Robert A. Holman

Practical use of video imagery in nearshore oceanographic field studies

An approach was developed for using video imagery to quantify, in terms of both spatial and temporal dimensions, a number of naturally occurring (nearshore) physical processes. The complete method is presented, including the derivation of the geometrical relationships relating image and ground coordinates, principles to be considered when working with video imagery and the two-step strategy for calibration of the camera model. The techniques are founded on the principles of photogrammetry, account for difficulties inherent in the use of video signals, and have been adapted to allow for flexibility of use in field studies. Examples from field experiments indicate that this approach is both accurate and applicable under the conditions typically experienced when sampling in coastal regions. Several applications of the camera model are discussed, including the measurement of nearshore fluid processes, sand bar length scales, foreshore topography, and drifter motions. Although we have applied this method to the measurement of nearshore processes and morphologic features, these same techniques are transferable to studies in other geophysical settings.

Extreme value statistics for wave run-up on a natural beach

Abstract Statistics of wave run-up maxima have been calculated for 149 35-minutes data runs from a natural beach. During the experiment incident wave height varied from 0.4 to 4.0 m, incident wave period from 6 to 16 s, and beach face slope from 0.07 to 0.20. Four extreme statistics were calculated; the maximum run-up height during each run, the 2% exceedence level of shoreline elevation, the 2% exceedence height for individual run-up peaks, and the 2% exceedence level for swash height as determined by the zero-upcrossing method. These statistics were best parameterized when normalized by the incident significant wave height and plotted against the Iribarren number, ξ = β/(H/L 0 ) 1 2 . The swash data (with set-up removed) showed less scatter than total run-up (with set-up included). For Iribarren number greater than 1.5 the run-up was dominated by the incident frequencies, for lower Iribarren number longer period motions dominated the swash. A reasonable value of wave steepness for a fully developed storm sea is 0.025 so that a storm Iribarren number can be estimated as 6.3 times the beach slope. Using this and an offshore design wave height, the included graphs may provide guidance in determining a design run-up height.

Swash zone sediment suspension and transport and the importance of bore-generated turbulence

A study of swash zone sediment transport was conducted at Gleneden Beach, Oregon during February 25–28, 1994. The data collected included suspended sediment concentration (SSC), sea surface elevation, and velocity (initially 4 and 8 cm above the bed) at three cross-shore locations within the swash zone spanning high tides. Ensemble averages of 6, 9, and 12 s duration swash events showed that the uprush suspension was high, concentrated in the leading edge, and nearly vertically uniform above the lower 1–2 cm of the water column. Shortly after the sensors were inundated by run-up, the sediment rapidly settled out of the water. During flow reversal the SSC was small but increased again in the backwash. Backwash vertical profiles were markedly different from uprush profiles with much of the suspension being confined to very near the bed where strong vertical gradients in SSC existed. These marked differences show that the backwash is not simply the reverse of the uprush, implying significant differences in the underlying fluid dynamics and sediment transport mechanisms. Backwash sediment suspension increased with flow duration. However, ensemble-averaged SSC profiles of varying duration showed that the backwash concentrations were not consistent at the same temporal phases, which suggests that water depth, in addition to flow duration, may be a controlling factor. Strong cross-shore gradients in SSC suggest that bore-derived turbulence may affect local sediment transport. Specifically, our data show this bore-generated turbulence (turbulent kinetic energy) directly influences local sediment suspension, hence, standard bed shear (Bagnold-type) sediment transport models may no longer be valid in the vicinity of the bore. In the vicinity of the bore a higher correlation between bore-generated turbulence and suspended sediment transport was found than between a Bagnold-type formulation and suspended sediment transport.

Estimation of wave phase speed and nearshore bathymetry from video imagery

A new remote sensing technique based on video image processing has been developed for the estimation of nearshore bathymetry. The shoreward propagation of waves is measured using pixel intensity time series collected at a cross-shore array of locations using remotely operated video cameras. The incident band is identified, and the cross-spectral matrix is calculated for this band. The cross-shore component of wavenumber is found as the gradient in phase of the first complex empirical orthogonal function of this matrix. Water depth is then inferred from linear wave theorys dispersion relationship. Full bathymetry maps may be measured by collecting data in a large array composed of both cross-shore and longshore lines. Data are collected hourly throughout the day, and a stable, daily estimate of bathymetry is calculated from the median of the hourly estimates. The technique was tested using 30 days of hourly data collected at the SandyDuck experiment in Duck, North Carolina, in October 1997. Errors calculated as the difference between estimated depth and ground truth data show a mean bias of-35 cm (rms error - 91 cm). Expressed as a fraction of the true water depth, the mean percent error was 13% (rms error - 34%). Excluding the region of known wave nonlinearities over the bar crest, the accuracy of the technique improved, and the mean (rms) error was-20 cm (75 cm). Additionally, under low-amplitude swells (wave height H _<1 m), the performance of the technique across the entire profile improved to 6% (29%) of the true water depth with a mean (rms) error of-12 cm (71 cm).

Intertidal beach profile estimation using video images

Abstract In this paper, we present a technique suitable for measurement of intertidal bathymetry over a broad range of length scales (10 1 to 10 3 m) and time scales (days to decades). A series of time-averaged images of the swash zone are used to map contour lines of the beach surface. In each image, contours are identified using bands of maximum brightness associated with breaking waves at the shoreline. By mapping the location of these bands in a sequence of images collected over one tidal cycle, contour maps of the intertidal bathymetry are generated. We expect this technique to work best (smallest absolute error) under waves which are nearly reflective at the shoreline, but break enough to be observed visually. This is typical of a barred beach since the wave height at the shoreline is limited by wave breaking over the bar crest. The ability of the measurements made with this technique to resolve actual beach elevation variation depends on the ratio of the measurement error variance to the true beach elevation variance. Thus, large measurement errors may be compensated by either large tidal ranges or large temporal changes of the beach itself. In a comparison to bathymetry surveyed using a Differential Global Positioning System (DGPS) during the Duck94 experiment, in Duck, N.C., the image-based elevation estimates were well correlated with the actual bathymetry. The deviations (imagebased vs. DGPS measurements) may be partially attributed to effects scaled by wave height at the shoreline, wave-induced setup, and wave height saturation over the sand bar. In particular, setup was important during dissipative conditions. The rms deviation (vertical) between the DGPS and image-based bathymetry was reduced from 0.24 m to 0.06 m by correcting for the systematic deviations due to variations in setup and wave height saturation. Further improvement of the elevation estimates resulted from parameterizing the actual bathymetry with a simple plane beach surface, which reduced random (or unresolvable) measurement errors. This led to estimates of the beach slope that were accurate to within 10% of the actual slope and estimates of the cross-shore location of the mean sea level line accurate to about 0.50 m.

Applying video sensor networks to nearshore environment monitoring

Environmental monitoring is an important emerging application area for pervasive computing. We describe shore-based sensing using standard video cameras and measurement techniques for important variables such as wave and ocean current conditions. We apply networked sensors for monitoring environmentally sensitive beaches and nearshore coastal oceans. We give some steps to improve the Argus video sensor networks functionality to quantify the time-space characteristics of the imaged world. We also discuss future system architectures, on the basis of the experience with a sensor network that have already deployed.

Runup kinematics on a natural beach

Runup kinematics on a gently sloping natural beach are examined with detailed measurements from video images, resistance wires deployed at five elevations (between 5 and 25 cm) above and parallel to the beach face, and pressure sensors located in the inner surf zone. As suggested in a previous study comparing a single-level resistance wire and manually digitized films, runup measurements are sensitive to the sensor elevation above the bed, owing to the elongated shape of the runup tongue. The measured mean runup elevation (setup) and vertical excursion increase as the sensor elevation decreases, with the video-based runup estimates having the maximum means and variances. For the six data runs the average ratios of the videobased setup and significant runup excursion to estimates based on wires elevated 15 cm above the bed are 2.7 and 1.5, respectively. These trends, combined with the high coherence and small phase difference between the video and the lowest wire, demonstrate that the video-based estimates correspond to a very near-bed (less than a few centimeters elevation) wire measurement. The measured increase in runup excursion with decreasing sensor elevation and the cross-shore variation in the amplitudes of pressure fluctuations at infragravity frequencies, are consistent with the theory for linear, in viscid, normally incident standing waves. For example, valleys in the pressure spectra occur at approximately the predicted standing wave nodal frequencies. Also in accord with small-amplitude wave theory, observed swash excursions are nearly identical to pressure fluctuations at the location of the measured runup mean (for pressure sensors located seaward of the most offshore bed-level rundown). However, at very low frequencies, where reflection is typically assumed complete and dissipation negligible, the observed, near-bed swash magnitudes are overamplified relative to a best fit of the linear standing wave model based on the amplitude and phase of the seaward observations.

Storm-induced response of a nearshore-bar system

Abstract A nearshore-bar system was surveyed periodically through a storm and the following recovery period. The data showed a very rapid response of morphology to changing wave conditions and allowed various models on bar formation to be tested. Under low-energy conditions prior to the storm a small bar was surveyed 13 m offshore. Both the high reflectivity of the beach and the cross-shore distance to the bar are consistent with a model of sediment convergence at the node or antinode of a standing wave of incident period. Such a small-scale bar may be a common feature on beaches with steep foreshores and more gentle offshore slopes. With the increase in wave height during the storm, the bar became better developed and migrated offshore at rates up to 2.2 m h−1. The bar maintained its form in that the ratio of trough depth to crest depth ( h t h c ) remained roughly constant. The bar was in no way related to processes which would cause the convergence of sediment in the breaker zone; through most of the storm the bar-crest distance offshore was typically only 10% of the surf-zone width. Analysis of the bar distance offshore in terms of a standing wave motion showed that the causative wave period must have been much longer than that of incident waves, probably on the order of a minute. Surf-zone wave data showed significant energy in the infragravity band at these periods although no definite link has been made. After the height of the storm, the bar had a crescentic morphology. The development of this morphology occurred very rapidly with parts of the bar migrating onshore at rates up to 1.2 m h−1. In contrast to the storm, during the recovery period h t h c varied by nearly a factor of three. Analysis of the offshore and longshore length scales showed the bar to be similar to one which would be generated by a standing mode 1 edge wave of period on the order of one minute.

SWASH ON STEEP AND SHALLOW BEACHES

This report will update the coastal zone practitioner on the National Flood Insurance Program (NFIP) as it affects the implementation of manmade changes along the coastline. It is our intent to place in proper perspective this fast-changing and often difficult to interpret national program. Readers will achieve an overall understanding of the NFIP on the coast, and will be in a position to apply the programs requirements in their efforts. We will begin with a history of the application of the NFIP to the coastal zone. The history of the problems encountered will lead into current regulations, methodologies, and the changes the Federal Emergency Management Agency plans for the future.The spatial variability of the nearshore wave field is examined in terms of the coherence functions found between five closely spaced wave gages moored off the North Carolina coast in 17 meters depth. Coherence was found to rapidly decrease as the separation distance increased, particularly in the along-crest direction. This effect is expressed as nondimensional coherence contours which can be used to provide an estimate of the wave coherence expected between two spatial positions.Prediction of depositional patterns in estuaries is one of the primary concerns to coastal engineers planning major hydraulic works. For a well-mixed estuary where suspended load is the dominant transport mode, we propose to use the divergence of the distribution of the net suspended load to predict the depositional patterns. The method is applied to Hangzhou Bay, and the results agree well qualitatively with measured results while quantitatively they are also of the right order of magnitude.

Wave run-up on a high-energy dissipative beach

[1] Because of highly dissipative conditions and strong alongshore gradients in foreshore beach morphology, wave run-up data collected along the central Oregon coast during February 1996 stand in contrast to run-up data currently available in the literature. During a single data run lasting approximately 90 min, the significant vertical run-up elevation varied by a factor of 2 along the 1.6 km study site, ranging from 26 to 61% of the offshore significant wave height, and was found to be linearly dependent on the local foreshore beach slope that varied by a factor of 5. Run-up motions on this high-energy dissipative beach were dominated by infragravity (low frequency) energy with peak periods of approximately 230 s. Incident band energy levels were 2.5 to 3 orders of magnitude lower than the low-frequency spectral peaks and typically 96% of the run-up variance was in the infragravity band. A broad region of the run-up spectra exhibited an f -4 roll off, typical of saturation, extending to frequencies lower than observed in previous studies. The run-up spectra were dependent on beach slope with spectra for steeper foreshore slopes shifted toward higher frequencies than spectra for shallower foreshore slopes. At infragravity frequencies, run-up motions were coherent over alongshore length scales in excess of 1 km, significantly greater than decorrelation length scales on moderate to reflective beaches.