- SPECIAL REPORTS
- THE MAGAZINE
The village of Bald Head Island, N.C., has been monitoring its beaches for about 10 years. Up till now, Brunswick Surveying (Supply, N.C.) had been cross-sectioning the beach every 400' for approximately 5 miles from the beachfront sand dunes to as far as a surveyor could wade through the surf at low tide to a depth of about 6' (approximately 200' from the high tide mark on the beach).
Bald Head Island has a tidal fluctuation of about 5.5'. It became evident that we were monitoring the erosion of the beach but not the cause of the erosion. Nor did we know where the sand was going. The village decided to expand the project to 3,000' offshore or to a water depth of 30'. The objective of the survey was to monitor the beach erosion and determine where the sand was going, where additional sand was coming from and what factors contributed to the erosion. We were then charged with suggesting measures that could be implemented to help control the erosion.
The magnitude of the tide fluctuation and the size of the expanded project posed some unique problems. We wanted to overlap the wading survey with the hydrographic survey, and we wanted to work at any stage of the tide. Since it's dangerous to operate a full-size survey boat near the surf zone, our work hours would be limited to a three-hour window. In addition, the wading survey would then have to be done at low tide to get an appropriate survey line overlap. Of course, high tide could fall midday one week, and low tide could fall midday the next week. On top of this, we needed relatively calm weather and sea conditions. It became obvious that we needed to devise a system that would enable the survey to take place at either high or low tide, thereby allowing us to work anytime with appropriate weather conditions.
Brunswick Surveying along with Innerspace Technology (Waldwick, N.J.) developed the Personal Watercraft Surveying System (PWSS) to fulfill the requirements of the project. The PWSS was designed as a highly mobile, hydrographic surveying vessel especially useful in shallow water areas, such as surf zone, coral reefs, berths and docking areas, ponds, canals and marinas. It is operated by one person and is easily transported from one site to the other. A Yamaha Wave Runner was selected for this particular project because of its 85 HP jet drive and adequate storage space for electronics.
The equipment selected included an Innerspace 455 survey depth sounder, 453A RF modems, remote indicator and datalog with Guidance software. The GPS used, an Ashtech GG 24 RTK system (Ashtech Precision Products, Magellan Corp., Santa Clara, Calif.), provided horizontal and vertical position. Innerspace developed a special multiplexer, the Innerspace 905, to synchronize the depth and RTK position data at the 1PPS (one pulse per second) available in the GPS receiver. A bi-directional voice radio system was also included. Balance of the equipment included system batteries for power, and associated charger and wiring junction box. All of the equipment was mounted out of sight in closed compartments except for the GPS and RF antennae and the depth sounder transducer. The instrumentation was mounted with quick-release mechanisms to facilitate easy removal so the equipment could be used on other projects when the PWSS was not needed.
On Land and at SeaOn Land
Equipment on shore:
- datalog with Guidance software on a single port notebook computer
- Ashtech RTK reference station
- one-half of the bi-directional hydrographic UHF data link
- the transmitter half of the Ashtech RTK UHF data link
- one-half of the bi-directional voice radio link box:
On the PWSS
Equipment on the watercraft
- survey depth sounder
- depth sounder "kickup" _transducer
- bi-directional RF UHF modem
- Ashtech GG24 RTK GPS
- bi-directional voice radio VHF
- remote indicator
- GPS and RF antennae
The PWSS is operated by one person who simply has to control the speed of the craft and the left/right steering as indicated by the easy-to-read display. There are no electronic systems to adjust while underway--everything is automatic. Via the Innerspace 603 display, the operator has an analog left/right steering indication and actual distance of line, distance to end of line, distance to start of line, bearing to start of line and depth. The 455 depth sounder, because of its design, is both small and lightweight, allowing it to fit inside the PWSS. It provides digitized depth for the data collection software. An analog mission storage feature also allows the LCD chart screens to be stored on a single-chip FlashDisk. If anomalies in the digitized data are noticed, the analog charts can be recalled for closer inspection of the area in question.
Data collection was via Innerspace's datalog with Guidance software, which was located in a shore-based computer. The bi-directional RF modem system was the data link that received depth and position data from the PWSS and sent pertinent information to the operator's display. This information, including trackline guidance, is displayed on both the shore-based computer and the PWSS display at the same time. RTK corrections are sent to the PWSS via the Ashtech RF modem. To ensure depth/position accuracy, depth is time-tagged to be synchronous with the 1PPS furnished by the GPS system. The third RF communications system is a voice link to enable shore station personnel to communicate to the PWSS operator.
The hydrographic data is converted into x, y, z comma-delimited files for AutoCAD Land Development Desktop (Autodesk, San Raphael, Calif.). A digital terrain model is generated from the coordinate files. Cross-sections and topographic maps are generated from the terrain model. Cross-sections from future surveys will be overlaid on the profile sheets for analysis to determine sand movement.
Brunswick Surveying was pleased with the performance of the PWSS because it enabled us to plan our survey day without consideration for the prevailing tide stages. This provided for tighter control of costs and enabled a reasonable estimate of when a particular series of survey lines could be completed.
Safety is by no means a minor concern. We believe that the PWSS is quite safe because of maximum operator visibility, excellent maneuverability and the fact that the operator is on not in the craft and can get off easily if necessary. Ease of transport and deployment are also significant advantages over a full-size survey boat. Finally, the PWSS operator has only to steer the craft and does not have to operate any electronic devices, eliminating potential distractions. Once the PWSS equipment is turned on, the system is automatic and survey management is in the hands of the shore-based survey control manager.
For more information on the PWSS, go to Innerspace Technology's website at www.innerspacetechnology.com and click on "Custom Products."