Journal ofCoastal Research 141 146 FortLauderdale, Florida Winter 1996 Shoreface-Connected Ridges in German and U.S. Mid-Atlantic Bights: Similarities and Contrasts Effiom E. Antia] Institute ofOceanography University ofCalabar PMB 1115 Calabar Cross River State, Nigeria ABSTRACT _ ANTIA, KE., 1996. Shoreface-connected ridges in German and U.S. mid-Atlantic bights: similarities and .tllllllll:. contrasts. Journal of Coastal Research, 12(1), 141-146. Fort Lauderdale (Florida), ISSN 0749-0208. :;,;rr:r::y. Shoreface-connected ridges in the German Bight (southern North Sea) and in the U.S. Mid-Atlantic Bight .hav~ pr~viously been 8ho~ to eX~ibit some similarities. This report, based on sedimentological ~. l~vestIgat~ons III,the German Bight, outlines additional similarities as well as significant differences in '--¥_C=:_ rl~ge attrl?utes. In both bights. Ridge migration, orientation, and vertical grain size patterns in both bights m:e Ident~cal. although rid~e .migration fates are up to 200% higher in the German Bight. However, ..., S-- c?ntr~tmg attributes such as mrmmum water depth of occurrence and longitudinal grain size trend of ridges In the German Bight are inconsistent with ridge generation models in the Mid-Atlantic Bight. ADDITIONAL INDEX WORDS: Shoreface-connnected ridges, German Bight, U.S. Mid-Atlantic Bight, grain size, storm flow. INTRODUCTION though the precise mechanism involved is still a Shoreface-connected ridges are well developed subject of speculation (McBRIDE and MOSLOW features in the German Bight (southern North 1991). A detailed review of the existing models of Sea) and in the U.S. Mid-Atlantic Bight. Since shoreface ridge evolution is contained in pioneering studies of shoreface ridges were con­ PATTIARATCHI and COLLINS (1987) and McBRIDE ducted in the Mid-Atlantic Bight, particularly in and MOSLOW (1991). A major problem associated respect of their characteristics, evolution, and with understanding ridge genesis is the difficult geological significance (e.g. DUANE et al., 1972; task of distinguishing between ridge character­ SWIFT et al., 1972; SWIFT and FIELD, 1981), it was istics which existed at the time of formation vis­ appropriate to compare their attributes with those a-vis after formation. of their counterparts in the German Bight and The comparative study approach of SWIFT et elsewhere (SWIFT et al., 1978). at. (1978) is particularly instructive in the above SWIFT et at. (1978) documented a number of respect; differences in ridge characteristics caused attributes common to ridges in both bights, name­ by local or regional setting and physical processes ly: (a) shoreline-oblique orientation, (b) shore­ can eventually be identified leading, ultimately, acute angle opening into the direction of predom­ to a better understanding of mechanisms of ridge inant sediment transport or major storm flow, and development. Until recently, the report of SWIFT (c) out-of-phase coast-normal relationship be­ et at. (1978) was the major source of information tween mean grain size and ridge topography. Most on shoreface-connected ridge characteristics in the of these observations are consistent with, and are German Bight. This present study follows the ap­ corroborated by, recent sedimentological inves­ proach of SWIFT et at. (1978). However, the main tigations of shoreface-connected ridges in the objective here is to update and reevaluate the German Bight (ANTIA, 1993a,b). observations of SWIFT et at. (1978) in light of sed­ Such a marked similarity in ridge characteris­ imentological investigations ofthe shoreface ridge tics from different environments would suggest a of Spiekeroog Island (German Bight). similar origin and process of maintenance, al- STUDY AREA The German Bight is situated at the southeast 93019 received 27 February 1993; accepted in revision 18 February 1995. t Former address: Marine Geology Program, Geosciences Institute, Uni­ corner of the North Sea Basin (Figure 1, inset). versity of Bremen, Bremen, Germany. At the southern margin of the bight is a chain of 142 Antia N SWDYAREA i I + N O RTH SE~~ 53° G ERMANY 30' N 20 KM 7° 00 ' E 8° 00 ' Figure 1. The general geomorphology of the German Bight showing the Fr isian barrier island s and inlet ebb-tidal deltas as well as the main study area of Spiekeroog Island (after FITZGERALD et al. 1984). drumstick- and linear-shaped barrier islands, re­ during winter months in the inner German Bight, ferred to as Frisian islands (Figure 1). These is­ most of which originate from NW to SW. Signif­ lands are seperated, respectively, from each other icant wave height (H.i. ) of 2.3 m and periods of 6 by inlets which are 15-20 m deep and about 2 km sees are considered typical for the shoreface of wide, and from the mainland coast of Netherlands Norderney at 20 m water depth (KLEIN and and Germany by up to a 7 km wide back barrier MITTELSTAEDT, 1992). In 10 m water depth off region or Wadden Sea. Most of the inlets are char­ Norderney, H.i• of 1.8-4. 5 m was documented by acterized by well-developed ebb -tidal deltas pro­ NIEMEYER (1979), which under extreme storm truding 3 km seawards (into water depth of 6 m) conditions may reach a height of 9 m and period from the E-W oriented barrier island shoreline. of 12 sees (NIEMEYER, 1976). The maximum water depth in the German Bight is about 30 and its seabed gradient varies between DATA BASE 1:600 and 1:1000. According to STREIF (1989), the The main data from the German Bight shore­ East Frisian barrier islands have retreated at a face-connected ridges were obtained offshore of rate of 1 m/yr over the last 1500-2000 year s. Spiekeroog Island. In addition to repetitive cur­ The physical oceanography of the southern rent measurements and analyses of detailed North Sea is detailed by OTTO et al. (1990). Re­ sounding charts, sediment size statistical param­ sidual currents attain a velocity of 10-15 cm/s eters were studied using 690 grab , 9 vibrocore and (NEUMANN, 1966) and flow eastward, but gradu­ 60 boxcore samples. The boxcores were taken re­ ally veer offshore along the Frisian islands. Tides petitively along two coast-normal transects . Field in the German Bight are semidiurnal and show logistics and laboratory procedures are detailed mean tidal range increasing eastward along the in ANTIA (1993a). Preliminary results of surficial Frisian islands from about 1.4 m at the extreme and vertical grain size patterns are given in ANTIA west to about 2.7 m around Wangerooge. Fair­ (1993b) and ANTIA (1994), respectively. weather peak tidal current velocities (varying be­ tween 30-60 cm/s) measured at 1 m above sea bed RIDGE CHARACTE RISTICS and in water depths < 20 m showed the easterly For clarity, ridge characteristics are detailed flood currents to be 30-50% stronger than their under two subheadings: morphology and sedi­ westerly ebb counterparts (ANTIA, 1993a). KLEIN ments. and MITTELSTAEDT (1992) reported a peak (flood) current velocity of 78 cm/s at 1 m above the bot­ Ridge Morphology tom on the shoreface-connected ridge of Norder­ The shoreface-connected ridges in the German ney Island during a violent storm. Bight are at least 10 km long, 1-2 km wide, 3-5 Storms occur at least 10 to 30 days annually m high, 0.5-1° steep, and are mainly composed of Journal of Coastal Research, Vol. 12, No.1, 1996 Shoreface -Connected Ridges 143 no 51' !'Ii -,----- -------------, - 1Wa!i "' . __ . .",3 - .-. --." .. ~~ --'... ... S P I t: K E RO n(; r .aG' [ ". ... r oo' [ .\0 ' ... roo ' ". Figure 3. Headward trough erosion and coastward migration Figure 2. The German Bight showing the shoreface-connected of a shoreface-connected ridge morphology, Spiekeroog Island. ridges in 10--25 m water depth. The ridges are typically linear in shape, oblique to the coastline trend and are composed of medium- to coarse-grained quartz sands (modified after the D ElITS CHES H VDROGRAPHIS CHES INsTITUTchart No. 2900, 1:250 000, 1981). Contours are in meters. Studied ridge is boxed. Mid-Atlantic Bight counterparts reported by SWIFT and FIELD (1981). A fundamental differ­ ence, however, is the much higher rate (as much as 200-500 % ) of ridge migration in the German coarse- to medium-grained quartz/feldspathic Bight, over comparable time intervals, than in the sands (Figure 2). SWWf et al. (1978) noted that U.S Mid-Atlantic Bight (ANTIA, 1993a). the acute angles of shoreface ridges relative to the Ridges in the latter environment, based on ex­ adjacent coastline open into the direction of major isting literature reports such as DUANE et al. storms or mean flow in both the German and U.S. (1972), SWIFT and FIELD (1981), rarely exhibit Mid-Atlantic Bights. The shore-acute angle in­ long-term coastwise and coast-normal mean mi­ creases in a northerly direction in the Maryland gration rates > 5 m/yr; whereas, coast-normal sector of Mid-Atlantic Bight from 10°to 40"(SWIFT ridge migration rates of 20-30 m/yr and coastwise and FIELD, 1981). However, DUANE et al. (1972) counterparts of > 100 m/y are common in the reported a decrease in shore-acute angle in a German Bight (ANTIA, 1993a). An additional dif­ southerly direction over a larger area, from 50° to ference in the coast-normal migration pattern of 20° between Delaware Bay and Chincoteague shoreface ridges in both bights was noted. In the Shoals. In the German Bight, the shore-acute an­ Mid-Atlantic Bight, the coast-normal migration gle decreases in a westerly direction from about of shoreface ridges is predominantly offshore 14° near Spiekeroog to 40° near Juist (c{., Figure (SWIFT et al. 1978; SWIFT and FIELD, 1981). By 2). On a regional scale, the shore-acute angles of contrast, a diabathic (oscillatory) pattern of mi­ the ridges decrease down-current of the major gration is typical of shoreface ridges in the Ger­ storm in both bights.