Jack A. Puleo

Analysis of the scale of errors in nearshore bathymetric data

Most studies of nearshore hydrodynamics, sediment transport, and morphology focus on bathymetric variability within a narrow band of spatial and temporal scales. Typically, these studies rely on bathymetry estimates derived from field observations consisting of discrete samples in space and time with varying degrees of measurement error. Sampling limitations, which result in aliasing, and measurement errors can significantly contaminate variability at resolved scales, and may lead to large errors in the representation of the scales of interest. Using a spectral analysis, interpolation errors were analyzed for three different nearshore bathymetric data sets, each of which targeted a different range of spatial scales. Bathymetric features that were unresolved or poorly resolved (e.g. beach cusps) introduced the potential for contamination in two of the data sets. This contamination was significantly reduced using an appropriate scale-controlled interpolation method, leading to more accurate representations of the actual bathymetry. An additional benefit of using scale-controlled interpolation is that interpolation errors may be estimated independently of actual observations, which allows one to design bathymetric sampling strategies that ensure that dominant scales are either resolved or largely removed. Finally, interpolation errors corresponding to a particular sample design can be used to determine which interpolated values contribute usefully to a band-limited analysis of bathymetric variability.

Estimating swash zone friction coefficients on a sandy beach

Abstract Video-based swash motions from three studies (on two separate beaches) were analyzed with respect to theoretical swash trajectories assuming plane beach ballistic motions under quadratic friction. Friction coefficient values for both the uprush and backwash were estimated by comparing measured swash space–time trajectories to these theoretical expectations given an initial velocity and beach slope. Observations were made spanning high tides, and in one case, during a light rain. Analysis of over 4500 individual swash events showed that the uprush friction coefficient was nearly constant during all three studies with a mean value of roughly 0.007 and showed no trends over a tidal cycle. In contrast, backwash friction coefficient values varied over the tidal cycles ranging between 0.01 and 0.07 with minimum values corresponding to the highest tides. Although these values are close to the theoretical estimates based on a Law of the Wall formulation and values commonly referenced in the literature, these observations show a consistent tendency for backwash friction estimates to greatly exceed uprush friction estimates. The disparity between uprush and backwash friction coefficients can be partially attributed to the exclusion of a pressure gradient term in the ballistic model. However, results indicate that backwash friction coefficients adjusted to account for this effect may be three times larger than the uprush friction values during lower tides. This tidal dependence for backwash friction coefficients is attributed to a complex interaction between swash infiltration and entrained sediment loads. These findings imply that friction estimates (necessary for sediment transport calculations and hydrodynamic predictions) based solely on grain roughness may not be correct for backwash flows.

Quantification of swash flows using video-based particle image velocimetry

Understanding of fluid flows and sediment transport in the foreshore has been severely hampered by the difficulty of obtaining swash flow velocity measurements in this dynamic and extremely shallow region. We present a digital imaging method, known as particle image velocimetry (PIV), to quantify the horizontal flow structure of swash. This technique exploits similar patterns of image intensity in multiple images sampled sequentially to identify spatial offsets corresponding with maximum correlations between image subregions. These offsets are used in conjunction with the sampling interval to derive velocity vectors describing the horizontal flow structure. Pre-processing methods to geo-rectify oblique imagery to a planar surface and post-processing methods of correcting spurious vectors are described. The PIV method overcomes many of the limitations of in situ sampling of swash flows and is shown consistent with results from a previously tested remote sensing technique for measuring swash edge velocities. In general, this technique provides a unique capability for spatially extensive and well-resolved quantification of swash flows.

Developing Terrestrial-LIDAR-Based Digital Elevation Models for Monitoring Beach Nourishment Performance

Abstract Since the completion of a 398,000 m3 nourishment project along 2 km of Rehoboth Beach, Delaware, in August 2005, the subaerial volume and area of the northern 25% of the beach has been monitored monthly through the use of terrestrial-based light detection and ranging (LIDAR) surveys. Traditionally, the Delaware Department of Natural Resources and Environmental Control and the U.S. Army Corps of Engineers use analyses of beach width from aerial imagery and volumes estimated from widely spaced profile surveys to assess nourishment performance and assist in determining renourishment quantities. However, these survey methods lack the spatial and temporal resolution needed for short-term management strategies. Recent efforts at monitoring Atlantic Coast beaches using airborne LIDAR show the potential of this technology for providing more detailed representations of beach volumetric change over time, but the operational costs still limit the frequency of surveys. Alternatively, our terrestrial LIDAR study allows for the development of models of subaerial beach topography with both high temporal and spatial resolution. Although geographically less extensive than airborne surveys, the digital elevation models from our data (1) allow for a better understanding of the range in variation in beach area and volume, especially that due to storm events, (2) provide more accurate volume estimates than traditional profile surveys by as much as 8%, and (3) indicate that the area and volume do not covary, limiting the usefulness of using aerial imagery in estimating volume.

Comprehensive Field Study of Swash-Zone Processes. II: Sheet Flow Sediment Concentrations during Quasi-Steady Backwash

AbstractSheet flow sediment concentration profiles were measured in natural conditions for the first time as part of a comprehensive field study on swash-zone hydrodynamics and sediment transport. Three conductivity concentration profilers (CCPs) measured the sediment concentration profile in the sheet flow layer with a 1-mm resolution in the swash zone of a dissipative beach. This paper focuses on sheet flow during quasi-steady backwash events generated by infragravity motion when the effects of phase lags, surface-generated turbulence, and accelerations are small. The sheet flow sediment concentration profile has a linear shape in the lower section of the profile and a power-law shape in the upper section, with the transition occurring at sediment volume fractions of 0.20–0.30. The shape of the concentration profile is self-similar for measured sheet flow layer thicknesses ranging from 6 to 18 mm. Because of the self-similarity, a single concentration profile curve can be used to describe the normalized...

Quantifying Seasonal Shoreline Variability at Rehoboth Beach, Delaware, Using Automated Imaging Techniques

Abstract We describe a seven-camera video system deployed at Rehoboth Beach on the Atlantic coast of Delaware that was used to monitor and quantify the evolution of a nourished beach. Shorelines along 6 km of beach were automatically identified using pixel-intensity gradients from time-exposure and variance imagery. Correlations between automatically identified and user-defined shorelines from images with various wave and atmospheric conditions exceed r2 values of 0.92. Small variations in camera azimuth and tilt were found to significantly affect apparent shoreline locations and were automatically corrected through image-correlation procedures. Hourly shoreline data were tidally and seasonally averaged to quantify seasonal morphodynamic variability. Over a summer-winter-summer monitoring period, the shoreline exhibited erosional and accretional variations of ∼10 m dependent in the alongshore direction on permanent man-made structures and subject to seasonally characteristic littoral transport variations. A comparison of mean seasonal shoreline locations revealed a mean erosion value of ∼0.8 m between the summer of 2006 and the summer of 2007; this erosion rate is consistent with previous studies of this area. The temporal history of the planform area, obtained from shoreline position, showed weak correlation with volumetric measurements and thus could not be used as a proxy for volumetric change at this site.

The Application of Bagnold-Type Sediment Transport Models in the Swash Zone

Abstract Measurements of cross-shore velocity (u) at z = 3 cm and suspended sediment concentration (SSC) at z = 1–33 cm were collected from the swash zones of gently sloping (Perranporth, UK) and steep (Sennen, UK) beaches, during morphodynamic conditions ranging from highly dissipative to almost reflective and breaker heights ranging from 0.8 to 2.2 m. Nine swash-zone data collection runs were used to compare the performance of simplified versions of Bagnold-type energetics sediment transport models on the two different beaches. Models originally designed for suspended load, where the sediment transport (I) is approximately proportional to u4, showed no apparent improvement over those intended for bedload, where I ∝ u3. It is suggested that the lowest SSC sensors could be measuring high concentrations in the sheet-flow layer, where previous laboratory results suggest a u3 model is most appropriate. The overall performance of the model is better (average r2 = 76%) on the gently sloping beach than on the steep beach (average r2 = 49%). Modifications to account for processes, such as bore turbulence, may improve the models performance, particularly on steep reflective beaches.

A Conductivity Concentration Profiler for Sheet Flow Sediment Transport

A new sensor for measuring sediment concentration under sheet flow conditions is presented. Electrical conductivity, measured using a four-electrode method, is used as a proxy for sediment concentration. The relationship between conductivity and sediment concentration was calibrated using known masses of neutrally suspended sediment in a heavy liquid and agrees well with existing linear and power-law relationships (r2 >; 0.98). A 29-point conductivity profile at 1-mm resolution is generated by multiplexing through a vertical array of 32 plate electrodes. Numerical simulations of the voltage field around the sensor indicate that the horizontal extent of measurement volume is 1.5 times the sensor width. The finite extent of the measurement volume leads to smoothing of the vertical concentration profile. The sensor resolves sheet flow layers with a thickness greater than 3.5 mm, and a correction formula is introduced to correct the measured sheet thickness for the smoothing effect. Initial field results in the swash zone of a natural beach quantify sheet flow processes with unprecedented detail. Short-lived sheet flow with a maximum thickness of 19 mm was observed during the uprush, and a longer duration sheet flow with a maximum thickness of 8 mm was observed during the backwash.

Comprehensive Field Study of Swash-Zone Processes. I: Experimental Design with Examples of Hydrodynamic and Sediment Transport Measurements

AbstractA comprehensive study of swash-zone hydrodynamics and sediment transport was conducted on a macrotidal beach in Perranporth, United Kingdom. The unique study is the first to simultaneously measure suspended sediment and sheet flow sediment concentrations, water depth, near-bed velocity profiles, and high-resolution swash surface and bed-level changes on a natural beach. Data collected during the study are used to quantify the vertical profile of cross-shore and alongshore velocities and the importance of sheet flow sediment processes in the swash zone. The swash-zone boundary layer for cross-shore velocities is observed to generally occur over at least the lower 0.06 m of the water column. Alongshore velocities are often the same order of magnitude as the cross-shore velocities and are dominant near cross-shore flow reversal. Flows are often logarithmic in profile, but the instantaneous nature of the measurements renders application of the logarithmic model difficult. When valid, the logarithmic m...

Tidal Variability of Swash-Zone Sediment Suspension and Transport

Abstract Ensemble-averaged suspended sediment concentrations (SSC) and transport from the swash zone of a high-energy beach are investigated with respect to the time-averaged setup level obtained from calibrated video records. Ensemble-averaged data indicate variability in SSC between both uprush and backwash during rising and falling tides. Mean ebb-tide SSC exceeds mean flood-tide SSC during both the uprush (by 28–34%) and backwash (by up to 25%). Suspended sediment concentration also varies between swash phases over a tidal cycle with mean uprush SSC exceeding mean backwash SSC by 46% to 108% depending on tidal level, with a general percentage decrease during the falling tide. The net ensemble-averaged transport (velocity-SSC product) magnitude displays a decreasing trend from roughly 30 to 20 kg m−1 during the rising tide and from roughly 45 to 15 kg m−1 during the falling tide that cannot be explained based on the percentage of foreshore immersion (water depth) alone. Thus, relative tidal level (cross-shore instrument position within the swash zone) and the phase of tide (rising or falling) within the swash zone play an important role in determining the magnitude of sediment suspension and transport.