Edward B. Thornton, Timothy P. Stanton
Office of Naval Research, Coastal Dynamics
US Army Corp of Engineers, Coastal Research and Development Program
Long-term goals are to predict the wave-induced three-dimensional velocity field and induced sediment transport over arbitrary bathymetry in the near shore given the offshore wave conditions. During DUCK94, suspended and bedload sediment transport, hydrodynamic forcing and bathymetry are measured at various locations across the surf zone from a moveable sled.
Direct measurements were made of sediment flux, stress, and dissipation over the vertical. It is planned to measure both suspended and bedload transport. The basic instrument to be used is a Coherent Acoustic Sediment Probe (CASP) operating at 5 MHz in both monostatic and bistatic doppler modes. CASP measures three components of velocity at 4 mm resolution with a 44 Hz response and a 75 db dynamic range. The sediment concentration over the vertical is inferred from the acoustic backscattered signal by range gating (1.2 cm vertical resolution) the three acoustic 5 MHz beams plus an addition 1 MHz beam. The two acoustic frequencies allows sediment size discrimination. The combined three components of velocity and sediment concentration allow calculating the sediment flux. The CASP will be mounted on a specially designed sled to allow making measurements anywhere in the nearshore. The CASP will be attached to a hydraulic, remote controlled arm to position over the vertical to obtain profiles of sediment flux, Reynolds' stresses and dissipation. Also mounted on the sled will be vertical stacks of eight electromagnetic current meters, 8 OBS's (collaborative with White), eight conductivity cells, an array of seven pressure sensors, an acoustic altimeter (collaborate with Elgar), and a scanning acoustic altimeter and video aimed at the bed beneath the CASP.
The CASP will make direct measurements of sediment flux and Reynolds' stresses. The bathymetry beneath the CASP will be measured with the array of four-beams of acoustics and scanning acoustic altimeter. The resolution of the CASP and the nonintrusive nature of the measurement will allow making measurements within the bottom boundary layer and bedload. It is noted that the CASP measures three components of velocity, which are required to resolve the velocity field over the variable bottom. These measurements will be used to test various suspended and bedload sediment transport and boundary layer models and form the basis for future theoretical developments.
The vertical array of eight em's will be used to measure the mean current profiles (collaborative with Guza). It was shown that during DELILAH the mechanism most responsible for the sediment transport based on a Bailard type energetics model was the mean current.
The Coherent Acoustic Sediment Probe (CASP) was calibrated and used during Duck94 to measure sediment flux, three velocity components and Reynold's stresses (Stanton and Kohanowich, 1995). The CASP uses four acoustic beams at two frequencies (1.3 and 5.3 MHz) to measure the backscatter from sediments and infer concentration in 1.7 cm bins out to one meter range. It also measures three velocity components and sediment concentration in the same intersection volume (O(1 cm3)) at a sampling rate of 36 Hz. Linear calibrations are obtained over the range O(1X10-3) to 25 kg/m3 at 1.3 MHz and up to 8 kg/m3 at 5.3 MHz, both at an accuracy of 10%. A grain size discrimination capability, based on comparison of measurements by the two frequencies and the assumption of log-normal sand size distribution, can detect size variation of +/-0.05mm. The CASP was mounted on a remotely controlled boom on the sled to vertically profile the water column. Field data during time of narrow banded, nonlinear wave forcing on 20 Oct. show that episodes of high sediment concentration correlate well with maximum onshore velocities within 2 cm of the bottom, but decorrelate above this level. Data are ensemble averaged with a phase mapping technique to illustrate this correlation (Figure 1). These first time ever direct field measurements of near-bed sediment flux calculations reveal strong shoreward transport within the wave boundary layer, and weaker seaward transport higher in the water column. Similar results were obtained by Ribberink et.al.(1995) in a laboratory water tunnel under sheet flow conditions.
Thornton, Soares and Stanton (1995) show the vertical profile of longshore current measured with a vertical array of 8 electromagnetic current meters is well described by a logarithmic profile for strong longshore current days (Figure 2) during Duck94 (0.98 mean linear correlation coefficient for all profiles). Equating the bottom shear stress determined from the log profile with a quadratic bed shear stress formulation, bed shear stress coefficients, Cf, were calculated. Measured Cf values varied by an order of magnitude across the surf zone. The Cf values were found proportional to the measured rms bed roughness, R, as measured by a sonic altimeter and bed form type identified by a side-scan sonar mounted on the CRAB.
NEED FIGURES from flat scanner
Figure 2. Vertical profiles of mean (60 minute) longshore currents measured (*'s) during time of storm (8m depth significant wave height 2.2m) at Duck94 experiment compared with log profile model (upper panel). Bed forms measured with side-scan sonar mounted on CRAB and bed shear stress coefficients, Cf, measured across the surf zone are indicated. Bottom profile is combined CRAB survey and sonic altimeter mounted on CRAB. Rms bed roughness (calculated every meter averaged over 20m) is shown in lower panel. Cf's vary by an order of magnitude across the surf zone and are directly correlated with rms bed roughness.
Figure 1. Sediment concentration and horizontal velocity measured using Coherent Acoustic Sediment Probe, CASP, during time of long period, narrow-banded waves at Duck94 experiment. a) Averaged (over 80 waves) sediment transport determined using zero-down-cross method and phase mapping for various elevations above the bottom. b) Simultaneously phase mapped horizontal velocities at various elevations. Sediment concentration correlate well with maximum onshore velocities within first 2 cm above bed within the wave boundary layer, but decorrelated above.
Stanton, T.P. and K.M. Kohanowich, 1995, Calibration and Application of an Acoustic Doppler Sediment Flux Meter, submitted to the J. Coastal Engineering. Thornton, E.B., C. Soares and T.P. Stanton, 1995, Bed shear stress coefficients related to bed roughness across the surf zone, submitted to the J. Geophysical Research. Collaboration: Guza, White, Elgar