Gerard P. Lennon

Hydrogeologic properties of a thrust fault within the Oregon Accretionary Prism

Two sets of hydrogeologic tests conducted at Ocean Drilling Program (ODP) Hole 892 on the Oregon Accretionary Prism provided the opportunity to determine hydrogeologic properties of an active accretionary prism fault zone. The first set of tests consisted of shipboard packer tests conducted during ODP Leg 146 (fall 1992), while the second set of tests were constant-drawdown and constant-discharge tests conducted in fall 1993 using the submersible Alvin. Pressure response during the first set of tests suggests that fractures remained open until excess fluid pressure (relative to hydrostatic) dropped below 0.315 to 0.325 MPa (λ* ∼ 0.53 to 0.54, where λ* = (pore pressure - hydrostatic)/(lithostatic-hydrostatic)). Analysis of the packer test data suggested an apparent background pressure of 0.25 MPa (λ* ∼ 0.42 to 0.50). Because the borehole had been open for 12 hours prior to the packer tests, formation pore pressures may have exceeded this value prior to drilling of the borehole. These overpressures dissipated by the time the second set of tests were conducted. One possible explanation for this decay is that the borehole may provide a vertical conduit between the overpressured zone and overlying or underlying sediments that had previously been hydraulically separated from the overpressured zone. The second set of tests were conducted at pressures (≤0.019 MPa or λ* ∼ 0.03) below that estimated to maintain open fractures and yielded transmissivities 1 to 2 orders of magnitude less than estimated for the packer tests (when fractures were open). Constraints on fluid flow rate along the fault are provided by observed displacement in a bottom-simulating reflector (BSR) at its intersection with the fault zone. The closed-fracture transmissivities are insufficient to produce flow rates capable of displacing the BSR; therefore open-fracture transmissivities under conditions of elevated pore pressure are inferred to be necessary for the observed BSR displacement. In addition, calculated rates of specific discharge through the fault zone are 2 to 3 orders of magnitude lower than discharge measured at an associated seafloor vent site; fluid flow must become spatially or temporally focused as it moves up the fault zone toward the seafloor.

Hydrodynamic model of Great Sound, New Jersey

Abstract The two-dimensional, hydrodynamic HYDTID model was used to calculate velocities, flows and tidal heights for a variety of tidal conditions in Great Sound, New Jersey. Required input data include forcing tides, bathymetry, friction characteristics, wind data and any external inflows. Evaporation and rainfall were negligible for the short duration simulations conducted in this study. The model was calibrated against observed discharge and tidal elevation data for selected spring- and neap-tide events. Subsequently, the model was verified by simulating a mean tide event using an independent set of data. To demonstrate the predictive capability of the model for variations in properties such as bathymetry, a hypothetical simulation was conducted assuming that the Intracoastal Waterway was not dredged. Such simulations provide useful information on the system that is impossible to investigate by field data acquisition alone, and can be useful to investigators interested in flow patterns that control sedimentation and biological processes. Great Sound, a back-bay located behind the barrier island of Avalon and Stone Harbor, is connected to the Atlantic Ocean by Great Channel and Ingram Thorofare. The sound is surrounded by an expanse of marshland which is regularly inundated and drained by a network of feeder channels. The Intracoastal Waterway, an important hydrodynamic feature of Great Sound, enters from the south through Great Channel, passes through the eastern part of the sound, and exits through Ingram Thorofare. Because spring tide in Great Sound involves inundation of a vast expanse of marshland, a unique method of modeling the flooding and drainage of a very large area of marshland was developed. A single wide channel is used to model several feeder channels in order to model the system. Test simulations indicated that adequate representation of the overall hydrodynamics was achieved with this technique at a greatly reduced computational effort. The predicted flows and water levels were sensitive to variations in marsh cell elevation, feeder channel arrangement and allocation of forcing cells, and were relatively insensitive to variations in friction characteristics and bathymetry.

Hydrodynamics and sedimentation in a back-barrier lagoon-salt marsh system, Great Sound, New Jersey — A summary

Southern New Jersey is a barrier island coast, characterized by a tide-dominated hydrographic regime. Great Sound, a shallow, open lagoon which is fed by tidal channels within the back-barrier salt marsh complex, is a sediment sink, apparently for detritus imported from the inner continental shelf through two tidal inlets. Study of the system tidal hydrodynamics and sediment accumulation patterns provides the basis for a numerical sedimentation model. This model predicts rapid accumulation of coarse-grained (> 20 μm) sediment near the Intracoastal Waterway which cuts through Great Sound, and dominance of storm-related sedimentation events. Observations generally confirm the model predictions. Sands are deposited rapidly on flood tidal deltas associated with the two major channels, Great Channel and Ingram Thorofare, and along the Intracoastal Waterway. Finer detritus is transported predominantly as organic-mineral aggregates, and accumulates slowly (< 2.7 mm/yr) in the southwestern and eastern parts of the sound. Resuspension of bottom sediments is common in the shallow (0.6 m) sound due to wave action and flood tidal currents on the deltas (U0.4 d max ≲ 42 cm/s). Low tidal flow velocities (U0.4 d max < 18 cm/s) over much of Great Sound and the presence of macroalgae in some locations, however, promote net accumulation. Although sediment deposition and accumulation data are variable, the range of accumulation rates suggests that recent accretion in Great Sound is approximately equivalent to the local sea-level rise of 4 mm/yr.

Modeling deposition of suspensate in Great Sound, New Jersey

Abstract A sediment deposition model is developed for application to Great Sound, New Jersey. A determination of the average annual accumulation rate is of primary interest. The settling tank concept is used for the model, employing a plug flow approach to model the tidal hydrodynamics. Assumptions inherent in this modeling technique include no mixing between plugs, a uniform vertical velocity profile and simplified geometry. Model inputs were based on hydrodynamic and suspended sediment data obtained for Great Sound during other investigations, including initial volume in the sound at mean low water, the inflow hydrograph and tidal range, the sediment sizes, concentrations and settling velocities, and a frequency versus concentration relationship. The model simulates a single tidal cycle in Great Sound for spring, neap or mean tidal conditions for a specified sediment concentration. Three tests were run to define the sediment deposition characteristics of the sound. The first test defined the relative impact of spring, neap and mean tidal ranges on the deposition. Deposition during mean tide was found to be the average of the spring and neap tide deposition. Concentration hydrographs for ebb flow were determined. The second test determined the average annual sediment accumulation rate in Great Sound to be 8.9 mm/yr by running multiple tidal cycles for fair, pre- and post-storm, and storm conditions. Model predictions compare favorably with predictions of other researchers. The distribution of the average annual accumulation across Great Sound is also defined. In the third test, the relative influence of storm conditions versus predominant fair-weather conditions was established. Only 8 storm days are required to match a year of fair-weather deposition.