Wave Study at Taharoa Beach

The study was undertaken due to a lack of information on inshore sea state. Needed to know likely prevalent sea state and the typical beach profile changes generated. Also needed some forecasting of swell for safe anchoring of tankers. Beach profile data was collected, but is not reported in this thesis, seemingly as the Auckland of University (Civil Engineering Dept) had reservations about the data. Over the four months recorded, average (“characteristic”) wave height was 5 ft, and period of 10.5 seconds. Study concluded that the Sverdrup-Munk-Bretschneider (S.M.B.) method of forecasting (described in Chapter 11) works fairly well in this region and could be used to give advanced forecasts of heavy swell or wind seas arriving at Taharoa. This technique utilised daily newspaper weather maps to find wind speed, direction and fetch parameters necessary to obtain wave forecasts. The author developed graphs (Plate 11-1, 11-2) to translate isobaric spacing to geostrophic wind speed, and then to wind speed at sea level. Further graphs are used to assess fetch and duration factors. The approach is described in more detail in the thesis. Purpose: This thesis is linked to the development of iron sand mining on the west coast of New Zealand, in particular the Taharoa field. This had lead to problems including pipeline design and the mooring offshore of bulk tankers. For the installation and subsequent life of pipelines used to pump sand along the seabed, information was needed on beach profile changes due to offshore sea and surf conditions.

Contents: 1. Introduction 2. General theory 3. Hydrodynamic filtering 4. Theory of deep water wind wave generation 5. System used at Taharoa 6. General results 7. Spectral analysis of records 8. A use of power spectra 9. Decay of deep water wind waves 10. Waves in shallow water 11. Use of S.M.B. Forecasting method 12. Results at Taharoa 13. Conclusions Figure 6-1, 6-2, 6-3 and 6-4 summarise results of wave characteristics in June 1971, July 1971, August 1971 and September 1971. Includes average wave period, characteristic height(average of highest 1/3 of waves), and wind speed. Appendices include computer programs used in the I.B.M. 1130 for processing data obtained at Taharoa.

Immediately offshore of Taharoa Beach

Information was gathered between June and September (1971)

Instruments: strain gauge bottom mounted transducer. Detailed description of devices and installation/recording design is recorded within the thesis (Chapter 5). Information was gathered between June and September (1971) using a pressure transducer recording system (42 ft water depth. 4-hourly measurements of wave height. Continuous anemometer recording of local weather during this time. Wind recorded on a New Zealand Meteorological Service anemometer at the weather station ¾ mile from the sea at a height of approximately 100 ft above sea level. Also used maps printed daily in the NZ Herald.

Limitations: system often overloaded during storms so didn’t record the highest real time wave heights.

Assessments of wave characteristics.