On the Importance of Tidal Bores on Estuarine Systems
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Vulnerability of coastal ecosystems to global change and extreme events – Biarritz 2011 On the importance of tidal bores on estuarine systems P. Bonneton, N. Bonneton, J-P. Parisot, and A. Sottolichio EPOC,METHYS team, Bordeaux Univ., CNRS http://www.epoc.u-bordeaux.fr/methys/ Introduction Tidal bore Tidal bore (TB) : • fascinating hydrodynamic phenomenon positive surge propagating upstream observed worldwide that may form when a rising tide with significant amplitude enters shallow, • significant impact on estuarine ecosystem gently sloping and narrowing rivers - sediment transport - turbulent mixing - larval dispersal and feeding zone for Garonne River - 2010 several forms of wildlife Introduction Objectives Few quantitative field experiments: Simpson et al. (2004), Wolanski et al. (2004) and Uncles et al. (2006) Estimation of tidal bore (TB) occurrence is generally based on qualitative visual observations (Chanson (2005)) Common view: TB propagation is a fascinating but uncommon phenomenon limited to macrotidal environments, spring tide conditions and low summer river discharges New high frequency measurements in the Garonne and Seine Rivers TB occurrence has been strongly underestimated Pressing need to reevaluate the role of TB on estuarine ecosystems Garonne River - 2010 Field sites Garonne and Seine Rivers Le Verdon Pk 93 Pauillac Pk 49 Bec d’Ambes Pk 26 Bordeaux Pk 0 Val des Leux field site Pk 03 Pk 265, 100 km from the estuary mouth 50 km La Reole Tr[3, 8 m] Pk -67 Seine Podensac field site Langon Pk -32, 130 km from Pk -47 the estuary mouth Tr[1.5, 5.5 m] Gironde/Garonne Field campaigns Garonne River: 3 intensive field campaigns in 2010 and 2011 around the spring (high river discharges) and autumn equinox (low discharges) 24th Feb. - 15th April, 2010 (TBG1): 97 tides 1st Sep. - 22nd Oct., 2010 (TBG2): 99 tides 30th Aug. - 8th Sep., 2011 (TBG3): 17 tides Seine River: water depth measurements in 2011 around the autumn equinox 25th Aug. - 13th Sep., 2011 : 38 tides Results Tidal bore occurrence Example of elevation profiles when the tide flow turns to rising for successive increasing tides from neap to spring tide (TBG2) Results Tidal bore occurrence Garonne River TBG1: tidal bores were identified for 63/97 tides 65% TBG2: tidal bores were identified for 89/99 tides 90% Contrary to the common view tidal bores in the Garonne River form for a large majority of tides even close to neap tides Seine River Identification of TB of significant amplitude at spring tide Results TBG1/TBG2 campaigns Large tidal range (spring tide) at the estuary mouth: TRv=5 m TBG1 • High Q 0.6m R •Fr=1.06 •TRp=5 m • U 1 m/s TBG2 • Low QR 1.3m •Fr=1.17 •TRp=6.3 m Results Fresh water discharge effect Estuary convergence tidal wave amplification River discharge tidal wave damping The competition of the 2 processes drives the tidal wave amplitude in the river Results Tidal bore intensity in function of TRp h D1 The tidal bore intensity is mainly driven by the local tidal amplitude Results Secondary wave steepness h h1 /h1 LSW HSW Lemoine’s theory F1 Results Tidal bore regimes No tidal bore F< 1 LSW tidal bore 1 < F < 1.15 HSW tidal bore « mascaret » 1.15 < F Conclusion Tidal bore occurrence is usually underestimated, because for 1< Fr < 1.15 (LSW), even if the jump is significant, wave steepness is low and visual observations are not straightforward TB analysis requires high frequency measurements: f > 2 Hz In the Garonne River, tidal bores form for a large majority of tides: For the first time, large amplitude TB were measured in the Seine River LSW regime Mechanisms which control TB formation and intensity impact of estuarine development and climate changes (sea level rise and Qr) on TB occurence Perspectives Role of tidal bores on estuarine ecosystem sediment transport see N. Bonneton et al. (CFM 2011) J-P. Parisot et al. (poster session) geochemical processes estuarine and river species M. Lepage, CEMAGREF Thank you for your attention Results Secondary wave steepness 2A h A/h1 h1 F1 /h1 LSW HSW F1 Results Tidal bore occurrence Examples of elevation profiles when the tide flow turns to rising for successive increasing tides from neap to spring tide (TBG1) Fr=1.07 TBG1 tidal bores were hardly visually observed FIG. 2 – Evolution sur un cycle de marée à Podensac de la hauteur d’eau et de la vitesse horizontale intégrée suivant la verticale, moyennée sur 5 minutes. a, b : le 25 février (TGB1), marnage de 4.4 m (2.7 m au Verdon) ; c, d : le 3 mars (TGB1), marnage de 5.1 m (5.1m au Verdon) ; e, f : le 9 septembre (TGB2), marnage de 6.3 m (4.9 m au Verdon), f : les données de courant à marée haute sont à prendre avec précaution, elles sont parfois manquantes en raison de la forte turbidité du milieu. Campagne de mesures - septembre 2010 Pêche dans le mascaret En haut, différents modèles de filets. A gauche prélèvement de 3 min (la plupart des poissons sont des gobies). A droite, les différents sites de prélèvement (ronds blancs). Bilan : beaucoup de poissons après le mascaret et très peu dans le mascaret 20.