, it may occur in estuaries and propagate up rivers. The present study was conducted in the Garonne River (France) in the Arcins channel. Using an ADV unit and further recording equipments, experimental data are collected during a flat undular bore with a bore Froude number close to unity. Velocity analysis and sediment characterisation revel a slight rise in water elevation starting about 70 s prior to the front and a flow reversal about 50 s after the bore front. The turbulent transport of suspended sediment is presented in term of mass flux per unit area highlighting a negative (upriver) sediment mass transfer
Ătude expĂ©rimentale de la turbulence de grille sur le transport sĂ©dimentaire
Lâobjectif de cette Ă©tude est double : caractĂ©riser le transport sĂ©dimentaire dans un Ă©coulement turbulent homogĂšne et isotrope afin de valider les modĂšles utilisĂ©s dans les simulations numĂ©riques sous des conditions dâĂ©coulement contrĂŽlĂ©es et stabilisĂ©es ; dĂ©velopper une mĂ©thodologie de mesures transposables Ă un canal long (10 m) dans le cas dâun Ă©coulement turbulent Ă surface libre gĂ©nĂ©rĂ© par une onde de propagation reprĂ©sentant les conditions dâun mascaret. LâĂ©coulement est gĂ©nĂ©rĂ© dans une veine hydraulique rectangulaire de 1.5 m de long de section carrĂ©e (10 cm de cĂŽtĂ©). La turbulence est dĂ©veloppĂ©e par une grille Ă mailles carrĂ©es (maille de 6.6 mm) choisie de maniĂšre Ă obtenir une faible dĂ©croissance spatiale de lâintensitĂ© turbulente soit environ 4-5% sur la majeure partie de la veine. Dans une premiĂšre partie, une analyse sans particules est faite sur la base de mesures LDV (Laser Doppler Velocimetry) afin de qualifier lâĂ©coulement de transport. Le taux de dissipation dâĂ©nergie turbulente est estimĂ© et les diffĂ©rentes Ă©chelles de la turbulence sont analysĂ©es. Le mouvement des particules injectĂ©es (PMMA) est ensuite quantifiĂ© par des mesures PIV (Particle Image Velocimetry) pour diffĂ©rentes concentrations. Une technique de mesures par fluorescence permet de distinguer la contribution de chaque taille de particule injectĂ©e. Une analyse en termes de flux de concentration, champs de vitesses et structure de lâĂ©coulement lors du transport sĂ©dimentaire de deux tailles caractĂ©ristiques de particules (20 et 200 ”m) est prĂ©sentĂ©e. Les rĂ©sultats sont comparĂ©s avec des donnĂ©es numĂ©riques
Experimental study of erosion of cohesive sand massif by monochromatic waves
La plupart des cĂŽtes de la Terre reculent et 80% sont rocheuses. La prĂ©vision du recul des falaises littorales est primordiale afin dâanticiper les risques futurs pour les amĂ©nagements littoraux. Cependant, la comprĂ©hension de ce recul est difficile car de nombreux paramĂštres le contrĂŽlent. Des expĂ©riences en canal Ă houle de petite Ă©chelle ont Ă©tĂ© effectuĂ©es oĂč nous avons mis en place un massif de sable humide soumis Ă lâattaque des vagues par sapement. Le but est de comprendre comment lâeffet des vagues contrĂŽle lâĂ©rosion des falaises. La technique de mesure par ombroscopie a Ă©tĂ© employĂ©e et nous a permis de dĂ©tecter la surface du sable et la surface libre en fonction du temps. Nous avons ainsi analysĂ© lâinfluence du forçage des vagues (F, Ο) (oĂč F est le flux dâĂ©nergie des vagues incidentes au large et Ο est le paramĂštre de similitude de âsurfâ) sur la vitesse de recul de la falaise et sur la profondeur des Ă©vĂšnements dâeffondrement. La vitesse de recul de la falaise augmente linĂ©airement avec le flux dâĂ©nergie F. Les dĂ©bris de falaise Ă©rodĂ©s changent la morphologie du fond, les types de morphologie du fond dĂ©pendent fortement du paramĂštre de similitude de âsurfâ au dĂ©ferlement, ou encore du paramĂštre de Dean Ω. Des profils du fond instationnaires prĂ©sentant une oscillation auto-entretenue de la barre sĂ©dimentaire ont Ă©tĂ© observĂ©s. Nous avons de plus Ă©tudiĂ© lâeffet de la granulomĂ©trie du sable utilisĂ© : pour un sable plus fin, la falaise est plus cohĂ©sive et sâeffondre au cours dâĂ©vĂšnements de plus grande ampleur. Etonnamment, le recul de la falaise est plus important pour du sable fin. Ceci est probablement dĂ» Ă une modification de la morphologie du fond conduisant Ă une dissipation de lâĂ©nergie des vagues moins importante. Le volume de sable injectĂ© dans le systĂšme a finalement Ă©tĂ© quantifiĂ©, la barre sĂ©dimentaire a dâabord Ă©tĂ© prĂ©levĂ©e pĂ©riodiquement et il a Ă©tĂ© observĂ© que la vitesse de recul de la falaise vr est constante. Puis, la hauteur de falaise a Ă©tĂ© modifiĂ©e, le recul des falaises est plus important pour des petites falaises. Il semblerait que lâinstationnaritĂ© dâun profil du fond se dĂ©clenche Ă partir dâun volume seuil de sable Ă©rodĂ©.Most of the Earth coasts recedes and 80 % are rocky. Prediction of sea-cliff recession is essential to anticipate future risks for coastal development. However, it is difficult to understand this recession because many parameters control it. In addition, both the space and time scales are too big for the different mechanisms of cliff erosion to be fully analysed. Experiments in a small-scale wave flume were conducted in which a massif made of wet sand is submitted to wave attack. The aim is to understand how cliff erosion is wave-controlled. The technique of shadow graph measurements was used to detect the time evolution of sand and water surfaces. We have analyzed the influence of wave forcing (F, Ο) (where F is the incident offshore wave energy flux and Ο is the surf similarity parameter) on the cliff recession rate and on collapse event size. The cliff recession rate increases linearly with the wave energy flux F. The eroded cliff materials change the bottom morphology ; the types of bottom morphology strongly depend on the surf similarity parameter at the breaker point, or the Dean parameter Ω. Bottom profiles characterized by unsteady self-sustained sandbar oscillation were observed. In addition, we studied how sand granulometry change the system evolution. Finer the sand is, more cohesive is the cliff and bigger are cliff collapses. Contrary to what was expected, cliff recession is more important for a finer sand : this could be due to a more dissipative bottom morphology built by fine sands. The sand volume within the system changes following cliff collapses and a sandbar removal during particular experiments. The cliff recession rate is constant when the sandbar is removed and decreases with cliff height. It seems that the unsteadiness of the bottom profile is activated when the volume of eroded sand exceeds a threshold value
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Sediment processes and flow reversal in the undular tidal bore of the Garonne River (France)
Cliff erosion: self-organisation of sand cliff material eroded by monochromatic waves
Laboratory experiments on cliff erosion were carried out in a monochromatic wave flume. Natural coarse sands are used to represent cliff erosion and bottom morphodynamics with a reasonable time scale. A bottom typology is established as a function of wave forcing, through the wave energy flux F and the surf similarity parameter Ο. The bottom types strongly depends on the surf similarity parameter at the breaker point Οb . Steep terraces (Οb > 0.48), one-bar profiles (0.42 < Οb < 0.48), gentle terraces (0.38 < Οb < 0.43) and double-bars profiles (Οb < 0.38) were observed. It can be translated into a Dean parameter Ω vs. Shields number Îb space to take into account sediment granulometry. Sediment grain diameter change has no noticeable influence on bottom typology. The bottom types depends more on the Dean parameter Ω than on the Shields number Îb . Finally, we explored cliff height effect: it does not modify the bottom typology established