John Brubaker

Control of Estuarine Stratification and Mixing by Wind-induced Straining of the Estuarine Density Field

Observations from the York River Estuary, Virginia, demonstrate that the along-channel wind plays a dominant role in governing the estuarine exchange flow and the corresponding increase or decrease in vertical density stratification. Contrary to previous findings that suggest wind stress acts predominantly as a source of energy to mix away estuarine stratification, our results demonstrate that the wind can play a more important role in straining the along-channel estuarine density gradient. Down-estuary winds enhance the tidally-averaged vertical shear, which interacts with the along-channel density gradient to increase vertical stratification. Up-estuary winds tend to reduce, or even reverse the vertical shear, reducing vertical stratification. In two experiments each lasting approximately a month, the estuarine exchange flow was highly correlated with the along-channel component of the wind. The changes in stratification caused by the exchange flow appear to control the amount of vertical mixing as parameterized by the vertical eddy viscosity. The degree of stratification induced by wind straining also appears to play an important role in controlling the effectiveness of wind and tidal mixing.

Acoustic-microwave water level sensor comparisons in an estuarine environment

Microwave water level sensors offer certain advantages over the acoustic sensor, the present standard for water level measurements obtained in U.S. coastal areas by the National Oceanic and Atmospheric Administration (NOAA). These include high reflectivity of microwave radiation from the target medium (water), low sensitivity to variations in air temperature and humidity, and open-beam transmission eliminating any contact between the device and the water. The latter feature has raised the question of possible interaction between time-of-flight microwave measurements and wind wave motion at the air-water interface. A field comparison between a microwave sensor and the NOAA acoustic water level sensor at Yorktown, Virginia revealed close agreement between sensor measurements in an operational setting and produced no evidence of an dasiaoffsetpsila in the presence of irregular surface gravity waves. However, unlike the acoustic sensor which has a mechanical filter (stilling well) to eliminate wave motion above a fixed dasiacutoffpsila frequency, microwave sensors operate without a stilling well and require numerical filtering to obtain water level measurements in the frequency range of interest; i.e., tidal and sub-tidal frequencies for the classic dasiatide stationpsila. Numerical methods now offer greater choice in deciding where to make the cutoff while reducing measurement error.

Flow convergence and stability at a tidal estuarine front: Acoustic Doppler current observations

Characteristics of the flow field in an estuarine frontal zone have been investigated in a field study in the lower James River estuary. Underway sampling with an acoustic Doppler current profiler (ADCP) on repeated transects across the front provided information on the structure of the flow field near the front and its evolution in time. As this tidal intrusion front advanced up the estuary during the flooding tide, prominent and consistent features in the velocity field included a localized zone of convergent flow beneath the visible surface line and a stratified shear layer just upriver of the front. Within the shear layer between the buoyant surface water and the faster, higher-salinity undercurrent, gradient Richardson number estimates suggest that the flow was at or near the threshold for shear instability. Another shear-type gradient in the flow field, the across-front variation of the along-front velocity component, strengthened over a sequence of transects, with intensity increasing toward the surface. Tracking of the front was then interrupted when the identifying line of foam and accumulated material on the surface, previously sharp and well defined, broke up and dispersed to such an extent that the visible signature of the front was lost temporarily. A visible frontal expression later reappeared, and propagation upriver continued. Lower bound estimates of downwelling flow in the frontal zone were determined by continuity considerations.

Underwater sunlight maxima in the Menai Strait

In a tidal sea, the time variation of underwater daylight depends on the rise and fall of the tide as well as the motion of the sun across the sky. If conditions are right, more than one maximum in underwater daylight can occur in a single day. In this paper we apply a simple analytical solution for the number of maxima and their timing to observations in the Menai Strait in Europe. The number of maxima depends on the value of the parameter γ = kRω2q2 where k is the diffuse attenuation coefficient, R is the tidal range, ω the angular frequency of the tide and q is a factor proportional to the day length. If the value of γ is greater than a critical value, and the day is long enough, then a double maximum in underwater sunlight is formed. The maxima lie between noon and low water, and their time relative to noon is given by tL = γ/(γ+4) where tL is the time of low water relative to noon. The critical value of γ varies from 4 to about 18, depending on the time of high water. The necessary length of day to produce the double maximum depends on the time of low water relative to noon, the required condition being |tL|<3q/2+6q/γ, where tL is the time of low water in hours relative to noon. The observations reported here are in agreement with these predictions. In the high tidal ranges of the Menai Strait, γ exceeds the critical value for about half the spring–neap cycles in summer, and the daily pattern of illumination switches from a double peaked to a single peaked curve. Maxima occur as early as 6 am and as late as 7 pm. Over this complete range, the times of the observed maxima are close to those predicted, the rms difference between times of observed and predicted maxima is about 45 min and the mean difference is less than 10 min. We conclude that the analytical theory presented here is a useful practical tool for predicting maxima in underwater sunlight in a tidal sea.

Three Dimensional Hydrodynamic Modeling Study Craney Island Eastward Expansion, Lower James River and Elizabeth River, Virginia

Recommended Citation Wang, H. V., Kim, S. C., Boon, J. D., Kuo, A. Y., Sisson, G. M., Brubaker, J. M., & Maa, J. P. (2001) Three Dimensional Hydrodynamic Modeling Study, Craney Island eastward expansion, lower James River and Elizabeth River, Virginia. Special report in applied marine science and ocean engineering ; no. 372.. Virginia Institute of Marine Science, College of William and Mary. https://doi.org/10.21220/V5372G

Analysis of extreme water levels in the lower Chesapeake Bay

The lower Chesapeake Bay (LCB) experiences coastal flooding due to both tropical and extratropical systems. Water levels produced from these events is often called storm surge; however, storm tide (a combination of the predicted tide, storm surge, and local anomaly), is a more appropriate term. Storm tide levels vary spatially in the LCB. Here we assess this spatial variability relative to local datums, such as highest astronomical tide (HAT) and mean lower low water (MLLW). We determined whether there was any difference in hours above HAT between each of our stations for each storm. We revealed a trend that central bay stations, such as Windmill Point and Lewisetta, spend more time above HAT than the stations in the south bay and Washington. The maximum water level above MLLW and HAT was determined for each station and storm and then analyzed for spatial trends. We found that, in general, central bay stations have smaller maximum heights above HAT than the south bay stations. Washington does not follow the pattern of other large tidal-range stations, such as the south bay stations.