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

Abstract

Flow hindrance by salt-marsh vegetation is manifested in the structure of the tidal current; it has a significant impact on sediment transport and causes increased sediment accretion. The flow characteristics in 3 different vegetation types (Spartina maritima, Sp. anglica and Salicornia/Suaeda maritima) were measured on 3 salt-marshes in Portugal and England.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 characterised 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 will be increased in that zone; this affects the sediment dynamics.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.

Hurricane-induced failure of low salinity wetlands

“…Based upon the hydrodynamic conditions (significant wave height, H s ; peak period, T p ; and water depth, h) and an appropriate wave friction factor (f w ), calculations were made to determine the maximum wave shear stresses acting on the vegetated marsh platform (25,36). The wave friction factor parameterizes the drag related to vegetation and is a function of the roughness length (Z 0 ), which is influenced by stem density, canopy height, and leaf geometry (37)(38)(39)(40)(41). Although, when emergent, vegetation dramatically reduces flow velocities (4,40,42), when fully submerged it produces a two-phase flow; speeds are reduced within the canopy, while a faster skimming flow occurs above the canopy (38,41).…”

“…When water depth becomes significantly deeper than the canopy height, as occurred during Hurricane Katrina, vegetation can be considered as a bed roughness element, retarding flow and increasing shear in the near-bed boundary layer (40,41). Two characteristic roughness lengths (17 and 30 cm) were used to calculate maximum wave shear stresses, representing the upper and lower limits of vegetation conditions (37)(38)(39)(40). Calculations were made at seven sites on either side of the Terre aux Boeuf distributary (see SI Text B and Table S2).…”

Impact of anthropogenically created canopy gaps on wave attenuation in a Posidonia oceanica seagrass meadow

“…At Stn ST1, the apparent canopy height during the spring and summer periods was ~0.45 m (10% shorter than the leaf length) while in winter it was 0.20 m; almost all leaves had undergone seasonal dehiscence from the individual shoots. To measure velocities within the meadow at ST1, 4 to 7 plants were removed to prevent leaves from blocking the sampling volume (Neumeier & Amos 2006).…”

Laboratory investigation of lateral dispersion within dense arrays of randomly distributed cylinders at transitional Reynolds number

“…This value corresponds to x = 30 m, for example, in a random array with the same d͑=3 cm͒ and ͑Ϸ0.1͒ as measured by Lightbody et al 36 in a constructed wetland ͑Re d =30-40͒. This is a non-negligible distance in many aquatic systems ͑e.g., constructed wetlands, 37 mangrove swamps, 38,39 and salt marshes 40 ͒, where homogeneous vegetation conditions extend no more than about 200 m perpendicular to land, and in some systems 37,40 much less. In particular, constructed wetlands typically comprise multiple cells of variable dimensions, and the shortest cell may only extend 30-300 m ͑Refs.…”