The influence of vegetation on turbulence and flow velocities in European salt‐marshes

Flow hindrance by salt-marsh vegetation is manifested in the structure of the tidal current; it has a significant impact on sediment transport and it has been related to increased sediment accretion. The flow characteristics in three different vegetation types (Spartina maritima, Sp. anglica and Salicornia sp./Suaeda maritima) were measured on three salt-marshes in Portugal and England. These in situ measurements differ from laboratory flume experiments with 'clean' vegetation by the complexity of natural canopies. Skimming flow develops above the Spartina canopy when the vegetation is fully submerged. In this situation, a low turbulence zone with nearly constant velocity in the denser canopy is separated from the skimming flow above by an interface characterized by high Reynolds stresses. In the low turbulence zone, a positive relationship exists between turbulence intensity and shoot density, which is due to wake turbulence generated locally in the canopy. The rate of particle settling should be increased in that zone. The lower limit of skimming flow is best predicted by the height within the canopy that includes 85% of the biomass. For emergent Spartina canopies and the short Salicornia/Suaeda marsh, the maximal velocity-gradient is shifted upwards compared to a standard boundary layer over bare sediment and the turbulence is attenuated near the bed, but to a lesser extent than for fully submerged Spartina canopies. A turbulence reduction near the bed was observed in all measured profiles; that should enhance sediment deposition and protects the bed against subsequent erosion.

[1]  Urs Neumeier,et al.  Velocity and turbulence variations at the edge of saltmarshes , 2007 .

[2]  A. Hine,et al.  Surficial Sediment Transport and Deposition Processes in a Juncus roemerianus Marsh, West-Central Florida , 1995 .

[3]  John Robert Lawrence Allen,et al.  Medium-term sedimentation on high intertidal mudflats and salt marshes in the Severn Estuary, SW Britain: the role of wind and tide , 1998 .

[4]  Z. Shi,et al.  Near-bed currents and suspended sediment transport in saltmarsh canopies , 2000 .

[5]  O. Polunin.  Flowers of Europe: A Field Guide , 1969 .

[6]  P. Wiberg,et al.  Flow and Sediment Transport on a Tidal Salt Marsh Surface , 2000 .

[7]  Sue Brown.  Sedimentation on a Humber saltmarsh , 1998, Geological Society, London, Special Publications.

[8]  Michael S. Kearney,et al.  SEDIMENT TRANSPORT AND TRAPPING IN MARSH SYSTEMS: IMPLICATIONS OF TIDAL FLUX STUDIES* , 1988 .

[9]  Iris Möller,et al.  Wave Transformation Over Salt Marshes: A Field and Numerical Modelling Study from North Norfolk, England , 1999 .

[10]  H. Nepf.  Drag, turbulence, and diffusion in flow through emergent vegetation , 1999 .

[11]  G. Csanady.  Turbulent Diffusion in the Environment , 1973 .

[12]  J. Bartholdy,et al.  Storm surge effects on a back-barrier tidal flat of the Danish Wadden Sea , 2001 .

[13]  Grid turbulence in air and water , 1979 .

[14]  C. Amos,et al.  Observations of some sedment-transport processes over intertidal flats, the Wash, U.K. , 1981 .

[15]  J. Lester,et al.  The Value of Salt Marsh as a Sea Defence , 1995 .

[16]  R. Soulsby.  Dynamics of marine sands : a manual for practical applications , 1997 .

[17]  O. Madsen,et al.  Combined wave and current interaction with a rough bottom , 1979 .

[18]  Richard P. Stumpf,et al.  The process of sedimentation on the surface of a salt marsh , 1983 .

[19]  T. Spencer,et al.  Dynamics of sedimentation in a tide-dominated backbarrier salt marsh, Norfolk, UK , 1993 .

[20]  Kenneth Pye,et al.  Flow Structure in and above the Various Heights of a Saltmarsh Canopy: A Laboratory Flume Study , 1995 .

[21]  Cheryl Ann Zimmer,et al.  LARVAL SWIMMING OVERPOWERS TURBULENT MIXING AND FACILITATES TRANSMISSION OF A MARINE PARASITE , 2003 .

[22]  M. Luther,et al.  Flow hydrodynamics in tidal marsh canopies , 1995 .

[23]  W. Sikora,et al.  Intertidal Marsh Suspended Sediment Transport Processes, Terrebonne Bay, Louisiana, U.S.A. , 1993 .

[24]  P. Wiberg,et al.  Sedimentation and Boundary Changes of Virginia Salt Marshes , 1996 .

[25]  S. L. Yang,et al.  Morphological response of tidal marshes, flats and channels of the Outer Yangtze River mouth to a major storm , 2003 .

[26]  U. Neumeier.  Quantification of vertical density variations of salt-marsh vegetation , 2005 .

[27]  K. Dyer.  Sediment processes in estuaries: Future research requirements , 1989 .

[28]  Enrique R. Vivoni,et al.  Flow structure in depth-limited, vegetated flow , 2000 .

[29]  Josef Daniel Ackerman,et al.  Mechanistic implications for pollination in the marine angiosperm Zostera marina , 1986 .

[30]  K. Dyer,et al.  Coastal and Estuarine Sediment Dynamics , 1986 .

[31]  J. Pizzuto,et al.  A Model for the Evolution of Marsh Shorelines , 2000 .

[32]  Peter G. Appleby,et al.  Accretion of a New England (U.S.A.) Salt Marsh in Response to Inlet Migration, Storms, and Sea-level Rise , 1997 .

[33]  D. Reed.  Physical Contexts for Saltmarsh Conservation , 2001 .

[34]  M. Collins,et al.  Tidal characteristics of an accretional tidal flat (The Wash, U.K.) , 2000 .

[35]  Tom Spencer,et al.  The Sea‐Defence Value of Salt Marshes: Field Evidence From North Norfolk , 2001 .

[36]  J. R. Allen,et al.  Principles of physical sedimentology , 1985 .

[37]  P. Carling.  Temporal and spatial variation in intertidal sedimentation rates , 1982 .

[38]  Tom Spencer,et al.  Geostatistical Analysis of Sediment Deposition In Two Small Tidal Wetlands, Norfolk, U.K. , 1995 .

[39]  H. F. Erchinger.  Intaktes Deichvorland für Küstenschutz unverzichtbar , 1995 .

[40]  T. Spencer,et al.  Marsh surface sediment deposition and the role of tidal creeks: Implications for created and managed coastal marshes , 1999 .

[41]  Paolo Ciavola,et al.  Flow Resistance and Associated Sedimentary Processes in a Spartina maritima Salt-Marsh , 2004 .

[42]  Z. Shi,et al.  Velocity profiles in a salt marsh canopy , 1996 .

[43]  M J Tucker,et al.  WAVES IN OCEAN ENGINEERING , 2001 .

[44]  R. Devoy,et al.  Contemporary process controls on the evolution of sedimentary coasts under low to high energy regimes: western Ireland , 1998 .

[45]  D. Reed,et al.  Hydrodynamics and Sediment Transport Through Tidal Marsh Canopies , 2002 .

[46]  C. Stace,et al.  New Flora Of The British Isles , 1998 .

[47]  A. D. Abrahams,et al.  Estimating shear velocity and roughness length from velocity profiles , 1992 .