William F. Tanner

Origin of beach ridges and swales

Abstract Sandy beach ridges occur in four main categories: (1) Swash-built, (2) Settling lag, (3) Eolian, (4) Storm surge. Ridges in the first two classes are geometrically regular, only a few tens of centimeters above adjacent swales, and commonly in ridge sets and systems (tens to hundreds of ridges each). Individual sets (of 5–25 ridges) tend to stand 0.5 m to 2 m above (or below) adjacent sets. Ridges in the third and fourth classes do not have these characteristics, do not occur in sets, and are generally not suitable for detailed historical studies. No one of these ridges is the same as the storm-built berm, which is almost never preserved. Swash-built sandy beach ridges have diagnostic (1) map spacing, (2) accretion rate, (3) periodicity, (4) crossbedding and (5) granulometry. The last two indicate fair-weather waves on a sandy beach, in contrast with settlinglag ridges (Postma style), which have the same external geometry but which were deposited without important wave work; and in contrast with storm-built berms, generally seen on eroding coasts, but rarely or never in beach ridge plains. The grain-size kurtosis of beach sand is an excellent index to near-shore wave energy density. Changes in kurtosis at ridge set boundaries mark changes in long-term wave energy density, hence in sea level. The latter can be deduced also from set height differences and from spacing differences. Periodicity is typically 30–60 years, map spacing 25–50 m and accretion rate fairly close to 1 m/yr. A few ridge sets, with intervals of 3–7, 10–12, or 18–19 years, have smaller spacings. This is not what can be done, if each ridge is built by one storm. If each ridge had been built by one storm, wide beach ridge plains (like one in Denmark, dating from about 11,700 yr B.P.), would have been formed in the last 30–50 years, at accretion rates of hundreds of meters per year. Instead, each swash-type sandy beach ridge was made by a sea-level rise-and-fall couplet (amplitude, 5–30 cm). The swale marks the lower position. This mechanism reflects the fact that the transverse profile, from beach to sea, is gently concave upward, with maximum curvature close to shore; this is a shape which is out of adjustment with the shoaling wave system shortly after a small sea-level change. Settling-lag ridges were built without waves, hence the ridge-and-swale sequence in this case cannot be attributed to waves, either fair-weather or storm. These ridges show the same historical pattern as do swash-built ridges; this fact suggests that both were controlled by the same mechanism (sea-level change).

Preliminary study of transverse bars

Abstract Transverse bars (finger bars) are extremely common along sandy beaches in low-to-moderate wave energy regimes. They are typically oriented normal or nearly normal to the beach toe. Each transverse bar acts as a “focusing lens” for advancing waves, controlling wave refraction in such a way that: (1) the transverse bar is maintained; (2) sediment is transported along the long axis of the bar; and (3) a subtle but nevertheless effective circulation pattern is established, whereby water is transported seaward from the surf, without rip currents. In areas where transverse bars are numerous and well developed, the onshore or offshore transport of sand along the bar axes may be more important than littoral drift. Field experiments delineated a subtle nearshore current pattern, caused by unequal distribution of wave energy over the foreshore. Currents of this type were studied also in model wave tanks. These studies indicate that a transverse bar causes shallow waves to be refracted so that wave energy increases over the bar and decreases between the bars. Bottom friction reduces the wave energy more rapidly over the crest than in the adjacent deeper water. If the bars are relatively short this loss of energy through bottom friction has little effect. The wave energy concentrated over the crest results in an onshore current along the axis. The onshore current divides near the breaker zone and returns seaward slowly between the bars. The currents form two meshed gyres of horizontal circulation, one on each side of each relatively short transverse bar. Over relatively long transverse bars, the greater loss of wave energy through bottom friction over the crests, compared with the deeper wave between the bars, results in a reduced amount of wave energy over the shoreward portions. The areas between these bars provide a greater capacity for onshore movement of water caused by the mass transport associated with shallow water waves. The seaward return of this water takes place over the crests of relatively long transverse bars. These bars produce four meshed horizontally circulating gyres with an offshore current over the shoreward portions and an onshore current at their seaward ends. Preliminary data indicate that current strength is a direct function of the depth of water over the bars and an exponential function of the initial wave height. Additional work is in progress.

Sabellariid Worms: Builders of a Major Reef Type

ABSTRACT Sabellariid worms build extensive wave-deforming reefs along tropical and subtropical coasts; they may be, in fact, among the important reef-producing agents in the world. A stretch of worm reefs built by Phragmatapoma lapidosa has been studied in detail along 320 km of the Lower East Florida coast, (U.S.A.), where the reef system is essentially continuous. Related species are reported from other places in Florida. The ability of the worms to thrive under high-energy breaker conditions and to extend their colonial tube masses upward and seaward by extraction and agglutination of littoral drift materials, makes them highly important factors in the development of the coastline. Beachrock, converted from the reefs, and materials impounded on their landward sides, provide for actua progradation of the beach. The activities of these organisms are, therefore, important to the zoologist, oceanographer, coastal engineer, and geologist. The latter may find that the same or closely related forms have also been instrumental in building and protecting beaches of the geological past and in exerting control over the evolution of shorelines.

Late Holocene sea-level changes from grain-size data: evidence from the Gulf of Mexico

Grain-size kurtosis of sand in the beach ridge plain of St. Vincent Island, Florida, is numerically reasonably uniform within any one ridge set, but changes at important set boundaries. Where these changes are found, there are also differences in air photo and field appearance and in measured ridge-set heights. Sets with low kurtosis (∼3.0) stand topographically high, and sets with higher values (∼3.6) stand low. Study of wave energy levels and beach sand grain-size kurtosis along modem Gulf of Mexico beaches shows that kurtosis is an inverse function of surf-zone energy density. Storms and storminess (for the study area) can be evaluated in the framework of the selective- transport model of May (1973); the actual data sequence is greatly different from what is required by the model, and therefore storms and storminess cannot be invoked to explain the history of the area. Instead, the data sequence matches results predicted by the model for small sea-level changes. Storms and tectonism do not have appropriate durations and rates of operation, hence did not cause changes in kurtosis across St. Vincent Island. Slow land elevation changes (warping, isostatic adjustment) do not take place repeatedly up and down like the granulometric parameters. Tidal effects, in the study area, are both much too small and too frequent to have been important. The changes in kurtosis therefore indicate water level changes, and provide a sea-level history, including three sea level drops and four rises, of 1-3 m each, since about 3000 BP. Similar results have been obtained elsewhere, so these results are not local.

The zig-zag nature of Type I and Type IV curves

ABSTRACT Many distributions of sedimentologic (and other geologic) data are non-Gaussian. Of nine examples studied in detail, seven are Pearsons Type IV, and two are Pearsons Type I. Each plots as a zig-zag line composed of two or more straight-line segments on probability paper. The zig-zag pattern appears to be caused by a deficiency of particles of certain sizes. This is perhaps a general, rather than a local, condition, as has been pointed out by many workers.

An occurrence of flat-topped ripple marks [Florida]

Flat-topped ripple marks from the Pennsylvanian rocks of Oklahoma have been compared with similar ripple marks formed at low tide along the modern Florida coast. It is concluded that flat-topped ripple marks indicate bevelling, by wave-lets, in water approximately two inches, or less, deep.

The River Profile

The long profile of a representative natural stream is composed of segments, each of which is exponential, or nearly so. Each segment tends to plot on semilog paper (distance vs. log elevation) as a straight line, with a faint upward convexity near the upper end of the segment, and a faint upward concavity near the lower end of the segment. These small, but real, departures from the straight-line plot require that we undertake a refinement of the classical profile equation, dA/dz = kA. Several modifications are proposed, and are explored mathematically. One attempts to separate dynamic sorting of bed load from attrition effects. Another (expressed initially as Lagranges linear equation) treats the development of the two-dimensional profile with time-varying uplift. A third is the Leopold-Langbein random-walk model. A fourth is a modified random-walk model which specifically includes the upstream convexity and which can be altered to include the downstream concavity. A fifth is Devdarianis heat-flow analogy, which seems to be too general, but which can be forced, arbitrarily, to describe (without explaining) many anomalies of curvature. Three more are nonlog profiles based on reasoning associated with precipitation or discharge effects. No one of the attempts is completely successful, but each sheds a little light on the overall problem.

Eolian ripple marks in sandstone

Eolian ripple marks, formerly supposed not preservable, were observed at eight localities in the Colorado Plateau, in the De Chelly, Wingate, Entrada, and Navajo sandstones. The ripple marks could be seen only when the sun struck the surface at a low angle, throwing the faint ridges into sharp relief. At one locality in the Entrada, the average wave length was 15 cm, height 0.4 cm, and ripple index 35. Average ripple indexes at the other localities ranged from 22 to 75(?). Shear ridges on subaerial sand do not produce eddies in the air, hence the ridges are built only by direct drag or bed shear. This limits their height and keeps the ripple index high.

Hydrodynamic Implications of Beach, Beach Ridge and Dune Grain Size Studies

ABSTRACT Stepwise linear discriminant and canonical analyses of beach, beach ridge and coastal dune (slip-face) sands from localities around the Gulf of Mexico show that beach ridges have textural affinities with both beaches and coastal dunes. However, the internal structure shows that they were deposited largely by swash action. Washover and slip-yface bedding are, in general, not present in beach ridges. Skewness is the most important moment measure entering the discriminant functions and the cannonical plots; standard deviation is the next most important. These two measures for beach ridge sands show both swash-zone and dune affinities. In these samples kurtosis reflects shell content. Mean grain size, an indicator of local availability, is of minimum importance for determining hydrodynamic conditions of transport and deposition in such a regional study. A progression of moment measures was determined for the surf zone, the upper wash zone, and the coastal dune. Changes from zone to zone are largely confined to the coarse tail; however, individual sample grain size distributions (plotted on probability paper) have important but subtle characteristics for each zone. Differences in skewness values bear this out. Our beach ridge data generally fall between the upper swash zone and the coastal dune (in most instances closer to the upper swash zone). The statistical process, from surf to beach ridge to coastal dune, is one of filtering (but not mixing). Each filter is truncated. The total result is to improve the internal sorting from zone to zone, but the terminal distribution is nevertheless still not simple. The lack of simplicity aids in making an hydrodynamic interpretation.

Tertiary sea level symposium—Introduction

Abstract Eleven papers making up the Tertiary Sea Level Symposium (spring 1967) are presented herewith. Their combined effect is to produce a “first approximation”, Tertiary history of sea level involving a more-or-less steady drop, of 70–100 m, beginning about the middle of Miocene time. This history requires a careful reconsideration of identifications of high marine terraces as having been formed in the Quaternary Period. The concept of possible glacio-eustatic oscillations, in the Tertiary, is explored in one paper. Data from Antarctica and the surrounding ocean show that a major influence on Miocene and later sea levels was the advent of extensive glaciation.