Ldo. Geología

Bathymetric Survey of Cardiff Grounds

Introduction
The aim of the study was to understand the sea-bed profile and how the data gathered differs from existing data shown on the chart. The total vertical uncertainty was also calculated in order to measure accuracy.
The location of the study was at Cardiff grounds sand banks, these sand banks are located approximately 2.5 nautical miles fromthe coast, south east of Cardiff in the Bristol Channel. The Bristol Channel is a high energy environment with a tidal range of 15meters, tides go up the Bristol Channel at 2-4 knots on the flood and back down the Bristol Channel at 2-4 knots on the ebb. This high energy means that the profile of the sea bed is almost always changing, especially in areas such as Cardiff grounds, due to the highmobility of the sand banks.

Location of study

Figure [ 1 ] (EDINA 2011)

The accurate profiling of the sea bed is vital for safe navigation for commercial and private vessels, and this is particularly important in the Bristol Channel due to the large amount of shipping traffic but even more important at Cardiff grounds as the sandbanks lie in the approaches to Cardiff bay and Cardiffdocks. This need for accurate bathymetric surveying is proven by the amount of vessels that have run aground Figure 1 above shows the area of study as well as the close proximity to the entrance to the bay.
The study was carried out on December 3rd 2010 between the hours of 13:00 – 15:00, the weather conditions in the morning were calm and visibility was normal, temperatures were very low with anaverage reading of less than 5⁰C, however during the afternoon wind speeds picked up to force 3 ( 12km/h) and sea conditions became rougher. Precipitation was light but almost constant.

Methodology
The survey was carried out on the 35ft University research vessel the Guiding Light,
A series of parallel transects were inputted into the Hydropro system so that the vessel would be able to hold astraight line along the chosen transect, if the vessel strays from the straight line then the amount of correction needed is shown on the Hydropro GPS and adjusted by the Captain of the vessel accordingly. (see fig3)
Trimble Ag122 – this is a Differential Global Positioning System used for accurate positioning. The centre of GPS is the use of triangulation to calculate the users location, this isachieve through the use of  a network of 24 satellites placed into orbit by the U.S. Department of Defence. Differential GPS is an improvement of normal GPS as DGPS requires a GPS receiver be set up at a precisely known location, this stationary base station receiver calculates its exact position based on satellite signals and compares this given location to the known location. The differencebetween the data is applied to the GPS data recorded by the second GPS receiver which is located on the vessel. This makes this positioning system highly accurate however, US official documents and IALA state that the achievable accuracy degrades at an approximate rate of 1 m for each 150 km (80 nm) distance from the broadcast site
Odom Hydrotrac – this is an echo sounder designed for use on smallsurvey boats. The equipment finds the distance between the transducer and the sea bed by transmitting an ultrasonic wave through the water, when this sound come in contact with the seabed or other object then some is reflected back to the transducer. The depth of the object or sea-bed can be determined by calculating the time difference between the sending of a sound wave and the reception of thesound, this is possible because the speed of sound in water is a known value of 1500m/s.
Coda Otopus F180 motion sensor- this piece of equipment is used to accurately monitor the heave, roll, pitch, heading and positioning information in real time. Data which is vital for survey work..
Valeport Midas- this sound velocity probe was deployed in order to measure the velocity of sound in water...