John D. Boon

On basin hyposmetry and the morphodynamic response of coastal inlet systems

Abstract The exchange of water and entrained material between coastal basins and the inner continental shelf reduces to the problem of polarized transport (flood +, ebb —) in the conveyance channels routing flow through a coastal inlet. Here the net long-term movement of materials is largely a function of the fluid velocity and discharge — variables whose time history at-a-station contains both periodic and aperiodic elements. In this paper the importance of the major periodic elements in coastal inlet flows and their potential contribution to the net transport of bedload materials is discussed. With the aid of a numerical model featuring a closed hypsometric (area-height) representation of basin storage-volume—channel-flow relationships in a typical basin and inlet system at Wachapreague, Virginia, on the East Coast of the United States were studied to determine present mechanisms for inducing net flood or ebb bedload transport. The basin hypsometry and channel dimensions of the prototype system were then varied in the model to simulate other conditions that may have prevailed during earlier stages of basin evolution. Given a sine wave input to the model, the principal lunar semidiurnal constituent (M2) and its first-harmonic overtide (M4) respond in a predictable way to differences in basin hypsometry and channel configuration, and these constituents in turn account for distinctive rise and fall duration differences observed in the mean tide within the basin. These distortions in basin tides are also reflected at-a-station in the time histories of channel discharge and velocity in the form of greater peak magnitudes during ebb or flood depending upon which phase has the shorter duration, and depending in some instances on a further modulation by basin hypsometry and channel cross-sectional area. A consistent imbalance between peak flows of opposing direction is a plausible mechanism for long-term, net transport of bottom sedimentary materials. Thus it appears that a systematic contribution toward either flood or ebb dominance in channel flows can arise through complex periodic tides that exist as a result of specific morphological features in basin and inlet systems. The primary significance of these periodic mechanisms may lie in their apparent sensitivity to changes in basin hypsometry.

Modes of cross-shore sediment transport on the shoreface of the Middle Atlantic Bight

The mechanisms responsible for onshore and offshore sediment fluxes across the shoreface zone seaward of the surf zone were examined in a 3-year field study. The study was conducted in the southern part of the Middle Atlantic Bight in the depth region 7–17 m using instrumented tripods supporting electromagnetic current meters, pressure sensors, suspended sediment concentration sensors, and sonar altimeters. The observations embraced fairweather, moderate energy, swell-dominated, and storm conditions. Cross-shore mean flows ranged from near zero during fairweather to > 20 cm s−1 during the storm; oscillatory flows were on the order of 10 cm s−1 during fairweather and 100 cm s−1 during the storm. Suspended sediment concentrations at about 10 cm above the bed were 5 kg m−3 during the storm. Three methods were applied to evaluate the relative importance of incident waves, long-period oscillations, mean flows and gravity in effecting shoreward or seaward sediment flux: (1) an energetics transport model was applied to instantaneous near-bottom velocity data, (2) higher moments of near-bottom flows were estimated and compared, and (3) suspended sediment fluxes were estimated directly from the instantaneous products of cross-shore velocity and suspended sediment concentration. The results show that measurable contributions were made by all four of the processes. Most significantly, mean flows were seen to dominate and cause offshore fluxes during the storm and to contribute significantly to onshore and offshore flux during fairweather and moderate energy. Incident waves were, in all cases, the major source of bed shear stress but also caused shoreward as well as seaward net sediment advection. Low-frequency effects involving wave groups and long-period waves made secondary contributions to cross-shore sediment flux. Contrary to expectations, low-frequency fluxes were just as often shoreward as seaward. Whereas cross-correlations between suspended sediment concentration and the instantaneous near-bottom current speed were high and in phase under storm conditions, they were weak and out of phase during fairweather conditions. This suggests that simple energetics models are probably inadequate for predicting fairweather transport of suspended sediment.

Evidence of Sea Level Acceleration at U.S. and Canadian Tide Stations, Atlantic Coast, North America

ABSTRACT Boon, J.D., 2012. Evidence of sea level acceleration at U.S. and Canadian tide stations, Atlantic Coast, North America. Evidence of statistically significant acceleration in sea level rise relative to land is found in a recent analysis of monthly mean sea level (mmsl) at tide stations on the Atlantic coast of North America. Serial trend analysis was used at 11 U.S. Atlantic coast stations and 1 Canadian station (Halifax, Nova Scotia) with record lengths exceeding 75 years to examine change in the linear trend rate of rise over time. Deriving trend estimates that apply in the median year of fixed-length mmsl series, reversals in rate direction (increasing or decreasing) were observed around 1939–40 and again in the mid-1960s except at the northeasternmost stations in the latter period. What has not been observed until recently is a sharp reversal (in 1987) followed by a uniform, near-linear change in rise rate that infers constant acceleration at eight mid- to NE Atlantic tide stations, change not seen at SE U.S. Atlantic stations. Quadratic regression and analysis of variance applied to mmsl series over the last 43 years (1969–2011) confirms that addition of a quadratic term representing acceleration is statistically significant at 16 tide stations from Virginia to Nova Scotia. Previous quadratic model studies have focused on sea level series of longer spanning periods with variable serial trends undermining quadratic expression of either accelerating or decelerating sea level. Although the present 43-year analysis offers no proof that acceleration will be long lived, the rapidity of the nascent serial trend increase within the region of interest is unusual. Assuming constant acceleration exists and continues, the regression model projects mmsl by 2050 varying between 0.2 and 0.9 m above mean sea level (MSL) in the NE region and between −0.3 and 0.4 m above MSL in the SE region.

The measurement of constituent concentrations in nonhomogeneous sediment suspensions using optical backscatter sensors

Abstract A method is proposed for estimating the concentrations of the constituents of nonhomogeneous sediment suspensions using optical backscatter sensors. The method is predicated on the assumption that constituent interaction (e.g. grain shielding and multiple scattering) does not significantly affect sensor response. This fundamentally linear approach was verified by laboratory tests of a backscatter sensor in a range of silt/sand mixtures. Implementation of the method will require further development of the optical backscatter sensor or possibly the pairing of an acoustic backscatter sensor with an optical sensor. The method has the potential for resolving small-scale spatial and temporal changes in the suspended-sediment population, and therefore could be employed in studies of winnowing and bed armouring, for example.

Spatial variability of bottom types in the lower Chesapeake Bay and adjoining estuaries and inner shelf

Abstract The spatial distributions of the bed textural and morphologic properties that influence boundary-layer roughness characteristics in the lower Chesapeake Bay, the lower portions of the York, James and Elizabeth Rivers, and the adjacent inner continental shelf were systematically mapped. A high resolution, fully-corrected side-scan sonar mapping system (100 kHz) was used for remote acoustic detection of bottom roughness, supported by ‘ground-truthing’ by direct in situ observations by divers. These complementary methods proved to be especially effective in detecting a wide range of roughness-controlling bed surface properties at various scales. Fine-scale variations in sediment size and associated bottom texture are considered to be the main source of heterogeneity in Nikuradse (skin friction) roughness. A wide variety of small- and intermediate-scale morphologic elements provide meso-scale and small-scale distributed (form drag) roughness. Depending upon location, the distributed roughness may be either biogenic or hydrodynamically induced (by currents and waves), although anthropogenic roughness prevails in certain instances (e.g. port areas). In terms of particular combinations of roughness scales and types, combined sonar and diver observation data allow the beds to be systematically but qualitatively classified into 10 bottom types, each of which is associated with a particular type of subenvironment.

The morphodynamic effects of incident wave groupiness and tide range on an energetic beach

Abstract The roles played by daily tide range and incident wave groupiness in affecting the short-term morphodynamic responses of an energetic beach are examined. Demodulation of original tide and wave time series from the moderate energy Narrabeen Beach, Australia, resulted in new time series of daily tide range and a daily grouping factor which expresses the relative amplitude of the alternation between groups of high and low waves. Using these new time series together with time series of daily beach state and changing breaker conditions, statistical analyses were performed aimed at determining the contributions made by tide and groupiness in explaining time-varying beach state additional to those made by time-varying wave conditions. Eigenvector analyses of monthly surveyed beach and surf zone profiles provided a set of time series on modes of profile configuration change which were also analyzed to determine the relative effects of waves and tide range on profile shape. Both tide range and groupiness are significant determinants of beach state. The six beach states are reflective, low-tide terrace, transverse bar and rip, rhythmic bar and beach, longshore bar trough, and dissipative (Wright and Short, 1984). Provided that wave conditions are within the appropriate range to favor beach states at the reflective end of the sequence, spring tides will tend to favor the low-tide terrace over the transverse bar and rip state. Higher tide ranges are also associated with more subdued bar-trough topography; the more accentuated bar-trough profiles are developed when tide range is minimal. Overall, higher relative incident wave groupiness favors the more dissipative states. Incident wave groupiness is the dominant factor determining whether the low-tide terrace state or transverse bar and rip state prevails, provided incident wave steepnesses permit one or the other of these states. Groupiness is also important in discriminating between the transverse bar and rip and rhythmic bar and beach states.

Acoustic-microwave water level sensor comparisons in an estuarine environment

Microwave water level sensors offer certain advantages over the acoustic sensor, the present standard for water level measurements obtained in U.S. coastal areas by the National Oceanic and Atmospheric Administration (NOAA). These include high reflectivity of microwave radiation from the target medium (water), low sensitivity to variations in air temperature and humidity, and open-beam transmission eliminating any contact between the device and the water. The latter feature has raised the question of possible interaction between time-of-flight microwave measurements and wind wave motion at the air-water interface. A field comparison between a microwave sensor and the NOAA acoustic water level sensor at Yorktown, Virginia revealed close agreement between sensor measurements in an operational setting and produced no evidence of an dasiaoffsetpsila in the presence of irregular surface gravity waves. However, unlike the acoustic sensor which has a mechanical filter (stilling well) to eliminate wave motion above a fixed dasiacutoffpsila frequency, microwave sensors operate without a stilling well and require numerical filtering to obtain water level measurements in the frequency range of interest; i.e., tidal and sub-tidal frequencies for the classic dasiatide stationpsila. Numerical methods now offer greater choice in deciding where to make the cutoff while reducing measurement error.

Chapter 7 Sediment Transport Processes in Coastal Lagoons

A review of sediment transport in lagoons provides a better understanding of how processes act to modify, retain and accumulate sediment. The lagoon transport system is examined as a series of processes that distribute fine-grained sediment between sources and sinks. The processes cycle sediment from one part of a lagoon to another with small amounts being added intermittently from diverse sources to balance amounts removed from the system or that go into storage. Residual transport of fine suspended sediment is regulated by tidal pumping, shear transport or time-flow asymmetry. During transport and recycling, fine particles are modified by aggregation, break-up and reforming. After deposition, benthic fauna further modify the sediment by changing its stability, geotechnical properties, and erosion resistance. Additionally, wind waves winnow fines from shoals thus modifying textural characteristics, while tidal mechanisms have selective effects on the particle composition and size distributions. Lagoon sinks incorporate a number of fill components reflecting multiple sources and fluctuations in energy dissipation interacting on the sediment supply. Climate mainly influences the source material and the sediment character of intertidal zones. Although sediments are extensively modified, recycled and reworked, especially by storms, lagoons primarily function as net sediment sinks in which the accumulation rates adjust to submergence. Sediment processes are a crucial link to understanding the fate of materials in lagoons since they modulate the chemical reactivity and biological productivity of lagoons. Our knowledge, however, is still imperfect and sediment processes therefore warrant increased study and scrutiny.

Nonlinear Change in Sea Level Observed at North American Tide Stations

ABSTRACT Boon, J.D. and Mitchell, M., 2015. Nonlinear change in sea level observed at North American tide stations. The rate at which coastal sea level is expected to rise or fall is of considerable interest to coastal residents and managers who view changes on the time scale of a 30-year mortgage. Analysis of historical records at North American tide stations provides evidence of recent nonlinear sea-level change at this scale using relative mean sea-level (RMSL) observations. RMSL tracks local inundation risk directly without the need to correct an accepted worldwide geocentric measure—e.g., global mean sea-level rise—with locally estimated vertical rate adjustments. Published RMSL linear trends provide essential information but are routinely compared between tide stations with widely varying record lengths, thereby obfuscating nonlinear change (acceleration or deceleration) over a specific period of time. Here monthly averaged RMSL data from 45 U.S. tide stations and one Canadian tide station are analyzed from 1969 through 2014, extending a definitive period of acceleration previously noted along the U.S. NE Coast. Using a Bayesian approach to determine the joint probability of paired regression parameters for RMSL quadratic trends, probabilities for forward projections to the year 2050 based on these trends suggest continued sea-level rise will be aided by acceleration presently on the order of 0.1 to 0.2 mm/y2 in the U.S. NE and Gulf Coast regions. Deceleration ranging from −0.1 to −0.4 mm/y2 is likely to reinforce falling sea levels at specific locations on the U.S. West Coast in the near term.

CARIBBEAN BEACH-FACE SLOPES AND BEACH EQUILIBRIUM PROFILES

The Labrador Sea Extreme Waves Experiment (LEWEX), is an international basic research programme concerned with full-scale measurements, analysis, modelling and simulation in test basins of 3-dimensional seas. The research is carried out in order to assess the significance of 3-dimensional sea states in engineering applications. The first phase of the programme full scale wave measurements in the North Atlantic Ocean was performed at a site and time that had a high probability of encountering severe sea states. The present publication shows examples of measured bi-modal directional sea spectra obtained with the WAVESCAN buoy and directional sea spectra measured with an airborne Synthetic Aperture Radar (SAR). Directional spectra of gravity waves are obtained with the SAR both in open waters and below an ice cover. Further work is needed in order to verify SAR-measurements with in-situ observations. In-situ measured directional spectra are also compared with hindcast spectra from the 3G-WAM model. Hindcast significant wave heights were found to be lower than the in-situ measurements.The littoral drift model developed at DHI and ISVA, see Deigaard et al. (1986b) has been extended to include the effects of the irregularity of the waves, of a coastal current and a wind acting on the surf zone. Further, a mathematical model to simulate the near-shore current pattern along a barred coast with rip channels has been developed. The influence on the littoral drift of the irregularity of waves, wind, coastal current, and rip channels is discussed. It is concluded that irregularity of waves and presence of rip channels must be considered while coastal current and wind action are of minor importance.At present, the Prodhoe Bay oil field in Alaska contributes a substantial amount of the domestic oil production of the United States. Oil is also expected to be present on the continental shelf of Alaska, and it is estimated that approximatedly 28 percent of the total U. S. reserve is located beneath the shallow ice covered seas of the Alaskan continental shelf. To expolre and to exploit these oil rich resources, engineers are confronted with hostile oceanographic conditions such as high tides, waves, strong currents and sea ice. The same area is also rich in fishery resources. Being one of the most productive fishing grounds in the nothern Pacific, the potential ecological impact due to an oil spill is of a major concern. This paper describes the methologies used for the development of a modeling system for the oil risk analysis. The system is designed with generality in mind so it can be used for other coastal areas. The development of three dimensional models used in the modeling system described here have been published in the earlier International Coastal Engineering Conferences (Liu and Leendertse, 1982, 1984, 1986) and a report published recently by RAND (Liu and Leendertse, 1987). In the oil-spill risk analysis, these three dimensional hydrodynamic models are coupled to a two-dimensional stochastic weather model and an oil weathering model.The two projects (LUBIATOWO 79 and LUBIATOWO 86) were aimed at study pore pressure behaviour in natural sand bed in the coastal zone of the Baltic sea under the action of storm waves. During both projects, the wave induced pore pressures at the various levels in the sea bed were measured. The collected data were used to verify the applicability of the various theoretical approaches. In the conclusion, the range of the application of the particular analytical method is given.The rapid recession of the shingle bank of Hurst Beach (up to 3.5m/yr) makes it an excellent natural laboratory for the study of the factors which influence the stability of shingle beaches. Studies have included: the significance of long period, high energy, swell waves the classification and quantification of overwash processes run-up and seepage characteristics the effect of settlement of the underlying strata and the implications for practices in shingle nourishment. The studies have revealed the distinctive character of shingle beaches as compared with the more fully researched sand beaches. More detailed research on shingle beaches is justified particularly in relation to (i) the run-up characteristics including its interaction with swash cusps and (ii) the influence of the subsidiary sand fraction on the beach characteristics.Environmental assessment, engineering studies and designs were completed for a new 26.5 m3/s seawater intake system in the Persian Gulf. The original intake facility consisted of a curved, 60m breakwater with one end attached to the shoreline, a settling basin immediately adjacent to the shoreline and dredged to a maximum depth of approximately 5m, and a pumphouse structure located on shore such that the seaward wall formed one side of the settling basin. The facility located on an island in the Gulf, which served multiple seawater uses, had experienced both structural and operational problems, the latter consisting principally of excessive ingestion of sediment and seaweed. These factors plus the requirement for additional demands for seawater beyond plant capacity caused the owner to initiate a study of alternative intake systems, produce a design for the most effective solution and construct the new intake system.A two-dimensional wave prediction model suitable for use on personal computers is described. The model requires the twodimensional time-dependent wind field as input. Output consists of wave height, wave period, and wave direction estimates at all grid points on a computational grid representing an enclosed or semi-closed basin. Model predictions compare favorably with observations from a wave research tower in Lake Erie. A formula is provided to estimate how long a model simulation would take on a personal computer given the surface area of the computational domain, the grid size, and the computer clock speed.Cullera Bay is a neritic ecosystem placed on the Spanish Mediterranean Littoral largely influenced by the Jucar River, that brings about lower salinities than surrounding waters, and broad variations of its values. An extensive research, with 9 samplings throughout the year, was carried out, measuring both physical and chemical parameters, and the planktonic communities. The trophic status of the ecosystem, the spatial and temporal variations of the nutrients and the planktonic communities were studied, evaluating the influence of the river loads and the littoral dynamics. Some essential basis to allow a suitable emplacement of waste waters disposals along the Valencian littoral are set up in order to minimize the gradual eutrophication of this coast.In the last two years a whole of studies was realized in order to determine precise solutions to the regeneration of Villajoyosas beach, in the Spanish mediterranean coast. Investigations were carried out to the surrounding coastal areas based in field investigations and laboratory analyses of the beaches materials.