# How much can the shape of the oceans affect the tides (on my earth-like planet)?

I have been concerned about the effect of tidal forces on my earth-like moon (0.7 earth masses and 0.88 earth radii), specifically given that because of the primary's mass (3 Jupiters, 1.04 Jupiter radii, at a distance of 1003917 kms) there would be hundreds of meters high or even kilometrical waves assuming earth-like oceans. Recently however I learned that there are several factors that can affect the tides besides the masses and gravity involved, with the main one being the shape and topology of the oceans.

My main questions are: how much does the shape of an ocean on an earth-like planet actually affect the effective height/strenght of the tides? Could it make the tides more manageable with increases of tens of meters rather than hundreds?

And to evaluate an idea that I had already, what if the oceans of my world weren't as contingous as earth's? What if they resembled more interconnected seas, resembling more the topography of south-australia, the carribeans or the Melanesia? They'd also be shallower on average (about 2 kms), would that help?

As illustrated here:

*Note that the moon isn't properly tidally locked, it's in a 5/2 spin orbit resonance.

• The "astronomical" tidal range on Earth, that is, the tidal range in the middle of a notional ocean spanning the world, is about 0.6 meters (2 feet).

• Of course, near the coast this gets amplified by decreasing depth, so that the "default" tidal range on the coast is about 1 to 1.2 meters (3 or 4 feet).

• Note that in most places on the coast of the ocean the tidal range will be much larger that what the astronomical calculations will give you.
• The real tidal amplitude on various coasts varies between just about zero (for example, throughout most of the Mediterranean and the Black Sea) to about 16 meters (52 feet).

• But for example in the supposedly tideless Mediterranean there are places, such as the Gulf of Sidre (known as Syrtis Major in the days of the Roman Empire) or the port of Gabes, where tides are very noticeable, with tidal range reaching about 2 meters (6 or 7 feet), whereas in most places around that sea the tidal range is less than half a meter or one foot.

For illustration, here is a map of the tidal range on Earth taking into consideration only the tidal force of the Moon:

(Map created by the (United States) National Aeronautics and Space Administration. Public domain. Available on Wikimedia.)

• That is a trip. I had no idea the force of the moon was so heterogenous. I am going to need to read more on that. Sep 7, 2021 at 14:53
• @JuimyTheHyena: Doesn't the answer say that the actual tidal range as observed at various points on the coast varies between just about zero and 15 meters, whereas the theoretical tidal amplitude in an uniform ocean is about 0.6 meters? This is how much local factors can affect the tides -- from making them insignificant to massively amplifying them. (The area and depth of the basin only count if they are very small; if the basin is reasonably large and reasonably deep then the shape of the basin is vastly more important.) Sep 7, 2021 at 18:02
• Hydrodynamic person here. Local factors are very important in tide and coastal current calculations. Coastal shape, underlying bathymetry and even the wind fetch (stretch of water the wind has to flow over without land/friction interruption) all play a dominant role in calculating local conditions. Unfortunately you can't say that all funnel shaped sunken valleys will always cause high tides if the local current and winds don't reinforce that condition. It's very much a complex scenario of factors that converge and diverge creating the final "tide" that you experience. Sep 7, 2021 at 22:11
• +1. A good example to work backwards from is to look at the "bay of fundy" with some of the highest tides. Look at the shape characteristic and then look at other regions of the world with matching valley shapes and investigate why those matching shapes don't return the same high tides! Then look at other regions with high tides that have different shape-characteristics and look at their similar yet different counterparts. It's not the same reason for all situations... it does mean work for you, but I don't know of a simple summary wiki that sets it all out in a simple Venn diagram :) Sep 7, 2021 at 22:18
• @EveryBitHelps: Are the comments intended for me or for the original poster of the question? (With no @ tag, they only pop up for my attention.) Sep 7, 2021 at 22:20

So the tides will not only be effected by the planet but also whatever star that planet is orbiting, you'll need to know how big that is and it's tidal effect to estimate your actual tidal regime. It is better to think of the tides as giant standing waves that the world you are calculating for moves through rather than the water moving around the the world. As a general rule the tidal range of deeper, and/or smaller bodies of water is lower than that of larger/shallower basins. A world of deep lakes will have very little tidal variation on those bodies of water compared to a world of wide shallow seas. Local variations, i.e. bay tides, are governed by extremely complex interactions between the size and shape of the bay and their distance from the closest amphidromic point. Artifexian has a good, if quite simple, video about tides here that sketches planetary tidal ranges and the factors effecting the local ranges.

You'll also need to take into account the effect of rotation speed on the depth differences between equatorial and polar oceans.

The relative distances and positions of the sun, moon and Earth all affect the size and magnitude of the Earth's two tidal bulges. At a smaller scale, the magnitude of tides can be strongly influenced by the shape of the shoreline. The distortion of water and earth that we call a "tidal bulge" is the result of deformation of earth and water materials at different places on earth in response to the combined gravitational effects of moon and sun. Because the Earth's surface is not uniform, tides do not follow the same patterns in all places. The shape of a seacoast and the shape of the ocean floor both make a difference in the range and frequency of the tides. In a confined area, such as a narrow, rocky inlet or bay, the tidal range could be many meters. The lowest tides are found in enclosed seas like the Mediterranean or the Baltic. They rise about 30 centimeters (about a foot). The largest tidal range is found in the Bay of Fundy, Canada. There, the tides rise and fall almost 17 meters (56 feet).

• Whilst quite interesting, and the point about the shape of the sea-floor is well observed, this doesn't answer the particulars of the question. You know this is a site for the creation of fictional worlds right? Sep 8, 2021 at 8:05