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Real time flow information system
Abstract
Ships and their pilots have difficulties to enter a harbour during periods of relatively high flow velocities. We developed a system that gives real time depth average flow velocity and direction vectors over a large area. The system also predicts the flow in that area for 24 hours, allowing ships to plan their voyage more accurately and safely. The systems consists of a horizontal current profiler which is mounted at the side of the river. A computer flow model computes the depth average flow in the area based on measured flow and on measured water levels. The flow prediction is also based on measured currents and on measured water levels. The Port of Rotterdam has deepened the river Rhine and the Meuse branch, which are the main access channels to the Waalhaven. This allows large container ships to enter the harbour. Currently the ships and their pilots are only allowed to enter the harbour during periods when the water level is sufficiently high and the current velocity in the river is smaller than 0.5 knots. This limits the time to enter the harbour to 1 hour every tidal cycle and makes planning of a trip difficult.
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Nowcasts of the surface velocity field in Monterey Bay are made for the period August 1–9, 1994, using HF radar observations blended with results from a primitive equation model. A spectral method called normal mode analysis was used. Objective spatial and temporal filtering were performed, and stream function, velocity potential, relative vorticity, and horizontal divergence were calculated over the domain. This type of nowcasting permits global spectral analysis of mode amplitudes, calculation of enstrophy, and additional analyses using tools like empirical orthogonal functions. The nowcasts reported here include open boundary flow information from the numerical model. Nowcasts using no open boundary flow information, however, still provide excellent results for locations within the observation footprint. This method, then, is useful for filtering high-resolution data like HF radar observations, even when open boundary flow information is unavailable. Also, since the nowcast velocity gradient fields were much less noisy than the observations, this may be an effective method for preconditioning high-resolution observation sets for assimilation into a numerical model.
We describe a nontraditional method for processing data derived from quasi-Lagrangian tracers. The approach relies on a representation of the thermohydrodynamic field as a set of a finite number of functions (modes) with the subsequent computation of mode amplitudes from quasi-Lagrangian data. This requires special methods for calculating mode components of the thermohydrodynamic field from Lagrangian observations. Also, because of the quasi-Lagrangian nature of the data there are special problems caused by the need to filter in both space and time and to account for motions unresolved by such observations. The former problem is addressed by a filtering procedure based on a variational criteria and Pontryagin's principle. To handle the latter problem, a subgrid parameterization scheme appropriate for Lagrangian data is proposed.
The numerical electromagnetics code (NEC) is a general purpose computer
program for modeling antennas by the numerical solution of an integral
equation. NEC solves an electric field integral equation for wire
structures and a magnetic field integral equation for surfaces, and can
evaluate the effect of an interface between air and a lossy half space
by means of a reflection coefficient approximation or the rigorous
solution involving Sommerfeld integrals. This report describes an
extension of NEC to model an insulating sheath on wires in air or buried
in a lossy half space including, if desired, the effect of the
air-ground interface.
This paper deals with an estimation of the effect of sea state on HF and
VHF ground-wave propagation loss across the ocean. For this estimation,
an expression is employed that was derived in part 1; it gives the
effective surface impedance in terms of the spatial height spectrum of
the surface. Two empirical models are employed for the height spectrum
of the ocean, the directional Neumann-Pierson model and the isotropic
Phillips wind-wave model. These effective surface impedances accounting
for sea state are then inserted into a standard ESSA computer program
giving the basic transmission loss above a spherical earth. The
dependence on sea state is plotted in curves which show the excess loss
in decibels over a smooth sea. The propagation loss to points at various
heights above the sea is also calculated. Normal sea state variations
are seen to be negligible below about 2 MHz but produce a maximum excess
loss at about 10 to 15 MHz.
HF radio waves backscattered from the ocean surface can be used to measure ocean surface currents. Measurements of the range-Doppler spectrum of these signals yield the wavenumber k and the frequency ω of an ocean surface wave and its phase velocityν = ω/κ relative to the radar. Subtraction of the phase velocity of the wave in still water, c = √(g/k), yields a measure of the average current from the surface to a depth of the order of (2k)−1. A measure of the current shear is obtained by observing at more than one radio frequency. To test these ideas, surface currents were measured using both a conventional and the HF technique, and reasonable agreement was found.
During the summer of 2000, a bistatic UHF radar - the RiverSonde - was designed, built, and tested on rivers and canals in the Central Valley of California. The transmitter and receiver were on opposite banks. They simultaneously transmit to and receive from elliptical time-delay cells that span the river, with the transmit and receive antennas as their focal points. With 30 MHz bandwidth, the cell span up/down-river is ∼10 m. A three-element receive array employs the direction finding music algorithm to determine echo bearing. Velocity along the river channel is measured vs position across the river from the first-order Bragg-echo Doppler shifts. Radiating less than 1 w power, received surface-echo signal-to-noise ratios of 40 dB were received, both across narrow canals and across the American River that was 80 meters wide. Our tests and analyses were sponsored by and conducted along with the U.S. Geological Survey in Menlo Park, CA. "Surface truth" velocity profiles were established by current meters suspended from a boat, from a bridge, and from timing the drifts of tennis balls between two transverse cuts. RMS velocity differences between 6%-13% of the typical average flow velocity were observed. The rms differences between the three "surface truth" measurements themselves also fell within the same span. From the velocity profiles across the river, estimates of total volume flow for the four methods were calculated based on a knowledge of the bottom depth vs position across the river. The flow comparisons for the American River were much closer, within 2% of each other among all of the methods. Sources of positional biases and anomalies in the RiverSonde measurement patterns along the river are identified and discussed.
To fully understand the hydraulic properties of natural rivers, velocity distribution in the river cross-section should be studied in detail. The measurement task is not straightforward because there is not an instrument that can measure the velocity distribution covering the entire cross-section. Particularly the velocities in regions near the free surface and in the bottom boundary layer are difficult to measure, and yet the velocity properties in these regions play the most significant role in characterizing the hydraulic properties. To further characterize river hydraulics, two acoustic instruments. namely, an acoustic Doppler current profiler (ADCP), and a "BoogieDopp" (BD) were used on fixed platforms to measure the detailed velocity profiles across the river. Typically, 20 to 25 stations were used to represent a river cross-section. At each station, water velocity profiles were measured independently and/or concurrently by an ADCP and a BD. The measured velocity properties were compared and used in computation of river discharge. In a tow-tank evaluation of a BD, it has been confirmed that BD is capable of measuring water velocity at about I I cm below the free-surface. Therefore, the surface velocity distribution across the river was extracted from the BD velocity measurements and used to compute the river discharge. These detailed velocity profiles and the composite velocity distribution: were used to assess the validity of the classic theories of velocity distributions, conventional river discharge measurement methods, and for estimates of channel bottom roughness.
Results of field tests of a RiverSonde streamflow radar are
compared with in-situ current measurements at a canal and a river in
central California during June, 2000. Typical water velocity in the
middle of the canal was about 0.45 m s-1 and 0.30 ms<sup>-1
</sup> at the edges. Velocity in the river was about 20% lower with
similar cross-channel variation. Differences between the RiverSonde and
in-situ velocities were 6-18% of the mean flow, with similar differences
among the various in-situ velocities. In addition to the surface
velocities, the total volume flow was estimated based on the in-situ
depth measurements. Volume flow for the canal was about 37 m3
s-1 and for the river was about 64 m3 s<sup>-1
</sup>, with differences between the various radar and in-situ
techniques of less than 10%
A high-frequency multifrequency coastal radar operating at four
frequencies between 4.8 and 21.8 MHz was used as part of the third
Chesapeake Bay Outflow Plume Experiment (COPE-3) during October and
November, 1997. The radar system surveyed the open ocean east of the
coast and just south of the mouth of Chesapeake Bay from two sites
separated by about 20 km. Measurements were taken once an hour, and the
eastward and northward components of ocean currents were estimated at
four depths ranging from about 0.5 m to 2.5 m below the surface for each
location on a 2 by 2 km grid. Direction of arrival of the signals was
estimated using the MUSIC algorithm. The radar measurements were
compared to currents measured by several moored acoustic Doppler current
profilers (ADCPs) with range bins 2-14 m below the water surface. The
vertical structure of the current was examined by utilizing four
different radar wavelengths, which respond to ocean currents at
different depths, and by using several ADCP range bins separated by 1-m
intervals. The radar and ADCP current estimates were highly correlated
and showed similar depth behavior, and there was significant correlation
between radar current estimates at different wavelengths and wind speed
Processing the signals received on an array of sensors for the location of the emitter is of great enough interest to have been treated under many special case assumptions. The general problem considers sensors with arbitrary locations and arbitrary directional characteristics (gain/phase/polarization) in a noise/interference environment of arbitrary covariance matrix. This report is concerned first with the multiple emitter aspect of this problem and second with the generality of solution. A description is given of the multiple signal classification (MUSIC) algorithm, which provides asymptotically unbiased estimates of 1) number of incident wavefronts present; 2) directions of arrival (DOA) (or emitter locations); 3) strengths and cross correlations among the incident waveforms; 4) noise/interference strength. Examples and comparisons with methods based on maximum likelihood (ML) and maximum entropy (ME), as well as conventional beamforming are included. An example of its use as a multiple frequency estimator operating on time series is included.