Three Dimensional Hydrodynamic-Sedimentation Modeling Study Hampton Roads Crossing, Lower James River, Virginia
Abstract
A three-dimensional hydrodynamic-sedimentation computer model, HYSED-3D, was used to evaluate the effect of bridge-tunnel infrastructure for a proposed highway crossing of Hampton Roads on the physical characteristics (tides, currents, circulation, salinity, and sedimentation) of the James River estuary in Virginia. Model-represented infrastructure included tunnel islands and bridges on pilings connecting the islands to interstate highways in Newport News, Hampton, Norfolk, and Portsmouth, Virginia. Combinations of these elements occur in each of three proposed crossing routes designated Alternative 1 (Hampton-Norfolk), Alternative 2 (Hampton-Norfolk, Norfolk-Portsmouth), and Alternative 9 (Newport News-Portsmouth-Norfolk). Simulation comparisons were made between the existing waterways and infrastructure in Hampton Roads (Base Case) and the proposed construction in a series of model test runs representing both normal and extreme hydrologic conditions. Variations in tidal range were simulated using a threeconstituent tide model. Three levels of freshwater inflow into the headwaters of the James River were represented using historical stream gauge data. The simulation of sedimentation was designed based on the existence of a 'turbidity maximum' upstream from the area of concern. Based on the hydrodynamic characteristics and the layout of the proposed crossings, nine and seven stations, respectively, were selected for examination of potential changes in tidal height and current velocity. No changes in tidal height were observed at any of the nine stations in response to the three alternatives over the range of conditions tested. Small changes were observed in time histories of surface and bottom current at three of the seven representative locations. The three locations of observed change are proximal to tunnel islands associated with Alternatives 2 and 9. An examination of the spatial (plan-view) distribution of the instantaneous current field revealed that most of the changes were caused by local deviations in the direction and phase, not the magnitude, of the current. Further examination of instantaneous and time-averaged (residual) current data shows that some aspects and features of the general circulation may also be altered by Alternatives 2 and 9. A recurring tidal front near Newport News Point showed a minor phase change in response to Alternative 9. A residual current 'eddy' in the Elizabeth River entrance diminished in response to Alternatives 2 and 9. Unlike the tidal front near Newport News Point, the existence of the eddy at the entrance to the Elizabeth River has not been confirmed by field observation. Salinity changes were observed in the vicinity of bridges on pilings, especially those for Alternative 9. This was primarily a response to piling-induced turbulence, increased vertical mixing and the elimination of surface-to-bottom salinity gradients immediately downstream. No changes were observed in the longitudinal salinity distribution along the channel axis of the James River in response to any of the alternatives under any of the conditions tested. Sedimentation was simulated in terms of both the sedimentation potential and the simulated accumulation of suspended material from a designated source upstream. Similar to bottom salinity, a small decrease in sedimentation occurred in the vieinity of bridge pilings for Alternatives 2 and 9 in response to turbulence-induced vertical mixing caused by these structures. A small increase in sedimentation potential was observed at the northeast end of Hampton Flats in response to structures added by Alternative 1. Conditions of high river inflow greatly increased the amount of fine-grained sediment reaching the lower James River from upstream.
