Description of sensor

The tide gauge consists of a Stevens 3-m-range pressure transducer arranged to measure piezometeric pressures below the shoreline sands and gravels. The current loop data (4-20 mA) are sampled for 30 ms every 3 minutes, digitally averaged for 15 minutes and recorded in a Hobo microstation data logger made by Onset Computer Corporation. The 15 minute data are transmitted via the Iridium satellite every 2 hours. the resolution of the measurements is 1 mm.

The pressure transducer input port is wrapped in copper mesh surrounded by a porous foam filter inside a 4-cm- diameter PVC tube m, 7-cm-long open at both ends and fastened to the base of a 61 cm long stainless steel bar. The bar is clamped to a 60 cm long helical screw pile whose base lies 1 m below beach level. The top of the bar was leveled relative to a nearby reference bench mark and to two microatolls 120 m to the north.

The first few days of data show that mean sea level is 83.6 cm above the zero pressure datum, i.e. 72.5 cm below the nearby bench mark, and 8.0 cm below the blue screw marker on the southern of the two microatolls, and 20.4 cm below the highest remaining point on the sampled northern microatoll.

Given a tidal range of more than 36 cm and assuming that the highest coral was immersed at low tide suggests that the microatoll here has been raised by 18-20 cm.

A confirmation of coral elevation relative to the pressure sensor is possible from the photograph. The tidal level was at 79.5 cm at 11 am 27 March, 4 cm below the 4-day mean. The highest point on the coral is approx. 24 cm above sea level when allowance is made for waves.

These coral emergence estimates (30 March 2010) are likely to be refined as more data become available.

We thank DataGarrison (Boston) who have kindly loaned a transmitter to the sea level monitoring project. The geodetic level was loaned to us by the USGS. The project is funded by the National Science Foundation.

Ca Ira Tide Gauge

18.52499 deg N, 72.65077 deg W

Located at the northern corner of the Ca Ira Orphanage. The photo below shows a group of Ca Ira villagers who assisted in the installation, surrounding the local radio station (Radio Cosmos) engineer, Hilaire Jovaty, who provided considerable help in the installation.

Incoming 15 minute sampled data may be viewed and downloaded at www.datagarrison.com user "geo" password "hobo". The data are displayed in local time with a latency of up to 2 hours.

The Iridium transmitter (box near top of fence) is located inside the sea wall of the orphanage and the cable passes beneath the grass to the beach sand 3.2 m in front of the corner of the sea wall. The exposed coral sand platform to the NW now ponds sea water at most stages of the tide.

Fig 2 Ca Ira microatoll photographed at 11:04 am local time 27 March. The leveling rod was placed on the highest point near the abundant growth of weeds on the recently uplifted coral.

Installation map and datum

Fig 3. The rectangle shows the orientation of the geodetic map above. Spirals indicate tripod turning points, square is transducer, orange circle is a reference bench mark, and blue circles are exposed corals at 18.52588N -72.65102E. The height of the zero of the tide-gauge datum is 1.039 m below the highest point on the exposed, sampled microatoll.

Fig 4 Recent data from the Ca Ira tide gauge

Fig 5 Richard Boyer pointing to the control bench mark located at the center step of the doorway above the sea wall at the northern entrance to the orphanage.