MEPS 362:45-57 (2008)  -  DOI: https://doi.org/10.3354/meps07448

ABSTRACT: Our main objective was to investigate the effect of Spartina anglica stems on hydrodynamics, sediment dynamics and their influence on seasonal morphological changes on the upper shore and salt marsh of an intertidal mudflat in the Tavy estuary (southwest England). On an intertidal transect, the greatest seasonal changes in sediment levels occurred at the seaward edge and lower part of the salt marsh, and on the lower shore mudflat, while the smallest changes occurred on the mid- and upper-shore mudflat. Flume studies demonstrated flow attenuation with increasing Spartina stem density, but this was accompanied by an increase in turbulent kinetic energy and bed shear stress (τ₀). The critical erosion threshold (τ_e), however, remained relatively constant at 0.12 Pa. Field measurements showed that τ₀ was well below τ_e under calm conditions. During periods of wind-wave activity, when daily mean wind speeds were >8 m s^–1, τ₀ was greater than τ_e in the lower salt marsh for brief periods at the beginning and end of tidal inundation. At the seaward edge of the salt marsh, τ₀ increased to 1.0 Pa and τ₀ was greater than τ_e throughout the inundation period, causing significant sediment erosion. Wind-induced wave activity was the major driver of sediment erosion on soft muddy shores and this was enhanced by the interaction with S. anglica stems. The number of days per month with mean daily wind speeds >8 m s^–1 was inversely related to changes in sediment levels at the seaward edge of the salt marsh. During the summer, winds were below this threshold, and there was gradual accretion. Results are discussed in the context of climate changes involving increased storminess leading to increased erosion. We show that the S. anglica salt marsh should not be considered a ‘bio-stabiliser’ of fine muddy sediment.

KEY WORDS: Spartina anglica · Salt marsh · Intertidal · Hydrodynamics · Waves · Sediment erosion · Morphology