Diurnal Tides and All That

I’m confused. Down here on the Gulf Coast of Texas and had been planning a trip closer to Sea Rim State Park for tomorrow and the tide charts were diurnal - one low and one high in 24 hours.

Conditions have caused a change in plans, so now we are headed down to San Luis Pass on the west end of Galveston Isl and there are four tides tomorrow. Looking further back and forward - some days have 2 tides, some 3 and some 4. They are not evenly spaced in other words there is a low tide only 3 1/2 hours after a high tide tomorrow evening. Does it have something to do with the full moon?

One explanation
that I read explained the Texas coastal tides in terms of both a normal astronomical tide and some sort of resonance wave bouncing back and forth between Texas and Florida. This wave was described as similar to the sloshing wave you get in a bath tub if you push water up one end. The northern Gulf (Lousiana, Mississippi, Florida Panhandle) were supposed to be mostly diurnal while this rebounding wave made the Texas and more soutnern Florida Gulf coast tides oscillate between a 6 hr cycle and a 12 hr cycle.

If this is true it might explain why Sabine (more on the east/west running north Gulf coast) would be different from San Louis Pass (on the north/south running part of the coast). I never actually went and checked on the other portions of the Gulf coast to confirm the info, but it did describe the tide changes down here in Corpus Christi quite well.

Guess I should do some comparison of tide predictors on the various sections of the Gulf to see if that explanation really holds up.


diurnal, semi-diurnal, mixed, anomalous
dirunal - 1 High and 1 Low in a 24 hour period

semi-diurnal - 2 Highs and 2 Lows in a 24 hour period

mixed - usually 2 Highs and 2 Lows but the height difference between highs and lows can fluctuate greatly. You might have the first high at 6 feet, the first low at 3 ft, the second high at 4 feet, and the second low at 1 ft.

anomalous - when the tides vary from one of the above patterns usually due to complex coastline, marshes, inlets, or some other geographic feature. Obviously there are exceptions.

Keep in mind that heavy amounts of rain inland, high and low pressure systems, and a bunch of other factors can alter the predicted tide times and heights.

Generally speaking you get diurnal tides from about Cedar Key Florida to the Yucatan Peninsula and mixed tides from Cedar Key to the Upper Keys in Florida.

Thanks Mark
Makes sense and explains the difference. I was surprised at the difference given a relatively short distance between the points.

This east coast boy is used to 2 highs and 2 lows a day; threw me for a whirl.

Regardless, glad to be getting out in the fun stuff tomorrow.

One thing to be aware of on Tx coast
is that the “normal” tide change is so small that in the bays and lagunas it can be completely wiped out by wind and barometric affects. Down the coast in Corpus you often here fishermen talking about “high tides” and “low tides”. Note the plural form used for tide. What they are talking about is that the wind and barometric pressure is causing both the high and low tide to be much higher or lower than predicted. In some cases the actual hide tide level will be lower than the predicted low tide. And of course the actual low tide will be way below predicted. These wind and barometric effects can last for several days in a row so you get a series of tides, both high and low, that are either much lower than predicted (so called “low tides”) or higher than predicted (the “high tides”). I have seen some channels connecting the bay to the Laguna Madre where the current ran in one direction continuously for a couple of day through several cylcles of predicted high and low tides that would normally have reversed the flow.

Hope you are having a good time. Watch out for that high afternoon wind.


San Luis Pass
Wow - what a great day. Tides didn’t matter as it was building from low to high all afternoon. I was really impressed with San Luis pass as a spot to play, at least on a big day like yesterday. Really wide break zone to play it although as seas built more in the afternoon it got tiring to paddle out just because it was so wide. Nice sheltered area to play in on the Bay side and fun wind waves to surf getting back there.

The creme de la creme however was the big swell and breakers on the bar about 1/2 mile offshore. Legit 5 - 6 footers were steady with a few waves bigger than that. The really strong winds that were predicted never materialized, sustained was 10-15mph with gusts a little higher.

Driving in and out, the surf was impressive/intemidating from the road, but the gradual nature of the breaks made paddling out to get to the bigger area manageable. Fun day.