Bipod Instrumentation

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Introduction

Storm activity is often associated with erosion of the subaerial beachface (List and Farris, 1999) and inner surf zone. These same storms may also lead to erosion or accretion deeper on continental shelves due to exchange of sediments between onshore and offshore locations. Even with this sediment exchange, offshore decreases in profile variability (Nicholls et al., 1998) suggest that the inner continental shelf is responding to waves and currents at different temporal scales than the subaerial beachface and inner surf zone.

Many studies have concentrated on the forces which can initiate and sustain sediment transport on continental shelves, but the actual amplitudes and nature of seabed responses to storm events are not well constrained (Morton, 1988). In particular, processes controlling scour and creation of marine erosion surfaces are not well documented (Field et al., 1999).

With advances in technology, longer-term observations of seabed dynamics have the potential to increase our understanding of seabed elevation response to different types of storm events (Beavers et al., 1999). Field measurements of seabed elevation changes during northeaster storms (Wright et al., 1994a) and hurricanes (Beavers et al., 1999) have been documented on the inner continental shelf, but rarely have both types of storm events been documented at the same location on the shelf.

By maintaining instrument packages at the same location for several years (1994-1997), temporal patterns in seabed response and the spatial variability of hydrodynamic forcing can be studied for a variety of storms. In order to obtain continuous seabed observations and document hydrodynamic conditions throughout storm events, instrument packages were deployed in 5.5, 8, and 13 m water depths beginning in 1994. Designed to span the transition from the inner continental shelf to the outer surf zone, these packages occupy a dynamic zone where both wind and wave forcing may be important (Fig. 2.1). The 2 major storm systems responsible for producing this wind and wave forcing at Duck, NC, are hurricanes and northeaster storms.


Figure 2.1. Definition sketch of the inner shelf and adjacent surf zone regions of the shoreface (adapted from Wright et al., 1991).

Bipod Instrumentation

To study longer-term sediment dynamics on the inner continental shelf and outer surf zone, a multi-year monitoring program of near-bottom and interior flows and seabed elevation changes across the shoreface of the FRF was initiated in 1994 (Howd et al., 1994). Instrument packages to monitor waves, currents, and seabed elevation changes were deployed in 5.5 and 13 m water depths in September and October 1994 (Fig. 2.3). In May 1995, a third instrument package was deployed in 8 m water depth.



Figure 2.3. Location of bipod instrumentation (stars) at the FRF. Contours are in meters.

Instrument packages were secured on ‘bipod’ frames (Fig. 2.4) designed to sleeve over two 6.4 m long pipes jetted vertically 4 m into the seabed. Power and communications were provided from shore via armored multi-conductor cables. Except for sensor repairs or replacement, these instrument packages collected data during numerous storms from 1994-1997.

















Figure 2.4. Bipod instrumentation.

Current Meters

Each bipod (Fig. 2.4) initially included 3 Marsh-McBirney electromagnetic current meters located on the offshore end of the frame. The biaxial electromagnetic current meters were replaced in fall 1997 with non-invasive triaxial acoustic current meters. This end of the frame was deployed to the southeast to minimize interference of current meters and vertical support with wave orbital velocities during northeast waves. Current meters were initially deployed at nominal elevations of 0.2, 0.55, and 1.5 m above the seabed to permit calculation of bed shear stresses associated with different flows by the velocity profile method (Drake and Cacchione, 1992). With a shoreline orientation of approximately N20W, longshore currents flow toward 340° (i.e. northward) or toward 160° (i.e. southward). Similarly, cross-shore currents are either onshore at 250° (westward) or offshore at 70° (eastward).

Pressure sensor

A pressure sensor (Fig. 2.4, P), sonar altimeter (S), and electronics housings (A, B, and C) were secured to the frame crossbeams. Current meters and Sensometric strain gauges were sampled at 2 Hz. Pressure fluctuations were measured to allow calculation of the wave spectrum and water elevation (tides). Initially, an analog Sensometric strain gauge (Fig. 2.4, P) was deployed with each instrument package. These sensors were relatively inexpensive and reliable, but often exhibit mean pressure drifts over long time periods, such as 10-20 cm in a month. In September 1997, digital Paroscientific gauges replaced the strain gauges for more precise and stable pressure measurements. These gauges output voltage signal with a frequency proportional to the pressure and operate at a nominal 38 kHz. The Tattletale Model 8 operated in a frequency-count mode to measure the Paroscientific signal over a 50 ms averaging interval, at a 2 Hz rate. This sample interval was determined to be short enough to have negligible filtering effect for wave measurements (2+ s), and long enough for an accurate pressure (frequency) measurement of better than 1 mm.

Wave height, Hmo, was computed as an energy-based statistic equal to four times the standard deviation of the sea surface elevations. Wave height reported from the pressure gauge has been compensated for hydrodynamic attenuation using linear wave theory.  Wave variance is computed from energy spectra and band limited to frequencies > 0.05 Hz (period <20 s) with a high frequency cutoff based on wave attenuation where linear theory amplitude correction is 10. Wave period is identified from the computation of a variance (energy) spectrum with 60 degrees of freedom calculated from a 34 minute record. Peak wave period, Tp, is defined as the period associated with the maximum energy in the spectrum.

Sonar altimeter

The Datasonics altimeter (Fig. 2.4, S) transmits a 210 kHz acoustic pulse once per second (1 Hz) with ‘bottom’ return echoes detected after each pulse. Returns are range-binned for 34 minutes. The bin with the maximum number of returns is recorded as the seabed elevation during that 34-minute period.In laboratory tests, the mean distance to the bottom (Fig. 2.4, d) measured with the altimeter was accurate to + 1 cm of an independent distance measurement. The altimeter transducer beamwidth is approximately 10° and results in an approximately 20 cm diameter footprint at 1 m range. The footprint of the sonar altimeter is too large to resolve short wavelength (1-5 cm) ripples (Gallagher et al., 1996); instead, larger scale patterns of erosion and deposition are resolved.


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The content for this web page was taken from the Dissertation Storm Sedimentation on the Surf Zone and Inner Continental Shelfof Rebecca Lenel Beavers in the Department of Geology of Duke University

This web site was created by Doug Call (Contract Student, University of Virginia) on July 31, 2001.