Shallow water surface gravity wave imaging, spectra and their use in shallow water dredging operations

Abstract

Imaging of shallow waters using high resolution video imagery is described. Common to mono, stereo and trinocular imaging approaches from ground and airborne platforms is the need to validate the surface water wave field measurements, particularly the amplitude and specular reflectance of water surface small gravity waves. A technique for calibration and validation of water surface gravity wave field energy spectra is described. Results demonstrate the value of video imagery where water level staff gauges with approximately with 0.5 cm wave height accuracy are easily sensed using high definition videography. Essentially, a staff gauge placed in shallow water constructed from PVC materials with custom colored line coding are imaged at 30 H or high frame rates, followed by frame by frame analyses in order to detect the water level measured at 0.5 cm height intervals. The image based time series allow the development of shallow water gravity wave energy spectra using standard FFT analysis procedures. Spectral models based upon peak frequency, for example, are then used in a two dimensional water surface wave simulation model that generates radiative transfer based hyperspectral images of the water surface wave field. The simulated and observed water surface wave patch fields are compared by extracting vertical or horizontal transects within observed and simulated imagery. The approach allows one to developed spectral energy model probability distributions at low cost. The novel noncontact video sensing and image analysis methodology used to calibrate and validate shallow water gravity wave models yield a means for ultimately calculating bottom boundary velocities under measured or simulated wave fields. These boundary layer velocities can cause migration and horizontal particle fluxes (g cm -2 s -1 ), resuspension, settling, and increased turbidity during dredging operations, but not necessarily due to waterway dredging operations and activities.

Video Measurements and Analysis of Surface Gravity Waves in Shallow Water

“…Video imaging of bottom features such as seagrasses and sand bottom features has also been demonstrated using practical imaging techniques [16]. The imaging technique described above is being used to assist in assessing moving fluid mud and muck in Indian River Lagoon and Banana River estuarine areas and tributaries [2]. In many shallow-water estuarine regions, wind-driven gravity waves are responsible for resuspension and transport of muds and decaying organic matter within the lutocline [17,18].…”

“…Sonde array [20] deployment was 16-17 knots, with east winds in a shallow ≈0.7 m water column and 20-30 cm amplitude wind-driven gravity waves at the location in Banana River near Pineda Causeway [2,16] near the Atlantic Ocean in Florida (28 12 0 33.02 00 N, 80 38 0 14.72 00 W). The use of the staff gauge video imaging and Sondes shown in Figure 11 showed that the shallow-water gravity waves increase the bottom water fine particulate transport or flux of 15,071 g m À2 day À1 dry weight-nearly 450 times greater than the surface values of 13.1 g m À2 day À1 .…”

“…A wave energy density spectrum of a wind-driven surface gravity wave patch E can be expressed in units of W=m 2 and is related to the wave height or the variance of the wave displacement, given as E ¼ 1…”

“…ðÞ is the conjugate of Z n (dimensionless), n is the Surface Waves -New Trends and Developments number of the data point in the measured sequence (dimensionless), t is the instant time corresponding to the data point in the unit of second (s), and T is the length of measurement time in the unit of second (s). A two-parameter Weibull distribution function was used as a test model for mathematical expression of the spectrum [2]. The Weibull probability density function (PDF) and cumulative density function (CDF) are given by [9]:…”

“…In water the wind stress or friction causes the downward movement or transport of momentum from the water surface to the water column. The downward transport of momentum causes internal friction in the water in the form of turbulent friction or viscosity, and the associated circular eddy-type motions influence the bottom lutocline within the bottom boundary layer [2,3]. The resulting shearing forces at the bottom in shallow water cause resuspension of and fluidization of bottom sediments and associated nutrients and/or trace metals.…”