Hydrography

Background and Development

Information on wave conditions is generally only available at some distance from the coastline, obtained through satellite observations, measurements or wave prediction models. Such numerical modelling and measurement of currents, water levels and waves, particularly in areas with a complicated bottom topography is often difficult, due to a lack of information on currents and bottom topography. Further, calibration of such models is often performed via the conventional method of using point measurements, such as wave buoys. While this method provides good estimations of the height (and amplitude) of waves, it is based on a parameterisation of the hydro-dynamic processes and as such, it is difficult to validate the 2D model output.

A new methodology which uses remote sensing by marine radar for measurement of hydrographic parameters has made it possible to obtain spatial and temporal information on the frequency and directional properties of the wave spectra. This methodology used by SeaDarQ has considerable advantages to in situ numerical modelling and can be used to calibrate and verify wave models and to also integrate measurements and numerical models.

SeaDarQ uses remote sensing by marine radar and has developed a complex processing system which extracts hydrographical and Oceanographic parameters from the sea surface for hydro-dynamic study. Sequences of X-band radar images provide information on the movement of wave crests. Analysing the speed, length and direction of these wave crests provides information on the directional spectrum and surface current speeds. The principle of this wave and current analysis of the X-band radar is to measure the movement (speed and direction) of wave crests. In this way, the spatial pattern of individual waves can be determined. From this, a Fourier Analysis obtains the ocean directional wave spectra. The hydrographical component of the SeaDarQ system is therefore capable of deriving quantities of current, water depth and bottom topography from the ocean directional wave spectra in the measurement domain, and can provide spatial information on directional and wave periods 360 degrees around the centre of the radar. This information can be used effectively for Rapid Environmental Assessments.

Requirements and Specification

One requirement for certain functionalities of the SeaDarQ system and of X-band radars in general is that wind generated capillary waves need to be present - i.e. a disturbance of the water surface is needed in order to make certain measurements. With capillary waves it is possible to measure:

• Surface boundary conditions (fresh and salt water)
• Internal wake (shear effect)
• Land-water borders
• Bottom topography
• Current Seams
• Surf Zones

Equally, high winds which cause whitecapping and distort the reflection against the capillary waves limit the capability of the X-band radar to measure properties of the wave spectrum. As a guideline, wind speeds greater than 2 m/s are required for the measurement of the wave spectrum, and wind speeds above 20 m/s represent a practical upper limit for calculation of hydrographical information.

For the measurement of certain hydrographical parameters, such as wave information and current, water waves of 15 meters wave length and 0.5 meters wave height are necessary. When water waves are present, it is possible to measure:

• Wave frequency/length
• Wave speed
• Wave direction
• Wave height (with calibration using a wave buoy)
• Water depth
• Current speed vector

Measurement of Hydrographical and Oceanographic Parameters

Surface Current [m/s or kn]

• Measured water current parameters represent the average in the upper 5 meters of the water column.
• The dispersion relationship (relation between the wave length and wave period, and water depth and current) is used to measure the magnitude and direction of the water current.

Wave Height

• The SeaDarQ system provides an indication (not an absolute value) of wave height, which requires initial calibration with a wave buoy.
• The wave height measurement depends on the wind speed and the reflection from the surface.

Water Depth

• The water depth is determined using the SeaDarQ system by applying the dispersion relation and measuring the wave spectrum of the surface, which is influenced by current and water depth.
• The low frequency part of the dispersion relation is used, and by fitting a curve through frequencies smaller than 0.15 Hz the water depth is determined.

Bottom Topography

The mechanism by which topography manifests itself in the radar image consists of three steps:

• Bottom height variation will not only change the water current in the close vicinity of the bottom, but also at the water surface.
• Water current variation will disturb the roughness of the water surface.
• Change of the roughness of the water surface will modulate the radar reflections which become visible in the radar images as light and dark patches. These roughness changes are related to topography changes.

The SeaDarQ system measures the bottom topography gradient by imagining the same area for a relatively long time. It has proved capable of accurately measuring bottom topography in a number of instances. It has been used, for example, to reveal submarine sand waves off the coast of Ameland, The Netherlands.

Characteristics of the System

Region of Interest:

• The particular area of interest can be chosen by utilising the graphical functions of the SeaDarQ system.
• A circular, a pie, a rectangular area or a virtual wave buoy can be selected.

Resolution:

• The complete area, or a selective part of this area around the radar is divided into smaller overlapping squares. These squares have a configurable size, typically 750 by 750 meters.
• The centres of these squares are equally spaced with a configurable distance (grid) of 100 by 100 meters to 1000 by 1000 meters.
• SeaDarQ can measure a huge amount of points in the grid -the system calculates 300 points at high resolution.

Range:

• The range can be manually selected and maximised according to the waves present in the radar image.

Enhanced Options:

• Radar information can be set at an optimal for both a monitor and human eye combination.
• A histogram function is further available to change the systems response to the sea state conditions automatically or manually.

Storage

Hydrographic and Oceanographic data can be stored in three different ways:

• Real time storage from raw radar lines
• Geo Snap Shot - full resolution images from the system independent of zoom or contrast enhancements applied.
• Snap Shot - to capture images from the system

Testing and Validation

Validation and calibration of the SeaDarQ radar system and the hydrographic functionalities with in situ point measurements such as wave buoys and current poles has been completed.

As an example, the SeaDarQ system has used information from the radar located at Radar Post 02 on the Maasvlakte in the Port of Rotterdam from 2008 to date. In this case, the SeaDarQ system was validated with the Dutch Ministry of Water and Traffic Management, who used the system for measuring hydrographic information. The SeaDarQ system obtained spatial and temporal information and the results showed that at both 200 ns and 50 ns the SeaDarQ system correlated well with the current measurements and had an accuracy of 96 percent.

In this particular example, the presence of a busy shipping lane renders it impossible to measure in situ and validate numerical models which traverse the shipping lane. This testing proved that the SeaDarQ system can be effectively used in locations where point measurements are impracticable and is additionally capable of producing reliable and accurate measurements of the current in practical situations.