Fetch Lengths Research Articles

A GIS-based step-wise procedure for assessing physical exposure in fragmented archipelagos

Wind and waves are major forces affecting the geomorphology and biota in coastal areas. We present a generally applicable method for measuring and calculating fetch length, fetch direction and wave exposure. Fetch length and direction, measured by geographic information system-based methods, are used along with wind direction and wind speed data to estimate wave height and period by applying forecasting curves. The apparent power of waves approaching the shore, used as a proxy for wave exposure, is then calculated by a linear wave model. We demonstrate our method by calculating fetch lengths and wave exposure indices for five areas with varying exposure levels and types of meteorological conditions in the Finnish Archipelago Sea, situated in the northern Baltic Sea. This method is a rapid and accurate means of estimating exposure, and is especially applicable in areas with geomorphologically varying and complicated shorelines. We expect that our method will be useful in several fields, such as basic biogeographical and biodiversity research, as well as coastal land-use planning and management.

Estimating palaeowind strength from beach deposits

Abstract The geological record of past wind conditions is well expressed in the coarse gravel, cobble and boulder beach deposits of Quaternary palaeolakes in the Great Basin of the western USA and elsewhere. This paper describes a technique, using the particle‐size distribution of beach deposits, to reconstruct palaeowind conditions when the lakes were present. The beach particle technique (BPT) is first developed using coarse beach deposits from the 1986–87 highstand of the Great Salt Lake in Utah, combined with instrumental wind records from the same time period. Next, the BPT is used to test the hypothesis that wind conditions were more severe than at present during the last highstand of Lake Lahontan (≈ 13 ka), which only lasted a decade or two at most. The largest 50 beach clasts were measured at nine beach sites located along the north, west and south sides of Antelope Island in the Great Salt Lake, all of which formed in 1986–87. At these sites, the largest clast sizes range from 10 to 28 cm (b‐axis), and fetch lengths range from 25 to 55 km. Nearshore wave height was calculated by assuming that the critical threshold velocity required to move the largest clasts represents a minimum estimate of the breaking wave velocity, which is controlled by wave height. Shoaling transformations are undertaken to estimate deep‐water wave heights and, ultimately, wind velocity. Wind estimates for the nine sites, using the BPT, range from 6·5 to 17·4 m s−1, which is in reasonable agreement with the instrumental record from Salt Lake City Airport. The same technique was applied to eight late Pleistocene beaches surrounding the Carson Sink sub‐basin of Lake Lahontan, Nevada. Using the BPT, estimated winds for the eight sites range from 9·7 to 27·1 m s−1. The strongest winds were calculated for a cobble/boulder beach with a fetch of 25 km. Instrumental wind records for the 1992–99 period indicate that wind events of 9–12 m s−1 are common and that the strongest significant wind event (≥ 9 m s−1 for ≥ 3 h) reached an average velocity of 15·5 m s−1. Based on this preliminary comparison, it appears that the late Pleistocene western Great Basin was a windier place than at present, at least for a brief time.

A sea spray generation function for fetch-limited conditions

Abstract. This paper presents a sea spray generation function for aerosols produced by bubbles bursting that accounts for the effects of fetch. This is achieved by introducing a fetch-dependent model for the whitecap fraction, which is valid for fetch-limited conditions, i.e. steady-state conditions of the wave field in the whitecap droplet flux derived by Monahan et al. (1986). The aerosol generation fluxes calculated by this method show an enhancement of the aerosol production with increasing fetch. However, the predicted generation fluxes are lower than those calculated by using the classical model for the whitecap fraction proposed by Monahan and O’Muircheartaigh (1980). The results are then compared to aerosol size distributions measured in a Mediterranean coastal site at various fetch lengths. The data confirm the role of fetch, through its influence on marine aerosol generation and subsequent particle dispersion. The aerosol size distributions are divided into "fine" and "coarse" fractions. Submicrometer particles decrease in concentration at larger fetch, while the concentrations of super micron aerosols increase with increasing fetch.Key words. Atmospheric composition and structure (aerosols and particles) Meteorology and atmospheric dynamics (waves and ties) Oceanography: physical (air-sea interactions)

Detecting fragmentation of cover in desert grasslands using line intercept

Changes in the amount or spatial distribution of grass plants are thought to be indicative of the stability of desert grasslands. This study assessed, through simulation, the sensitivity of statistical properties for distance between plants (fetch length), measured with a line intercept transect, to changes in the spatial distribution and amount of plant cover. Monitoring plots, 30 X 30 m, were simulated for 1, 2.5, 5, 10 and 15% grass cover with random and fragmented spatial distribution. Fetch lengths were measured on 2 randomly placed 30 m transects. In addition to the median and interquartile range, the asymmetry of the sampling distributions was measured with a ratio [(maximum-median)/(median-minimum)] that would identify the presence of at least 1 large open space. The accuracy of the fetch length method was confirmed by the similarity of its sampling distribution to that for the well known random point-to-plant sampling procedure. In both the fetch length and the point-to-plant measures, the median and interquartile range increased with decreasing cover for random and fragmented distribution. The asymmetry estimate increased sharply with increasing cover for the fragmented distribution but asymmetry was nearly constant with increasing cover for the random distribution. The results suggest that the evaluation of changes over time at a monitoring site could use fetch lengths measured along a line intercept transect to detect changes in both absolute and spatial arrangement of cover.

Detecting Fragmentation of Cover in Desert Grasslands Using Line Intercept

Changes in the amount or spatial distribution of grass plants are thought to be indicative of the stability of desert grasslands. This study assessed, through simulation, the sensitivity of statistical properties for distance between plants (fetch length), measured with a line intercept transect, to changes in the spatial distribution and amount of plant cover. Monitoring plots, 30 X 30 m, were simulated for 1, 2.5, 5, 10 and 15% grass cover with random and fragmented spatial distribution. Fetch lengths were measured on 2 randomly placed 30 m transects. In addition to the median and interquartile range, the asymmetry of the sampling distributions was measured with a ratio [(maximum-median)/(median-minimum)] that would identify the presence of at least 1 large open space. The accuracy of the fetch length method was confirmed by the similarity of its sampling distribution to that for the well known random point-to-plant sampling procedure. In both the fetch length and the point-to-plant measures, the median and interquartile range increased with decreasing cover for random and fragmented distribution. The asymmetry estimate increased sharply with increasing cover for the fragmented distribution but asymmetry was nearly constant with increasing cover for the random distribution. The results suggest that the evaluation of changes over time at a monitoring site could use fetch lengths measured along a line intercept transect to detect changes in both absolute and spatial arrangement of cover. DOI:10.2458/azu_jrm_v54i1_kuehl

Wind wave characteristics at Lake Dunstan, South Island, New Zealand

Information on wave statistics, seasonal characteristics, and their distribution about lacustrine shorelines is virtually non‐existent. The absence of such data limits the effective management of lake shores where issues of shoreline erosion are no less significant than on open ocean coasts. Results from instrumentally measured waves during seven storm events in 1995 are presented for the Clutha Arm of Lake Dunstan, South Island, New Zealand. The significant wave height ranged from 0.07 to 0.57 m with a mean of 0.28 m, whereas the maximum wave reached 1.05 m. Peak spectral wave periods ranged from 1.7 to 3.6 s with a mean of 2.46 s. The largest and most destructive waves are observed along the southern shore of the Clutha arm where fetch lengths are at their maximum. These waves exhibit the longest and widest range of periods and are generally the steepest waves, making them highly erosional at the shore. Correlations of waves with those predicted by NARFET, a deepwater wave‐hindcasting computer model, revealed reasonable predictions of the wave height (R = 0.77–0.81), particularly for sites exposed to longer fetches, while the correlations with the wave period were lower (R = 0.56–0.69). Wave hindcasting indicates that the wave regime in the Clutha Arm is bi‐directional, with most waves arriving from the north or south. Findings from Lake Dunstan have important implications for larger lakes located throughout New Zealand, where considerably larger waves can be expected to occur during severe storm events.

Application of the Generic Spectral Formula to Fetch-Limited Seas

A deep-water spectral formula, based on the Weibull probability distribution of wave periods, is modified to satisfy fetch-limited conditions for wind-generated seas Results of this generic spectral formula are compared with those obtained using the specific JONSWAP formula, in which the empirical parameters resulting from specific wind and fetch conditions are used. The comparisons are shown to be excellent for three of the five cases studied. For the last two cases, the generic formula is shown to be somewhat nonconservative near the spectral peak. In addition to the comparisons with the specific JONSWAP formula results, the generic formula results are compared with those obtained from the standard JONSWAP expression, where the Pierson-Moskowitz parametric values and averaged peak-enhancement values are used. Except for the greatest fetch length (37 km), the standard JONSWAP formula is shown to significantly under-predict the peak spectra values. The generic spectral formula is found to well-predict the spectra for fetch lengths of 11 km and greater.

Inhomogeneity of the Land Surface and Problems in theParameterization of Surface Fluxes in Natural Conditions

Eddy correlation techniques to determine the turbulent fluxes of heat, moisture and momentum in the near-surface atmospheric layer rely on the Monin-Obukhov similarity theory, which requires stationarity and horizontal homogeneity. Experiments at specially selected sites over land and particularly over sea are used to develop this concept. Recent experiments, deliberately conducted in non-ideal conditions, show an underestimation of turbulent fluxes. Results from the field experiments FIFE, KUREX, TARTEX and SADE, point to a relationship between the underestimation of turbulent fluxes and terrain inhomogeneity. In order to systematically correct for this effect a scheme is suggested which uses fetch lengths of different types of surface in the sites surrounding the environment. In addition, horizontal differences in atmospheric stability above different surfaces are included in the correction scheme. This scheme might be useful for the design of validation experiments in non-homogeneous terrain.

Local advection of sensible heat in the snowmelt landscape of Arctic tundra

The spring landscape of the Arctic tundra is dominated by a snow cover which is highly variable in depth owing to redistribution by wind. Because of different energy dynamics, this heterogeneous land cover produces a horizontal transfer of energy at a small scale, a process termed local advection. An advection efficiency term (F S ), which represents the fraction of the sensible heat from snow-free patches which is advected to snow patches, was determined from field studies and published model results. Energy balance calculations demonstrated the strong contrast between the two surface cover types that drive advective processes, and F S was found to decrease exponentially with decreasing snow cover fraction. The field results suggest higher values of F S compared with the model results for single snow patches of varying size, but similar in magnitude to F S for multiple small snow patches. Utilizing exponential best-fit relationships between F S and fractional snow cover shows an increase in sensible heat flux of over 100% for low snow cover fractions. When considering the average flux over a composite snow and snow-free surface, the average sensible heat flux obtained from weighting the fluxes for each surface by their respective areas underestimates the composite flux when compared with when advection is considered. This work provides a simple method to estimate the effect of local advection on sensible heat to snow patches and the average flux from a composite surface during the snowmelt period, using only fluxes calculated independently for 0% snow cover and 100% snow cover and an estimate of F S . It demonstrates a good first estimate of the role of advection, but for future study the influence of wind speed, patch distribution patterns and fetch lengths needs to be considered more explicitly. This has important implications in studies of areal energy fluctuations over melting, patchy snow covers, basin water balance studies and regional and global climate modelling.

Controls on Formation and Form of Barred Nearshore Profiles

Linear regression is used to examine effects of maximum fetch length and nearshore slope in controlling characteristics of barred nearshore profiles. Data came from twelve sites in the Great Lakes and two in the Gulf of St. Lawrence. Fetch length is a good predictor of height of the outer bar, depth to the crest, and offshore distance; bar number correlates significantly with decreased nearshore slope. Incident waves are the dominant control on nearshore bar formation and maintenance during storms. THE purpose of this article is to examine the extent to which the variation in several morphological parameters of barred profiles can be explained by fetch length and nearshore slope. The approach is empirical, with data from sites in the Great Lakes and the Gulf of St. Lawrence, and the strength of the explanation is tested by linear regression. Barred profiles exist year-round at the two locations. This condition allows definition of a characteristic profile for each site and avoids complication induced by the seasonal change from a barred to a nonbarred profile that occurs on some oceanic coasts. Data for the analysis came from twelve sites in the lower Great Lakes and two in the Gulf of St. Lawrence (Fig. 1). Thirteen of the sites have one or more nearshore bars year-round, but the other site has a nonbarred profile throughout the year. My research was the source of data for ten sites; data for three other sites were taken from published studies (Hands 1976; Owens 1977; Gillie 1980), and John Coakley of the Canada Centre for Inland Waters provided data for one site. Typical profiles for each site appear in Figure 2, and selected characteristics of each site and profile are contained in Table I. The sites were chosen to encompass a wide range of fetch lengths, nearshore slopes, and nearshore-profile characteristics. However, short, steep storm waves dominate at all sites, swell waves having limited importance. During a period of years, the wind climate at all the Great Lakes sites should be very similar, and conditions at the two gulf sites are probably comparable. The actual wave climate at each site varies with orientation and fetch, but the magnitude of the dominant storm waves is controlled largely by fetch length and is generally independent of orientation. The Great Lakes sites are virtually tideless, although water level fluctuates seasonally by 0.3 to 0.4 m. Both sites in the gulf are microtidal, with a mean tidal range of less than 1 m. * Grants from the Natural Sciences and Engineering Research Council of Canada since 1977 supported the fieldwork on which this article is based. * Department of Geography, University of Guelph, Guelph, Ontario, Canada N1G 2W1. This content downloaded from 207.46.13.101 on Sat, 08 Oct 2016 05:42:06 UTC All use subject to http://about.jstor.org/terms 186 THE GEOGRAPHICAL REVIEW

Gravel Ripples on the Inner Scotian Shelf

ABSTRACT Sidescan sonar and photo/video surveys have revealed the presence of symmetrical gravel bedforms, interpreted as wave-generated ripples, with wavelengths of 1.3 to 3.0 m, occurring in patches and narrow ribbons in depths of 15 to 65 m on the inner Scotian Shelf. Direct observations and sediment sampling from the Pisces IV submersible have been used to ground-truth surface-based survey dam and to quantify relationships between the bedforms and the processes responsible for their formation. The ripples are formed in polymodal sandy gravels with modal sizes ranging from 0.1 to 45 mm. Evaluation of threshold criteria for coarse sediments together with an 11-year record from a nearby wave-rider buoy suggests that existing models for wave entrainment of gravel on rippled surfaces ove estimate the critical velocity by a factor of approximately 2 and that conditions suitable for transport of the coarsest (45-mm) size mode sampled in the ripple crests occur, on average, 43 hr/yr. Analysis of gravel ripples on the inner Scotian Shelf and of other examples noted in the recent literature extends the range of empirical data for oscillation ripple wavelengths by a factor of 3 and for ripple grain size by a factor of 5. The gravel ripples display a linear scaling to wave-orbital diameter similar to that reported for orbital ripples in sand. Use of such relations for paleoenvironmental interpretation of gravel ripples preserved in the rock record is complicated by the difficulty of selecting appropriate grain size and wave parameters. Conditions suitable for potential formatio of large-scale, symmetrical ripples in coarse sediments extend from the outer continental shelf to shallow shore face settings with possible fetch lengths as short as 20 km under intense storms.

Wave-built shelves on some low-energy coasts

A nearshore terrace has formed in unconsolidated deposits of the low-energy coast of the Strait of Georgia. Scale-modelling concepts have been used to draw similarities between this small-scale terrace and the continental shelf. The beach grades seaward onto an upward-concave shelf; the outer edge of the shelf is marked by an abrupt increase in gradient on the steep offshore slope. The morphology of the shelf varies systematically with the degree of exposure to wave energy; the width of the shelf and depth of water over the shelf break increase with effective fetch length. Sediments grade seaward across the shelf with a transition from sand to mud at the shelf break. Calculated near-bed velocities under waves, recorded at six sites of differing fetch length within the Strait, show the shelf break has formed in a transitional zone between high and low “wave effectiveness”. The profile form, shelf sediments and nearshore dissipation of wave energy suggest that the shelf evolves as an equilibrium form in response to contemporary wave processes. Similar terraces have developed on sheltered lake shores and on the Fiordland coast of New Zealand suggesting a general model of terrace formation at effective fetch lengths up to 50 km. Data from the Great Lakes indicate that the model may be extended up to 100 km, but break down at larger fetch lengths where effective fetch is probably an unreliable indicator of wave conditions. Similarities between the restricted fetch model and the continental terrace suggest there may be a continuum of shelf forms from small lakes through to proto-continental shelves.

Modeling of Structures Subjected to Wind Waves

The difficulties inherent in the direct determination of loads on off-shore structures which are exposed simultaneously to wind and waves make it desirable to test a model of the structure in the laboratory. It is shown herein that scaling of the structure and the loads due to waves is possible by using waves which are generated by blowing air over the surface of a laboratory channel, and by choosing a model material with an appropriate modulus of elasticity. Fully developed wind generated waves are found to be determined in magnitude, frequency, and spectral shape by Froude number scaling. The structural response obeys Cauchy number similarity, which is interpreted as implying equality of the ratios of peak frequency of the spectrum to natural frequency of the structure. The wave modeling aspects of the method are illustrated with an example. Some consideration is also given to dynamic wind load modeling and to the question whether simultaneous modeling of both the wave and the wind forces is possible in a laboratory. In principle, this appears feasible, but for sufficiently large scale models, fetch lengths may be required which are larger than typical laboratory channels.

A laboratory investigation of wind‐generated waves

Waves were generated by forcing wind through a covered laboratory flume. For steady‐state conditions, measurements by means of an oscillograph were made of the wave characteristics at various points in the flume. The significant heights and periods were determined and related to the fetch and wind speed. The height and period of the individual waves for various fetch lengths were compared by means of joint frequency diagrams. The orbital motions of particles throughout the water depth were determined from motion pictures. Particle velocities below the crest are compared with theoretical relationships. Some information on wind set‐up and shear‐stress coefficients are presented.