Marsh Margin Research Articles

Patterns of flow and suspended sediment concentration in a macrotidal saltmarsh creek, Bay of Fundy, Canada.

Abstract Measurements of velocity and suspended sediment concentration were carried out in a saltmarsh tidal creek network in the Cumberland Basin, Bay of Fundy, Canada. The study area was located on the NW shore of the basin in part of an undyked marsh that is about 200 m wide with a simple reticulate creek network. The area is macrotidal with spring tides greater than 12 m and suspended sediment concentrations in the basin characteristically range from 150–300 mg l −1 . The purpose of the study was to determine vertical and along channel variations in these two parameters over individual tidal cycles and to use these data to assess the role of the tidal creeks in the import and export of water and sediment from the marsh surface. Measurements using a vertical array of co-located electromagnetic current meters and OBS probes for measuring suspended sediment concentration were carried out over four spring tides at a cross section in the lower part of Middle Creek. Six sets of measurements were carried out at four locations along the length of the creek, a distance of about 200 m over six tides ranging from spring to neap. Maximum mean velocities measured over sampling times of eight minutes did not exceed 0.1 m sec −1 in Middle Creek and 0.15 m sec −1 in Main Creek. Transient high velocities associated with the overbank flows were weakly developed as a result of the absence of significant levees or embankments on the marsh surface. Suspended sediment concentrations in the creek generally decreased steadily over the period of inundation. Flow across the marsh margin occurred simultaneously with the achievement of bankful conditions and the creeks themselves appear to play a relatively minor role in the movement of water and sediment onto and out of the marsh. Despite the fact that the marsh surface is still low in the tidal frame and active sedimentation is still occurring, the low flow velocities and observations in the field suggest that the tidal creek network is unable to flush itself and that it is contracting.

Water exchanges in a shoreline Typha marsh on Lake Ontario

A Typha-dominated marsh bordering Lake Ontario had a strong hydraulic connection with the lake during spring 1991, when the water level was more than 10–20 cm above the marsh surface. During this period, water-level changes were dominated by the general decline of Lake Ontario, by short-term fluctuations caused by seiche activity (1.7 h periodicity) and by setup (diurnal). These were transmitted into the marsh at 65–135 m h −1. The water surface profile was generally flat over the marsh during this period but water depth was uneven owing to peat surface adjustment. High buoyancy of the surface in the 30 m margin adjacent to the lake minimized water depth there. By 21 June, water depth at the margin was essentially zero, but elsewhere was 0.07–0.1 m; all water exchanges between the marsh and lake were thereafter constrained to relatively slow subsurface flow (approximately 10 −3–10 −4 m h −1). Short-term fluctuations caused by lake-level variability were then restricted to the margin. The Lake Ontario water level then dropped faster than that on the marsh, as did the water table on the adjacent mineral terrain, resulting in an upwardly convex water surface profile on the marsh, where water flow was away from the marsh. Between 14 June and 6 August, 116 mm of rain and 264 mm of evapotranspiration resulted in 30 mm of lateral water loss from the marsh centre; only 1 mm was lost at the marsh margin, reflecting the frequent reversals of hydraulic gradient there.

Erosion susceptibility of the Virginia Chesapeake Bay shoreline

This study defines the susceptibility of differing shoreline types in the Virginia Chesapeake Bay to erosion. There is a high variability of the coastal environments in this area influenced by: (1) the relict Pleistocene high-order dendritic drainage system, resulting in a large diversity of shore orientations; (2) the moderate, but highly variable wave energy in the Bay system and highly variable submergence rates, resulting in a spectrum of shoreline transformation from primary to secondary types; (3) exposures of coastal plain sediments of widely varying composition and volume; and (4) diverse salt marsh development. Eighty percent of the shore is composed of three morphologically distinct beach environments, each reflecting different susceptibilities to erosion: (1) Permeable beaches, composed entirely of sand-sized material, comprise 59% of the beaches (mean erosion = 0.85 m/yr) and have the largest vertical and horizontal dimensions. This provides the largest vertical buffer to the effects of storm surge and waves. (2) Impermeable beaches, composed of a veneer of sand overlying impermeable, pre-Holocene sediments, comprise 24% of the beaches. The combined influence of low swash infiltration, low beach elevation and groundwater effects result in the highest mean erosion (1.14 m/yr). (3) Marsh barrier beaches (17%), composed of a veneer of sand overlying peat, contains a resistant rhizome framework, resulting in the least erodable beach environment (0.66 m/yr). Marsh margins, the remaining 20% of the shoreline, are the least erodable shore environment (0.54 m/yr). There is a large variability in the local subsidence rates along the shoreline. The subsidence highs correspond to the areas with largest nearshore terraces and the main concentrations of salt marsh development. A balance in the shore stability exists as a result of salt marsh development: The marsh retards shore erosion, yet it occurs at the areas of highest local submergence where the highest shoreline retreat is expected.

The significance of reed beds on siltation in the Tay Estuary

SynopsisThe Phragmites communis marshes occupy 250–330 hectares bordering the northern tidal flats of the Tay Estuary. The outer edge of the marsh forms a step up to 50 cm in height. Reed stems grow with densities of 90–150/m2 and break to produce upward facing cylinders which entrap sediments. Measured daily accretion rates in broken stems varied from 001 g to 0.07 g. Sedimentation was greatest during spring tides, but decreased with falling tidal range. Accretion of 16 g per stem is normal and many stem plugs have greater weight. Annual rates of contribution from instem accumulation vary between 0.9 and 2.6 kg/m2 on the marsh margin. Stem fall provides cylindrical sedimentary lumps to the marsh sediments, whose internal structure reveals nodules and thin laminae juxtaposed.