William T. Fox

Comparison of Ridge and Runnel Systems in Tidal and Non-Tidal Environments

ABSTRACT Beach and inner nearshore areas of Lake Michigan are basically the same as northern Massachusetts except for scale of the morphologic features and tidal range; in Lake Michigan spring tides reach 0.25 feet whereas in Massachusetts they reach 13 feet. Ridge and runnel topography is developed in the inner nearshore zone at both locations is the result of storm activity. These ridges migrate shoreward during low energy conditions and eventually weld onto the beach. Overall morphology, surface features and internal structures are quite similar in both areas. The only appreciable differences between the two areas are the scale and rate of migration. Apparently tides have no appreciable affect on the sediment sequence that accumulates as ridges weld to the beach although tides are significa t in determining the rate at which the shoreward migration of the ridges takes place.

Coastal Processes and Nearshore Sand Bars

ABSTRACT Detailed daily topographic maps of beach and inner nearshore areas indicate a cyclic pattern to processes and responses in this non-tidal environment. This pattern is the result of complex interaction between changes in shoreline configuration, discontinuous nearshore sand bars, and environmental variables such as barometric pressure, wind velocity, breaker height, and longshore currents. Of the 18 variables measured, barometric pressure appears to provide the best index for changes in coastal processes. The results of these variations are morphologic changes in the beach and inner nearshore area. As a low pressure system approaches the coast there is an increase in wind velocity, breaker height and longshore current velocity as barometric pressure drops. When the low pressure system passes barometric pressure rises and there is a reversal of wind direction with an accompanying reversal of longshore current direction. This cycle in conditions is accompanied by a pattern of responses in the position and morphology of the shoreline and sand bars. During high pressure and low energy conditions shallow discontinuous sand bars have somewhat regularly spaced rip channels. The shoreline is slightly sinuous with protuberances behind the sand bars. Longshore currents are slow and small waves break on the bars causing their shoreward migration. Shoreline sinuousity increases as protuberances grow and embayments are slightly eroded. An approaching low pressure system causes increase in wind, waves and longshore current velocity. As the pressure system passes there is reversal of longshore current direction at the time of maximum wind velocity and wave height. During this time rapid longshore currents are defl cted by the sinuous shoreline such that strong rip currents are formed. These rip currents pass over sand bar crests and excavate channels. At the same time new bars are forming as sediment accumulates in relatively quiet areas between rip channels. As a result there is apparent migration of the bar form. The return to low energy conditions yields a pattern much like that during the previous time of low energy. In response to these coastal processes, the bar form oscillates back and forth alongshore rather than migrating down the beach. although the sediment does move alongshore.

Stratigraphy and Paleoecology of the Richmond Group in Southeastern Indiana

The Richmond Group (Upper Ordovician) in southeastern Indiana is subdivided into rock-stratigraphic units (formations and members) and biostratigraphic units (assemblage zones). The formations and members established by earlier workers on the basis of faunal zones are either redefined as mappable rock units or discarded. A newly proposed formation replaces the lower three “formations,” Arnheim, Waynesville, and Liberty, which are based on paleontologic criteria and are lithologically indistinguishable. The new unit consists of thin layers of fossiliferous and barren limestone alternating with layers of calcareous shale. Two barren shale beds in conjunction with silicified fossil and pyritic limestone horizons are used to subdivide the new unit for a quantitative study of its lithologic variation. The Whitewater Formation above is redefined and subdivided into five rock units on the basis of lithology: (1) a lower unnamed member of massive earthy limestone, (2) a black carbonaceous shale bed, (3) the lower Saluda Member, which is a northward thinning wedge of mud-cracked slabby limestone with layers of massive dolomitic limestone at the base and top, (4) the upper Saluda Member, lithologically similar to the lower wedge but separated from it by a layer of massive dolomitic limestone, and (5) an upper unnamed member of massive earthy limestone. Four faunal assemblage zones are recognized in the newly proposed unit: (1) Resserella meeki, (2) Leptaena richmondensis, (3) Sowerbyella rugosus, and (4) Strophomena planumbona. The assemblage zones become thin and pinch out to the south, and their boundaries cross lithologic marker horizons within the newly proposed rock unit. The Homotrypa wortheni and Tetradium minus assemblage zones within the Whitewater Formation are split into upper and lower parts by the barren Saluda Member. The numbers of individuals in each species are counted in small unit areas and are statistically analyzed to determine the distribution of species within the assemblage zones and the geographic extent of the zones. The following interpretation of depositional environments is based on the distribution of the rock types and faunal assemblage zones. The lower unit of the Richmond Group was deposited in a shallow, inner sub-littoral, marine environment. The Whitewater Formation was formed during the regression and transgression of a shoal area with the lower and upper unnamed members representing the submerged mud flats, and the Saluda Member, the exposed mud flats.

Simulation Model for Storm Cycles and Beach Erosion on Lake Michigan

A mathematical simulation model is used to study the relations among storm cycles, beach erosion, and nearshore bar migration. The model is based on Fourier analysis of weather and wave data collected on Lake Michigan during the summers of 1969 and 1970. In the simulation of coastal processes, barometric pressure is used as the independent variable with longshore current velocity computed as the first derivative and breaker height as a filtered version of the second derivative of barometric pressure. The simulated curves are used to compute wave and longshore current energy for each storm cycle and poststorm recovery. Daily profiles across the nearshore area provide data for topographic maps and maps of erosion and deposition. For simulation, the nearshore area is broken down into five components including beach, foreshore, plunge zone, trough, and bar. A gently sloping linear plus quadratic surface is used to represent the barless topography, with bars and troughs generated by normal curves. Bar distance is computed as a function of wave energy and bottom slope. Position of the bar and trough along the shore is determined by wave and longshore current energy. Simulated maps are produced for each storm cycle and poststorm recovery.

Interaction between wave- and tide-generated processes at the mouth of a microtidal estuary: Matanzas River, Florida (U.S.A.)

Abstract The zone of transition between the wave-dominated open coast and tide-dominated mouth of estuaries is complicated. A time-series study of wave parameters, longshore currents and tidal parameters at the mouth of an estuary on the northeast coast of Florida has demonstrated that tidal influence on nearshore processes is modest and localized. Tidal currents are nearly undetectable less than 0.5 km from the estuary mouth along the open coast. Tidal currents reach 80 cm/s in the throat of the main ebb channel but immediately seaward of the terminal lobe of the ebb delta the tidal component is barely detectable with longshore currents predominating. Lateral flood channels adjacent to the estuary mouth show that tidal currents are significant and dominate longshore currents near low-tide stage, but are suppressed by longshore currents near high tide. Weather patterns may have an important effect on tidal processes at the estuary mouth because they control wave impingement and therefore longshore current speed and direction. Longshore currents may reinforce or suppress tidal currents, particularly in lateral flood channels.

Comparison of Late Ordovician epicontinental seas and their relative bathymetry in North America and China

Six widely separated areas with Upper Ordovician strata in Canada and the United States are compared with six localities in southern China. Cyclic sedimentation, including evaporites, carbonates, and phosphatic black shales, occurred in relatively shallow epicontinental seas of North America. As many as three Ashgill cycles of regionally different styles may have been coeval throughout North America in response to modest changes in sea level. Environments in South China primarily included carbonates and black shales with a far more uniform distribution. Absence of comparable sedimentary cycles confirms that the platform bathymetry of South China was consistently deeper than in North America. The almost complete exposure of North China (Sino-Korean Plate) by Late Ordovician time underscores the fact that independent cratons have different bathymetric histories. By mid-Silurian time, South China also was fully exposed. Such hypsographic variation is critical to the intercontinental correlation of features related to eustasy, water chemistry, and even the patterns of extinctions.

Computer model of wind, waves, and longshore currents during a coastal storm

A mathematical model has been developed to forecast or hindcast wind, waves, and longshore currents during the passage of a coastal storm. Storm intensity is a function of the barometric pressure gradient which is modeled by rotating an inverted normal curve around the center of an ellipse. The length and orientation of the major and minor axes of the ellipse control the size and shape of the storm. The path of the storm is determined by a sequence of storm positions for the hindcast mode, and by interpolated positions assuming constant speed and direction for the forecast mode. The site location, shoreline orientation, and nearshore bottom slope provide input data for the shore position. The geostrophic wind speed and direction at the shore site are computed from the latitude and barometric pressure gradient. The geostrophic wind is converted into surface wind speed and direction by applying corrections for frictional effects over land and sea. The surface wind speed and direction, effective fetch, and wind duration are used to compute wave period, breaker height, and breaker angle at the shore site. The longshore current velocity is computed as a function of wave period, breaker height and angle, and nearshore slope. The model was tested by comparing observed data for several coastal locations with predicted values for wind speed, wave period and height, and longshore current velocity. Forecasts were made for actual storms and for hypothetical circular and elliptical storms.

Coastal processes and depositional patterns on Cape Ann, Massachusetts

ABSTRACT Four beaches on the southeastern coast of Cape Ann were studied to determine sediment source, transport, and depositional patterns. Methods of analysis included beach profiling, sediment sampling, and simulated wave refraction. Wave energy, resulting primarily from northeast storms and the prevailing southwesterlies, concentrates on nearshore island and headland complexes. Due to wave refraction, northeast flowing longshore currents are found at Long, Cape Hedge and Pebbly Beaches. At Good Harbor, longshore drift is predominantly southwestward. The wave energy and refraction patterns determine beach gradients and grain size distribution. Predominate direction of longshore drift and systematic internal decreases in grain sizes support a spit formation for the baymouth barrier beaches. and-size sediments are primarily glacial in origin and secondarily a result of erosion of the crystalline headlands. The source of cobbles at Cape Hedge and Pebbly Beaches is believed to he a moraine which intersects the shoreline at Cape Hedge.

Beach and Nearshore Processes and Morphology in Nontidal Environment: ABSTRACT

Recent detailed studies of the beach and nearshore environments of eastern Lake Michigan have revealed almost no significant differences compared with similar marine environments, except for the absence of marine tidal fluctuations. The morphology and the processes operating in both areas are remarkably similar; however, the rates at which these processes operate appear to be more rapid in Lake Michigan. Beach profiles reflect environmental conditions which may or may not be associated with seasonal cycles. Storm conditions yield nearly identical flat profiles in both areas with characteristic lag deposits of heavy minerals in the back-beach zone. Quiescent conditions produce accretionary beaches except when lake levels rise gradually for prolonged periods. The inner nearshore profile in both Lake Michigan and marine areas is commonly characterized by an ephemeral bar which migrates shoreward and is welded to the beach. The bar forms during the waning phase of a storm and migrates shoreward during low-energy conditions. Migration of the bar generally proceeds more rapidly in Lake Michigan than in tidal areas. The crest of the bar is not exposed in Lake Michigan until welding occurs, whereas, it is exposed during low tide in comparable marine environment. Farther from shore are relatively stable bars whose number and position are controlled largely by the slope of the nearshore bottom. These features also show generally comparable morphology in both areas, although they seem somewhat less stable in the marine environment. End_of_Article - Last_Page 335------------