While it's not spelled out explicitly that you take no damage, the intention seems to be that you take no damage.
You fall 30ft per round, which is about 5ft per second, which is much too slow to be taking falling damage. (Even slower than Feather Fall, which is 60ft/round)
Keep in mind that you're descending at a regular walking pace, and you are free to land on your feet at the end.
Do keep in mind that the "slow fall" feature only works while you are using the flying option of the boots. If you just drop off a cliff without flying, the boots will not protect you.
This method nets you more damage
Falling damage is dealt at the end of a fall.
PHB 183, Falling
At the end of a fall, a creature takes 1d6 bludgeoning damage for every 10 feet it fell, to a maximum of 20d6. The creature lands prone, unless it avoids taking damage from the fall.
So in order for Thunderwave to stop you from taking this damage, it has to end the fall first and then begin a new one, wherein you fall at a height of 15 ft or less. Furthermore, it must end the fall without dealing damage to you.
If you are falling and then are subjected to Thunderwave such that your fall stops and is reversed (ie, you are tossed upwards), you should take falling damage then. And now you begin a new fall from your new height.
For example: you fell from a height of 100 ft. After falling 90 ft, your Cleric (who is on the ground, right at the spot you are going to land on) casts Thunderwave, sending you back 15 ft upwards. Then, your fall must have ended at the moment you were 10 ft away from the ground when you were hit by Thunderwave.
You take 9d6 + 2d8 damage for this. Then you begin a new fall from a height of 25 ft (the 10 ft you were at, plus the 15 ft from TW), dealing you another 2d6 damage when you land.
All in all, this nets you extra damage equal to 1d6 bludgeoning, for the extra 15 ft fall, and 2d8 thunder, from the Thunderwave.
Response to Objections:
Thunderwave causes forced movement, and forced movement doesn't trigger damage effects. Is there a rule that Thunderwave must trigger the damage?
A spell does only what it states it does. Thunderwave does not say it prevents falling damage. Moreover, the saying goes "It's not the fall that hurts you; it’s the sudden stop" which I feel is applicable here. The rules for taking falling damage state that you take damage "at the end of the fall." If Thunderwave causes your fall to end, then you must take damage.
There is no rule that says forced movement never triggers damage.
Is there a rule that the falling counter isn't reset once pushed back up? Take Feather Fall -- you don't take fall damage by just waiting to cast it.
- There are no rules about "resetting the counter of fall damage"; however, Feather Fall specifically negates fall damage.
So going by (2), then if you cast Fly on yourself while mid fall, do you take damage because your fall ended?
- PHB 191, Flying Movement seems to suggest that if you are held aloft by magic, you are not considered falling. A non-falling creature does not take Fall damage. See this answer which discusses this specific objection in more depth.
"At the end of the a fall... the creature lands prone..." Thus the fall ends when the creature lands. That is not to say that the creature didn't fall 100', but that the thunderwave doesn't itself precipitate falling damage
- These are two separate sentences with independent thoughts. More accurately, it's "At the end of the fall, you take damage equal to X" and "If you take damage after falling, you land prone." If Thunderwave causes the fall to end, damage is still taken. But landing becomes inapplicable as you do not land when hit by TW in this way.
Best Answer
Updated Answer - NO DAMAGE!
Crawford confirmed that the wording of Earthbind was "probably too subtle" and that the intent is for no damage.
This makes sense with the use of DESCEND vs FALL.
Below is my original answer which covers some good discussion, so I'm leaving it there.
The plus side is that if you have the time, Earthbind can be used as an Action-Cost Featherfall.
PREVIOUS ANSWER, NOW INCORRECT
No Damage
The language of Earthbind creates a problem in interpretation. The Spell states(from Elemental Evil 17):
What's unclear is what is happening when the speed is reduced to 0 and what "descends at 60 feet per round" means in terms of reaching the ground.
My assessment below is based on a literal RAW reading of the spell against similar abilities and spells along with the general rules of 5e.
Flying Speed Reduced to 0
The PHB, 191 has the rules on Flying Speed:
It is clear here that when a flying creature has it's speed reduced to 0 and has no means of hover/magical flight then they fall.
Unfortunately, the language of Earthbind doesn't allow for Hover/Fly to bypass falling. The spell is actively pulling a creature to the ground.
This leads to the question of the type of descent (controlled vs uncontrolled) and what happens when they reach the ground.
Hitting the Ground
This is the problematic part of the spell due to it's lack of clarity. It doesn't say that there is no damage upon 'impact', but it also doesn't say that it would deliver damage.
What we can do is look at some relevant mechanics
Monk's Slow Fall - PHB 78
and
Feather Fall - PHB 239
and
Wind Walk - PHB 288
The implication of these three is that when a creature falls from a height, it takes damage unless the language specifically states to mitigate it.
Earthbind doesn't have any language to mitigate damage.
Is Descending considered Falling?
Given that the creature has a fly speed of 0, is actively being forced to the ground, and has no way to maintain or prevent hitting the ground (due to speed of 0), and the differences in mitigation language between other 'falling' type spells strongly suggest yes.
However, "Descent" could also be considered Controlled and therefore wouldn't be an impact, but instead be a landing.
The big arrow pointing to "Falling" is mostly because it doesn't state "landing safely" or "takes no falling damage".
Discussion
Clearly, the big assumption made here is that being pulled to the ground is the same thing as falling. Given the language of the spell and it's harmful nature, it is not a wrong assumption to make. But because it doesn't clearly state that is the case, it does allow for the interpretation that the creature isn't Falling and would just be placed on the ground. If your DM rules as such, it is a reasonable call to make - but it is worth having a discussion about the reasoning as other spell/ability language seems to suggest otherwise.
We've also been given an unofficial 'window' into the design intent of Mike Mearls. Please note that, unlike Jeremy Crawford, his tweets are not considered Official by Wizards of the Coast.
Mike Mearls, via Twitter
The implication is that most creatures will take damage upon reaching the ground. Hovering or creatures who can magically Fly would normally resist "falling", but that active pull of Earthbind prevents them from doing so and drags them to the earth, kicking and screaming until impact.
However, allowing creatures who have Magical Flight or Hover abilities (not just a flight speed) to arrive safely would be reasonable as well.
Falling Damage
Falling Damage is listed in the PHB 183.
Calculating how much damage may be confusing. When doing so, there are a couple of factors to keep in mind. You can look at the total distance the creature was pulled, or you could do just the final distance in the turn.
5e doesn't have a mechanic to calculate velocity, the damage is purely based on distance.
The Simplicity of 5e vs the Complexity of the Real World
It's important to note that 5e is not a simulationist RPG and that the ruleset is purposefully simplified. Trying to integrate and rationalize real-world physics to 5e rules simply doesn't work. It doesn't mean that you can't adjust your damage totals at your table, but the simplicity often means exactly what it says in the book. In this case, the creature has dropped X feet from point of spell to ground and for every 10' of descent they get 1d6 bludgeoning damage. Falling damage in 5e is calculated based on distance, not on speed.
The designers made things simple. But doing so means the rules often don't mesh with the real world. Adjusting at the table is fine, but then that's a houserule to add a complexity that doesn't exist in the base rules.
Feather Fall (1st level) vs Earthbind(2nd Level)
Another thing to consider is that if you do rule that there is no damage upon reaching the ground, then this becomes a Feather Fall Variant. It does all the same things, except it's an action not a reaction, requires a save, and it can drag someone down rather than simply soften their landing. If you've got the time (1 action) before you hit (if you're high enough), you can cast this on yourself/someone to act like a featherfall. I'm not sure that's the intent, but it's another way to use it IF you rule no damage.