W. Frank Bohlen

Stability of giant sand waves in eastern Long Island Sound, U.S.A.

Abstract A combination of a highly accurate bathymetric surveying technique and in-situ submersible observations and measurements were used to assess the migrational trends and morphological changes of large sand waves (Ht ≤ 17 m) in eastern Long Island Sound. Although residing in a high-energy tidal environment characterized by a net westward sediment flux, the large bedforms are relatively stable over the short term. Over a 7 month period, 55.1% of a total 2942 m of sand wave crestline lengths migrated less than the horizontal accuracy limits of navigation (2 m). Approximately 35% of the remaining sand wave crests migrated less than 4 m. Net migration of the sand wave crests in the study area was 0.2 m. In addition, the bulk form (center of area in profile view) or the base of the sand waves showed little, if any, movement. These data, in conjunction with flow data within the sand wave field, suggest that net migration rates are greater than the time span of this study and/or the sand waves move in response to large residual flows created by high-energy, aperiodic storm events. The latter scenerio suggests that day to day processes only serve to rework and modify the sand waves.

Suspended material distributions in the wake of estuarine channel dredging operations

Field sampling of the suspended material field downstream of a large volume bucket dredge operating in the Lower Thames River estuary near New London, Connecticut was conducted in order to examine the magnitude and character of the dredge-induced resuspension and to evaluate typical operational efficiency. These data indicate that approximately 1.5 to 3% of the sediment volume in each bucket-load is introduced into the water column producing suspended material concentrations adjacent to the dredge of 200 mg l −1 to 400 mg l −1 . These values exceed background levels by two orders of magnitude. Analysis of particulate organic carbon and grain size characteristics indicates that resuspension also alters suspended load composition increasing the percentage of inorganic materials and median grain size. Proceeding downstream, material concentrations along the center-line of the dredge-induced plume decrease rapidly approaching background within approximately 700 m. Compositional variations display similar trends with the major perturbations confined to the area within 300 m of the dredge. The observed spatial distributions indicate the dredge-induced resuspension is primarily a near field phenomenon producing relatively minor variations as compared to those caused by naturally occurring storm events. Previous work ( Bohlen & Tramontano, 1977 ) has shown that these latter systems can produce estuary-wide variations in suspended material concentrations, increasing the mass of material in suspension by at least a factor of two. This increase in total suspended load is nearly an order of magnitude larger than that produced by the dredge. These field observations also show that there is a distinct physical similarity between dredge and storm induced resuspension and provide some useful indications of the probable response of the larger scale coastal suspended material field to a variety of natural, high energy disturbances.

The Physical Oceanography of Long Island Sound

Coastal ocean ecosystems are strongly influenced by circulation, tides, waves, and the rates of mixing of the water. Many shoreline communities are increasingly threatened by the same phenomena, most notably through flooding and coastal erosion. In this review we summarize the observations that have been acquired in LIS to describe and explain the magnitude and variability of these physical processes. We also comment on the status of our theoretical understanding of the links between them and some of their consequences. Analysis of available buoy and coastal wind observations suggests that the shear stress due to wind over LIS is under-predicted by a factor of between 2 and 3 if shore station winds are used to make the estimates. This difference is significant to both water quality and wave forecasting. We describe the magnitude of seasonal variations in wind and waves and use long-term records from coastal stations to show that there are decadal-scale variations in both wind speed and directions. Available wave data from two buoys suggest that the wave field is consistent with that predicted by fetch- limited wind forcing. Semi-diurnal tidal sea level variations and vertically averaged currents are well described by theoretical models, however, recent observations show high amplitude over tides in the western LIS that remain to be explained. The vertical structure of the tidal currents is much more complex and a closer examination of model predictions, particularly of the across Sound velocity components, should be conducted. The interaction of the vertical structure of tidal currents and the salinity and temperature distributions may lead to significant heat and salt transport vertically and horizontally. Observations of the mean density, temperature, and salt distributions and the mean circulation in LIS are qualitatively consistent with several models and we summarize the recent work. A more critical evaluation is now appropriate. We also discuss evidence of long-term trends. The role of shorter time-scale meteorological forcing and the bathymetry of the Sound on the structure and variability of the circulation is summarized using observations and simulation. Long-term observations of both hypoxia duration and hypoxia areal extent in western and west central Long Island Sound are analyzed to determine the directional response to wind forcing. We show that a substantial fraction of the inter-annual variability in area and duration can be explained by the directional statistics of wind. Using simulation, we demonstrate that the geometry of the basin and across isobath flow may be significant. The response of the Sound to severe storms is outlined and the technical developments in simulation and observation that are necessary to the improvement of model predictions are suggested.

The Mysound Project: Building an Estuary-Wide Monitoring Network for Long Island Sound, U.S.A.

A network of five water quality monitoring stations has been established in Long Island Sound, measuring temperature, salinity, and dissolved oxygen since 1999. The stations are located in areas of extreme water quality degradation (western Long Island Sound) as well as in pristine areas (eastern Long Island Sound). The data from these stations are collected every 15 minutes and posted to the project web site in real time as provisional data. After subsequent quality assurance procedures, the data are archived to the project File Transfer Protocol (FSP) site for downloading by the user community. The network of stations is in part supported logistically by a number of partners, including state and local agencies, schools, and non-governmental organizations. Data from the monitoring programs of some of these partners are also published to the project website providing a more comprehensive and complete picture of the status of the Sound than can be provided independently. This repository of information is used by marine educators, resource managers, scientists, and the general public, each with a different end purpose. We use the data from two of the stations to show that these high frequency time series measurements can be used to complement and enhance other monitoring programs within the Sound, documenting in greater detail the occurrence and duration of hypoxic events.

Storm enhanced bottom shear stress and associated sediment entrainment in a moderate energetic estuary

Several important mechanisms for storm-induced entrainment of estuarine cohesive sediments are analyzed using field measurements collected in a moderately energetic estuary, central Long Island Sound, U.S.A. The sediment concentration and hydrographic data were obtained by an array of sensors mounted at 1 m above the bottom. The bottom sediment in the study site composed mostly of silt and silty sand. The study showed that the bottom shear stress, computed using a wave-current interaction model, increased significantly during the episodic wind events. A large resuspension event was triggered by a frontal passage when strong wind-driven currents augmented the tidal currents. The timing of storm waves with respect to the tidal phase also was a critical factor. Based on the changes of suspended sediment concentration, the bottom appeared to respond to the shear stress in two phases: the tidal resuspension and the storm-induced erosion. During each tidal cycle, entrainment was associated with resuspension of high water content, loosely consolidated material. During episodic events, a thin layer of more consolidated bed below the sediment-water interface was eroded by the enhanced bottom stress.

FACTORS GOVERNING THE DISTRIBUTION OF DREDGE-RESUSPENDED SEDIMENTS

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.

A Comparison Between Dredge Induced Sediment Resuspension and That Produced by Natural Storm Events

Parameter Identification (PI) algorithm is an optimization procedure that systematically searches the parameters embedded in a mathematical model. These parameters are not measurable from a physical point of view. The optimization is based on the minimization of a selected norm of the differences between the solution of the mathematical model and scattered observations collected from the system. Parameter identification (or inverse problem) has been studied in groundwater systems extensively for the past decade (15), and it has also drawn many researchers in the fields of open-channel flow and estuarine modeling since 1972 (1,2,9,17). All the past estuarine PI works in the literature are confined to the one-dimensional case, and hydrodynamics and transport equations are treated separately. This study deals with PI in a two-dimensional vertically-averaged estuarine salinity model. The salinity transport equation is coupled with the hydrodynamics equations. The coupled relationship introduces extra density terms in the hydrodynamics equations, which must be solved simultaneously with the transport equation. One of the most difficult problems in PI is the collection of needed observations from the system which is being modeled. With limited exception, the currently available data from the prototype estuaries are not adequate for the purposes of developing a PI algorithm. This is usually critical in quantity (the number of stations and/or the period of time) and in quality (noise of data). However, if an operational hydraulic model is available, the data could then be obtained economically and accurately under an ideally controlled environment. The large amount of data that can be collected from a hydraulic model of an estuary will provide a sufficient number of observations and the required initial and boundary conditions for the development of a PI algorithm. The use of the estuary hydraulic model could provide a better source of prototype data than would be available from the real estuary. It will be much easier to distinguish between the inadequacy of the mathematics and the inadequacy of our understanding of the prototype. Thus, it will give us an idea of how well we could expect to mathematically model the real estuary if we had an unlimited amount of prototype data. Additionally, when these types of data are used in PI, parameters can be optimally identified and the mathematical model can then be used conjunctively with the hydraulic model for prototype applications, provided that the mathematical model is consistently formulated. How well a hydraulic model simulates the prototype estuary is not considered in this study.The problem of buoyant surface jet (BSJ) is relevant from the practical standpoint to the discharge of cooling water of power plants into the receiving water bodies. The buoyant surface jet has extensively been studied by numerous investigators both theoretically and experimentally. Most studies have been concerned with the problems of BSJ discharged horizontally into a surface of a deep ambient water with or without cross current. From a practical standpoint, however, the design engineers are often confronted with the design of thermal outfalls in the coastal regions which are frequently shallow and have the boundary effects. Few investigators have studied the problem of BSJ discharged horizontally over slopping bottom into quiescent receiving water (1), (2), (4), (5), (6), and (7). However, no information on the foregoing problem with moving ambient water is available. The purpose of this paper is (a) to present the experimental results of BSJ which is discharged over slopping bottom into moving ambient water, and (b) to see the degree of error which is introduced by applying the deep water integral models to the case of a buoyant surface jet with a bottom boundary.This study deals with the statistical properties of the group formation of random waves determined by the zero-up-cross method. Probability distributions about (1) the run of high waves (2) the total run (3) the run of resonant wave period are derived theoretically providing that the time series of wave height and wave period form the Markov chain. Transition probabilities are given by the 2-dimensional Rayleigh distribution for the wave height train and the 2-dimensional Weibull distribution for the wave period train. And very good agreements between data and the theoretical distributions have been obtained. Then the paper discusses those parameters which affect the statistical properties of the runs and shows that the spectrum peakedness parameter for the. run of wave height and the spectrum width parameter for the run of wave period are the most predominant.One of the characteristics of the North Sea between the British Isles, the Netherlands, Germany and Denmark is the occurrence of heave storm surges especially in autumn and winter with heights of about 4 m above spring highwater. Coastal areas and especially the estuaries of the tidal rivers are hit by these storm surge events. The mean tidal range at the German coast comes to about 3 m with relatively low daily and semimonthly inequalities of less than 0.5m. Within the framework of long-term developments of the navigation channels of the estuaries as well as of the storm surge protection works, physical model tests had to be carried out in order to predict the influences of such measures on the storm surge heights to be expected.A two-dimensional laboratory investigation of sediment transport, induced by shallow-water waves, showed that the sediment motion over suspension-dominant asymmetric ripples is closely related to the development of eroding beaches. High-speed motion picture analysis revealed that vortices, formed over this type of ripple, play a crucial role in transporting the sediment to the offshore region. A relation for net offshore sediment flux was formulated for sand 0.02 cm in diameter. A simple model for eroding beaches was proposed and its validity was checked by using two existing data sets for 0.02-cm sand beaches; the model could predict fairly well profile and shoreline changes in the early stages.Queenslands Beach Protection Act (1968-1974) resulted, inter alia in the formation of the Beach Protection Authority, which is responsible for investigating coastal erosion, planning remedial works, recording and evaluating results of investigations, and various other functions. Control of windblown sand and retention of vegetated and naturally stable coastal sand dunes are valuable means of decreasing coastal erosion and because of this the Authority implements a broad research program into the management of coastal dunes in Queensland. Field trials are carried out at the Authoritys Dune Management Research Station on South Stradbroke Island to determine methods of repairing, stabilizing and managing coastal dunes. The research program conducted so far consists of fifteen separate field trials within four general areas of investigation:- (a) Dune Forming Fences Two trials were installed in blown-out sections of the frontal dune to compare different types of semi-permeable fences (eg. wooden slats) and brush matting (a surface mulch of tree branches) on the basis of their ability to accumulate windblown sand and initiate dune formation. (b) Dune Stabilization Techniques Three trials were installed on bare dunal areas to evaluate methods of temporary sand surface stabilization (organic mulches and spray-on materials) as an aid in establishing dune vegetation. (c) Dune Vegetation Two trials were conducted to produce and compile information on the performance of important or potentially useful dune plants. (d) Plant Nutrition Eight trials using different combinations and rates of fertilizer were used to study methods of improvement of establishment and early growth of dune stabilizing plants, particularly sand spinifex grass (Spinifex hirsutus). Good establishment and rapid early growth is required in order to accelerate, improve, and decrease the costs of, the stabilization process.Construction of the Jebel Ali Port complex, 35 km southwest of Dubai, in the United Arab Emirates, has included the disposal of 110 Million cubic metres of excavated spoil. Disposal of dredged carbonaceous breccia raised particular problems due to the high proportion of fines generated and which, unless properly controlled, could cause wide scale environmental damage in the coastal zone. At the same time it was necessary that acceptable reclamation should be created. An extensive data collection exercise was carried out in order to monitor the geotechnical, hydraulic, meteorological, marine climate and marine biology aspects of the operation.Tung-Kang Fishing Harbor, which is about 16 km to the south of Kaohsiung Harbor, is a river harbor on the south-west coast of Taiwan. This harbor is located at the estuary of the Niu-Pu River, which meets the Tung- Kang River and the Kao-Ping River on the north side, (see Fig. 1) The original north and south jetties were constructed in 1959. Because the entrance is located at the meeting of the three rivers and the water depth at the entrance is shallower than that in the breaking zone, the entrance is easily chocked with sand during the summer season when the south-west wind and waves are strong. Therefore, dredging is always necessary to maintain the required depth. On. the other hand, because of the increasing number of fishing boats and deeper draft, the port cannot function effectively. There-fore, how to keep the required water depth at the entrance and to obtain a wider and stable water basin is an urgent problem with this harbor. Based on the sounding of 1973, the littoral drift is mainly from the south. In the next year the construction of a 176 m long new south jetty was begun to protect the entrance and to facilitate the sedimentation study. In 1975, the Taiwan Fisheries Consultants was appointed to undertake the investigation and long-term planning work. This project includes littoral process study, planning, model test and design. Finally it is recommended that an adequate layout of south and north jetties can solve the problem of accretation of the harbor entrance. The purpose of this paper is to describe some aspects with emphasis on how to prevent the shoaling of the entrance channel located at the meeting of the rivers.