SEDIMENTOLOGIC ARCHITECTURE OF THE SHOREFACE PRISM, RELATIONSHIP TO PROFILE DYNAMICS, AND RELEVANCE TO ENGINEERING CONCERNS - DUCK, NC

Robert K. Schwartz, Derek W. Cooper, Paul H. Etheridge
Department of Geology
Allegheny College, Meadville, PA 16335

ABSTRACT

Forty nine oriented vibracores were collected in July 1993 along two shore-normal transects extending from the beach out to ~8 m water depth at the US Army Corps of Engineers Field Research Facility at Duck, NC. Close vibracore spacing and detailed hierarchial sedimentologic analysis enabled "high-resolution" definition of the textural and structural architecture of the shoreface prism which was then compared with biweekly survey data representing a 12+ year period. The shoreface prism erosionally overlies an inlet-associated tidal complex. Maximum shoreface thicknesses of 3-4 m occur below the beach through middle profile with seaward thinning down to 30 - 40 cm between 7.5 and 8.5-m water depth. Gravel-rich swash zone and swash-trough transition facies record episodic seaward progradation of beach-to-inner trough flank surfaces over cross-stratified finer sand of downflank and trough axis settings. Cross-stratified sand and gravel also occurs in the lower half of the shoreface prism below bar, upper ramp, and middle platform sectors of the profile documenting the migration of relatively large three-dimensional ripples and bars in historical current-dominated trough settings. A horizontal-laminated fine-sand facies overlies the trough-associated facies representing shoal zone buildup under high-velocity plane-bed conditions. A bioturbated horizontal-laminated fine to very fine-sand facies underlies the lower ramp sector of the profile also indicating buildup under high-velocity conditions. However, profile data documents that the lower ramp builds up in response to storm events which simultaneously causes erosion and outer-trough development along the landward-adjacent profile. Conversely, the lower ramp undergoes slow subelevation during long fairweather stretches as the storm-trough sector undergoes buildup.

Greater than 80% of the shoreface prism lies within the envelope of surveyed profile change with the surveyed lower limit of erosion (LLe) serving as a close approximation to the sedimentologic lower limit of the shoreface (LLs). The LLe is nearly coincident with the LLs below the lower ramp and rises above the LLs in the landward direction where deeper erosion usually predated surveys. Profile data show that the lower ramp tends to erode and build up in form-parallel fashion, contrary to the upper-ramp and middle profile sector which undergo localized cut and fill. Along the lower ramp, "fairweather erosion surfaces", similar in shape to the LLe and LLs, cut down into storm-accretion sequences. The LLe and LLs below the lower ramp primarily reflect a maximum erosion surface which developed about 6 1/2 years prior to vibracoring. Lower ramp deposits above the LLe record the occurrence of 4 accretion trends each associated with periods of greater-than-unusual storm activity and each truncated by progressively decreasing erosional maxima. Shoreward of the lower ramp, the LLe and LLs represent spatial composites of localized longshore trough-scour events which occurred at various times throughout the 12+ yr survey period.

Combined profile and sedimentologic data document that trough-associated scour occurs across a 250 - 300 m wide zone down to ~-5 m NGVD resulting in a concave-shaped composite-trough fill. The seawardmost location of trough-associated facies corresponds the break-in-slope between the middle platform and lower ramp, and serves as an estimate to maximum width for longshore current depositon during extreme events. Textural data indicates that medium sand to gravel tends to remain within the longshore current domain, even during extreme storm events, with no evidence for significant quantities of material coarser than fine sand being lost to the offshore. Short-term closeout depths (e.g. 4 - 6 m) as well as the calculated depth of annual closure (dl) coincide with the break-in-slope and facies boundary. Thus, the lower ramp facies coincides with a time-dependent zone of profile closure wherein durations of closure increase in the seaward direction and intervening changes in bed elevation decrease. The zone of closure may be thought of as a zone of decreasing seaward transport in which fine to very fine sand is the typical bedload material, even during high energy storms. Textural distribution within the entire shoreface mass indicates net longterm transport direction and loci of deposition for different sized material, thus providing guidelines for project fill and expected retention.

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