Ebb Delta Research Articles

Vertically Accreted Foreshore to Shoreface Deposits of Sego Sandstone (Campanian), Northwest Colorado: ABSTRACT

Exposures of the Sego Sandstone, on the northern flank of Rangely dome, Colorado, represent a thick (50 m [165 ft]) sequence of shoreline sandstone. The Sego Sandstone generally represents a progradational sequence. However, the interplay between sediment supply and subsidence resulted in stabilization of shoreline position. Deltaic distributaries were not observed, suggesting interdeltaic deposition. The initial progradational phase within this sandstone complex is represented by three facies. (1) Basal facies composed of bioturbated shale with occasional silt stringers. (2) Medial facies, averaging 5 m (16 ft) in thickness, grading upward from the underlying shales, and composed entirely of ripple-stratified fine sandstone. Starved ripples at the base of this sequence grade vertically into flaser and amalgamated ripples. Channels filled with clay and ripple-stratified sand oriented south-easterly are present. (3) Upper facies is composed of fine-grained sandstone exhibiting basal hummocky cross-bedding that grades vertically into small-scale troughs and planer stratification. This facies is capped by a thin (35 cm [14 in.]) coal. Maximum observed thickness for this facies is 7 m 23 ft), with thinning toward the northwest. Facies 1 represents the marine shale deposits of the Buck Tongue. Facies 2 is an ebb delta system believed to have functioned concurrently with the prograding shoreface (facies 3). Stabilized shoreline conditions are characterized by four facies. (1) The basal unit is composed of fine to medium-grained sandstone. Trough and tabular cross-bedding is abundant, as is ripple stratification. Individual beds are 10 to 50 cm (4 to 20 in.) thick and the unit thickness is 10 to 20 m (33 to 66 ft). No apparent vertical trends with respect to either grain size or sequence of sedimentary structures were observed. (2) The second facies is fine grained, predominantly ripple stratified with some tabular and low angle bimodal cross-beds. Sand-filled channels with Ophiomorpha are present. This facies is 9 m (29.5 ft) thick and onlaps with a portion of facies 1. (3) The third unit is fine to medium grained, and as with facies 1, contains abundant trough and tabular cross-bedding s well as abundant ripple stratification. Paleocurrent data suggests a northwesterly transport direction for facies 3 as opposed to a southwesterly direction for facies 1. Bedding is between 10 to 50 cm (4 to 20 in.) thick with unit thickness of from 0 to 12 m (0 to 39 ft). This unit thins northwestward. (4) The upper unit thickens northwestward and is composed of bioturbated shales, humates, and very fine-grained sandstone. Facies 2 represents a sand tidal flat situated behind the coastal barrier (facies 1). The lack of well-developed trends either in grain size or sedimentary structures within facies 1 indicates that the bar formed from the amalgamation of diverse environments. Overlying both the bar and tidal flat facies is the spit (facies 3) and lagoonal muds and washovers (facies 4). This shoreline sequence is overlain by fluvial deposits of the Mesaverde Group. End_of_Article - Last_Page 551------------

NATURAL BAR-BYPASSING OF SAND AT A TIDAL INLET

Captain Sam's Inlet, a shallow unstable inlet, periodically migrates up to three kilometers (km) alongshore over a 30- to 40-year period. As the inlet migrates, sediment accumulates at the seaward terminus of the tidal inlet to form a wave-modified ebb-tidal delta that trends downdrift. Continued sediment accumulation on the tidal-delta shoal and subsequent lengthening of the tidal channel in a downdrift direction results in an unstable channel configuration. The ebb delta is eventually breached on its updrift side, releasing a sediment package for inlet bypassing. The sediment-bypassing process was initiated at Captain Sam's Inlet after the final landfall of Hurricane DAVID in September 1979. Initially, the newly formed updrift hurricane channel scoured 1.1 m, migrated updrift, and became the predominant tidal channel at the inlet with eventual abandonment of the prehurricane, main ebb channel. These two channels outlined a portion of the ebb-tidal delta that was freed for bypassing. The initial sediment volume contained in the bypassing shoal, above the -0.6 m (-2 ft) MSL contour, was 47,000m3. The sediment volume of the bypassing shoal did not change significantly until final attachment to Seabrook Island. This channel dominance and the wave-induced migration of the bypassing sediment package aided the bypassing of sand at the inlet. Immediately after filling of the prehurricane, main ebb channel on the delta, the downdrift beaches began accreting. The accretion continued throughout the bypassing process. The tidal prism and cross-sectional area reflected little change during the bypass, showing evidence of the system's overall stability.

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

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.

MECHANISMS OPERATING AT A JETTIED RIVER ENTRANCE

Between October 1976 and July 1977 a northern rubble mound jetty was constructed at the mouth of the Hastings River, transforming the entrance from a single to a double jettied system. Prior to the jetty construction the entrance was characterised by the presence of a substantial swash bar (alternatively called an ebb delta marginal shoal) which was a continuous feature over 100 years of hydrographic survey records. However, construction of the northern jetty triggered an unprecedented onshore movement of the swash bar. This movement was well documented by a field monitoring programme incorporating hydrosurveys, aerial* photographs, tidal gaugings, sediment sampling, float tracking and nearby wave rider buoy information. A semi-quantitative model was developed to aid understanding and quantification of the macro sedimentary processes associated with this phenomenon. The model demonstrated that the sudden reduction of the swash bar was due to the disruption of a circulation of sand which had previously aided the dynamic stability of the bar. The quantitative predictions of the model agreed well with subsequent entrance behaviour. The philosophical development of the model and its findings are discussed in detail. In the literature there is a general lack of attempts to quantify the sediment transport relationships between the gross morphologic features of tidal entrances. This paper presents a methodology for assessing the sedimentary process at tidal entrances.

Geomorphic Cycles in Ebb Deltas and Related Patterns of Shore Erosion and Accretion

ABSTRACT The interrelationship between inlet flow and delta morphology has an important effect upon the bypassing of sediment across inlets. Along the Sea Island Section of the Coastal Plain Physiographic Province the nature of inlet drainage and sediment bypassing profoundly influences patterns of beach erosion and accretion. Small coastal plain inlets generally have arcuate ebb deltas transected by a radially distributed pattern of channels. This arrangement permits an efficient flow of sediment across the inlet with little disturbance to the sediment budget of the adjacent shorelines. Several small inlets also have spits parallel to the shore that divert the flow of river water into the downdrift shore causing erosion. Accretion at the distal ends of these spits takes place at the expense of the proximal end of the spit where the shoreline erodes. Erosion eventually reopens a new channel across the proximal end of the spit and produces a second drainage reentrant. The major inlets along the coastal plain shoreline exhibit shoals with pronounced shore-normal orientations. The proximal ends of these spits may be attached or separated from the shore. When spits are attached to the shore, erosion is observed along the inlet margin and deposition is apparent at the distal end of the spit. When spits are separated from the shore, accretion is observed at shores adjacent to the inlet.

CHANNEL STABILITY IN TIDAL INLETS: A CASE STUDY

Wachapreague Inlet, a downdrift offset inlet in the barrier island complex of the mid-Atlantic U.S. coast was studied during the period 1971-1973. Elements of the study included: (1) the inlet morphometric history (120 years), (2) assessment of surficial and sub-bottom sediments within the inlet complex, (3) response of the channel cross-sectional area to short-term variations in wave activity and tidal volumes, and (4) the distribution of tidal flows within the channel. It is concluded that: (1) a qualitative correlation exists between short-term channel cross-sectional area change and the ratio of ebb tidal power to wave power. It is inferred that the important element is the direct wave activity on the ebb-tidal delta; (2) duration differences in rising and falling phases of the tide (flood longer than ebb) lead to an ebb dominence in bedload capacity at the inlet with the result that this inlet has a natural flushing ability; (3) there is pronounced sand circulation within the inlet complex via a sediment flow loop which is driven by wave refraction and lateral inflow on the updrift side. The sand volumes thus delivered annually to the inlet channel from the ebb delta appears to far exceed the estimated littoral drift. The local sand circulation should, therefore, be considered in engineering design for inlet control structures.

The Mississippi river improvements

The morphodynamic response of a mixed-energy ebb-tidal delta (Guadiana estuary, southern Portugal) and its down-drift barrier island (Canela Island) to channel relocation and stabilisation by jetties is examined using a series of sequential bathymetric maps and vertical aerial photographs spanning five decades. Morphological analysis indicates that the ebb delta is in an immature state, characterised by weak sediment bypassing. Landward-migrating shoals on the swash platform have been produced by the jetty-induced artificial bank-breaching and by the collapse of the eastern portion of the delta. The welding of these shoals has largely controlled the evolution of the coast, with local accretion and erosion lasting for years, and large amounts of regional accretion occurring over decades due to sand accumulation against jetties located further down-drift. These observations provide insights into the potential response of a coast to very large, locally concentrated sand nourishment in the form of shoals. The main effects of the jetties on the coast are observed at the centre of Canela Island, with the production of an erosion hot-spot associated with a temporally persistent and divergent longshore transport providing sand to the adjacent areas. Significant accretion is anticipated for the next decade along the entire island due to the ongoing attachment of the presently observed shoals. After the depletion of this sediment source, and in the context of weak sediment bypassing, the most severe down-drift erosion induced by the jetties is predicted to occur some decades after their construction. This study demonstrates that the geomorphic response of an ebb-tidal delta to jetty construction must be considered at multiple temporal and spatial scales when assessing the impacts of jetties on the down-drift coast.