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Download Download Journal of Coastal Research Charlottesville The Beach: Where Is the "River of Sand"? William F. Tanner Geology Department Florida State University Tallahassee, FL 32:106,:,026 USA ABSTRACT _ TANNER, W.F., 19H7. The beach: where is the "river of sand"? Journal of Coastal Research, 3(3), :177-:lH6. Charlottesville, ISSN 07 49-020H. I ••••••••••• The "river of sand" concept is basically a model of sediment transport; sand is carried 4""""" lengthwise along a beach from "input" end to "output" end, like a slurry in a pipe. Such ~ transport can be seen locally, in nature, but the primary sand path is at right angles to the river ofsand line. Thousands ofbeach ridges have tips terminating in marshlandor swamp, with no attachment to any possible source ofsand, commonly both ends. The termination is susss" ---- ~ 'it' generally by tapering, not truncation, Others are attached at both ends, but in a pattern such that neither end was the source. Wide inlets on both sides ofmany beach ridge plains = * show that the only source ofsand was offshore. Grain size studies do not indicate shore­ parallel motion, but show sine-like variations in size parameters: patches ofcoarser sand must have been derived from offshore, here and there, The dq/dx model (shore-parallel movement) fails over long distances: it may hold for short segments. The dq/dy model (shore-normal) is more successful. Parallelism ofridges also requires an offshore source. The "river of sand" idea is a gross oversimplification. In the areas studied it is gen­ erally wrong. ADDITIONAL INDEX WORDS: Beach beach ridge, beach ridge plain, dq/dx model; offshore sand; parallel beach ridees. ridge tips, "river of sand, " shore-parallel transport; unde inlet INTRODUCTION identified in the text. Other beaches, not on beach ridge plains, have been investigated also. The"river of sand" model for sand transport on beaches is, in its simplest form, a scheme in which GEOMETRY sand is placed at one end of a long beach and transported to the other end, where it vanishes (as Beach Ridge Tips far as the model is concerned). This is shore parallel transport, The concept is simple, elegant and easy Beach ridges can be built in several different to understand. Furthermore, it has been promoted ways (TANNER, in press). A common variety is the in an attractive color movie (ENCYCLOPAEDIA BRIT­ swash- built ridge, having only minor amounts of TANICA EDUCATIONAL FILMS, 1965), The basic idea eolian or overwash deposits in it. This type may oc­ has been used widely, but commonly not explicitly cur in sets of five to 200 or more individual ridges, identified as the" river of sand." However, there is parallel, or nearly so, with their neighbors. A pop­ muchevidence that many beaches, if not most, were ular idea is that each ridge is attached, at one end or built in some other way, and the river of sand is lo­ the other, and that the point of attachment permits cal, temporary or minor, The present note sum­ us to identify the source of sand. Transport, in this marizes some of this evidence, model, was from the point of attachment to the Beach ridge plains on which this study is based other end of the ridge, are located in various parts of Canada, the USA, St. Vincent Island, Florida Panhandle, has been Mexico, Colombia, Venezuela and Brazil. Specific studied intensively for two decades (TANNER, areas, selected from more than 60 such plains, are 1974; DEMIRPOLATetal, 1986), It consists of some 86034 received 5 October, 1986; accepted in revision 6 July 180-200 individual beach ridges (Figure 1), a few 1987 marked by eolian decoration. Most ofthe ridges are 378 1 anner GULF OF ,I .., o o '" z o o '"<0: KM o~ ~ J c-= ~~W ATE R :::::::------.. BEACH RIDGES Figure 2. The northern half of St. .Ioseph peninsula. showing Figure I. St. Vincent Island (small figure, left center) and beach representative beach ridges. (For location, cf Figure 6.) Oeep ridges on two parts of the island (approximately to scale: for loca­ water, near C, was at one time the inlet between two of the three tion in Florida see Figure 6). Representative beach ridges are original islands. The northern island started with a nucleus near H. shown. The upper figure maps part of the eastern third of the where the ridges grew in both directions, until the system became island; ridges terminate at A, S, and C, and points between. This integrated. The middle island started north of the old inlet. The termination is not by truncation, but by lack of deposition. Seven southern island started with a nucleus below the southern edge of ponds are shown; around them is a marsh. The lower figure shows the map; curvature of ridge ending at (' shows that the inlet was the western ends of selected ridges: note points A, R, C, 0 and be­ active. tween. The area to the left of points A-0 is marsh, mud flat or very shallow water, with no ridges, In these IRO or so ridges, none indi­ cate significant shore- parallel transport. The southern part of the is that a later inlet has destroyed the pointof attach­ island (X and Y, small map) contains beach ridges built of sand ment. Another is that the eastern edge ofthe island derived from the east, later in its history; they belong to a dif­ is fault- controlled, and that the point of attachment ferent system. is now below lagoon waters. Neither of these can be defended. The inlet east of the island is a kilometer faintly concave seaward. To the east is an active or more away from the ridge tips, and has not been inlet. To the west is the bay head. In the "river of migrating, and the ridges taper, eastward, to points, sand" model, transport was from east to west, rather than showing signs of truncation. The fault across the inlet. suggestion likewise fails at these points. However, no single ridge on the island is attached The oldest beach ridge on St. Vincent Island was to anything except some other part of the same built without any attachments. It formed in open island. Ridge sets A to G have eastern tips that end water, probably on a long narrow shoal. The ter­ in the marshland, many of them one km or more minations show that there was no significant east­ from the beach to the east. The seven sets cover ward or westward transport. The younger ridges in almost the entire history of the island. the seven sets listed above show the same pattern. Various hypotheses have been voiced to me, by The explanation is that the sand in these ridges was persons who feel that there has to be an attachment, derived from offshore. The only objection to this so that there can be an up drift source of sand. One suggestion is the "river of sand" model itself. .Iournal of Coastal Research, Vol. :J. No. :J, 1987 Where Is the"Riverof Sand"? 379 WIDE INLET GULF OF MEXICO o 2 '-----~~-~. Figure4. Part of the CayoCosta Island (westcoast of Florida: see Figure6); and Johnson Shoal, in the Gulfof Mexico, to the west.Twopositions ofthe shoalare shown in 1944 and 1960. Be­ Figure 3. Southernend ofLaguna Madre, Texas(near Browns­ tween these dates the rate of migration was 50-100 rn/yr. The ville), the RioGrande, and the naturalleveeon its southernside. original shoal, or island.wasnot a dredgespoilpile,but a natural Fourbeachridgeplainswithrepresentativeridgesare shown; the feature; itmayhavebeenadunecomplex. AtpointA arethetaper­ largest is Mesadel Gavilan, studied in detailin 1986. The ridges ingtips ofbeachridgeswhich extendnorthward, but which didnot formed insidethe lagoon, and post-date the barrierislandto the bridgetheinlet AtB isarepresentativeearlyridge, terminating in east. The sequenceis: oldest to the southeast, youngest to the both directions, and not due to shore-parallel transport. At C northwest. Each beach ridgeon each islandhas tapered ends in younger ridgeslikewise terminatein both directions. Divergence bothdirections; therewasnoshore-parallel transportofsand,but at D suggests a shift in location of the offshore sourceof sand. rather, sedimentwas derived fromthe adjacent lagoon floor. at least two older islands, which were connected Little St. George Island, across the inlet to the later by plugging the inlet between them (Figure 2). eastfrom St. Vincent, is muchyounger, butconsists Clear beach ridge patterns show that, during the of some two dozen ridges. Some ofthese have been history ofaccretion, eachisland started as a nucleus truncated by wave erosion, but many ofthem taper in open water, and grew in three directions: toward off to the southeast and to the west. The oldest was both ends, and seaward. The beach ridge tips were formed in open water, without attachment of any unattached With the closing ofthe inlet, growth of kind. Therefore this younger island cannot have ridges into the inlet ceased, but the tips are still been the updrift source for St. Vincent. visible. Gap Island, some kilometers farther east, is made Mesa del Gavilan, and similar beach ridge plain of a few beach ridges, some of them showing evi­ islands in the Laguna Madre, east of Brownsville, dence oflater erosion. However, ends in both direc­ Texas (Figure 3), are made of beach ridges each of tions show taper and termination. Dog Island which has its ends preserved and visible (TANNER (SPICOLA, 1984) is made ofseveral beachridge sets. and DEMIRPOLAT, in preparation). There is no Except for the modern beach, which is eroding and attachment to any other sand body; the material in in places truncating older ridges, depositional ter­ the ridges was derived from the adjacent shallow minations are easy to locate. They were at no time lagoon, as is shown by bedding, shape and attached to any other island, spit, point, cape, or granulometry.
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