Karl F. Nordstrom

'Low energy' sandy beaches in marine and estuarine environments: a review

This review was undertaken to identify locations where low energy beaches may occur and their diagnostic forms and process controls, including waves, tides and water levels. Examples are drawn from the sheltered coastline of Western Australia near Perth and fetch-limited estuarine environments on the northeast coast of the United States. We suggest that the term low energy be used in locations where: (1) non-storm significant wave heights are minimal (e.g. <0.25 m); (2) significant wave heights during strong onshore winds are low (e.g. <0.50 m); (3) beachface widths are narrow (e.g. <20 m in microtidal environments); and (4) morphologic features include those inherited from higher energy events. Micro-topographic features can persist in the swash zone of low energy beaches under non-storm wave conditions. There is little evidence of cyclic cross-shore sediment exchange. Bars, excepting transverse forms, located seaward of low still-water level do not appear to be part of the sediment exchange system with the foreshore. Developing a better definition of the term low energy requires understanding the occurrence and duration of morphological characteristics and the type, magnitude and frequency of hydrodynamic controls that are responsible for these characteristics. Efforts also should be directed toward: (1) discriminating between processes generated within basins (in true fetch-limited environments) and processes generated outside basins (that affect sheltered environments); (2) identifying the relative contributions of tide- and surge-related water level fluctuations on low energy beach shape; and (3) estimating thresholds for beach change.

Estuarine Shores: Evolution, Environments and Human Alterations

Partial table of contents: Environments, Processes and Interactions of Estuarine Shores (C. Roman & K. Nordstrom) EVOLUTION OF ESTUARINE SHORES Evolution of Estuarine Shoreline Systems in Sierra Leone (E. Anthony) Shoreline Changes in the Bodden Coast of Northeastern Germany (R. Lampe) Late Quaternary Infill of Macrotidal Estuaries in Northern Australia (C. Woodroffe) ENVIRONMENTS AND PROCESSES Hydraulic Processes Affecting the Morphology and Evolution of the Westerschelde Estuary (J. van den Berg, et al.) The Role of Seagrasses in Nearshore Sedimentary Processes: a Review (M. Fonseca) Ecological Dynamics of a Tropical Intertidal Mudflat Community (J. Vargas) HUMAN ALTERATIONS AND MANAGEMENT Management and Use of Dynamic Estuarine Shorelines (J. Doody) Natural and Legal Shoreline Buffers (J. Phillips) Index.

Effects of Time-dependent Moisture Content of Surface Sediments on Aeolian Transport Rates Across a Beach, Wildwood, New Jersey, U.S.A.

A one-day field investigation on an unvegetated backbeach documents the importance of surface sediment drying to aeolian transport. Surface sediments were well sorted fine sand. Moisture content of samples taken in the moist areas on the backbeach varied from 2·9 to 9·2 per cent. Lack of dry sediment inhibited transport prior to 08:50. By 09:10 conspicuous streamers of dry sand moved across the moist surface. Barchan-shaped bedforms, 30 to 40 mm high and composed of dry sand (moisture content <0·10 per cent), formed where sand streamers converged. The surface composed of dry sand increased from 5 per cent of the area of the backbeach at 09:50 to 90 per cent by 12:50 Mean wind speeds were beetween 5·6 and 8·6 m s−1 at 6 m above the backbeach. Corresponding shear velocities were always above the entrainment threshold for dry sand and below the threshold for the moist sand on the backbeach. Measured rates of sand trapped (by vertical cylindrical traps) increased during the day relative to calculated rates. The measured rate of sand trapped on the moist foreshore was higher than the rate trapped on the backbeach during the same interval, indicating that the moist foreshore (moisture content 18 per cent) was an efficient transport surface for sediment delivered from the dry portion of the beach upwind. Measured rates of sand trapped show no clear relationship to shear velocities unless time-dependent surface moisture content is considered. Results document conditions that describe transport across moist surfaces in terms of four stages including: (1) entrainment of moist sediment from a moist surface; (2) in situ drying of surface grains from a moist surface followed by transport across the surface; (3) entrainment and transport of dry sediment from bedforms that have accumulated on the moist surface; and (4) entrainment of sand from a dry upwind source and transport across a moist downwind surface.

Beaches and dunes of human-altered coasts

Landforms are created, reshaped or eliminated to suit human needs. These alterations affect the mechanisms of change, freedom of movement, locations of sources and sinks for sediment, internal structure, outward appearance and spatial and temporal scales of landform evolution. The processes by which landscapes are transformed by human agency follows a progression of alterations that may be subtle or overt, planned or unplanned, but most of them are predictable. Models of change for human-altered coasts may be formulated by viewing them as open or closed systems. Alternative methodologies for examining evolution of these coasts include: 1) comparing and contrasting a developed area with an undeveloped area that is assumed to have the same process controls; 2) assuming that the kind of shoreline change that occurred in the recent past will continue unabated by local actions; or 3) basing predictions on probabilities of future human action. Evidence suggests that human alterations are an integral component of landscape evolution. Future challenges for scientists include: 1) formulating conceptual and predictive models of landform dynamics that evaluate humans as an endogenic process and include assumption about human actions; and 2) providing scientific criteria for maintaining landforms in developed areas in ways that safeguard or promote an optimal diversity of landforms, species and ecosystems. Controlled disturbance may be required to create landforms compatible with natural landforms in appearance and function if not in genesis.

The role of bore collapse and local shear stresses on the spatial distribution of sediment load in the uprush of an intermediate-state beach

Abstract Spatial variations in sediment load in the swash uprush and textural properties of sediment in transport were evaluated to investigate the mechanisms responsible for sediment transport during wave uprush. Four streamer traps were deployed at 2.0-m intervals across the swash zone of a sheltered, microtidal sandy beach at Port Beach, Western Australia, over a 4-day period. During these trapping experiments, offshore significant wave heights were 0.3–0.5 m and wave periods were about 10 s. The average width of the uprush zone was 6.9 m and the average uprush duration was 5.9 s. Cross-shore distributions of sediment load for 70 uprush events reveal a maximum in sediment load landward of the base of the swash (at about 20% of swash width) during single events and a maximum closer to mid-swash (at about 40% of swash width) during multiple events characterized by swash interactions. Settling velocity distributions of trap samples during individual uprush events are similar to distributions found on the beach surface, with the lowest settling velocities (finest sediments) near the base of the swash zone and maximum settling velocities (coarsest sediments) around the mid-swash position. It was found that sediment transport during wave uprush occurs through two distinct mechanisms: (1) sediment entrainment during bore collapse seaward of the base of the swash zone and subsequent advection of this bore-entrained sediment up the beach by wave uprush; and (2) in situ sediment entrainment and transport induced by local shear stresses during wave uprush. Both mechanisms are considered important, but the first mechanism is considered most significant during the early stages of wave uprush when sediment is transported mainly in suspension, while the second mechanism is likely to dominate the mid- to later stages of wave uprush when sediment is transported mainly by sheet flow. The relative importance of the two mechanisms will vary between different beaches with the morphodynamic state of the beach (reflective versus dissipative) expected to play a major role.

Beach and dune restoration

Preface Acknowledgements 1. The need for restoration 2. Beach nourishment and impacts 3. Dune building practices and impacts 4. Restoring processes, structure and functions on developed coasts 5. Options in spatially restricted environments 6. A locally-based program for beach and dune restoration 7. Stakeholder interests, conflicts and cooperation 8. Research needs References Index.

Aeolian transport of sediment on a beach during and after rainfall, Wildwood, NJ, USA

Abstract A field investigation was conducted 9 March 1994 on a dissipative beach to compare wind characteristics and aeolian transport during and after a light rain. Winds blew alongshore during the monitoring period. Moisture content of surface sediment on the backbeach was above 7% during rain and 4% four hours after rainfall ceased. Trapping rates, measured using a cylindrical vertical trap, were 14.1 kg m−1 h−1 during rainfall and 140.2 kg m−1 h−1 for a period beginning 3 h 33 min after rainfall ceased. Results indicate that light rain, resulting in bulk surface moisture values over 7%, is not sufficient to eliminate transport, but the rate of transport is lower than can occur on the beach three to four hours following rain. The amount of sand trapped at Wildwood during rainfall is relatively high when compared to many previous studies on beaches in the absence of rain. The great length of the beach as a source area, combined with parallel or oblique winds are significant in increasing rates of sediment transport.

Geomorphology and natural hazards

Abstract Natural hazards research was initiated in the 1960s by Gilbert White and his students who promulgated a research paradigm that involved assessing risk from a natural event, identifying adjustments to cope with the hazard, determining peoples perception of the event, defining the process by which people choose adjustments, and estimating the effects of public policy on the choice process. Studies of the physical system played an important role in early research, but criticismsof the paradigm resulted in a shift to a prominence of social science. Geomorphologists are working to fill gaps in knowledge of the physical aspects of individual hazards, but use of the information by social scientists will only occur if information is presented in a format that is useful to them. One format involves identifying the hazard according to seven physical parameters established by White and his colleagues: magnitude, frequency, duration, areal extent, speed of onset, spatial dispersion, and temporal spacing. Geomorphic hazards are regarded as related to landscape changes that affect human systems. The processes that produce the changes are rarely geomorphic in nature, but are better regarded as atmospheric or hydrologic. An examination of geomorphic hazards in four fields — soil erosion, mass movement, coastal erosion and fluvial erosion — demonstrates that advances in those fields may be evaluated in terms of the seven parameters. Geomorphologists have contributed to hazard research by focusing on the dynamics of the landforms. The prediction of occurence, the determination of spatial and temporal characteristics, the impact of physical characteristics on peoples perception, and the impact of physical characteristics on adjustment formulation. Opportunities for geomorphologists to improve our understanding of geomorphic hazards include research into the characteristics of the events particularly with respect to predicting the occurence, and increased evaluation of the impact of human activities on natural systems.

Depth of activation of sediment by plunging breakers on a steep sand beach

Abstract Field data were collected over a lunar tidal cycle on a meso-tidal medium-sand estuarine beach in Delaware Bay, USA to document the relationship between wave height and depth of sediment activation in a low-wave-energy environment. Dominant energy was at the frequencies of locally-generated waves and ocean swell. Significant wave heights ranged from 0.06 m to 0.52 m. Periods of locally-generated waves averaged 4.0 s during strong onshore winds and 2.6 s during low-speed winds. Waves on the foreshore were always plunging. The foreshore maintained a slope of approximately 6.0° throughout the study period. Mean grain size at mid-foreshore was 0.46 mm. The greatest net surface-elevation change in response to storm erosion and post-storm recovery was 0.26 m. Results confirm previous observations that depths of sediment activation are greater for steeper beaches. The spatial variability in depths of activation across the foreshore is comparable to results of tidal cycle studies, where the ratio of depth of activation to wave height is great because a large percentage of the foreshore comes under the influence of the breakers. The ratio of depth of activation to wave height on the portion of the foreshore that is not transgressed by the breakers is lower and comparable to results of short-term investigations, provided that the influence of beach slope is considered in the calculation of breaking wave height.

The Use of Grain Size Statistics to Distinguish Between High- and Moderate-energy Beach Environments

ABSTRACT Swash zone sediments on two oceanside and two bayside beaches on Sandy Hook spit are examined to determine whether distinct differences exist among grain size parameters on beaches which have very. different wave regimes. The differences are subtle due to the inherent similarity of swash zone processes and the overall similarity of the source sediments. Differences in grain size statistics are noticeable, however, and they appear to be related to differences in wave energy and beach mobility on each beach. Measures of dispersion offer one means of discriminating among beaches. Examination of plots of changes in grain size statistics through time are also useful. Bivariate plots appear to be of limited value, however.