Friday, October 29, 2021

ENC Object: Tidal Stream, Flood and Ebb (TS_FEB)

 We have two ways to learn about tidal currents in a typical navigation program, such as qtVlm and OpenCPN that we use in our online courses, and numerous other commercial versions. 

First, we have the overlay of current and tide predictions that the navigation program itself computes for us, based on the harmonic constants we have loaded.  This source of tide and current data is our main workhorse for navigation, but it has nothing at all to do with the ENC charts themselves. It is a program function, unique to each program in the details of how it presents the results. These current predictions work perfectly well looking at ENC or at an RNC, but we can also shut off the charts completely and just use the  base map to obtain currents over the areas covered by the harmonics loaded.

Completely independent of those forecasts are the tidal current summaries that are included in the ENC and indeed in many cases on the related RNC as well, and it is these ENC current data that are the subject at hand.

The ENC object is called Tidal stream, flood and ebb (TS_FEB), the symbol is a current arrow, usually in pairs, one showing the flood direction, the other showing the ebb direction. On the RNC these are labeled with an F and and E, but not on the ENC—but  it is easy to tell because: "the Flood has the Feathers."  You can cursor pick these objects, but we do not learn more than shown, namely the average direction of the flood and ebb along with the average maximum speeds.


A cursor pick gives the report below.

There are a couple nuances to the use of this parameter. One is the S-57 standard for this current velocity is the average of the maximum spring currents, whereas the NOAA standard is to use the full average of all maximum currents. Thus we must keep in mind when reading these averages that during a full moon or new moon we can expect the peak max flow to be about 20% larger than we read on this ENC object. In principle this is not the case when viewing this object on the charts of other nations—with Canada being an exception for adjacent waters, because of our historic sharing of this data.

The second, more important point to keep in mind is these predictions on US and adjacent Canadian charts are at present not the latest values available—so the distinction between full average and spring averages is not really significant. It appears there is some internal NOAA communications catch up called for. One division makes the charts and codes the data we read here, but it is another division that makes the tide and current predictions that the chart makers depend upon. 

All of the related NOAA divisions are undergoing major changes these days—not to mention that they are also mostly working from home—so it is not too much a surprise that we experience a transition here, and indeed this is not a serious issue. But somethings can and likely will change soon with the ENC. The blue link in the ENC cursor pick report shown above, for example, brings up the text message shown, which refers to an official NOAA book that no longer exists. The last issue of that book was 2020. The new reference should be something like "For exact predictions see" — or for completeness, there may in fact be 2021 books with a similar title that are no longer approved by NOAA that include the old 2020 corrections.

One of the reasons given by NOAA for discontinuing the annual printed current tables is the increasing use of digital sources of these predictions based on the harmonic constants that can be used to compute the predictions. And, indeed, NOAA makes these constants available to the public. The navigation program we use, qtVlm, is just one example of how convenient it is to get current data at any time, now or into the future. We also illustrate how you can confirm that the harmonics you have are valid.

Historically, the TS_FEB objects were placed on the RNC and ENC at each of the locations of the reference stations (now called harmonic stations)  and  subordinate stations. But over the years, NOAA has discontinued some of these stations, but some TS_FEB objects are still there, even through there are are no longer any official predictions for those locations.

It seems that the US ENC chart makers are aware of these issues and the role of the TS_FEB object is likely in a state of flux. For example, if we look at all US charts in the Eastern Strait of Juan de Fuca that are north of the ENC cell boundary at 48º 20' N, there were at one time about 100 of these objects. And indeed if you look at the latest Canadian charts that cover this region they are all still there, using the original US tidal data. But the US has removed all of these objects from the ENC in that region, in part, I suspect, because the data are no longer the latest values.

To be more specific, NOAA has current predictions at multiple depths, and we cannot just say "take the one closest to the surface," because at some stations there are multiple forecasts within 30 ft of the surface. In short, we have to look now to see what might be best. 

Also there is the much more practical point that many of the currents shown at the TS_FEB locations are not purely reversing currents as might be implied by just two arrows. Newer NOAA data shows that many of the stations are more rotary than thought some years ago. A pure reversing current has just two directions, with speed diminishing to zero as it changes from one to the other, whereas as a pure rotary current does not change speed, but just rotates from flood to ebb direction. Real currents are a  combination of both, showing an elliptical pattern in the plot of the current vector—typically in between pure rotary (ellipse pulled out into a circle) or pure reversing (ellipse squashed flat into a line). 

From our text Inland and Coastal Navigation. As a rule, coastal currents are more rotary than inland, but there are notable exceptions.

In short, these TS_FEB data in the region of our training charts (Eastern Strait of Juan de Fuca) can serve as a quick guide to current flow at peak strength, but the values shown may not reflect the latest knowledge. A couple examples are below.


1. From cursor pick of the ENC symbol

2. Data from the last edition of these books. These data will become increasingly less dependable as NOAA updates their stations with new measurements. 

3. We get the average directions from a stations daily report; and then we figure the average speeds by downloading the full year of data and averaging the year's ebb and flood speeds in a spreadsheet. (We do not know where the average speeds are given, now that Table 2 is no longer supported.)

4. This is one of the objects removed from US charts, but still appearing on Canadian charts.

5. This is a random check of one station on the East Coast, using the Atlantic 2020 Table 2 .

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