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The Challenge of Measuring Water Currents

The first indication that current measurements are a challenge to obtain is the fact that to deploy and maintain a current meter is anywhere from four to ten times as expensive to do as similar activities to measure water levels. This has important implications for the quality and breadth of current observations and tidal current predictions available today to the Nation's mariners.

This increased expense can be readily appreciated by noting a few sharp contrasts between the behavior of water levels and currents. Water level is relatively the same over a wide area; therefore, water level measurements can be made from the relative convenience of dry land along a nearby shoreline. By contrast, current speed and direction can be very localized, varying greatly over short distances as bottom contours and shoreline configuration alter both the current's speed and direction of flow as well as spinning off eddies. Given these circumstances, if you wish to know the current at a particular locationin the bay or channel you must leave the comfort of the shore and accept the expense and endure the effort to place your instrument exactly "there" or perform a parallel measurement to estimate being exactly "there."

Getting "there" illuminates further challenges. One can load the workings of a water level station (tide house, water level sensor, electronics, etcetera, all valued at about $15,000.) into a large truck and drive to your preferred shoreline location for a few hundred dollars per day. Installation is done mostly from the safety and working convenience of dry land. By contrast, the equipment and deployment of current measuring devices is more expensive and involved. First, the equipment to measure currents (current sensor, electronics and various bottom anchors, cables and floats, etc.) is valued at about$40,000. Next, your truck will only get you to the ship's dock. You and yourequipment need to be out on and in the water and the boat to do that will typically cost several thousand dollars per day.

Staying "there" long enough to obtain a meaningful observation reveals additional challenges. Most of the components of a water level measuring system (tide house, electronics, sensor) are on dry land and thus subject to slow corrosion and weathering. Routine maintenance on such an installation typically occurs once each year. By contrast, all of a current measuring system is typically in salt water and thus subject to both rapid corrosion and fouling by marine growth. Such an installation must routinely be visited at least four times per year for cleaning and inspection. And remember, each visit requires a boat and divers to perform even the simplest inspection.

Some of the forgoing explains why the current observations which we do have are of shorter duration, at fewer locations, and less up-to-date than we have for water levels. In fact, continuous current observations only began a few years ago. Previously, current observations were typically made for only a few days, at most a month, at any location. By contrast, continuous water level observation at many locations go back to the mid 1800s. In addition, most of the current observations were made so long ago that the technology for measurement, though sophisticated at the time, is quite primitive by today's standards.

Moreover, as stated above, currents are strongly influenced by local conditions and can change in dramatic and unknown ways when those local circumstances change. In fact, such changes occur all the time. For example, shipping channels are dredged deeper and wider, or natural processes move sand bars or reshape the bottom. These changes will alter the current strength and direction in unknown ways and tidal current predictions and forecasts based upon older observations are at least questionable and may no longer be valid. The only way to know for sure is to reoccupy the site and make new current observations.

As a result of these challenges, current observations are important for shipping, commercial fishing, recreational boating and the safety of life, property and natural habitats both on the water and on shore. A knowledge of predicted, real-time and short-term forecasted currents is critical to safely docking and undocking ships, maneuvering them in confined waterways and making safe passage through our coastal waterways. With this knowledge commerce and people arrive on schedule. Lack of the knowledge can have serious consequences.

Revised: 10/15/2013
NOAA / National Ocean Service
Web site owner: Center for Operational Oceanographic Products and Services