Donald K. Stauble, J.Bailey Smith, William A. Birkemeier
Geologic Analysis of Shelf Beach Interchange Work Unit,
US Army Corps of Engineers, Coastal Geology and Geotechnology Program
To quantify sediment distribution response to forcing functions on the foreshore, bar/trough and offshore along a three-dimensional beach. Sediment grain size distributions of the different beach environments change as a function of wave and tidal forces as the beach erodes and accretes through time. A deficit of knowledge exists in relating sediment movement on the foreshore due to swash processes both during fair weather and storms with a) the bar and trough movements due to wave breaking, b) longshore currents and/or cell circulation (rip currents), and c) the nearshore slope out to closure and beyond due to wave, tidal and wind driven circulation. This research will examine the interaction of sediment along the entire active profile during both erosional and accretional events. A better understanding of the dynamic processes of sediment deposition and interaction with profile volume change on a natural beach will be documented. Our ultimate objective is to understand the fate of beach fill material placed anywhere on the profile.
Surface sediment samples, box cores, and vibracores will be collected. The experimental design includes collection of sediment data along three profile lines, approximately 50 to 100 m apart straddling the FRF's north property survey line (Line 230) and extending from the dune base to the 8m depth contour.
Surface sediment samples will be collected at dune base, berm crest (HT), mid-tide, swash (LT), bar trough, bar crest, -4m, -5m, -6m, -7m, -8m (and -9m if possible). Sediment samples to be collected by surface scoop to wading depth and grab sampler offshore. Samples will be collected during both the fair weather period (August) and during storm conditions (October). The sampling schedule includes daily profile and sediment collection at low tide, coordinated with other experiments, for a duration of around 14 days. The sediment sampling schedule will be flexible in order to bracket individual storms and to minimize use of the CRAB.
Nearshore boxcores collected on the Duck, NC shoreface since September, 1994 offer a unique opportunity to study sedimentologic data including bedding structures, grain size characteristics, and bed level changes, and their relation to profile survey, hydrodynamic and bed-elevation data. Boxcores and reference rod measurements have been taken at 14 locations along three cross-shore transects to measure cross-shore and longshore variations in sedimentologic data (Figure 1). Measurements were taken at approximate monthly increments with more frequent monitorings before and after storm conditions. Boxcore/reference rod stations extend from 580 to 1600m from the shoreline (5.5 to 13.2m water depth) with cross-shore spacings ranging between 50 and 200m and alongshore spacings in 25 and 90m increments. Ongoing bi-weekly profile surveys to -8m (or shallower during storm conditions) with the Coastal Research Amphibious Buggy (CRAB) are situated 90m to the north of the data collection area. Wave height, period and direction; current velocity and direction; and bed elevation changes were measured continuously (see Howd and Hathaway-Experiment #22) commencing October 1, 1994 at a frequency of 2 Hz by two instrumented bipods at the two boxcore/reference rod stations situated at -5.5m and -13.2m. Analyses consider the relationship between these three data sets during both storm and fairweather conditions with special emphasis on the October 10-21 1994 northeaster and Hurricane Gordon of November 1994.
Storm-influenced shoreface sediment transport at Duck, NC was measured during the October, 1994 northeaster, which produced 4.0 m waves and mean currents exceeding 0.50 m/s, which were oriented to the south and slightly offshore for the initial nine days of the storm, and to the north and slightly offshore for the remaining three days. During the storm, 11 of the 14 boxcores recorded accretion (average net bed elevation change in all 14 boxcores was +0.05m), and 4 of the 14 reference rod measurements recorded accretion (average net bed elevation change in all 14 reference rods was +0.01m). Bed elevation changes at -5.5m, as recorded by the bipod sonic altimeter, included a fluctuation of 0.10m with no net change(Table 1). Boxcore and reference rod measurements showed accretion and erosion, respectively. At -13.2m, the sonic altimeter recorded a 0.30m fluctuation in the bed elevation with 0.10m of accretion over the event. Reference rod and boxcore measurements including 0.10m of recently deposited sediment consisting of southerly and onshore-dipping crossbeds support these data. The surveyed profile translated seaward in excess of 40m and pinched out (+/- 0.30m) at -5.5m (450m seaward of the shoreline).
|Table 1. October 10-21 1994 Storm Net Bed Elevation Changes (m)|
Fairweather conditions are typified by bioturbation of surficial sediments (to @ -0.07m below the sediment/water interface) throughout the boxcore cross-shore transect, and stable bed elevations (except for the deposition of fine-grained "fluffy" sand seaward of -9m) thus rendering no sedimentologic bedding information. The surveyed profile seaward of -5.5m tends to remain stable during fairweather conditions except for a post-storm landward translation.
Continuing field measurements will help to determine sediment transport directions and volumes in the vicinity and seaward of the "depth of closure" (defined as the depth at which sediment movement occurs but no apparent bed elevation change takes place) for this shoreface. For instance, a deposition of 0.03m of sediment (based on boxcore and reference rod measurements) at the field measurement area (1.37x105 m2) during the October 1994 storm translates to a volume of approximately 4.11x103 m3 of deposited sediment. Although this sediment may not imply a useful estimate for application to an entire shoreface system, continuing analyses will help to address the transport direction, origin, and exchange of this sediment with seaward and beach zones. These studies will ultimately help to better estimate offshore losses of sediment for beachfill design and evaluate the significance of beach losses over the life of a project.
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For a list of addresses of the investigators involved in Duck 94 click here.