It rained so hard that the water was ankle-deep on the sidewalk.
The heavy rain flooded the sidewalk so much that the water was ankle-deep.
1番目は一番短い言い方ですね。
so (形容詞) that 〇〇と言う文法のパターンですが、
大雨→雨が大きくて(It rained so hard that)足首まで水に浸かるほどになった(the water was ankle-deep)歩道に(on the sidewalk)
2番目は同じ意味ですが、より直訳的ですね。
直訳するとBecause of the heavy rain the sidewalk flooded, and the water was up to ankle-level.みたいな表現になりますが、
短縮するとThe heavy rain flooded the sidewalk so much that the water was ankle-deepになります。
ご参考になれば幸いです。