Measuring Water Levels

A tertiary tide gauge set up.
A tertiary tide gauge set up consists of the gauge in a black weather-proof case, solar panel, and nitrogen tanks that feed gas to the orifice. The gauge transmits data once and hour to COOPS via GOES satellite antenna .

Depths on NOAA nautical charts of tidal waters are relative to the local Mean Lower Low Water (MLLW) datum. Determining this datum and accurate water level correctors for hydrography requires a network of water level measurement stations. In remote areas, NOAA hydrographic field parties establish temporary land-based tide gauges, or tertiary control stations, to complement the permanent National Water Level Observation Network (NWLON) tide stations maintained by NOAA’s Center for Operational and Oceanographic Products and Services (CO-OPS).

The tide gauge set-up consists of a computer housed inside a weather-proof case, batteries and a solar panel, a nitrogen tank connected to an orifice on the seafloor, a graduated staff, and components for a satellite uplink. The gas-purged pressure gauge has a sensor to measure the water level above the orifice. In addition, a set of tidal benchmarks are installed around the gauge site so that the water level data collected by the gauge can be reduced to the tidal datum used for NOAA’s hydrographic survey operations. When the elevation of the primary benchmark relative to the orifice is known, the tertiary station data can then be reduced to tidal datum and compared to a NWLON station.

RAINIER Survey Technician Matt Boles prepares to install a benchmark. Benchmarks are usually placed in bedrock, and set with cement so that their position will be as stable as possible
Rainier Survey Technician Matt Boles prepares to install a benchmark. Benchmarks are usually placed in bedrock, and set with cement so that their position will be as stable as possible

The Benchmark (left) represents a vertical datum reference point and establishing its permanence is vital.

The orifice (right) must be attached to a plate, which is then weighted to the bottom with dive weights, chain, or rock. Nitrogen tanks on land are employed to feed gas to the orifice on the sea floor.

RAINER crew trained as a NOAA working diver and a picture of the bubbler orifice.
Rainier crew members, trained as NOAA working divers install the bubbler orifice. The neoprene bubbler tube running to the gauge is also secured by divers under the surface

A graduated staff is installed in the tidal zone by RAINER crew.
The graduated staff is installed in the tidal zone by Rainier crew.

The benchmarks are leveled to a graduated staff (right) that is also semi-permanently installed at sea level, on which water level observations are taken periodically during the station occupation. Leveling is conducted with a collimated optical instrument set on an undisturbed tripod (right), which allows the operator to read the level rod to the nearest millimeter.

RAINIER crewmeber helps level the graduated staff.
Rainier crewmeber helps level the graduated staff.