Jeff Ollerhead

Hydrodynamics and sedimentation in salt marshes: examples from a macrotidal marsh, Bay of Fundy

This paper presents an overview of sedimentary processes operating at the time scale of the individual tidal cycle in tidal creeks and on the marsh surface, with examples of data from an ongoing study of a high-macrotidal salt marsh in the Cumberland Basin, Bay of Fundy, Canada. Flow and suspended sediment concentrations were measured with electromagnetic current meters and OBS™ probes at stations set-up in the tidal creek system and on the marsh surface. Measured flow velocities in the tidal creek channels and over the marsh surface are generally low (<0.2 m s−1), reflecting the relatively short, straight creek geometry and absence of levees along the creek channels. At spring high tide, flow vectors are initially controlled by creek geometry and the topography of the marsh surface, but near high tide, when water depth over the marsh exceeds 1 m, flow vectors are controlled more by the general circulation in the Cumberland Basin and by the effects of winds and waves. Suspended sediment concentrations are in the order of 150–250 mg l−1 and are similar in the tidal creeks and over the marsh surface. Suspended sediment concentrations generally decrease over the tidal cycle but show little fluctuation in response to changing hydrodynamic conditions. Deposition on the marsh surface is highest in the low marsh area, except near the marsh cliff margin, and shows a significant positive correlation with distance from the marsh edge.

Topographic Steering of Alongshore Airflow over a Vegetated Foredune: Greenwich Dunes, Prince Edward Island, Canada

Abstract High-frequency measurements of airflow from ultrasonic anemometers and time-averaged cup anemometer profiles were taken during an oblique alongshore sand-transporting event (6.7 m/s) over a vegetated foredune in May 2002 as part of a larger study on the sedimentary dynamics of a beach-dune complex in Greenwich Dunes, Prince Edward Island National Park, Canada. Local flow and sand transport pathways deviated significantly from the regional wind because of topographic steering of oblique alongshore airflow on the beach back toward the foredune in the backshore. Flow decelerated on the lower seaward slope of the foredune because of flow stagnation and vegetation-induced roughness effects. On the upper seaward slope, flow veered crest-parallel because of secondary flow effects, including potential flow reversal, acceleration, and interaction with faster regional flow. An inflection point in velocity profiles indicates a momentum sink at plant canopy height, and above this height exists a distinct shear layer. Flow acceleration occurs only on the upper slope above the plant canopy as the shear layer intensifies. Flow steadiness, as indicated by the coefficient of variation in horizontal (U) and vertical (W) velocity, also declines up the seaward slope, in contrast to measurements of flow over unvegetated dunes, perhaps because of increasing vegetation-induced drag and turbulence. Gusting and vertical bursting was evident as honami motion in the beachgrass and was responsible for maintaining intermittent saltation into the foredune. Temporal correlations exist between U, W, and flow angle. As flow shifted onshore, both U and W increased because of enhanced topographic forcing and increased vertical lift on the lower seaward slope, whereas when flow shifted alongshore, U and W declined because of reduced forcing and acceleration effects because the dune is effectively less steep to flow. Although these dunes align well with the vector of the regional resultant sediment drift potential, sand transport pathways followed local flow vectors of varying magnitude and direction. Offshore to oblique alongshore winds are typical of the summer wind regime, and, although frequently incompetent, they contribute to dune maintenance by cycling sand to the backshore for incipient foredune growth, scarp infilling, or both. To date, the role of secondary flows under alongshore winds in foredune morphodynamics has not been well documented.

Application of a Remote Sensing Technique to the Study of Coastal Dunes

Abstract This paper reports on a remote sensing station specifically designed to investigate eolian processes at a beach–dune system. The monitoring station is located at Greenwich Dunes, Prince Edward Island National Park, Prince Edward Island (Canada), and it is the second, improved generation of a previous system using continuous video and photographs. The setup consists of three digital single-lens reflex cameras, a two-dimensional sonic anemometer, two safires, erosion–deposition pins, and an array of batteries and solar panels. The cameras run on a timer that takes pictures every hour. The images are rectified and analyzed using a combination of ArcMap 9.2 and PCI Geomatica software, which permits the generation of moisture maps, vegetation, ice and snow cover, shoreline position, and erosion–deposition processes. The two-dimensional sonic provides continuous wind speed and direction, and the saltation probes record the intensity of transport events. The result is a large geodatabase of a time series of factors affecting eolian processes at the beach at a variety of temporal and spatial scales. This geodatabase can be queried, and it is a valuable tool for studying the frequency and magnitude of events delivering sediment from the beach to the dune and thus for improving our knowledge of sediment transport at coastal areas. Although the remote sensing station was initially conceived as a tool to measure subaerial processes, a full year of measurements shows large potential for the system to provide information on processes at the nearshore environment and ice dynamics.

Measurement of Beach Surface Moisture Using Surface Brightness

Abstract This article describes the testing and calibration of a digital video camera system used to measure surface moisture over a section of beach extending tens of metres alongshore using the brightness of the sand surface. The system consisted of two networked video cameras mounted on a mast on the foredune crest about 14 m above the beach surface and oriented obliquely alongshore in either direction. Photographs and video clips were stored on a notebook computer. Over a period of 4 weeks in May and June 2006, a series of field tests was carried out; these tests were designed to calibrate the surface brightness recorded at locations visible in the digital images against gravimetric surface moisture measured by scraping the surface at the same location. While there was a significant correlation between surface brightness and gravimetric moisture content, the strength of this relationship was improved by normalising against a white board present in the field of view of the camera. An even stronger relationship was obtained by normalising against an area of dry sand present within a portion of the photograph. Rectification of the images and application of the calibration equation permitted mapping of surface moisture variations on a timescale of hours to days. The approach appears to provide a useful means of estimating surface moisture on the beach on a scale of tens of metres and over timescales of seconds to months.

Controls on suspended sediment deposition over single tidal cycles in a macrotidal saltmarsh, Bay of Fundy, Canada

Abstract A field study was conducted on a section of Allen Creek marsh in the Bay of Fundy to examine changes in suspended sediment circulation and deposition over single tidal cycles. Net flow velocity, suspended sediment concentration and sediment deposition were measured over 13 individual tidal cycles during the summer of 1998. A vertical array was deployed in the low marsh region, consisting of three pairs of electromagnetic current meters, OBStm probes and one pressure transducer. Sediment deposition was measured using full-cycle sediment traps. The temporal distribution of sediment deposition was monitored using sequential sediment traps exposed at different tidal stages. The data suggest that sediment deposition on the marsh surface is primarily controlled by the interaction of water flow, marsh morphology and vegetation. The highest amounts of sediment are deposited during conditions of high suspended sediment concentration and low wave activity, particularly when the relative roughness of the vegetation is the highest. Loss of suspended sediment from the water column was shown to be correlated with the sediment trap data; however, predictions of sediment deposition based on the variation in suspended sediment concentration were found to be valid only for conditions with less than 0.15 m high waves. For higher wave conditions, the use of suspended sediment loss calculations should be used primarily for estimating the relative rather than absolute values of deposition on the marsh surface.

Low-energy Morphodynamics of a Ridge and Runnel System

ABSTRACT A field experiment was carried out over a period of five weeks on an intertidal ridge and runnel system on the Northumberland Strait coast of Nova Scotia, Canada. The area is microtidal with a spring tidal range of just under 2 m. The main purpose of the research was to examine the effects of changing water depth and bar emergence on the morphodynamics of the system and to determine the controls on the stability of the ridges during non-storm conditions. The system is developed on a gently sloping platform over 300 m wide at spring low tide with an average gradient of 0.004. It is characterised by the presence of 5 or 6 ridges in the intertidal zone and 2 bars in the subaqueous zone. The ridges are 0.35–0.50 m in height, 50–60 m in wave length, and the continuity of the ridges alongshore is broken by drainage channels. The system at Linden Beach is similar to others that have developed in a number of areas along this coast on platforms resulting from recession of relatively weak sandstone cliffs. Topographic surveys were carried out along 10 profiles spaced 25 m apart using a total station and the position of the ridge crests and troughs was also mapped using a GPS system. Measurements of wave transformation, water motion and suspended sediment concentration over individual tidal cycles were carried out along a profile across the second ridge and associated troughs using electromagnetic current meters, resistance wave staffs and OBS nephelometers. There were no major storms during the monitoring period but there were a number of days with significant wave heights >0.4 m and the measurements spanned the full range from neap to spring tides. During the five week period the ridge crests exhibited a high degree of stability with maximum movement <5 m. The effects of tidal currents were isolated through measurements made during calm conditions with light winds. Measurements indicate that wave shoaling and breaking across the ridge crest at mid to high tide have the potential to transport large quantities of sediment landward and thus to induce landward migration under non-storm conditions. The stability of the bars appears to be controlled by a combination of offshore flows across the bar crests due to undertow and tidal currents near high tide, and through the transport of sediment alongshore in the troughs and offshore in the drainage channels on the ebb tide. The dynamics of the system more closely resembles that of sub-tidal multiple parallel bars than that of intertidal swash bars.

Dispersal of marine benthic invertebrates through ice rafting

Knowledge of dispersal vectors used by organisms is essential to the understanding of population and community dynamics. We report on ice rafting, a vector by which intertidal benthic invertebrates can be transported well outside their normal dispersal range during winter in temperate climates. We found multiple invertebrate taxa in sediment-laden ice blocks sampled in the intertidal zone. A large proportion of individuals were alive and active when freed from the ice. Using radio tracking, we found that ice blocks can travel over 20 km within a few days. Given the abundance of highly mobile ice blocks carrying viable invertebrates, we conclude that ice-rafting is likely an important dispersal vector, contributing to spatial community dynamics in intertidal systems. This mechanism helps explain observed genetic structure of populations, but it also raises concerns about potential negative impacts of climate change on connectivity between populations.

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.

Sediment budget controls on foredune height: Comparing simulation model results with field data

The form, height and volume of coastal foredunes reflects the long-term interaction of a suite of nearshore and aeolian processes that control the amount of sand delivered to the foredune from the beach versus the amount removed or carried inland. In this paper, the morphological evolution of foredune profiles from Greenwich Dunes, Prince Edward Island over a period of 80 years is used to inform the development of a simple computer model that simulates foredune growth. The suggestion by others that increased steepness of the seaward slope will retard sediment supply from the beach to the foredune due to development of a flow stagnation zone in front of the foredune, hence limiting foredune growth, was examined. Our long-term data demonstrate that sediment can be transferred from the beach to the foredune, even with a steep foredune stoss slope, primarily because much of the sediment transfer takes place under oblique rather than onshore winds. During such conditions, the apparent aspect ratio of the dune to the oncoming flow is less steep and conditions are not favourable for the formation of a stagnation zone. The model shows that the rate of growth in foredune height varies as a function of sediment input from the beach and erosion due to storm events, as expected, but it also demonstrates that the rate of growth in foredune height per unit volume increase will decrease over time, which gives the perception of an equilibrium height having been reached asymptotically. As the foredune grows in size, an increasing volume of sediment is needed to yield a unit increase in height, therefore the apparent growth rate appears to slow

Attenuation of Wave Energy by Nearshore Sea Ice: Prince Edward Island, Canada

ABSTRACT Manson, G.K.; Davidson-Arnott, R.G.D., and Ollerhead, J., 2016. Attenuation of wave energy by nearshore sea ice: Prince Edward Island, Canada. Sea ice is widely held to be decreasing in coastal waters where it is known to be effective in attenuating wave energy. This process is critical for understanding nearshore sediment transport and coastal change in ice-infested waters. We explore the attenuation of waves shoaling in nearshore ice using a simple attenuation model, hydrodynamic modelling, field studies, and daily charts of sea ice. The attenuation model is drawn from studies in deep water and modified for shallow water using field measurements. In a simple but common configuration where ice lies in a band onshore and waves enter from open water, the theory that wave energy decays exponentially as waves enter ice appears to hold true in shallow and deep water. The wave energy density in ice relates to the incoming wave energy density, attenuation distance, and an attenuation coefficient related to ice concentration and floe diameter. Much of the variability in measurements is explained by the theory, but substantial uncertainty remains. Prediction potential might be improved with higher resolution wave measurements and modelling, consideration of the rebuilding of attenuated waves in partial ice cover, and separate treatment of new ice. The adapted semiempirical theory is likely generally applicable to ice-infested coastal waters, but field studies in particular environments will be required to calibrate attenuation.