Issues with tide tables

jse · Joined: 17 Apr 1995 Posts: 1460 Location: Maui

Hi,

Maybe this has been covered before, but I noticed today that all the online tide charts, plus some of the printed tide charts disagree with the iWindsurf charts by an hour. This is at the Golden Gate station. Is this a daylight savings correction? I bring this up because the other day I drove to Treasure Island expecting to be able to ride the ebb out to the windline, only to see it was flooding an hour earlier than expected.

Also, one more thing. Can the tide tables on the web page be reformatted so the ebb, flood, slack, hi and low don't collide with the time table, rendering them unreadable?

Steve

victor · Joined: 03 Aug 1998 Posts: 581

ive seen a two hour difference in the tables and actual tides happening. unfortuantely, the bay doesn't read the tide tables. at any given moment it can be flooding on one side, ebbing in the middle and slack somewhere else.

also, the tide tables read fine on my system.

jse · Joined: 17 Apr 1995 Posts: 1460 Location: Maui

Victor,

Have a look at Crissy, Golden Gate or Sherman Marker 14. Then check out the currents below the tide table. That's where the fields overlay. Also, I'm comparing the tide at the Golden Gate on iWindsurf with other websites or publications at the Golden Gate. Tides will vary from site to site, but for the same site, all publications should be in agreement.

Steve

bil7y · Joined: 17 Dec 2002 Posts: 349

on them. If you follow the tables on the websites you can see they are frequently updated mainly because they are always wrong. Use your eyes check the buoys and always pay attention to the water. The tables are so wrong its not even funny. I just gave up on them

SWE106 · Joined: 14 Feb 2005 Posts: 264 Location: San Mateo

interesting, tides can be predicted 100yrs in advance with 99.9% accuracy, so how difficult should it be? I and others have noticed that PA (Palo Alto) often has a high tide before 3rd Ave has... that's physically impossible!

Use NWS' tide charts they are (more) correct: http://www.wunderground.com/MAR/PZ/530.html

pthorner · Joined: 29 Mar 1994 Posts: 164

The important thing to understand is that the tides are predictable but the currents are not. It is common to have a counter current that runs opposite of the main tide close to the shoreline. I have made it back up to Crissy East Beach from the St. Francis during a flood in sub planing winds by just tacking back and forth and letting the counter current pull me upwind. At TI you get counter current as well during a significant portion of the cycle and the sewer plant crew can tell you better than any chart when it works best. When a lot off fresh water comes down from the Sacramento River, it can even be ebbing and flooding in the same place with fresh water going out under the salt water that is coming in (which is the part you care about unless you are 10-20 under water). How the currents work follows general patterns, but it always going to be a bit different every day, so be ready be flexible and wear a warm wetsuit.

Peter

beallmd · Joined: 10 May 1998 Posts: 1154

Peter; great answer. I have always wondered about that, as have most above too. It does make sense although it is a bit tricky. Now can you run for Governor and straighten Cal out...

girsang · Joined: 16 Jun 2000 Posts: 52 Location: San Francisco

It's my understanding that the tide charts are based on astronomical factors and don't take into account the amount of rainfall for the season. Is that correct?

It's been my observation that after big snow years, the ebbs are stronger and the floods are weaker. And for the past two weak snow years, I've felt that the ebbs are weaker and the floods are stronger.

This has been my observation and it would seem to make sense that the more water the basin catches, the more we will have flowing out of the gate.

Does anyone have any science to add to this?

I sail Crissy almost exclusively and I imagine Crissy would be affected the most by this.

pthorner · Joined: 29 Mar 1994 Posts: 164

mac · Posted: Tue May 27, 2008 3:57 pm Post subject:

All written so far is essentially correct. There is an assumption in the tide table generation about delta outflow (river flow), and more outflow means greater (longer and stronger) ebbs. After a particularly rainy year, it almost never flooded at the gate. A lot of freshwater changes the dynamic in another way; since freshwater is lighter than saltwater, and rides on top. To amplify Peter's point about higher tide levels. Big changes in atmospheric pressure or wind also change water elevations. If there is a big low, it means less atmospheric weight on the water, and the water level is a bit higher. If there is a lot of freshwater, less dense, a slight increase in water level. Finally, if there is a big wind field (major Pacific Storm coming from the deep Pacific), the wind presses the water against the shore faster than it can run back. In the case of the big 1987 series of storms, this anomoly was as great as 3 feet! Combined with storms that happened at annual high tides, and 30 foot deepwater waves, lots of coastal damage. A typical El Nino means about a half a foot.