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).