Morphodynamic Behavior of Sorted Bed Forms on the Continental Shelf I Posters
Presiding: B A Murray, Duke University; P A Traykovski, Woods Hole Oceanographic Institution
OS23B-01 1330h
Ripple and Morphologic Behavior of Sorted Bedforms
Complex sorted bedforms or rippled scour depressions (RSDs) are common inner shelves features, yet understanding of their morphodynamic behavior is limited. Seabed measurements from the Martha's Vineyard Coastal Observatory (MVCO) are used to field test several widely used bedform models including Nielsen (1981); Wikramanayake (1993); and Wiberg and Harris (1994). Deployed quadpods supported acoustic sensors and provided time series on boundary layer processes, sediment transport, and bedform dynamics. Observations indicate that sorted bedforms form as windows through thin blankets of fine sand into underlying coarse sand. Elongate wave oriented narrow windows form and fade in response to wave events. Their location is closely tied to transitional boundaries between coarse and fine sand. A conceptual model of the evolution of such windowed features is presented. Alternating patches of coarse and fine sand beds characterized the study area. Patterns of bed roughness were mapped using autonomous underwater vehicle (AUV) side-scan sonar sonograms, diver observations, grab sampling, short cores, and acoustic images from stationary rotary sonar systems. The exposed coarse patches supported large wave orbital ripples. The fine sand areas possessed smaller anorbital ripples during calm periods, while during storm conditions occasionally developed into complex irregularly spaced low relief hummocky bedforms or cross ripples. During other storm events, flat bed conditions prevailed. The tested ripple models predicted some, but not all of the observed bedform variation between the coarse and fine sand sites. Specifically, the models tended to account for most of the evolution of large wave-orbital ripples within coarse sand deposits, but accounted for less of the evolutionary diversity of bedforms within the fine sand deposits. In particular, none of the models predicted that the hummocky bedforms would replace ripples on the fine sand bed during the storm. Instead the models all suggest that sheet flow conditions would wipe out the ripples leaving a plane bed. The ripple models tested each qualitatively predicted a strikingly distinct class of bedforms to exist between the coarse and fine sand sites such that only suborbital and anorbital ripples were predicted to develop over the fine sand site, whereas only orbital and suborbital ripples were predicted to develop over the coarse sand site. Continued observations and new models of bedform behavior particularly during storm events are critical in order to advance our understanding of bedform dynamics associated with heterogeneous sediment systems.
OS23B-02 1330h
Distribution and Morphology of Sorted Bedforms on the U.S. Atlantic Inner Continental Shelf: Implications for Regional Geology, Sediment Supply, and Sediment Transport
Regional seafloor mapping surveys show that sorted bedforms (grain-size-sorted features with wavelengths of 10s-1000s m and heights generally <1 m, oriented usually at a high angle to the local shoreline) are abundant, nearly ubiquitous morphologic features of the U.S. Atlantic inner shelf. Their occurrence appears to be controlled by several geologic factors, including: 1) regional geology, 2) shelf bathymetry, and 3) relative sediment abundance. Although sorted bedforms may develop as the result of emergent behaviors, sorted bedforms often occur where the underlying geologic framework contributes different sediment size classes to the shelf sediment cover in a spatially heterogeneous manner (e.g., braided glacial outwash deposits; complex fluvio-estuarine valley-fill deposits). These abrupt changes in grain size may initiate and enhance morphodynamic feedbacks. Sorted bedforms tend not to occur in areas with well-developed bathymetric features (e.g., ridge and swale topography with meters of relief over <1 km spatial scales) that trend at a high angle to their oft-observed shore-perpendicular orientation. Sorted bedforms also tend to be absent in locations where there are well-developed shoreface-attached sand bodies that reflect local- to regional-scale sediment abundance on this otherwise generally sediment-starved shelf. Where observed at storm-event to decadal time-scales, sorted bedforms are shown to be temporally persistent in their size, orientation, and location, and may reflect a preferred morphologic state of the seabed that is adjusted to regional physical and environmental variables. Examples from the Mid-Atlantic and South Atlantic Bights illustrate aspects of sorted bedform occurrence described here.
OS23B-03 1330h
Observations of Sand Transport Processes Over Sorted Bedforms
High resolution sonar surveys in the past decade have revealed that complex sorted bedforms (rippled scour depressions, RSDs) are ubiquitous features on many sandy inner shelves. These features consist of alternating bands of coarse and fine sand with along-shore scales of 10 to 1000 m and across-shore scales of 100 to 5000 m. Modeling approaches for these features have ranged from rules-based approaches to sediment transport physics based approaches. However, the sediment transport processes over fine and coarse sand with combined weakly non-linear waves and mean current forcing are poorly understood. Specifically, the relative roles of bedload and suspended load forced by non-linear waves and mean currents over large ripples in coarse sand and smaller ripples, or low-relief bed conditions in fine sand are not well understood. For instance, tripod mounted rotary sonar observations have generally shown onshore ripple migration forced by non-linear wave velocities to be the dominant transport process in coarse sand. However, larger-scale sonar and grab sample surveys have shown along-shore grain size variability that is presumably forced by along-shore mean currents. Over the past three years, observations were conducted at the Martha's Vineyard Coastal Observatory RSD field in attempts to quantify the relative roles of these processes. A quadpod, with a rotary sidescan sonar and a 2-axis pencil beam sonar to measure ripple morphology, was deployed in both coarse and fine sand in successive winter seasons. The quadpod also had a 3-axis bistatic pulse coherent acoustic Doppler profiling system that is capable of measuring near-bed (within 30 cm of the seafloor) suspended and bedload transport. Bedload was estimated using spectral processing on the bed and bedload backscatter, thus the high intensity returns from the stationary bed could be separated from returns from the moving bedload in the frequency domain. Preliminary analysis of the observations revealed that in fine sand the transport is dominated by suspended load forced by wave-resuspension and mean current transport. The observations in fine sand contained many storms of varying energy thus are fairly robust. The observations in coarse sand only had 4 hours of data during moderate-energy active conditions, due to a equipment failure, thus additional data and analysis is required to determine the amount of along-shore transport and the relevant forcing processes in coarse sand during more energetic events. From the available data in coarse sand, onshore bedload and near-bed suspended load transport (within 1 cm of the seafloor) forced by non-linear waves dominated the flux, consistent with prior observations of bedload flux estimated from ripple migration.
OS23B-04 1330h
Insights about the processes associated with the origin and evolution of Rippled Scour Depressions based on multibeam sonar imagery
We present observations of the textural and morphological characteristics of Rippled Scour Depressions (RSDs) identified during high-frequency multibeam sonar surveys at three inner shelf environments. RSDs are evident as distinct bathymetric depressions that are associated with high acoustic backscatter intensity and contain coarse sediment formed into shore-parallel ripples. Elongate shore-normal RSDs identified near Grays Harbor, Washington, are associated with a larger-scale bathymetric undulation upon which they are superimposed. Repeat bathymetric surveys indicate that the large-scale undulation with which these RSDs are associated was propagated in the alongshore direction by 10s of meters over 2 years indicating that they are driven by shore-parallel processes. Other RSDs, not associated with larger-scale bathymetric patterns, identified near Grays Harbor and off the South Shore of Long Island, exhibit along-shore textural asymmetry suggesting that they are transverse sorted bedforms. Smaller amorphous RSDs that are not associated with larger-scale bathymetric patterns can be created and destroyed over time-scales of less than 1 year, and do not appear to be sorted bedforms. The shore-parallel ripples contained within all of these features, and observations of shoreward evolution, indicate that RSDs are modified by shore-normal processes. We propose a classification of RSDs and a conceptual model that describes their creation and evolution based on their morphologic and textural characteristics and the larger-scale seabed patterns associated with them. By comparing RSDs observed at different sites we can begin to address the non-local significance of these features and gain important insight about the mechanisms associated with their creation and evolution.
OS23B-05 1330h
(Ir)regularity of sand ripple patterns - a field observation
High-resolution photographs of sea floor were obtained from a camera mounted on a tripod that was deployed in 10 meters of water on the shelf off the Santa Clara River mouth, southern California. During the month-long (mid-January to late February, 2004) deployment the camera was only able to record high-quality images of sand ripples for a 50-hour window of relatively calm seas (and lower turbidity). During the 50-hour period the significant wave height varied between 0.8 and 2.2 m, and peak wave period between 8 and 13 seconds. Two ripple patterns were observed - (A) long-crested, 2-dimensional vortex ripples, and (B) short-crested, 3-dimensional ripples, both of which have ripple spacings between 7 and 8 cm, indicating a case of anorbital ripples whose spacings are a function of sand grain size (median diameter d0 = 0.013 cm) and independent of the hydrodynamic forcings. While the main ripple orientations in both patterns very well conformed with the wave direction that was consistently from the west, varying between 263-271 degrees, the changes of the ripple patterns during the 50 hours do not appear to be associated with wave parameters that have been shown in lab studies to determine the ripple patterns. These parameters include wave height, wave period, near-bed shear stress, and wave ellipticity. Surprisingly, the ripple patterns seem to change in association with the tidal current, whose magnitude was always less than 10 cm/s, in comparison to wave orbital velocity of 40 - 50 cm/s. The 2-D ripples were mostly observed when the tidal current directed to the south during ebb tide, and the 3-D ripples during the flood tide when the tidal current directed to the north.
OS23B-06 1330h
A Refined Numerical Model for Sorted Bedform Formation and Evolution
Murray and Thieler (2004) hypothesized that a feedback operating on a poorly sorted seabed, and subsequent emergent interactions, lead to development of large-scale grain-size sorted patterns (`sorted bedforms') on inner continental shelves that have traditionally been referred to as `rippled scour depressions.' Unlike familiar bedforms that grow because of an interaction between bed topography and sediment flux, the initiation and evolution of these sorted bedforms results from a coupling between bed composition and sediment flux. In essence, larger wave ripples form in coarser patches, and these inhibit fine-sediment accumulation and enhance fine sediment flux over the patch by enhancing vertical mixing. As an initial test of the hypothesis, Murray and Thieler developed an exploratory numerical model that produced sorted bedforms exhibiting the main characteristics of the patterns observed in nature. Murray and Thieler's initial model parametrically treated the coupling between bed composition and sediment flux. We further developed the model, incorporating more detailed parameterizations, including: 1) empirical predictions of ripple dimensions as a function of bed composition and near-bed wave conditions; 2) near-bed wave-orbital motions that do not involve a shallow-water assumption; 3) calculations of the vertical profiles of suspended-sediment concentration and mean-current velocities that result from wave and current motions interacting with the ripples; and 4) bedload transport. We tested the sensitivity of model behavior to the parameterizations adopted by conducting numerical experiments using wave, depth, and current conditions measured off the coastline of Tairua (New Zealand). We performed two tests: a comparison of predicted ripple characteristics and sediment concentration profiles with tripod observations; and a comparison of the large-scale geometry emerging from model simulations with some of the features observed during bathymetric surveys. Further numerical experiments were aimed at determining which combinations of forcing conditions promote and which suppress the development of sorted bedforms, and how the seabed patterns develop under time-varying forcing. Unsteady wave and current conditions appear to substantially affect the growth of sorted bedforms, and can lead to the development of more complex patterns. Adding net sediment accretion to the model can, under some conditions, lead to the burial of sorted bedforms, which is in agreement with stratigraphic field observations from Tairua. The Andrew W. Mellon Foundation and the Foundation for Research, Science and Technology (contract C01X0401) supported this work.
OS23B-07 1330h
Distribution, Character, and Importance of Sedimentary Furrows in South-Central Long Island Sound
The U.S. Geological Survey, in cooperation with the Connecticut Department of Environmental Protection and the National Oceanic and Atmospheric Administration, is producing detailed geologic interpretations of the sea floor in Long Island Sound to improve our understanding of the processes that control the complex distributions of sedimentary environments and benthic habitats. Although the deeper waters of the south-central Sound are generally characterized by relatively weak bottom currents and by depositional conditions, multibeam data reveal the presence of sedimentary furrows. These erosional bedforms occur in fine-grained cohesive sediments and cover an elongate east-west trending area (~80 km2) that lies approximately 5 km off Herod Point, NY, in water depths of 31-41 m. The furrows are irregularly spaced, trend east-northeast, average 9 m wide and 0.4 m deep, and can exceed 1.9 km long. Although most of the furrows appear to taper out gradually, some furrows show a "tuning fork" joining pattern. Most of these junctions open toward the east, indicating net westward sediment transport, but a few junctions open westward suggesting that the tidal regime is important to furrow formation and that the furrows can form when water flows in either direction. The sedimentary furrows in south-central Long Island Sound, which are similar to those we have previously described along the Connecticut side of the estuary, form under recurring directionally stable tidal currents, constrained by the elongate geometry and regional bathymetry of the Sound. These conditions, in turn, produce the secondary helical and turbulent flow patterns conducive to the formation of erosional furrows. Concurrently, bioturbation by crabs not only suspends sediment, but also nutclam shells and other coarse biogenic debris, which are aligned by the secondary flow and abrade the furrows as they saltate in the oscillating tides. Through resuspension due to biological activity and the subsequent development of these furrows, fine-grained cohesive sediment can be remobilized and made available for transport farther westward into the Sound.