Lower wall at Angel Stadium will boost HRs

You can expect to see a nice increase in home runs in Anaheim in 2018, and it's not just because the Angels have added Shohei Ohtani, Ian Kinsler and Zack Cozart to Mike Trout, Justin Upton and Albert Pujols. It's because Angel Stadium is simply different than it was. It's now easier than ever to go deep at the Big A.
That's due to last week's announcement that the team would be lowering the height of the home run boundary in right field by 10 feet, cutting it by more than half from the previous 18 feet. Now, most of the outfield wall will be a consistent eight feet around, except for the small sections near the foul poles where it dips to five feet.
Now, let's clarify something about that: The wall itself isn't actually being cut down. It will still stand 18 feet, but the yellow line indicating the home run boundary will shift from above the out-of-town scoreboard to below it, just above the green padded wall. 
The question, then, is how many more homers the change will create, and whether it will benefit or hinder the Angels. By looking at the 3D trajectories of batted balls provided by Statcast™, and noting where they impacted the wall, we might be able to help answer those questions.
Based on the hundreds of batted balls we saw at Angel Stadium over the past two seasons, we found some extremely consistent results. In 2016, we might have seen 16 more homers in Anaheim, or an 8.5 percent increase on the 187 homers hit in the park. In '17, we might have seen 17 additional homers on the 202 homers hit there, a rise of an identical 8.5 percent. That's consistent. Consistency is good.
In the graphic below, every green dot represents a batted ball from the past two years that was not a home run then, but would be with the new dimensions.

This is more an educated guess than a scientific certainty, to be fair. Batted ball tracking isn't yet accurate down to the inch, and some plays are judgement calls as to whether it would have hit exactly on the line or just above or below it. But 8.5 percent sounds perfect, because it's consistent across the two years, and it aligns with the "seven to nine percent" estimated by MLB.com senior data architect Tom Tango as well as the eight percent estimate from ESPN fantasy expert Derek Carty. We'll go with it.
That means that a ball like this Luis Valbuena double from last year probably leaves the yard, as it were.

It means that when Keynan Middleton threw the first pitch of his Major League career last May, he might have watched Carlos Beltrán circle the bases, rather than cruising into second with a double.

This is what 8.5 percent more home runs in 2016 actually looks like, for what it's worth. It's relatively evenly split, nine for the Angels and seven against them, with no single pitcher allowing more than one probable new home run.

It might have especially benefited right fielder Kole Calhoun, who hit 18 that year, but might have had four more. Over the past two seasons, only nine lefty hitters have hit more balls in the air to their pull side than Calhoun has, making the lower boundary an enticing target.

In 2017, the results were similar, although it's worth noting that the Angels' offense could use all the help they can get; no American League team had a lower home slugging percentage than their .406, and only Boston had fewer home field home runs. Last year, the results weren't in the team's favor, as it looks like Angels pitchers would have allowed 11 more homers and their batters gaining only five -- two more from Calhoun.

As we said, some of this gets into a little bit of guesswork. When Jackie Bradley Jr. made this phenomenal catch to rob Yunel Escobar last season, was he robbing a double? Or a home run? How would it have played out with a lesser fielder than Bradley there? (The Catch Probability on that ball was a mere 7 percent, which includes our soon-to-be-unveiled update to account for the wall.)  

What about one of the most controversial plays of the year, when Calhoun appeared to have robbed Aaron Hicks to end a game in June? After a review showed that the ball had scraped the wall before Calhoun caught it, Hicks was given a double. The Angels still won, but that's a play that would potentially be called a home run in 2018.

There will definitely be more home runs coming, that's for sure. The impact of them may be less than you think, however; depending on how you feel about the Bradley play, 33 of our 34 "probably homer" plays were already hits. Of those 33, almost all (31) were for extra bases. A home run is better than a double, obviously, but it's not converting an out into runs on the board.
The balls will be flying out of Angel Stadium in 2018, because Ohtani, Kinsler and Cozart will bring a lot more power than, say, Ben Revere, Escobar and Cameron Maybin. The talent will be better. But the ballpark will be easier to hit in, too. There's so many reasons we're seeing record-setting home run totals in baseball. The shorter wall in the Angels' home park is another one to add to the list.