Hydro and Science

Regional Patterns and Local Variations of Sediment Distribution in the Hudson River Estuary
Nitsche, F.O., Ryan, W.B.F., Carbotte, S.M., Bell, R.E., Slagle, A., Bertinado, C., Flood, R., Kenna, T. and McHugh, C., 2007.
Estuarine, Coastal and Shelf Science Sedimentological and Ecohydrological Processes of Asian Deltas: The Yangtze and the Mekong, 71: pp 259-277.

The Hudson River Benthic Mapping Project, funded by the New York State Department of Environmental Conservation, resulted in a comprehensive dataset consisting of high-resolution multi-beam bathymetry, side-scan sonar, and sub-bottom data, as well as over four hundred sediment cores and six hundred grab samples. This detailed data set made it possible to study the regional pattern and local variations of sediment distribution in the Hudson River Estuary. Based on these data, we describe the distribution of sediment texture and process-related sedimentary environments for the whole 240-km long estuary, together with along-river variations in depth, cross-sectional area and grain-size distribution. We compare these parameters with changes in surrounding geology and tributary input and as a result divide the Hudson River Estuary into eight sections with distinct combinations of channel morphology, bedrock type, sediment texture and sediment dynamics. The regional sediment distribution consists of marine sand-dominated sediments near the ocean end of the estuary, a large, mud-dominated central section, and fluvial sand-dominated sediments in the freshwater section of the Hudson River Estuary. This regional trend is highly modified by small-scale variations in the sediment distribution. These local vari­ations are controlled by changes in morphology, bedrock and tributary input, as well as by anthropogenic modifications in the estuary. In some areas, these local variations are larger than the overall trend in sediment distribution and control the actual sediment type as well as the condition of erosion and deposition in the estuary.

Bathymetry Impacts on Water Exchange Modelling Through the Danish Straits
She, J., Berg, P. and Berg, J., 2007.
Journal of Marine Systems: Marine Environmental Monitoring and Prediction - Selected papers from the 36th International Liège Colloquium on Ocean Dynamics, 65: pp 450-459.

Deep and narrow channels in the Danish Straits are one of the governing factors for the Baltic North Sea water and salt exchange. The channels have a depth of up to 50m and a horizontal scale of a few hundred meters. The typical horizontal resolution used in current operational 3-dimensional Baltic North Sea models is one nautical mile (nm), which cannot resolve these deep channels well. In this paper, an alternative method is used to generate the 1nm resolution bathymetry so that the deep channel is well resolved and at the same time the total water volume is roughly conserved. The impact of the new bathymetry on modelling water and salt transports as well as temperature and salinity structure was assessed by comparing a three-year model run with the adjusted bathymetry and a control run with the averaged bathymetry. The temperature and salinity structure was significantly influenced by the bathymetry, especially during inflow events. The stratification was strengthened and the bottom salinity was increased in the Danish Straits and adjacent waters.
The bathymetry impact was significant throughout the entire three-year model run, and the signal was propagated to a large area covering the Stopple Channel. Comparison with observations showed that such changes are positive improvements to the models. The results suggested that the bathymetry of the deep channels in the Danish Straits have to be carefully resolved in order to correctly simulate the Baltic North Sea water exchange.


Efficient Algorithms for Finding Sills in Digital Topographic Maps
Otto, T.D. and Thurnherr, A.M., 2007.
Computers & Geosciences, 33: pp 678-684.

In stratified geophysical flows, the energetically optimal exchange of dense fluid across a topographic barrier generally takes place at the deepest unblocked connection, which is typically a saddle point (sill). The flow at or near a sill is often hydraulically controlled, in which case the sill is called a controlling sill. Oceanographic examples include overflows of newly formed dense water at high latitudes as well as sills in channels connecting major ocean basins, such as the Strait of Gibraltar. Controlling sills are usually associated with strong flows, making them ideal sites for monitoring transport and hydrographic variability. The locations and depths of controlling sills also provide strong constraints for the downstream hydrographic properties below sill depth. In this paper, two algorithms for find­ing sills in digital topographic maps are presented. The first approxi­mates the sill height to arbitrary precision in O( k ) steps, where k is the number of data points in the map. The second algorithm, which requires O( k log k ) steps, additionally returns the sill location. Several tests carried out with realistic problems from physical oceanography reveal that the second algorithm runs faster in practice, even though its worst-case behaviour is worse.

Accuracy Assessment of Ocean Tide Loading Computations for Precise Geodetic Observations
Zahran, K.H., Jentzsch, G. and Seeber, G., 2006.
Journal of Geodynamics, 42: pp 159-174.

The increasing resolution of ground-based gravity measurements (for example, by superconducting gravimeters) as well as satellite-based gravity field studies allows the study of very small signals, globally as well as locally. For detailed studies of the Earth's deep interior, the signals of gravity measurements must be corrected for disturbance related to ocean tidal loading.

Although new ocean tide models are being derived from current satellite missions, uncertainties remain. In this paper, we present an intercomparison of ocean tide models to test their fit to worldwide observations. For this, three TOPEX/POSEIDON (T/P) satellite-derived models (CSR3.0, FES95.2 and TPXO.2) and the classic Schwiderski (SCHW80) model were selected for an intercomparison test including tide gauge validation and comparison to 59 tidal gravity stations.

The tests showed that the T/P-based models gave correct results for the open ocean but, in the coastal regions, model and observations deviate. The tide gauge validation shows that the T/P-derived models fit tide gauges better than did the SCHW80 model, with a better fit for the semidiurnal constituents than for the diurnal constituents. The evaluation of the gravimetric ocean tide loading showed that the T/P-derived models worked better compared with the Schwiderski model, especially in M2.