Robert J. Byrne

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.

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.

FIELD STUDY OF BREAKING WAVE CHARACTERISTICS

The equations describing conservation of mass, momentum and energy in a turbulent free surface flow are derived for a controle volume extending over the whole depth. The effect of the turbulent surface oscillations are discussed but neglected in the following analysis, where the equations are applied to the energy balance in a surf zone wave motion. This leads to results for the wave height variation and the velocity of propagation. The results cannot be reconciled completely with measurements and the concluding discussion is aimed at revealing how the model can be improved.A three-dimensional morphodynamic model of sequential beach changes Is presented. The model Is based on variations in breaker wave power generating a predictable sequence of beach conditions. The spectrum of beach conditions from fully eroded-dissipatlve to fully accreted reflective is characterised by ten beach-stages. Using the breaker wave power to beach-stage relationship the model Is applied to explain temporal, spatial and global variations In beach morphodynamlcs.The agents of initial damage to the dunes are water, which undermines them, and animals (including man) which damage the protective vegetation by grazing or trampling. Of these, man has recently assumed predominant local importance because of the popularity of sea-side holidays and of the land-falls of certain marine engineering works such as oil and gas pipelines and sewage outfalls. The need is therefore increasing for active dune management programmes to ensure that under these accentuated pressures, the coast retain an equilibrium comparable with that delicately balanced equilibrium which obtains naturally at a particular location.

CHANNEL STABILITY IN TIDAL INLETS: A CASE STUDY

The steady state profile of the longshore current induced by regular, obliquely incident, breaking waves, over a bottom with arbitrary parallel bottom contours, is predicted. A momentum approach is adopted. The wave parameters must be given at a depth outside the surf zone, where the current velocity is very small. The variation of the bottom roughness along the given bottom profile must be prescribed in advance. Depth refraction is included also in the calculation of wave set-down and set-up. Current refraction and rip-currents are excluded. The model includes two new expressions, one for the calculation of the turbulent lateral mixing, and one for the turbulent bottom friction. The term for the bottom friction is non-linear. Rapid convergent numerical algorithms are described for the solution of the governing equations. The predicted current profiles are compared with laboratory experiments and field measurements. For a plane sloping bottom, the influence of different eddy viscosities and constant values of bottom roughness is examined.The calculation of turbulent flow using Naviers equations assumes the introduction of a turbulent viscosity coefficient the value of which is normally constant, conforming with Boussinesqs hypothesis. It was shown that setting aside this hypothesis, a velocity profile quite different to that resulting from the classic theory is obtained in the case of flow induced by wind. This result appears to be confirmed by the tests carried out in the Mediterranean. The advantage of this method is that it gives the vertical turbulent diffusion which is of particular interest to pollution studies.In the numerical method of prediction of wind waves in deep water, Hasselmanns nonlinear interaction theory is applied. This method assumes the energy balance of individual component waves. However, the total energy balance must exist in the transformation of irregular waves in shoaling water. In this investigation, experiments were carried out on the transformations in shoaling water of composite waves having two components and random waves having one or two main peaks. It was found that the elementary component wave height of the composite waves and the elementary peak power of the random waves decrease with decrease in the water depth. This reason can be explained qualitatively by the theory of the elementary component wave height change of finite amplitude waves in shoaling water. The secondary component wave height of the composite waves and the secondary peak power of the random waves increase with decrease in the water depth. This can be explained qualitatively by Hamadas theory of nonlinear interaction in uniform depth.Experiments have been carried out by using non-breaking waves and breaking waves to investigate the wave forces on a vertical circular cell located in the shallow water. Based on the experimental data, the drag coefficient and the inertia coefficient of a circular cylinder and the curling factor of breaking waves are estimated, and the computation methods of wave forces are examined. As a result, it is shown that the phase lag of inertia forces behind the accelerations of water particles should be considered for the estimation of the drag coefficient as well as the inertia coefficient. In addition the previous formula of the maximum breaking wave forces acting on a cell or a pile is revised by introducing the effects of the above-mentioned phase lag and another phase difference, both of which are functions of the ratio of the cell diameter to the wave length. • It is confirmed that the proposed formula is applicable even to the large cell with the diameter comparable to the wave length. INTRODUCTION Many studies have been done on the impulsive pressures acting on a vertical wall, but there has been very little investigation of breaking wave forces on a cell-type structure. The breaking wave forces should be taken into consideration all the same in the design of pile-type or cell-type structures in nearshore area, because breaking waves cause extreme shock pressures on a cell structure asThe air bubble plume induced by the steady release of air into water has been analyzed with an integral technique based on the equations for conservation of mass, momentum and buoyancy. This approach has been widely used to study the behavior of submerged turbulent jets and plumes. The case of air-bubble induced flow, however, includes additional features. In this study the compressibility of the air and the differential velocity between the rising air bubbles ,and the water are introduced as basic propertie s of the air bubble plume in addition to a fundamental coefficient of entrainment and a turbulent Schmidt number characterizing the lateral spreading of the air bubbles. Theoretical solutions for twoand three-dimensional air-bubble systems in homogeneous, stagnant water are presented in both dimensional and normalized form and compared to existing experimental data. The further complication of a stratified environment is briefly discussed since this case is of great practical interest. This paper is to be considered as a progress report, as future experimental verification of various hypotheses is needed.

An Inexpensive, Fast Response Current Speed Indicator

A low cost, fast response, ducted current speed sensor with digital readout was developed. The sensor output is linear over the calibration range of 0 to 150 cm sec−1. Threshold speed is about 1.5 cm sec−1. Field usage indicates the device will be useful for shallow water applications in hydraulics and ecology.

Shoreline erosion: a problem in environmental management

Abstract Finding the most satisfactory solution to a shoreline erosion problem, particularly one affecting several owners of private property, is a complex task. In addition to the expected technical‐physical analysis of the shore, an economic assessment of the direct and indirect costs of and benefits accrued due to each of several alternative measures or options is required. The efficiency of shore protection is significantly enhanced if entire shoreline reaches are managed with a coordinated strategy rather than with unrelated actions.

An economic analysis strategy for management of shoreline erosion

Abstract An economic framework for making management decisions on tidal shoreline erosion is provided. It provides for evaluation of the impacts of coastal erosion on shoreland property and structures. Coefficients for estimating erosion‐induced losses to property and dwellings were determined. Lots which contained a dwelling decreased in value by an average of

Surficial Sediments, Chesapeake Bay, Virginia: ABSTRACT

0.24 for each square foot of soil lost to erosion. Decrease in value of dwelling due to erosion‐induced loss of distance between dwelling and shoreline was an average decrease of

FIELD STUDY OF A TIDAL INLET, BIMINI, BAHAMAS

8.64 per foot of distance lost. These values plus other measures were applied to three case study coastal areas in Virgina. The evaluation procedure provided a realistic basis for comparisons: (1) of benefits and costs of selected erosion control measures; (2) between various levels of control; and (3) distribution of costs of controls among property owners and between the private and public sectors.

Processes Affecting Recent Estuarine Stratigraphy

Surficial sediments, Chesapeake Bay, Virginia, are significantly sandier than previously reported. Sixty-five percent of the area as determined from 2,000 grab samples are sands when plotted on the ternary sand:silt:clay diagram. The mean graphic-mean is 3.17^phgr. Distribution of sediments is, in large part, a function of geomorphology with an apparently good correlation between depth and sediment type. Finer grained sediments are usually confined to the deeper channels. The exceptions to the depth-size relationship are the presence of fines in the shallow, marginal embayments such as Mobjack Bay and the absence of fines in the deep channel in the southeastern section of the bay. The occurrence of sands here is a function of infilling with sands from the area of the bay outh and, perhaps, of scour into older (Pliocene?) materials. Sediment distribution also reflects the local source with the shallow-water marginal sands derived from erosion of the banks and relict features. Several large geomorphic features are distinguishable on the maps of sediment characteristics. These features include the deep channels, a large sand shield near Tangier Island, relict spits, the zone of influence of the bay mouth, and the possible existence of an ancient channel extending from Mobjack Bay. The number of samples in this study is an order of magnitude greater (2,000 versus 200) than previous studies, allowing a significantly better delineation of sediment types. 900 samples, biased away from the coarser sands, were analyzed for total carbon, organic carbon, and sulfur contents. There are strong correlations between these characteristics and sediment type, especially weight percent clay. Additionally, there is a good relationship between the organic carbon and sulfur content. Total carbon content reached 10% in some samples, however, the average was 1.5%. Average organic carbon and sulfur contents were 1.0 and 0.34%, respectively. End_of_Article - Last_Page 582------------