Onshore Direction Research Articles

Sediment transport in the Bristol Channel: A review

Investigations into the physical oceanographic characteristics of the Bristol Channel and Severn Estuary are reviewed; these include the general pattern of water movement and the influence of high freshwater discharges on the salinity distribution. Various approaches to the problem of defining sediment transport paths in the macrotidal estuarine system, for fine grained and coarse grained material, are summarised. Transport patterns are examined, for example, through: the distribution of surficial sediments and their associated bedforms; seabed drifter studies; the analysis of tidal current and wave data, separately and in combination, because of their influence on bedload transport paths; satellite imagery, within the visible part of the spectrum; and on the basis of output from depth-averaged numerical models. Comparison between the interpretations shows that there are discrepancies and contradictions in the derived sediment transport paths. In particular, it is difficult to explain the up-channel movement of fine grained material against a background of inferred offshore movement, shown by the orientation of sandwaves and the direction of movement of the surface waters. An early model to overcome this contradiction incorporated ‘two-way differential transport’ throughout the water column. Localised contradictions in the transport paths for sand and ‘fine sediment’ occur in the inner part of the Bristol Channel. The most recent conjectural models for sediment movement incorporate an ebb- dominated zone along the central axis of the Channel, together with flood-dominated coastal zones. This lateral variation in transport paths, across the Channel, constrasts with the earlier proposition for vertical differentiation; it is suggested by seabed drifter recovery patterns and confirmed by the output from one of the numerical models. Transport towards the flood-dominated zones at the boundaries is enhanced under the superimposed influence of waves, which is demonstrated through the use of sediment transport formulae and sand tracer experiments. Although the annual contribution of the rivers to the overall sediment budget is shown to be small, the time of retention of waters and sediment within the system could result in fluviatile sediment inputs being retained for tens of years. In an attempt to extrapolate observations of transport processes into the ancient record, a preliminary facies model for the macrotidal Bristol Channel is presented. The model shows the availability of sand increasing in an onshore direction; progressing in this direction, the facies pass from scoured bedrock and lag deposits, through cross bedded megaripple foresets and the topset planar bedding of intertidal flats, to Holocene fill and supratidal deposits.

A Wind-Driven Ice-Push Event in Eastern Lake Ontario

Ice on Lake Ontario at Kingston weakened rapidly in warm air temperatures to 17° C on the afternoon of 26 March 1986. Moderate onshore winds to 33 km/h changed to offshore winds during the night, and by morning most of the ice in the nearshore area had blown’more than 5 km offshore. After 0600 on the morning of 28 March winds shifted to an obliquely onshore direction, increasing to 28 km/h by noon and driving the ice onshore. A small ice-push event occurred at 1225 lasting 10 minutes, piling ice to a height of 2.5 m above the bank by successive overriding. Eleven stones of up to 206 kg were moved from the shore by the body of the ice and carried up, in some cases, to near the top of the ice pile, despite the weakness of the thin, candled ice sheet. The need for quantitative studies of these irregular and unpredictable events is demonstrated by these observations.

Internal Tides on the Australian North-West Shelf: A Preliminary Investigation

Abstract An analysis of current meter data and cross-shelf temperature measurements from the Australian North-West Shelf shows the existence of internal waves of semi-diurnal period. Vertical displacements of density interface am seen to reach ∼30 m, a value nearly half the water depth. Baroclinic currents are isolated from the measurements and have amplitudes reaching 0.2 m s−1 in the cross-shelf direction. The baroclinic motion is shown to be consistent with a first-mode internal wave propagating in the onshore direction at phase speed of approximately 0.4 m s−1 with wavelength of 20 km and being rapidly damped in amplitude while propagating across the shelf. The dissipation of the internal tide across the shelf appears to result from the influence of turbulent mixing processes. There is little coherence between the barotropic tide and the internal tide and it is suggested that this results from the internal tide being generated over a large section of the shelf slope down to a depth of approximately 90...

Drift velocity at the wave breaking point

Wave-tank studies were conducted on the measurement of the drift velocity at the breaking point under different types of breaking waves on a rigid, plane beach. The drift velocity has onshore direction near the surface and close to the bottom; in the main flow column, the drift velocity is always offshore. The offshore drift velocity shows a more uniform vertical distribution than that in the offshore region. The experimental data are compared with theoretical values of three different second-order constant-depth wave theories. Comparisons with data from other sources are also made.

Mid‐level intrusions at the continental shelf edge

Observations across the continental shelf offshore from New Jersey in late summer 1976 show an intrusion of saline water at the mid level of the water column across the shelf edge front, which appears in density only as an offshore thickening of the pycnocline. This internal density field produces horizontal pressure gradient forces within the pycnocline in the onshore direction. These forces, in the linearized equation of motion with a constant eddy viscosity, drive a circulation which resembles a double Ekman spiral for internal pressure vertical distributions which are thin with respect to the Ekman depth. For thick pressure distributions, the circulation is geostrophic. The resulting flow pattern has no net crossshelf transport. For the continental shelf edge in the example, a northward geostrophic mid‐level jet is predicted by the theory. Ekman depth, and thus the vertical coefficient of eddy viscosity, can be determined from hydrographic data describing an intrusion.

Sediment dynamics in the macrotidal Avon River estuary, Bay of Fundy, Nova Scotia

The Avon River estuary in central Nova Scotia is part of the northeastern arm of the Bay of Fundy. The estuary is macrotidal; tidal range is 15.6 m at lunar perigee. The large tidal range generates strong currents. Maximum current speeds increase from 0.6 m/s at the estuary mouth (seaward end) to 1.7 m/s at the estuary head (shoreward end). Every location in the system is either flood or ebb current dominant.There are six major intertidal sand bodies in the estuary. Three of these form an ebb tidal delta at the estuary mouth that resembles many mesotidal ebb tidal deltas. One of the sand bodies at the estuary head also is part of an ebb tidal delta, another is a flood tidal delta, and the third is a tidal point bar.Ripples, megaripples, and sand waves are the three major classes of bedforms. Flow regime increases in the onshore direction, and bed configuration reflects maximum flow conditions.Mean grain size decreases from the estuary mouth to the head, producing an inverse relationship between mean grain size and current velocity. Cumulative frequency curve analysis indicates that the inverse relationship is produced by hydraulic sorting, which excludes the coarsest grain population from the estuary head.

Adaptive visually‐mediated movements ofLittorina irrorata(Mollusca: Gastropoda) when displaced from their natural habitat

Littorina irrorata displaced from their natural location amid plant stems in the upper inter‐tidal zone to areas of intertidal zone devoid of vegetation, oriented onshore under all sky and light conditions tested. The responses of snails tested in experimental arenas in the upper intertidal zone indicated that the natural substrate slope of 3 degrees was ignored in favor of movement toward a black paper rectangle. Subsequent releases on natural substrate supported the conclusion that the onshore‐oriented response resulted from visual perception of a dark area (bushes and trees near the high tide line). Where vegetation was equally tall in all directions, snails oriented toward the closest vegetation regardless of the actual onshore direction.

TYPES OF SAND RIPPLES AND THEIR ARRANGEMENT OBSERVED ON THE TATADO BEACH, SHIMODA CITY, IZU PENINSULA

Sand ripples formed by oscillatory flow under waves have interested many researchers in hydraulics, and the hydraulic conditions for their formation havee been examined by using small scale experimental wave tanks. The results of these studies, however, are insufficient to explain the hydraulic conditions for the formation of many kinds of the ripple types under natural conditions, which have been described by physical geographers and sedimentologists, because the experimental studies have been concerned almost exclusively in the most simple ripple type, that is, a parallel type of ripples. The authors tried to examine some hydraulic conditions for the formation of various types of ripples by the observations and measurements in a real field. A series of observations was made from 19th to 22nd of July, 1969, on the shallow sea bottom of the Tatado Beach, Shimoda City, Shizuoka Prefecture, Central Japan. This coast is a sand beach with a length of 500 m and faces southwards directly to the Pacific Ocean. An observation range, along which observations and measurements of ripples, waves, and bed materials were made, was established perpendicularly to the shore line through the middle part of the beach (Fig. 1). The bed materials sampled along the range were medium sands (0.24mm in median diameter) with high content of shell fragments and did not show any certain tendency of size distribution and sorting along the range (Figs. 2 and 3). Keeping pace with the measurements of wave and tide conditions at an observational tower, observations of ripple forms were carried on by skin divers from shore to the point about 6 m in depth along the range, at intervals of one to two hours. The ripples here were classified into following ripple types based on the patterns of their crests and troughs and also on the relations between the directions of ripple crests and of surface wave ridges. 1) Parallel type (Type P). This type of ripples shows regular alternation of ripple crest and trough lines parallel to those of surface waves. This type may be subdivided into long crest parallel type (Type Pl) and short, sinuous crest parallel type (Type Ps). 2) Diagonal type (Type D). This type includes various patterns of ripples which have the crest lines diagonal to the wave ridge lines. Usually, this type of ripples consists of a set of ripple crest lines which cross each other, and are also subdivided generally into two types, long crest diagonal type (Type Dl) which reveals a ladder-like pattern, and short crest diagonal type (Type Ds). 3) Lunate type (Type L). This type shows lunate-shaped depressions with an offshore-faced convex arch. They are arranged discrepantly each other on a flat sea floor. Cross profiles in the off-and onshore direction of the depressions show a distinct asymmetrical form, that is, gentle offshore-faced slope and steep onshore-faced one. Three-dimensional forms of some typical ripple patterns were measured by the “comb method” which uses an apparatus shown in Photo 1, and contour maps of the ripple forms were shown in Fig. 4. By the repeated observations of the ripples at regular time intervals, it was made clear that these ripple types were always arranged towards the shore in the following regular order: parallel type, diagonal type and lunate type, and that their zone of occurrence fluctuated in such a general trend as they retreated offshore when waves increased in height or tide level dropped, and vice versa (cf. Fig. 5 and Table 1). This might indicate that each type of the ripples is formed closely in response to the water movements on the sea bottoms under the surface waves. Some hydraulic conditions for the formation of each ripple type were examined. During the observation period on the Tatado Beach, it was seen that the wave form in the offshore zone showed a transformation by shoaling.

COLLECTIVE MOVEMENT OF SEDIMENT IN LITTORAL ENVIRONMENT

Collective movements of sediment occur in the form of sand waves m the nearshore zone and affect beach topography to a significant extent. Bar-type sand waves move only in the onshore direction and account for various accretive profiles on the subaerial beach. When the beach is eroded, these sand waves are simply disintegrated on the subaerial beach, instead of migrating back in the form of sand waves. These characteristics of bar-type sand waves help explain dynamic behavior of beach profiles with respect to profile configuration, sediment storage, and beach width. Cusp-type sand waves are considered to be a product of interaction between longshore currents and an erodible bed. Their presence causes variability in beach profiles along the shore between those containing a prominent bar and those without a bar. Migration of these sand waves may well produce pulsational transfer of material along the shore.

CURRENTS IN THE SURF ZONE

Surface and bottom currents in the surf zone were measured at 15 equally spaced points along two straight beaches with approximately parallel bottom contours. The measurements showed that offshore currents predominate over onshore currents at the bottom, while at the surface there is a slight predominance in the onshore direction. With regard to the longshore component, it was found that surface and bottom currents have a similar velocity distribution. The variability of the longshore component as measured by its standard deviation is equal to or larger than the mean longshore velocity. This wide variation in longshore currents indicates the impracticability of estimating the mean velocity from a single observation of longshore current. It was found that the momentum approach to the prediction of longshore currents by Putnam, Munk and Traylor (1949) leads to useful forecasts provided the beach friction coefficient k is permitted to vary with the longshore velocity, V. The indicated relation is k~v^(-3/2).